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Aluminum Weldment Design Help
4

Aluminum Weldment Design Help

Aluminum Weldment Design Help

(OP)
I have recently moved from a mechanical design role into engineering at my small R&D company figured I would try reaching out to you folks for some help and direction as I do not have other colleges to consult with. I am looking to design a weldment between two 6061-T6 components supporting a very strong solenoid magnet. This had been a bolted connection but the bolts failed after the system was overloaded and now we would like to weld them together for a stronger, permanent connection. My thought was to chamfer the parts to make for a groove weld to hold them together but I am mostly unfamiliar with weldment design.

From what I have seen it looks like to be conservative I can expect the welded area to behave like 6061-O which is dramatically weaker. There is an axial magnetic force of about 250 kN downward and an outward radial force that is still being evaluated, for now I am treating it as 0.5 MPa acting on the inside wall of the support. Where should I start with evaluating the stress experienced at the weld and how it will behave? I have attached some snips of a simplified model for discussion.




RE: Aluminum Weldment Design Help

Hi Andre3

I assume you are hoping to weld around the perimeter where the two chamfers meet ( bottom pic) left hand side.
Well the problem I see is what about the distortion that the welding might cause due to the heat input and in addition will the heat do any damage to the solenoid magnet inserted within the enclosure, from the pics it appears you can only insert the magnet prior to welding.
It might help if you can tell us in detail how the bolts failed as we might be able to help keep the bolted cover if the points I have made above are a concern.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
Yes that was my thought with the weld geometry and I am definitely concerned with distortion and damage to the coil. We work with a good welding shop that I was hoping would weigh in on the likelihood of distortion of the cover and their ability to keep the coil cool while welding (going slow?).

There were many things at play leading to the failure of the 718 bolts that were used and I have not been able to sort it all out. The system has two of these magnet assemblies mirroring each other, the attraction force between them is about 250 kN at full current. During testing the structure failed well below its limit when the attractive force would be around 130 kN. After disassembly it was found that all of the bolts on one magnet assembly ruptured at the joint, and on the other magnet assembly half of the bolts ruptured at the joint and half pulled the helicoils out of the aluminum structure.

Some compounding issues:

The bolt preload was not controlled, tightened by hand.
Differential thermal contraction further reduced the preload.
The magnet assembly with failed helicoils had improperly short helicoils installed.
misalignment of the magnets could have been present and adding shear force to the joint.
The bolts had been reused from prior testing.
The magnitude of the radial force of the coil on the structure was not known.

The bolts should have been extremely strong, my attempts to analyze the joint still do not explain the failure at a relatively small load. We need to be completely confident in the strength of the joint moving forward so welding seemed to be the way to go.

RE: Aluminum Weldment Design Help

(OP)
@dvd Thank you, that will be useful

RE: Aluminum Weldment Design Help

Hi Andre3

A couple of things you mention Differential Thermal Contraction? i thought it would have been Differential Thermal Expansion?. I envisaged the bolt preload being to low and the solenoid cover forcing the bolts to fail as the thing heated up.
Also you mention the bolts being extremely strong however you had failed helicoils which might suggest the threads machined into the aluminium for the helicoil installation may have failed under the combined loads as the magnets got hotter.
Just my thoughts.

Coming back to the welding I guess once those magnets are welded in there is no requirement to ever get them out?
Just looking at the section view again though there appears to be a small gap at the top between the magnet and the bolted cover so unless that gap was taken up be the coil expanding I don't see how those bolts would get loaded up or am I missing something?
I always think that when something as failed you need to understand why it did so before implementing another solution because even if you weld this cover as the weld been sized for the stresses it might see?
If you have differential thermal stresses on the weld it still might fail.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
The magnets are conductively cooled to about 3K and don’t exceed 6K during operation. Questionable threads are definitely a possibility in contributing to the failure. I would really like to understand the failure better but given all of the factors it seems like a definitive quantitative answer is out of my reach. Is it possible for the bolts to be unevenly loaded enough that that quickly ‘unzip’, failing one at a time from taking a large percentage of the total load?

The magnets will not need to be removed so welding is okay there. The cover is fixed in place by more structure that is not shown and the coil is applying a force downward, putting the bolts in tension. There is a g10 spacer on top of the coil filling that gap also but it is not shown in that simple model.

RE: Aluminum Weldment Design Help

Without making any other changes the resilience of the joint could be greatly improved by drilling the bolt holes nearly through the body of the magnet and only threading the bottom. this will substantially increase the grip length of the bolt and reduce its sensitivity changes in tension due to differential thermal expansion. It's also a good practice to put a register or dowel pins to precisely locate components.

RE: Aluminum Weldment Design Help

Just recently looked at a magnetic attraction issue on a smaller scale - the magnetic force and relatively small mass result in a big accelerations/decelerations. Is there kinetic energy being absorbed, or is loading only due to forces?

RE: Aluminum Weldment Design Help

(OP)
@Tugboateng this is very interesting, I would think that would reduce the stiffness of the bolts. I will have to think more about it to understand how that would help. I definitely would add pins if we go to using bolts again, it would help with the shear force also.

@dvd the magnet does not move and is ramped very slowly, I have thought of it as quasi static forces.

RE: Aluminum Weldment Design Help

If you go the pin route hollow pins on existing bolt holes would have little effect on design. A register would provide the best shear resistance, though.

Think of the bolt as a spring. Your differential expansion is fixed. The bolt will see the same change in length whether it's long or short. However, with a longer bolt the change in strain will be less as increasing the length is in the denominator of the strain unit.

RE: Aluminum Weldment Design Help

I hadn’t realised the coils were cooled so low, why so low assuming Have read your post correctly 3K is according to my Celsius scale -270deg C is that correct?
I see why you said thermal contraction contraction now (lol). Yes it is very likely the bolts could be unevenly loaded and fail one at a time, I saw this happen once on a crane cab slewing gear where 48 bolts unzipped, I didn’t believe it at first but I saw all the evidence.
If the bolts weren’t controlled when tightened to achieve an equal preload then that could be a source of the problem and I have to say torquing the bolts would still produce a very uneven preload in each bolt because torquing bolts can be inaccurate by as much as +/- 25%.
The other thing here at such low temperatures what about the material properties? I imagine that some materials in the assembly might go brittle at the temperatures you mention.
Something else with the welding is whether it needs stress relieving are not to remove residual stresses from the weld as these we only add to any mechanical stress the unit sees in service.

I think we need to understand more about the operation of this magnet, it’s unlikely you will get the best answers if there is information we are not aware of like the the temperature you have brought to our attention.
I agree with TugboatEng using longer bolts will help but also I would take the bolt hole and tapped hole through the base and not leave it a blind tapped hole, the reason I say this is that it will reduce the stress concentration factor at the bottom of the threaded hole which you would normally get with a threaded blind hole, however that can come later I think you need to understand the failure more first.👍

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Quote (Andre3)

The bolt preload was not controlled, tightened by hand

This is a major problem. Before you go and redesign something that may not need it, you should run any quality control problems in the manufacturing process all the way to ground.

RE: Aluminum Weldment Design Help

Bolted/flanged connections are usually avoided when possible in cryogenic systems when using permanent connections; take a look at any tank fill connection flange or 4 bolt Mueller flange in cryo service, and they will usually leak over time from thermal cycling. Also most manufacturer recommendations are to slightly re-torque when assembly is cold.

Now to your problem. At 2-3 Kelvin I'm guessing you are working on a superconducting magnet in the vicinity of a bath of helium with subatmospheric pressures. At these ultra-low temperatures, you may have to use invar material with the bolted assembly to counteract thermal loss of fastener preload. Just a thought.

----------------------------------
Not making a decision is a decision in itself

RE: Aluminum Weldment Design Help

Are the housing parts too large to thread one in to the other?

Ted

RE: Aluminum Weldment Design Help

(OP)
Thank you for the replies, let me give some more information.

Yes these magnets operate at 3-6 Kelvin and are cooled conductively (cryogen-free) using GM cryocoolers. The entire system is contained in a vacuum chamber and takes serval man weeks to assemble. While it is operating we have have temperature data at select locations, voltage taps, and external magnetic field measurements, otherwise we do not have feedback on what is going on inside the chambers, especially structurally. This is all for a one of kind prototype machine. Future iterations of the design will be carefully thought out for manufacturability, reliability, etc. but for now the goal is to get the magnet together, ideally very overbuilt in this problem area, and functioning so the rest of the systems can be tested.

The bolts used were Inconel 718 which remains ductile and has been used for structural components in other cryogenic systems. How a welded 6061-T6 joint will perform I am less certain of. I would prefer to chase down the exact cause of the bolt failure but I don't see how a solution could be found with the number of unknowns. If anyone has ideas on how to better asses the failure I would like to know your thoughts. What I already know:
There are no signs of fatigue on the fractured bolts.
The surviving bolts were tensile tested on our Instron to failure reaching a peak load of 27.3 kN, which is higher than the nominal ultimate strength of the M6 bolts.
The attractive force between the coils could not have exceeded 200 kN at any time by geometric constraints.

Silly things like not controlling the preload and reusing bolts I did not have a part in and wont happen again obviously.

If the strength can be be massively improved by welding then that is the way we should go to keep the project moving. I prefer bolts so the joint is not permanent and I don't need to worry about but it is not a necessity. That leaves the question that I am trying to answer to be; How strong would a welded joint like the one shown be? Maybe the better solution would be to size up the bolts and make them long? I am going to take a look at that as well.

RE: Aluminum Weldment Design Help

(OP)

@hydtools I like that idea but I don't think it would be practical in this case.

RE: Aluminum Weldment Design Help

Andre3,

I looked very quickly at dvd's link, and it looks very useful.

When I consider designing an aluminium weldment, I assume that the welds will anneal heat treated material, making it weaker, and that the welding will cause distortion. Any critical machining must be done after welding, stress relieving, and heat treating. Soft aluminium probably is difficult to machine.

If you are bolting this together and running it at 3°K, you need to manage differential thermal contraction. You could use a stack of belleville washers to maintain an acceptable contact force, or you could use aluminium bolts.

--
JHG

RE: Aluminum Weldment Design Help

Use through bolts with nuts in tierod fashion.

Ted

RE: Aluminum Weldment Design Help

Hi Andre3

If the solenoid container is made solely from the Aluminium alloy then the weld will only have the same region of strength as the parent metal,so you still need to evaluate the stresses in service and not to mention distortion post welding with solenoid in situ and or any effects of post weld stress relieving . I can’t find any information on the coefficients of linear expansion for the Aluminium alloy or the Inconel bolts at the temperatures mentioned in your design but I did find a minimum working temperature of -250 degrees C for Inconel bolts and from the information you gave above the design exceeds that limit at 270 degrees C.
The bolt failures, some of which you informed us failed at the cover interface, I now believe failed in shear due to relative movement between the housing cover and body,due to differential thermal expansion on the diameter of the housing. Do you have any of the bolts? If so can you take some photographs of the failed bolts and post on here?
I can’t really comment on the bolt failures you mentioned due to incorrect helicoil installation and I am puzzled why tensile tests were done on the remaining bolts as I don’t now believe they failed in tension unless you were looking for changes in mechanical property of the bolts or testing to ensure you had what you paid for.
Below is a link for pictures of bolts failed in shear which might help https://www.onallcylinders.com/2014/03/13/diagnose...

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
Here are some images of the bolts:



This is what the they looked like during disassembly:


you can see on average they broke very close to the joint interface.

This also shows the steel flat and lock washer that have no place being there.

@hydtools would long bolts and nuts increase the strength?

RE: Aluminum Weldment Design Help

Looks like tensile failure through the thread root.

Ted

RE: Aluminum Weldment Design Help

At those temperatures, differential shrinkage between the bolt material and the joining parts may have created enough tensile strain to break the bolts. Do the the bolts and the other parts ahve the same reaction to your temperatures? A more basic question: are the bolts suitable for such temperatures or are they embrittled? Did they break as the temperature was decreasing or as it was increasing? Do the joining parts "feel" the temperature swings before or fater the bolts?

RE: Aluminum Weldment Design Help

It certainly looks like the aluminum shrank more than the magnet material putting the bolts in tension. I expect that the contraction is not linear for any of the materials involved, but what is the CTE for the magnet and how long it? Also to keep in mind, the aluminum cover is also shrinking radially without constraint of the magnet while the shell is constrained by the magnet - this adds a shear load as well.

RE: Aluminum Weldment Design Help

Hi Andre

Thanks for the photographs, those bolt failures do look brittle and did you get any documentation with the bolts because Understand Inconel bolts can suffer hydrogen embrittlement.In addition my last post I mentioned a working temperature limit of -250 and not -270 degrees C which might not help your cause.
However coming back to the bolt failures if the Aluminium as a higher coefficient of linear expansion than that of the bolts, I would have expected the bolt preload to reduce whilst cooling and thereby reduce the tensile stress in the bolts,in line with a comment you yourself made on your post dated the 18/12/20. Other than the mass of the solenoid what other forces could induce tensile stress on the bolts? You mention the force of attraction of 250kN but I cannot visualise the set up. I suspect that the bolts have failed in shear due to differential thermal movement between the cover and the container.
Through bolts with nuts are usually better for a joint in my opinion, particularly if there is cycling tensile stresses but if those bolts are failing in shear then I don’t see how they would help in this case, of course this is only my opinion and maybe you should get those bolts checked at a laboratory as they will give you a much better answer.


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Is it necessary to fully contain the magnet? Would it suffice to have end plates and long screws threaded into the opposite plate. The screws would have thermal characteristics similar to the magnet and no interfering aluminum.

Ted

RE: Aluminum Weldment Design Help

(OP)
Thanks everyone for the input.

The operation of the magnet starts with cooling everything down to its base temperature, around 3K, over ~30 hours. Then current is slowly supplied causing the magnetic field to rise. The temperature of the magnet also rises during operation but only slightly, never exceeding 6K, which negligible effects compared to the magnetic forces. The coil is wound and impregnated with epoxy, axially it contracts more than the aluminum and radially it contracts less. At room temperature there is a gap between the structure and the coil which closes during cooldown. The aluminum structure contracts more than the Inconel bolts, 0.435% and 0.238% respectively, so the bolts are loosing tension preload as it cools down.

The coils are attracting to each other, that is where the 250 kN axial force comes from. There is also radial force outward on the structure, the coil wants to expand, but this is more tricky to evaluate, I am trying to sort that out now.


I don't have anything that came with the bolts but I have reached out to the manufacturer to see what they say. This material is advertised as remaining ductile at cryogenic temperatures and has been used in other structural applications at low temperature so that doesn't feel like the solution to me. The bolts tested in tension to failure look very similar to ones that failed in service:



RE: Aluminum Weldment Design Help

(OP)
@htdtools yes we coil needs to be contained radially and the structure helps with cooling.

RE: Aluminum Weldment Design Help

28 bolts ? ~ 2000 lbs per bolt axial load?
What is the estimated "overload" load. tTere's your target for bolt preload at operating conditions
What size bolts ?
Are the axial and radial loads DC or AC ?

The attached image shows the cover with a close fitting ( nominal .002" Ø clearance for assembly ) pilot to resist the radial load applied to the cylinder.
Note there is only one mating face on the cover an cylinder.
Is there some kind of a gasket (indium) or sealant used ?


The bolt holes are much deeper for longer bolts. Bolts with grip length >7X the diameter would be a nice starting point, but through bolts would eliminate having to tap deep holes.
The bolt holes are counterbored deeply so the bolts don't handle shear loads. The bolts should have solid shanks in hte shear plane anyhow.

RE: Aluminum Weldment Design Help

Hi Andre3

I am studying your latest post and thanks for that, before I respond in any more detail I have several important questions and these are :- are the magnets arranged as shown in your sketch ie one mounted vertically above the other? Are the fasteners in both magnets failing are just the one in the top or bottom position?
Struggling to understand how the solenoid can can contract more in the axial direction but less in the radial direction when compared with the Aluminium?
Can you not just put a bigger clearance radially between the solenoid and aluminium body?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Longer bolts with may stretch more and be a bit more forgiving. Were longer bolts ever used or specified?

RE: Aluminum Weldment Design Help

(OP)
@ Tmoose
The overload would be 300 kN, so you propose that the bolts be preloaded so they will have 10.7 kN of tension when cold and ready to operate?
The bolts are M6x1.0 Inconel 718 age hardened.
The magnet is charged with DC current, I am not sure if that answers your question.

I like the pilot.
There is no seal indium seal there, the entire assembly is in a sealed and evacuated vessel, 10e-6 mbar.
Long bolts have been a common suggestion, it hasn't clicked with me yet what makes them so beneficial. Is it simply that they elongate more at the same load compared to a shorter bolt preload?

@mfgenggear
Good link thanks, they also list Inconel 718 for cryogenic service so that is encouraging.

@ desertfox
Yes the orientation is as shown and they both failed. The bottom assembly had the short helicoils installed on all threaded holes, 14 helicoils ripped out and 14 bolts broke. The top assembly had appropriate helicoils and all 28 bolts broke.

The images of the assemblies I have posted are simplified, the coil winding has spacers on the top and bottom made of composite which contract more than the superconductor-epoxy matrix of the coil.

The contact that the coil makes with the structure is part of the cooling path so we need to be making contact there to get the coil temperature low enough. In the future the cooling can be done differently to avoid the need for the radial contact.

@IFRs
No long bolts have not been tested but seem to be a common theme.

RE: Aluminum Weldment Design Help

I know less than most the point of the link I posted is at that low temperature the steel become very fracture brittle, and loss of physical properties.
the most that a cryogenic steels is rated for is -270 D F, and test have to be run which a charpy destructive test. I don't believe welding the aluminum will solve the problem.
I would suggest a failure analysis by a metlab, and may consider changing material of the bolts.

RE: Aluminum Weldment Design Help

may have to go with larger bolts, to allow for the strength difference.

RE: Aluminum Weldment Design Help

I'm wondering how the CTE for the materials involved were determined for the full range of temperatures seen in this assembly. Likewise how the strain calculations for room temp assembly were translated to operating temperature strains and material strength properties determined for the operating temperature.

How were any calculations done in the absence of this information?

In any case, a small test sample is easy to prepare - block of aluminum with an insert, aluminum block with a hole to match the cover dimennsions, and a bolt torqued as designed. Drop it into liquid helium to reach the operating temperature and see if it breaks the same as seen in the full assembly. If it does then the bolt is too brittle/strain is too much for some reduced strength. If it doesn't, then I'd say the estimate that the coil is shrinking more than the aluminum is wrong, but needs confirmation.

RE: Aluminum Weldment Design Help

(OP)
@ mfgenggear Yes I agree steels are not good for the temperatures we are working with but there is no steel in the assembly aside from the 304 stainless helicoils, the bolts are Inconel 718.

RE: Aluminum Weldment Design Help

(OP)
@3DDave

I was not involved with the design of the system and the designer is gone and with him so are many details of the process so I will do my best.

We have access to temperature dependent material property databases down to cryogenic temperatures such as cryocomp thermal analysis software and other sources that we use. The cryocomp software also helps with quick calculations of things like heat loads and length changes from thermal contraction as a percentage. The materials we use have been defined with these properties in ANSYS and have been used to analyze the system. As far as I can tell the bolts were not analyzed in ANSYS. So there was no consideration for the bolt strain at low temperature, or control of the room temp assembly.

We do not work with cryogens often but have done some thermal shock testing in liquid nitrogen and have a dewar. We could cool a test assembly to liquid nitrogen temperature and try to quickly transfer it to the Instron but I think the sample would heat up considerably before testing started. The thermal expansion curve is not linear at low temperature and is nearly flat for most materials from liquid nitrogen temperature to liquid helium temperature so that may still be useful depending on how warm it gets before the test starts.

RE: Aluminum Weldment Design Help

Andre3, about the benefits of longer bolts. You need to think about the strain in the bolt. The length of the bolt is the denominator in strain so increasing the length of the bolt reduces the strain. In your case you have a specific amount of thermal expansion you must deal with. A longer bolt will see less change in strain over the range of conditions.

RE: Aluminum Weldment Design Help

I think I can tell why the designer is gone - is their supervisor and the design review team also gone?

(fixed typo)

RE: Aluminum Weldment Design Help

Hi Andre3
Have a look at the links below, the first link on page 9 and 10 shows what an Inconel bolt tensile failure looks like. The second link shows the advantage of increasing bolt grip length.

Reading your latest post will respond later.




https://www.researchgate.net/publication/325520569...


https://www.nord-lock.com/insights/bolting-tips/20...

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
@ 3DDave yes and no, we are a very small group so the sr. engineer was almost entirely self regulated. Given the complexity of the system it is impressive how much he got right in the amount of time that it was done. We are changing things moving forward to prevent design misses like this, in the interim I am doing my best to move things forward and develop my own skills.

@ desertfox that is a great paper about failure analysis in general, even better that it discusses 718 bolts/studs. I think I will get some quotes for lab analysis, very valuable information.

RE: Aluminum Weldment Design Help

(OP)
@ TugboatEng The strain will remain fixed for a given force though, right? I can see how the strain would be reduced for a given amount of elongation, but when would that be the case? When we increase the grip length the change in length from thermal contraction will stay proportional so wont the stain remain the same?

RE: Aluminum Weldment Design Help

If the only contribution is the differential between the bolt and the aluminum then that is correct. In that case then the bolt should never break and the short helicoils would not fail.

RE: Aluminum Weldment Design Help

Your force isn't fixed. Your force varies greatly due to the differential thermal expansion. Increasing the length reduces the magnitude of that change in force. The aluminum has a higher CoTE so as it cools the bolts stay long while the aluminum shrinks causing them to lose tension. A longer bolt with more stretch will lose less tension for a given amount of shrinkage in the aluminum. When the bolts become loose they may not share the load evenly. Once the heavily loaded bolt fails, the zippering mentioned earlier begins.

RE: Aluminum Weldment Design Help

(OP)
@TugboatEng The point I am trying to make from what I have learned is that the amount of shrink is defined by the length of the grip. A longer bolt must to pass through more aluminum, the grip length is equal to the bolt length. The amount they both shrink is a fixed proportion, no matter the length, the aluminum will shrink 0.435% and the bolt 0.238%. Maybe it is factoring in the joint stiffness that is getting me confused

RE: Aluminum Weldment Design Help

If the preload is causing the bolt to stretch a finite amount, the longer the bolt the less each unit length of the bolt needs to experience strain. The total strain is the same but for a longer bolt the amount of pain felt is less.

RE: Aluminum Weldment Design Help

Tugboat - I'm talking about the endpoint not changing, not that the force is invariant over temperature. Doubling the length of the bolt doubles the length differential due to temperature change, which doubles the overall stretch but, because it's over double the length, the strain is the same and so is the tension change due to temperature change. This applies in the case where the bulk material is of much larger section and therefore produces a higher "k" than in the bolt, a situation that seems to apply here. It also assumes that the Young's modulus is constant over the range; I think that is not true, but I also think the contribution isn't key to causing this failure.

Typically lengthening the fastener makes the joint more resilient by reducing the spring rate and allowing more vibration/shock energy to be absorbed and offsetting preload loss from wear/embedment, but I think that does not seem to be the underlying problem here.

What I believe is true is that this problem description is missing critical information that is required to evaluate the state of strain over temperature. It should be a simple linear equation. Were I more motivated I'd create a spreadsheet, but it's easy enough no one here should have trouble doing so.

RE: Aluminum Weldment Design Help

Hi Andre3

I’ve been given this problem some thought and maybe you could test this physically at your place of work. If the Aluminium casing contracts sufficiently during cooling then all the bolt preload could be lost and that being the case, or the worst case, the bolts are loose and the casing can float, any external force acting like the magnetic force would transfer all the load onto the Inconel bolts and in addition to the external force the bolts would also experience the mass of the solenoid and the casing. Would it be possible to assemble a solenoid and casing and preload the casing bolts as the design stands at the moment and then check at the lowered temperature whether the bolts are loose or not? I have know idea what the mass of the components are so I might be way off port here. I do agree however that provided the 28 bolts share the 250KN evenly then the bolts should not fail. Contraction wise there is probably more going on in the radial direction than in the direction of the bolt axis and that’s why I think the bolts are shearing due differential contraction radially rather than vertically but additional tensile loading of the bolts will come from the mass of the components which is an unknown to me anyway, so the bolt failures might be a combination of tension and shear.
My bottom line I suppose, is that the bolt ,preloads have to compress the aluminium alloy casing lower than that of the free contraction that the aluminium would under go with the reduction in temperature,otherwise I think the joint will always fail. This is just my thoughts but I might have missed something in my thinking.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Daggumbit, I see my error now. Increasing the grip length increases the expansion. There is no difference in strain between short and long.

Another thought so maybe I can redeem myself, where does the force from the solenoid transfer to the plate? If the force acts in the center that can create a bending force about the bolts. A step ring on the that fits close to the bore would ensure the support is near the bolt circle minimizing the bending. The same step could also carry the shear as another user mentioned.

RE: Aluminum Weldment Design Help

If the bolts completely lose their clamping force, a couple things might happen. You could get some impact loading on the bolts or, due to vibration while cold, the bolts could tighten and then they break while warming up.

RE: Aluminum Weldment Design Help

let me just say it's awesome to be with a group that can help out each other.
If I may say finding the solution is not easy for this problem and it's way over my abilities.
however as a part of the failure analysis I believe a fish chart would be helpful.
to distinguish several issues at hand. It amazes me that that the bolts failed before the aluminum
housing. maybe a few changes may make a difference such as precise holes with shoulder bolts instead of fully threaded.
a more precise fit as not to allow the assembly to cause axial moment.
I sure some of the structural engineers can kick in comments on this. the threads are stress points.
and a fully shouldered bolt wit a slight press or very close slip fit may help.
I suggest maybe do an fea study on different configurations.

RE: Aluminum Weldment Design Help

If the coil temperature rises during operation, will the coil expand axially more than the presumed smaller gap? Thermal expansion of a nearly incompressible material, epoxy and conductors, can generate very high forces. Would this cause the tensile failure of the screws?

Ted

RE: Aluminum Weldment Design Help

Getting back to the original question, is there really any reason to not try welding the pieces together and not have any bolts at all, perhaps some blind pins for alignment?

RE: Aluminum Weldment Design Help

(OP)
@ desertfox the mass is negligible in reference to the magnetic forces, only about 50kg. I wish it were practical to cool the assembly down for testing the bolts but it takes a massive effort to assemble, and once it is together it is sealed inside a welded vacuum chamber so the bolts are not accessible. If we had a suitable container to bathe just the coil and structure with bolts in liquid nitrogen that would be almost as good, I might look into what that would take.

@ The force from the coil is simulated and fed into ANSYS mechanical as a body force density, I am not sure where the resultant is. There is definitely some bending which I think is a weak point of the design. I don’t have enough space in the bore of the coil to add any supports but I think that will be good to do in the future.

Thanks again! I am still going over the responses I will be back later today.

RE: Aluminum Weldment Design Help

Andre3

Presumably, your coils are superconducting. Am puzzled by an early comment you made that the coils expand, that only happens in rare circumstances.

RE: Aluminum Weldment Design Help

Andre3

I have been looking at thermal stresses in bolted joints and I have come to the conclusion that if you want have a bolted joint, then the bolts need a similar coefficient of expansion to that of the clamped components. Because the Aluminium has a much higher coefficient of expansion than that of the bolts, when it is cooled I believe the Aluminium contracts sufficiently to reduce the bolt preload to zero.
I don’t believe the bolts made from inconel can do the job especially with how low you are cooling, I think you need to find another bolt material


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help


Andre3,
you may not have a stress problem at all if the coil supply is improperly designed.

RE: Aluminum Weldment Design Help

(OP)
@ hydtools The temperature of the coil remains approximately constant, it rises less than 3K. There is a radial force that needs to be better understood. I am also leaving the door open to the possibility that what we are using for thermal contraction properties could be incorrect.

@ IFRs I am interested in this as well, I am concerned about the annealed aluminum, cracking, and distortion. If anyone has thoughts I would love to hear them.

@ hacksaw Yes they are superconducting and generate a radial hoop force.

@ desertfox I took another look at the thermal effects of the joint and I think this could be the root cause. My estimates have a preload reduction of 4-5 kN from room temperature which could possibly have allowed for separation of the joint. Add to that brittle bolts and I can imagine the bending and shear they would experience to be enough to cause failure. If anyone has resources on thermal contraction effects on bolted joints I would like to pursue this more.

@ hacksaw what do you mean by coil supply?

I am beginning to think a redesigned bolted joint may be a good solution. Possibly keep the 718 bolts and use washers of low a low CTE material like Invar.

RE: Aluminum Weldment Design Help

Hi. Andre3

Well I haven’t got the values of the expansion coefficient but I believe the Aluminium alloy contracts more than the inconel bolt and you cannot keep the inconel in contact with the aluminium and therefore the bolt preload ends up at zero, you also say the joint separates which in my book means there is no load on the aluminium components.
If you have some calculations could you share them? The inconel bolts should be okay at -250 degrees C but I have no info at the -270 C which is where you are at. (Think I have mentioned this before).
Just out of curiosity what are you preloading the bolts too? If the preload is to high initially then the aluminium alloy will be yielding under the screw head and that will mean that the preload value will not be what you intended but something much less.
When you say that you have a 4-5KN loss of preload at room temperature, I take to mean that using the linear coefficient of expansion value that’s given at room temperature you have a 4-5KN loss of preload.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Is there room for m8 bolts or larger?

Ted

RE: Aluminum Weldment Design Help

Hi Andre3

I am convinced now the problem is caused by the Aluminium clamped parts contracting during cooling to the point were the the inital preload generated by bolts on the clamped parts is reduced to zero because the linear expansion coefficient of the Aluminium alloy is greater than that of the Inconel, at this point the magnetic force is transferred to the bolts but because the load isn't equally shared the bolts fail sequentially. I had a look at the link which I have provided below dealing with compound bars.
http://gbreco.pl/images/Pdf/compositebars.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Since it appears the low operating temperature is causing the bolts to lose tension maybe another approach is to use friction to hold everything together. If a shoulder bolt were installed with a close clearance to the bolt hole and counterbore then as the unit cools the aluminum will shrink and form an interference fit on the shoulder of the bolts. This will also make the entire assembly stronger because it eliminates the diameter reduction from the threads. I don't know that this would work if there were a gasket or seal involved as it still wouldn't be able to maintain compression.

RE: Aluminum Weldment Design Help

(OP)
@ Hydtools I think I might be able to get away with M7 bolts (They will be custom made anyway so an odd size is fine) but since the aluminum will need to be Helicoiled I don't think I would have the thickness in that area to go any larger.

@ desertfox I will make some notes on my estimation of the preload loss from cooldown and post them. The surface of the cover was pretty heavily marred so there was definitely local yielding which is not helping the preload situation. I have to just estimate what the bolts had for preload since they were not controlled at all during installation. I have been using 8 kN as a room temp preload estimate but I will look more closely at that to see if it is reasonable. That pdf looks useful also thank you.

@TugboatEng That is a very interesting idea, I haven't seen it done but I like it. There are some tight clearance applications where this could be very useful. No gaskets or seals to worry about but we need a pretty tight positional tolerance.

RE: Aluminum Weldment Design Help

Hi Andre3

Thanks for your post.
Just an observation but I think the assumed 8kN preload is to low, if there are 26 bolts and you use 8kN preload that only gives a clampdown force of 208kN and the external load is 250kN according to your earlier posts. I estimate the bolts should be able to take a preload of about 30kN before yielding however that’s at room temperature and I haven’t given any consideration of the stress under the bolt head at that load so I don’t know if the aluminium alloy is yielding or not. I might have a look at the bolt head area and see what preload yields the aluminium but I still think you need bolts with a LCE that matches the Aluminium alloy at the required service temperature.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

It does not appear to me that the load path from the magnets goes through the bolts.

RE: Aluminum Weldment Design Help

Hi 3D Dave
where do you see the load path then? If the joint separates at -270 C Which I believe is happening I can’t see where else the load would go? Also if there is no load path through the bolts why are they breaking?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Andre3, for my suggestion to work, you would machine the two pieces together, bore the holes with the lid and body clamped together. This reduces the need to control the bolt hole location tolerance. The two parts will have to be clocked in that they be installed in the same orientation as machined. Think turn of the century precision oarts. A single dowel or some timing marks will facilitate this.

The weakness of my design as I speculate is that as the temperatures drop the aluminum will grip on to the bolt at a certain height and then both parts will shrink towards the center of that friction zone so they will shrink ways from each other. I'd you need the faces to seal this can cause trouble. An Inconel spring gasket with its lower COTE may be able hold a seal as the aluminum shrinks away.

RE: Aluminum Weldment Design Help

desertfox - though I mentioned it earlier, I think that the core is not shrinking as rapidly as the housing, regardless of the information supplied. The bolts are failed in a manner that matches a pure tensile failure, which means not primarily bending, so the lid isn't shrinking radially and bending them. If radial shrink was the only force, it would not pull the short inserts loose and one last bolt would remain unbroken. There is a static magnetic load being applied, as shown in the arrows of the original post. In the latest post he says the cover was heavily marred - a condition that a high bolt tension load would produce.

RE: Aluminum Weldment Design Help

Hi 3DDave

Well I thought at first the bolts were shearing due the shrinkage of the aluminium casing but has you say the bolts appear to be failing in tension and since I realised or believe that the bolts cannot prevent the separation of the joints due to the difference in expansion coefficient,then in my opinion the bolt will see the full magnetic load and possibly fail in tension one by one due to uneven load distribution, a heavy presence on the surface of the aluminium parts could be the bolts were over tightened at room temperature and possibly embedded into the aluminium alloy due to exceeding the aluminium yield stress.
Unfortunately it’s all guess work because we don’t have the expansion coefficient at the cryogenic temperature so my guess work is based on the coefficients at room temperature which would indicate to me that the aluminium alloy contracts and losses contact with the inconel. The magnetic force can then pull tension directly on the bolts and there might be some bending on the bolts which could also be tensile bending.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

It's clamping a magnet made of a material that I expect has a lower CTE than the aluminum. The magnet pulls against the bottom of the bore in the aluminum, according to the initial diagram, so the bolts are not in that load path.

RE: Aluminum Weldment Design Help

On the 22nd of December the OP posted a picture of two opposing magnets pulling at the base of the opposing aluminium housing, if the preload on the bolts is lost due to contraction each housing is being drawn to each other due to the attraction of the energised magnets, now he also stated the lids of the housing were held by brackets not shown but it’s clear to me that under that condition the load from the magnetic field will be transferred to the bolts

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

My understanding of the situation

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

If they failed under magnetic load it would have been an explosion as the two magnets would collide. I've seen the damage a small tool can do near one of these; full magnet release would be catastrophic.

RE: Aluminum Weldment Design Help

The casings are contained within another chamber read OP’s post 24th December, again if the bolts failed in tension and not in the load path then are you saying they failed during tightening? I doubt that is the case. So if it wasn’t during tightening and it wasn’t that the bolts were in the load path, then we’re did the tension come from?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
Yes this was definitely a catastrophic failure, it was quite the event. The magnets are in separate vacuum chambers with an air gap so they did not collide with each other but instead slammed into the bottoms of their chambers. The energy in the coils was quickly dumped into a locomotive resister and amazingly the magnets survived. The heat shields were smashed and they put some good dents in the chambers but otherwise the hardware was alright.

@ desertfox That is the right idea, notes coming on the contraction calculation. Keep in mind we know what the total contraction will be, the aluminum will shrink 0.435% and the bolt 0.238%, which should get around needing the CTE as a function of temperature.

RE: Aluminum Weldment Design Help

Thanks Andre3

That’s a bit more of the puzzle solved. Without the CTE at -250C how do you know the contractions? I guess you are basing it on room temperature CTE.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
@ desertfox I have the data in plots, tables, and software:



If we use the room temp CTE we would over estimate the total contraction quite a bit, almost zero contraction happens below 50 K.

RE: Aluminum Weldment Design Help

" The magnets are in separate vacuum chambers with an air gap so they did not collide with each other but instead slammed into the bottoms of their chambers"

So it wasn't bolts failing one at a time. I knew this was a wild goose chase.

In literature it's called "the unreliable narrator."

RE: Aluminum Weldment Design Help

(OP)
I don't recall claiming the bolts failed one at time other than them possibly rapidly unzipping and I don't see how the magnets being in separate chambers changes the problem I proposed at all. There is a lot going on with the system and I have been trying to keep from overloading with unnecessary information. This is my first post in the forum and I am new in the field, just doing my best and appreciate the help.

RE: Aluminum Weldment Design Help

The unreliable part is failing to provide: an accurate free-body diagram. The fact that the failure was explosive in nature. That the lid was in the load path. The total load capacity of the bolts. These should all be in the first post.

It's fun like a who-done-it mystery to pull these facts out one at a time.

Since there is room for the magnet to move there is a possibility have the bolts loose/ use precision spacers between the lid and the housing and to leave a 2 or 3 mm gap and try again, this time with a suitable spacer under the assembly to prevent more damage if bolts fail. If the bolts don't fail then the estimate of CTE of the magnet is likely wrong.

If the bolts do fail you probably need to find a different material for the bolts or to increase their number - it looks like there is enough room to triple the count.

RE: Aluminum Weldment Design Help

Hi Andre3
Thanks for the information, well we got there in the end, the bolts had to be in the load path for them to fail and I read all your posts very carefully, so eventually we came to the conclusion we agree on. You weren’t involved in the beginning of the design of this device, so I can appreciate that you inherited it and it’s hard after a failure to just become involved, under those circumstances I think you did your best to give as much information as you could. This is your first post to boot, so don’t be put off posting further problems,there are a good bunch of guys here that will help you.
Now the next problem is how to fix it, have you considered putting bolts, dowels at 90 degrees to the current bolts, this would mean putting them in shear but the bolt preload wouldn’t be affected by the contraction..

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Make the cup part with an OD flange. Drop the cup into a hole in a supporting structure. Take the magnet load through the flange. The bolted-on cover then carries no load.

Ted

RE: Aluminum Weldment Design Help

(OP)
Here are some notes on the joint :
https://res.cloudinary.com/engineering-com/image/upload/v1609286770/tips/Bolted_Joint_Notes_adveoh.pdf
resources:
https://roymech.org/Useful_Ta%20%20%20bles/Screws/...
https://mechanicalc.com/reference/bolted-joint-ana...

I made a guess at the input torque of 17 Nm after tightening one of the bolts down several times with an hex key and comparing it with a torque wrench. 17 Nm seems pretty reasonable to me, it was assembled by someone about my size so I think that is at least close. The bolts were not lubricated so I used a friction factor of .3, I think it was probably lower than this but that would help account for some of the local yielding of the aluminum reducing the preload.

With an estimated separation load of 5.2 kN and error bars from 107 N to 10.3 kN, it seems very likely that the joint had separated when it failed at 130 kN total attractive force or 4.64 kN/ bolt.

RE: Aluminum Weldment Design Help

According to this: https://www.specialmetals.com/assets/smc/documents... the strength goes up as the temp goes down. Now that we know the area is for M6X1 screws, and the tensile area is 20.1mm^2 per screw and we know the number of bolts is 28, for a total section area of about 560 mm^2.

Inconel rod has a room temp yield strength of about 160 ksi or 1100 N/mm^2; so it should resist (1100*560)N or about 600kN. It should be higher at the low temperature.

So that leads to a potential problem of stress concentration - like single point sharp root cut threads.

Did you put your area as mechanical because that's your engineering degree or because that's the job you have now?

RE: Aluminum Weldment Design Help

Another thought occurs to me - the magnetic force won't be constant if the coils fail to keep the separation.

If the initial failure was in a few of the short helicoils then some of the remaining short helicoils would quickly follow. Once that starts it's a race between the helicoils and the bolts on that side. Once enough of the short-helicoil side fasteners fail it will move towards the long-helicoil side, causing the force on both to go up and failing the remaining bolts on both sides.

RE: Aluminum Weldment Design Help

Hi Andre3

Thanks for the notes however I’m not sure I agree with some of the figures you have shown, firstly the bolt preload at 17Nm ie 9.4KN would not cause the bolt head to become Embedded in the
Aluminium at room temperature based on a T6061 276 N/mm^2 yield stress.So what value of yield stress do you have for the Aluminium Alloy?

On cooling the bolt preload would fall to zero in my calculation, assuming the bolt is 36mm in length (approx) and given a preload of 9.4KN the bolt stretch at room temperature when assembled would be approximately o.o5mm which would relieve itself on cooling of the components and reduce the bolt preload to zero as stated earlier, however I can’t see the actual bolt length in your calculations could you please provide it along with the actual length of the helicoil? There appears to be at least three lengths of helicoil 1.5D,2D and 3D. I was trying to find the pullout load for M6 helicoils but obviously the length plays a part.

I suspect the bolts did fail in a domino fashion because the 250KN external load would not have broken the bolts had it been equally distributed however as you have stated in some area’s it appears to be the helicoils which have pulled out rather than the bolt failing so it would be interesting to know what the difference in length of the helicoils which failed compared to those which didn’t, assuming of course the helicoil threads were fully utilised.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox
Radial pins might work, the total OD of the structure must remain fixed so the thickness of the flanges would have to be pretty small at about 9mm each. That may not be an issue though so I will look closer at that. I think I would use large diameter Aluminum pins and shrink fit them in place but I have to think more about it.

I have been using 270 N/mm^2 for 6061-T6 yield strength, the yielding I was considering was just surface effects. The top surface of the cover was fairly marred up after bolting and before the failure, I figured cutting the soft aluminum like that would also contribute to the preload loss, I believe I read that also but I am not sure where.

Thank you for pointing that out the bolt length, the grip length is not right for a joint with threads. Here https://mechanicalc.com/reference/bolted-joint-ana... they suggest using the length of the thru hole plus half of a bolt diameter for the grip length, so we would use 21mm for the grip length. factoring that in I get a separation load of 4.74 kN with error bars from 0 (totally loose bolts) to 9.69 kN. the bolts are 30mm long but I think it is the grip length that we use in these calculations.

The short helicoils were 1D and the long ones 1.5D. The 1D heilcoils should have failed around 15 kN and the long ones at around 25.5 kN. My understanding is that you want the strength of your threads to far exceed the bolt so even the 1.5D were marginal here.
https://res.cloudinary.com/engineering-com/image/upload/v1609343008/tips/Tensile_Strength_Bulletin_Metric_LR_1_pcqv5g.pdf

Also to be clear, the failure happened around 130 kN (±10%), but the joint must be designed to operate at 250 kN continuously.

Dave
Yes the bolts should have been able to take over 600 kN, that is what is puzzling and why I am here. The attractive force increases at 9 kN/mm as the coils move toward each other and the total spring constant of the bolts when loose (AE/L) would be 6.35 MN/mm so I don't think there was a runaway with the attractive force. What has confused me from the start of the problem is how any bolt could fail before a short heilcoil and how could any of the short helicoils fail first if the force per joint was about 4.6 kN. That is why I am thinking now that the bolts on the top (with 1.5D helicoils) could have failed first from some symptom of the joint separating like shear and bending on the bolts with maybe stress concentrations, all made worse by the brittleness of the material. Once the top failed completely it gets closer and raises the total force to about 220 kN and causes the bottom to fail with half the bolts breaking for the same reason as the top and the remaining half rip out the short helicoils.

RE: Aluminum Weldment Design Help

Hi Andre3
Thanks for the information, I used a bolt length to get an idea of how much the bolt will stretch under a torque of 17Nm, I had to guess the length because I had nothing else to work with. I am not sure where you are getting the separation force from? Once that magnet is cooled to its working temperature the bolts are free of any tension. I will have a look at the information you gave and comeback later.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Hi Andre3

There won’t be any preload left because the aluminium as contracted away from the bolt and the bolt stretch to get the preload you estimated is only 0.05mm, there is nothing to keep any tension in the bolt, what are the current depths of the parts being connected.

Ps the link you posted doesn’t work it just gives error.
“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox

Sorry about the link here is a good one:
https://roymech.org/Useful_Tables/Screws/Preloadin...

The stiffness of joint factors into the thermal loading not just the relative contraction and the initial strain, I think I used the formulas correctly to get my conclusion but let me know what you think. The cover is 18mm thick, the total height of the bobbin is 97mm with a radial thickness of 18mm at the joint. The total OD of the structure is 470mm.

RE: Aluminum Weldment Design Help

Quote "The overload would be 300 kN, so you propose that the bolts be preloaded so they will have 10.7 kN of tension when cold and ready to operate?
The bolts are M6x1.0 Inconel 718 age hardened.
The magnet is charged with DC current, I am not sure if that answers your question." Unquote

the fact is the bolts failed
so the conclusion the bolts have to be more robust such as minimum of M9 bolts or larger.
in the English version 3/8 or even better 1/2 inch, it is really very evident the bolts were inadequate.
Kindly advise if the aluminum cover distorted. from the moment.

RE: Aluminum Weldment Design Help

Make the cap in a conical shape to eliminate the "oil canning" effect of the flat plate shape to reduce or eliminate the bending load applied to the screws.

Specify rounded or radiused thread root on the screws to reduce the sharp notch effect in the screw thread.

How quickly is the magnetic field established? There could be a case of tensile impact on the screws.

Ted

RE: Aluminum Weldment Design Help

(OP)
mfgenggear

I agree, I came here looking to move past the failure to come up with a solution which I thought may be welding, but I am glad we have gone down this path-I think that I am understanding the failure better. I have very little flexibility on dimensional changes to the structure, so I am stuck with smaller bolts. The radial thickness of the bobbin at the joint is 18 mm and limits the bolt diameter, especially considering the aluminum will need to be helicoiled. The covers are minimally distorted, they rock slightly on a surface plate but I cant say that it is any more than when we received them.

hydtools
Is this sort of what you had in mind?:



I like that, to the extent that I have enough clearance I will try to incorporate that profile.
I Definitely will use bolts with round roots in the future, especially with more brittle materials like the 718.
The field is ramped very slowly over the course of hours so I wouldn't expect any impact from the ramp. We did hear some pinging/clicking noises accompanied by voltage spikes as we got close to the failure load which indicates some type of coil motion. It could have been bolts starting to pop or settling of the other structural components not shown but that could have been some shock to the screws.

RE: Aluminum Weldment Design Help

Hi Andre3

You haven’t shared the joint stiffness calculations but try to imagine this, when you tighten the bolt to get the preload you quote it needs to stretch about 0.05mm so now you have the 9.4kN and the bolt is clamping the Aluminium alloy, the joint is now cooled and the aluminium alloy contracts by 97 * 0.00435= 0.421mm but the bolt contracts 30* 0.00238 = 0.0714mm. Now the cover alone is 18mm thick and that contracts during cooling by 18 * 0.00435 = 0.0783mm.
If the bolt is a spring and it is stretched by 0.05mm and then released it will return to its equilibrium position and it tension force returns to zero, given that the lid itself contracts more than the original bolt was stretched suggests to me that on cooling the bolts have zero preload. I will look into it again in case I have made a mistake, however if you are correct the design you have might work but you would have to increase the preload at room temperature to compensate the loss you have calculated and without embedding the screw head into the aluminium alloy, personally I don’t think that’s possible but let’s see the joint stiffness calculations you have and go from there.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Andre3

The inconel 718 is not brittle at -250C according to literature I have seen.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Andree

let me try to under stand the issue.
excuse my terminology I am old school from the ranks.

even thou the assembly is in cryogen, the coils are at elevated temperature.
therefore it has thermal expansion radially and axially. if I am correct
then there is moment on the cap of the assembly. causing the bolts to fail.

therefore if the bolts size can not be changed how to eliminate the moment.
if the expansion of the magnets can be calculated. allow clearance between the
magnets and housing, and cap to allow for the expansion, is that possible
.
all speculation if there a instantaneous super heat from the magnets causing a rapid change in
size, it would put instantaneous force on the housing in all directions.
thus exceeding the ultimate stress of the bolts. rapid failure.

RE: Aluminum Weldment Design Help

(OP)
The formulas I used are shown in my first set of notes, I got them from https://mechanicalc.com/reference/bolted-joint-ana... . With a revised grip length of 21mm I get a bolt stiffness of 2.268e8 N/m and a grip stiffness of 4.575e8 N/m.

I will have to think more about your explanation on the thermal loading, I have trouble imaging how the the material below the joint factors into the loading. Using this estimation I would be thinking more like the cover and portion of the bobbin engaged with the bolt contacts 30*0.00435 = .1305mm and the bolt contacts 30*.00238 = 0.0714mm with a difference of .0591. Since the bolt stretched .030*9.4e3/(2.21e11*20.1e-6)= .0592mm we would still have .0001mm of stretch in the bolt.

*I made an error. When calculating the initial strain I entered E= 2.37e11 into the calculator (which is at 3 K) to get .0592mm. Either way gets the point across.*

I spoke to an engineer today with the company that manufactured the bolt and his qualitative take was the 718 is fairly brittle relative other metals at room temperature and that the brittleness would worsen at low temperature and could make them shock sensitive.

As a comparison here is the room temperature failure of 718 vs 316 SS :

RE: Aluminum Weldment Design Help

(OP)
mfgenggear

There is no cryogen and the coils do not heat up significantly, the source of the axial (dominant) and radial forces are from the magnetism. I don't think it is a differential thermal contraction problem with the aluminum structure and the coil. My thought was that the bolt failure caused a quench (when the coil goes resistive and you get massive heating) and I haven't thought of it much the other way around. We have quench protection that automatically starts dumping the current out the magnet if a resistance is measured so I think it is unlikely but I will think more about that.

RE: Aluminum Weldment Design Help

Hi Andre3

Thanks for the update, yes I can see that the stainless is more ductile than the inconel 718 but when I googled I found that inconel 718 was good at -423F see link attached courtesy of 3DDave https://www.specialmetals.com/assets/smc/documents...

Yes I can see the formula's you say you used but I meant see your workings because the formula only apply's if the two materials are in contact and in my opinion at the cooled temperature there is no connection.

Another question are the magnets operational during cooling or are the only turned on after cool temperature is reached?

If I take the residual stretch left in the bolt after cooling that you quote as 0.0001 then I get a residual preload of 19.3N, now that is an over estimate because I used the bolt O.D. and not the root diameter.



“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Andre3,
What is the critical temperature of the coil material, and does your power source have provision to quench the energy dump that takes place should excessive temperature rise occur?

RE: Aluminum Weldment Design Help

Andre3, as to shape, yes.

Similar to a tool I designed to generate 30,000 lbs of force resisted by the screws in the bolt circle.

Ted

RE: Aluminum Weldment Design Help

"My thought was that the bolt failure caused a quench (when the coil goes resistive and you get massive heating) and I haven't thought of it much the other way around. We have quench protection ..."

Andre3, if the quench circuit is inadequate you have an explosive release that will result in failure of the coiler enclosure,

Good Luck,

RE: Aluminum Weldment Design Help

(OP)
desertfox
I am confused on what you are looking for, I used those formulas to draw my conclusion and showed my work (other than the arithmetic) in the first set of notes. Unless I am misunderstanding something, it seems like formula shown in the thermal stress section would be how you determine if the joint stays in contact after cooldown. If F ≤ 0 then there is no contact F ≥ 0 then there is still load on the bolt. Am I missing something?
The magnets are not turned on until the base temperature has been reached, this takes many hours.

hacksaw
The critical temperature is field dependent and varies over the cross section of the coil. We have done well using voltage drop as detection method but our electrical engineer has written all the code and I am new to it all so I cant speak on it very much. We have talked about adding temperature triggers but thermally our systems have performed very well and we haven't needed it. Quickly dumping the current out of the coils should be avoided so if we see any temperature rise we tend to slow the ramp rate and let the cooling catch up.

RE: Aluminum Weldment Design Help

(OP)
desertfox

.0001/30 = 3.33e-6
F = EAe = 2.37e11*20.1e-6*3.33e-6 = 15.87 N
right?

RE: Aluminum Weldment Design Help

Hi Andre3

I think the bolt and Aluminium alloy are not in contact when the thing is cooled down, your conclusion is that they are so what I was requesting was your workings out so I could see how you arrived at that conclusion, you have better information than I but what I am also saying is that if you are correct and there is tension still in the bolt ie you say 3.4kN then maybe if we increase the bolt load on assembly at room temperature you might be okay with the design.
I am also confused by your last post where you state there is 0.0001mm stretch still in the bolt because that only equates to 19N preload and not 3.4KN.

yes I agree with your last post.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Hi Andre3

The bolt notes that show joint stiffness etc don't have anything to do with the temperature, the calculations are say correct at whatever temperature they are assembled at but any heating and cooling of the joint must have the new temperature stresses added to them, so for example in the link http://gbreco.pl/images/Pdf/compositebars.pdf exp 2.4 that only works because the brass or copper tube expands more than the steel, if you consider that instead of heating the rod and tube you cooled it down then the tube would freely contract and there would be no temperature stress and the rod/bolt would loose its tension and this is what I am saying is happening in your case. If the bolt were made from aluminium alloy and the housing from inconel 718 you could use those formula.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

desertfox

Not trying to ask a dumb question here.
Your the expert here, but how does the preload contribute to the bolts failing.
what puzzles me this
for example the yield stress of the aluminum is significant less than Inconel.
yet the bolts failed and not the cap. is this due to the Concentrated force (moment) on the bolts? and because the moment is
more spread evenly on the cap?

as an example bolts on an automotive engine head secured by bolts to the head, and preload on these bolts are important.
to maintain a seal by means of a head gasket. torque values(preload) will require retightening to maintain the exact preload.
due to thermal cycling of the engine, and expanding and contracting, also the temperature the bolts cycle thru.
but it is very rare if not at all these bolts will have catastrophic failure. not sure if this is a good example because
the aluminum heads and bolts are very robust. and with a large safety factors.

I am Intrigued that the original engineer calculated acceptable values. yet there was a missed stress value if that is the correct description.
again a failure analysis of the bolts is required by a metlap, to verify the bolts do not have a material defect, was it the result of hydrogen embrittlement.
or mis-heat treated parts. or was there a defect in the original parent material.
a formal electron microscope analysis , and a spectral analysis, and a verified the parts were heat treated to the correct hardness.
if the bolts pass with no defects, then a failure analysis of why the bolts failed.
the key is analyze if in fact the bolts were over loaded exceeding the yield stress values.
is there a hiccup of coils as noted above.

RE: Aluminum Weldment Design Help

(OP)
desertfox

I took some more notes that I hope clears things up. It is mostly the same information as my last set of notes but a bit more broken down. I used one website to get the stiffness formulas (the joint stiffness is roughly constant with temperature, less than 10% higher at 3K vs 293K) and the other to calculate the thermal loading.
https://res.cloudinary.com/engineering-com/image/upload/v1609369175/tips/Bolted_Joint_Thermal_Effects_c8pmcb.pdf

"I am also confused by your last post where you state there is 0.0001mm stretch still in the bolt because that only equates to 19N preload and not 3.4KN."
That was my attempt at showing that I don't understand how you came to the conclusion that the joint is not connected when cold by using your method to find the remaining bolt strain. Using your method I came up with 16N bolt tension and with the method I outlined I came up with 3.2kN

RE: Aluminum Weldment Design Help

Hi mfgenggear

What we believe as happened is this:- the whole thing was assembled at room temperature magnet,aluminium housing etc with the Inconel718 bolts; however there was no tightening control on the bolts so each bolt would have had a different preload of unknown magnitude but that isn't the big issue but does contribute to the failure.The whole assembly is placed in a sealed chamber which is then subject to vacuum and the whole thing cooled to around -275 degrees centigrade, unfortunately during this cooling the Aluminium Alloy has a higher coefficient of contraction than that of the Inconel bolts and we believe that all the bolt preload was lost or most of it due to the aluminium alloy being able to contract more and loose contact with the bolt, effectively making the joint loose. There are two sealed chambers both with assembled magnets and positioned opposing each other some distance apart, when the both magnets are switched on they act to attract each other with a force around 250kN and because the bolt preload was lost both magnets move toward each pushing on the aluminium covers and transferring the external load directly to the bolts and subsequent failure, now not all the bolts failed and those that didn't fail had the helicoil insert pull out of the housing, in preference to the bolt failing, the OP indicated that too short an helicoil was used or the helicoil was poorly installed.

I'm no expert by the way but this is my understanding thus farbigsmile

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Hi Andre3

Thanks for your last post, I think I understand what you were doing now in the first set of notes you posted but it wasn't clear to me then, I will look at what you have just posted and get back to youbigsmile

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Thanks for the reply Desert Fox, true Gentlemen. and you are an expert my friend.
I looked at the force applied and was also puzzled by that.
the force is applied on the opposite faces of the caps. unless it has a mechanism
that holds the assembly in place with the cap. the magnets are attracted as shown in the OP first post.
thus 250Kn force which exceeds stress values of the bolts.
unless I am still confused. :)

RE: Aluminum Weldment Design Help

Hi mfgenggear
I should of added that if the load was distributed evenly across all the bolts then the bolts should have held; however in practice loads are never spread that evenly and so whats happened is a few bolts have been overloaded and failed and then there are less bolts to take the full load, so the bolts fail progressively or the helicoils pull outbigsmile

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

From the previous messages it seems that the load is dynamic and the fix is some combination of more bolts, longer engagement, larger bolts, longer helicoils, better control of the pre-tension.

I'd be amazed if the attraction was truly 100% uniform - could that have a contribution?

Has welding been completely ruled out?

RE: Aluminum Weldment Design Help

"Quickly dumping the current out of the coils should be avoided so if we see any temperature rise we tend to slow the ramp rate and let the cooling catch up."

That's right, but loss of incoming power can also trigger the energy release, is your power source designed to trigger shunting the coil discharge to avoid mechanical failure?

RE: Aluminum Weldment Design Help

(OP)
IFRs
I think of the loads as quasi-static, if there are any rapid changes something is going wrong. I agree I think the solution will involve all of those characteristics.

The design is for everything to be very symmetrical but it cant be perfect. Especially with loose bolts that had uneven preloads to begin with, I think that contributed.

I am leaning away from welding now that it is clear the bolted joint was not properly designed and I am understanding how to fix it. I am still open to welding if anyone has a different opinion.

hacksaw
On a high level, the charging circuit and quench protection we use are handled by a 45+ year veteran physicist specializing in SC coils and a really good EE so I trust they have it all very well sorted out. To try answering your question, it is my understanding that we control our charging circuit and quench protection with custom in house software and it has been designed and tested to prevent excessive mechanical stress and heating from eddy currents.

RE: Aluminum Weldment Design Help


Andre3,
I am sure your system works as intended, but how can it fail?

The various failure possibilities need a thorough review as well. That takes time and open to addressing challenging possibilities.

RE: Aluminum Weldment Design Help

Hi Andre3

I agree with your calculations, I saw where I was going wrong I was just thinking that the free contraction of the aluminium was greater than that of the Inconel718 so sorry for the confusion, I calculated a different way and got a residual preload of about 2.967kN about 13% difference to your answer.
So either way there is still bolt preload present but not that it will do any good.
Now I calculated that you can preload the bolts to around 11kN at room temperature before the screw head embeds itself into the aluminium alloy but you would need to be much higher than that to get the lid to hold at the cooling temperature and the only way to do that is increase the number of bolts if you want to stay with the M6.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
hacksaw
I totally agree and will make sure we go over everything before implementing any new designs or testing.

desertfox
Great I am glad we are on the same page. I think I am going to propose the following revisions:
-Double the number of bolts to 56/ea
-specify Invar washers/spacers thickness such that the room temperature preload is maintained
-specify a room temperature preload that results in a separation force that exceeds the full attractive force while avoiding embedding into the aluminum cover
-specify 2D helicoils
-Make grip length at least 4 bolt diameters
-keep M6 diameter if all other conditions met

I like this because the covers can be modified and reused, the bobbins need to be replaced for revisions not involved with the bolted joint.

RE: Aluminum Weldment Design Help

Hi Andre3

Make sure you specify a preload about 50% higher on each bolt over what you actually need, this will allow for the error in using torque to preload the bolts.
I would also specify a tightening sequence for the bolts which I think is tightening in a star pattern for a bolted ring. A couple of other points, one if possible lubricate the bolt threads before tightening bolts this should reduce friction losses, I would also if possible drill a pilot hole all the way through the base rather than have a blind hole, this eliminates trapped air when assembling bolts it also reduces the stress raiser which you normally get with blind tapped holes.
Good luck and happy new year!👍

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Andre & Desert Fox

would a low strength Loctite help in this situation

RE: Aluminum Weldment Design Help

Hi mfgenggear

Not sure how adhesives fair in a vacuum.😀🤪

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

There would be no advantage using an adhesive since the screws do not unscrew loose.

Ted

RE: Aluminum Weldment Design Help

Hi Andre3

I have run a quick check with some of your proposals and I think you would be okay with 56 bolts at M6 to achieve correct clamping at 3K. If you have 8KN preload at the cooled temperature then the joint would be okay but that would mean a bolt preload of around 15KN at room temperature to allow for the bolt preload loss during cooling. (based on your last set of calculations posted on the 30th December).
At 15KN you would of course,according to my earlier calculation embed the bolt head in the Aluminium alloy, so you would need washers as you proposed to spread the load and prevent that but in using said washers what impact will that have on the bolt and joint stiffness?
In addition can you physically fit 56 bolts and washers on the given flange diameter?
If you can then this would be the easiest way to modify the current design as it stands and I believe you could use the 1.5D inserts although going to 2D is conservative but only if you use up the additional threads with a longer bolt and then its back to joint and bolt stiffness.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Up-size the screws. The screw capacity can increase by the square of the diameter change.

Ted

RE: Aluminum Weldment Design Help

The screws aren’t the weakest part, the limiting factor will be the tapped aluminium alloy and the helicoils, the design of the joint should allow the bolts to fail in preference to the helicoil inserts, now in the failure above that happened, the 1D inserts failed and the 1.5D didn’t according to the OP. That said there might be room to increase the bolt size it depends on the sizes of the magnet housing and whether they can physically fit in.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Yes, but
Larger screws require larger, stronger threads.
Are helicoils necessary?

Ted

RE: Aluminum Weldment Design Help

I once solved a problem. The impeller hold down bolt on a pump was failing. They couldn't increase the diameter because it would interfere with the keyway on the shaft. Their solution was to change to a hex shaft (no key). I suggested changing to a fine thread bolt. At 1/4 inch, the fine thread bolt was 70% stronger. Much money was saved that day.

RE: Aluminum Weldment Design Help

Are helicoils necessary?

yes, much stronger then the aluminum parent material threads

RE: Aluminum Weldment Design Help

But the helicoils failed. Ultimately the helicoil is threaded into aluminum thread.
Helicoils are good in aluminum for frequent assembly and disassembly of fasteners. A case could be made for spreading the thread loading.

I would prefer coarse threads in aluminum.

Ted

RE: Aluminum Weldment Design Help

the helicoil's failed because the wrong length was specified see this link copied from the OP's post 30.12.20 it gives pull out loads for the inserts, the design used the M6 * 1 insert. https://res.cloudinary.com/engineering-com/image/u...

We need input from the OP really to see what can physically fit in the existing housing, the problem with increasing screw size means the bolt might become stronger than the internal thread and we don't want that, it better that the bolts break as opposed to stripping the internal thread. Even with a bigger thread you still need the preload that I posted earlier because a lot of preload will be lost on the cooling cycle but if you change the screw size then that effects the stiffness of the joint and screw stiffness and without going through all the calculations for the stiffness's of these components its hard for me to say one way or another how much better or worse it would be. If a larger insert cannot be fitted without changing the magnet housing envelope then I would say its better to try increasing the number of M6 screws first. Lets see if the OP comes back with more information.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox
Ivar has extremely low thermal expansion, note the figure from 29.12.20, so I will use invar washers of a specified thickness such that the room temperature preload is not lost. Think of the washer as taking up the ‘slack’ from the contracting aluminum. I will need to calculate the required thickness and add it into the stiffness calculations to make sure everything is still good. Yes I do have space to fit 56 M6 bolts comfortably.

Hydtools
The helicoils give better pullout strength because of the increased shear area for a given amount of thread engagement and will be a benefit here. An exception would be if the size of the bolts must increase and I can’t afford the space required for helicoils, then long engagement in threaded aluminum could work.

The bolt pattern is centered 8.5 mm from the edge of the structure so there is not much room to work with. I am not sure if I can push the size, I need to check the machinery’s handbook to try finding the recommended minimum edge distance. Circumferentially I can also do 56 M7. I still prefer the smaller bolts if possible, I think they will be more desirable from a stiffness prospective and will give me more clearance above the cover which will also be helpful.

I am going to try to work out the particulars tomorrow, I have written a script that helps speed up the calculations but I will need to make adjustments to account for the invar washers.

RE: Aluminum Weldment Design Help

Another idea to look at is using a thread forming tap instead of a thread cutting tap. It produces no cutting chips and creates improved thread quality.
For what it's worth:In my referenced product I have 18 each 5/16-18 screws on a 5.5 inch bcd in the center of a 0.5 wall thickness (5 inch id, 6 inch od cylinder). The 1/4-20 screws I started with broke during testing.

Ted

RE: Aluminum Weldment Design Help

Hi Andre
Yes I agree with what you have written, normally as a rule of thumb the edge distance wants to be 1*D as an absolute minimum better figure is 1.5D, I assume from your post that the wall thickness is only 17mm that the helicoil is sat in? In which case I would stick with the smaller size.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox
The wall thickness is actually 18.4mm, why the bolts are not centered is still a mystery to me. Since I would prefer to reuse the cover if possible, I am somewhat tied to an edge distance of 8.5mm.

RE: Aluminum Weldment Design Help

(OP)
hydtools
I like to use forming taps when tapping small holes in C101 copper especially, without flutes the taps are much stronger and have a tendency to not break off. I have never tried them in aluminum or with anything larger than an M4 but I will look into it.

RE: Aluminum Weldment Design Help

Hi Andre3

Well I am in agreement with using the M6 screws/bolts its the easiest way to modify the existing design, as you said the other day though you need to include washers to spread the load under the screw / bolt head.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)

I wrote a script that uses the NIST cryogenic material data (https://trc.nist.gov/cryogenics/materials/material...) to calculate the separation force from room temperature down to 3 K for varying washer thicknesses, preloads, etc. . It works and I am closing in on the final design but I am a bit confused on how to specify the preload and further the torque wrench setting.

I plan on lubricating the threads with cryogenic grease, should I use a nut factor of .2 (Inconel bolt on 304 helicoil threads)?
If specify the torque setting to give a initial preload of 50% higher than needed, will I need still need to be at least 25% away from embedding in the aluminum? Does this already account for 10% loss from preload relaxation?

Between the nut factor and torque wrench uncertainty it seems like a bit of a shot in the dark for making a safe design. On the high end of the joint force I have to avoid embedment and on the low end I need to prevent separation with a good safety factor, with so much uncertainty I am concerned.

RE: Aluminum Weldment Design Help

Hi Andre3

Have just read your post and I need some time to think about what you have said, however the first two things I would say is that a nut factor of 0.2 seems a bit high if you are lubricating threads, also because of the error in torquing up the bolts it’s usual to do practical tests ie torque some rings up with a set methodology the you need to follow each time and then test the rings to see what force they separate and then adjust the torque etc.
Can you share your calculations?, have you increased the number of bolts?
I realise you would have to test at room temperature and then calculate down to the service temperature so I know of no other way.
One other possibility is to use studs in the ring joint and tension them with a bolt tensioner which is accurate to within about 3%.
I wonder if you can use load indicator washers but not sure they are suitable for sub zero temperatures.

Does the aluminium alloy increase in strength when it’s cooled if so if you avoid embedding at room temperature then you should be okay at the sub zero temperature because the preload relaxes.


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
Here is the current state of the design:
-56 bolts preloaded to 10.5 kN
-3mm thick invar washer, 12mm OD
-2 mm counterbore in bobbin before threads

I am calculating the joint stiffness in steps from room temperature down to 3 K using the NIST data, the joint constant at room temperature is .308.
At 3K the calculated separation load is 9148 N per bolt.
Here is the frustum that I am using in the calculation and an interesting 3D plot of the separation force vs temp and washer thickness:




I am still tweaking it though. The changes I am working on now are:
-changing the final frustum diameter to 2D instead of 1.5D to account for the helicoil
-redefine the joint stiffness to account for a truncated frustum due to the flange thickness (18.4mm)

RE: Aluminum Weldment Design Help

(OP)
If possible I would like to stick with tightening the bolt with an input torque, in its assembled state we wouldn't have space for a hydraulic tensioner or anything like that. The aluminum contracts more so unless you make a very thick washer, the preload goes down with temperature. I am not sure how load indicating washers would impact the stiffness here, but I cant use any magnetic materials so I think they are likely not an option. Also can you share your calculation for embedding?

RE: Aluminum Weldment Design Help

Hi Andre3

Thanks for the update, how much preload are you left with at 3K? You have said that the separation force at 3K is 9.148KN per bolt and therefore that exceeds the external force by a factor of 2 so I would say that was safe, however my approximate calculation suggested you would need a preload of 15KN at room temperature to enable you to achieve a 8KN preload at 3K, so how come your figure is much lower than mine?


“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Hi Andre3

Only just seen your post it must of crossed with mine, I will try and get some calcs on about the embedding but need dimensions of the washers mainly the diameter otherwise I only have embedding figure for bolt without washer.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox

At 3K there is still 6.35 kN of preload left, that is possible because of the invar washer. The washer is reducing the total thermal contraction of the joint to more closely match the bolt and therefore looses less preload. If the washer was very thick, over a centimeter, the preload would increase with decreasing temperature because the bolt would contract more than the joint.

RE: Aluminum Weldment Design Help

(OP)
desertfox

The proposed washer in this example has an OD of 12mm, and ID of 7mm and is 3mm thick.

RE: Aluminum Weldment Design Help

Hi Andre3

I can explain the embedding calculation, assume no washer and so the bolt clearance is hole 7mm diameter and the bolt head is 10mm diameter. Therefore area of bolt head in contact with aluminium ring is -


Pi*(10^2 -7^2 )/(4)= 40mm^2

Now using aluminium alloy at room temperature of 270 N/mm^2

Therefore preload at the point of embedding = 270 *40 = 10.8KN

So at room temp bolt preload without a washer shouldn’t exceed 10.8KN

The good news is the aluminium alloy increases yield stress etc as it cools, therefore if you avoid embedding at room temp it shouldn’t be a problem at the service temp.

This site mentions the aluminium alloy and states that the alloy improvs with cooling temp

http://www.totalmateria.com/Article23.htm











“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Hi Andre3

So with a od of 12mm then :-

Pi*(12^2-7^2)/ 4 = 74.61mm^2

Therefore preload for embedding = 74.61*270 = 20.1 KN

Hope this helps

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Torque + turn is the best intermediate tension control method as it directly controls elongation of the fastener independently of the lubrication of the threads and thread variation. Pick a torque that has only a small elongation but will elastically close any gaps and then the number of turns to produce the desired elongation.

RE: Aluminum Weldment Design Help

(OP)
desertfox
Shoot- the invar 36 I will use for the washers has a yield strength of 276 MPa so I am limited to 11 kN before the bolt will embed in the washer.

3DDave
I looked into this method and I think this will be our best approach, does ±15% error sound reasonable?


I realized that Inconel is actually a poor bolt material in this situation. The failure mode is separation of the joint, not bolt tensile strength, which means a less stiff bolt is desirable (decreases the joint constant).
Re-running the calculations for 316 stainless (A4-80 bolts), a 5mm thick invar washer, and an initial preload of 9.7 kN and after 10% relaxation that gives a separation force at 3 K of 8.212 kN. Custom inconel 718 bolts were going to cost about $5K with 4 week lead time, I can get stainless bolts tomorrow for 1% that cost. It seems counterintuitive that weaker bolts would be preferable, I am glad that I though to make the comparison.

RE: Aluminum Weldment Design Help

Hi Andre3

Well at least we caught it in time (the washer I mean), what is the drop in preload within the bolt, I ask because the separation force for the joint is always higher than the bolt pre-load or should be. I take it that you have checked the stainless bolts for linear expansion coefficient etc when you ran the calcs again.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox
The preload goes from 9.7 kN initially to 8.73 kN after relaxation, then from room temp to 3K it drops to 5.9 kN from the differential thermal contraction. The script I wrote with the NIST data makes switching between materials very simple so all of the correct material properties are applied to the new joint.

One puzzling thing that someone might be able to explain to me is how A4-80 bolts achieve so much more strength than what is listed for just 316 stainless steel. I can see that the compositions are essentially identical and from what I have gathered the elastic modulus and thermal properties are the same, is it just how they are treated? For this calculation I just need to be confident that the elastic modulus and thermal contraction match what is in the NIST database, I should be totally safe from yielding.

RE: Aluminum Weldment Design Help

Hi Andre

Well with those figures it seems you still have some safety margin in hand.

I think the A4-80 bolts get their additional strength by being cold worked however that cold working can leave them slightly magnetic.

https://www.schaefer-peters.com/uploads/tx_kkdownl...

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
I feel like I am ready to submit the design:

-56 X M6x1mm 35mm length DIN 912 A$-80 socket head cap screws.
-Preload to 9.6 kN by first torqueing to 2.88 Nm (~2400 kN preload) in a star pattern then repeat turning 19 degrees.
-minimum factor of safety on joint separation at 4K = 1.96


I think I did the torque angle calculation correctly, but it is a first for me and I couldn't find a formula:

load/turn = combined stiffness * pitch = (kbkj)/(kb+kj)*.001 = 1.3684e5 N/turn

7200*(1/1.3684e+5)*360 = 18.94 degrees



RE: Aluminum Weldment Design Help

Hi Andre3

Unless I am reading it wrong torquing to 2.88Nm is almost generating the preload you require at the end of tightening, I read 3DDaves post as setting a very low torque that just allows faces to contact each other ie “finger tighten nut on bolt” and then use the rotational angle to generate the preload, I think you are setting to high a torque at the first stage which will have all the friction error you are trying to avoid, also without the dimensions of the joint and the stiffness values you have calculated I can’t check the maths.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox
T = (.2)*(.006)*(2400) = 2.88 Nm
2400/9600 = 25% of the preload

kb = 1.88e08 N/m bolt stiffness
kj = 5.03e8 N/m combined stiffness of the washer and aluminum
pitch = 1mm

For greased threads I have seen .2 as a K factor but please advise otherwise. 2.88 Nm (~25 in-lbs) is near the bottom of most common 1/4 drive torque wrenches but I could go as low as 2 Nm. The covers are not perfectly flat so I think I will need at least 1 kN to be sure the gaps are closed.



RE: Aluminum Weldment Design Help

Hi Andre3

Okay I misread the units, however that 25% will be subject to an error of 25% and the turn of the nut angle method is +/-15% so if it were me I would reduce my first torque setting down to 5% or 10% of the required preload, that way the +/- 15% error on the final turning of the bolt angle method will be subject to a smaller error overall.
On the friction factor in my book is normal 0.2 for dry threads but here is a link for friction factor for bolt threads dry and lubricated https://roymech.org/Useful_Tables/Tribology/co_of_...

I will get back to you on the angle method of tightening shortly👍

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox

I see your point about the error, I will knock the initial preload to around a 1 kN if possible, thank you.

RE: Aluminum Weldment Design Help

Do you have a tightening pattern established? I imagine you'll have to go around more than once at each stage

RE: Aluminum Weldment Design Help

Andre3

Can you confirm bolt stiffness you quote 1.88 * 10^8 N/m that seems high to me ?

Also what is the grip length of the bolt you used in the calculation?the overall length you gave was 35mm and last but not least what values of modulus of elasticity for bolt and joint are you using

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
IFRs

No I do not have a pattern other than hitting them twice in a star pattern, where can I find information on establishing a pattern?

desertfox
The grip length is 24mm

Here is how I got Kb
Eb = 194.6 GPa 316 stainless at 293K
length of shank = 11mm
length of thread in grip = 13mm
K = A*E/L
Kb = Kshank*Kthread/(Kshank+Kthread)

Em= 70.4 GPa 6061-T6
Ewasher = 151.9 GPa invar 36

RE: Aluminum Weldment Design Help

Hi Ande3

From the information you have given I cannot get the same bolt stiffness as you so just post what length you used in the formula for stiffness along with the bolt area which I had from the inconel bolt as being 20.1mm2

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
I wrote the Kb formula wrong, I corrected it.

There are two areas we need, one for the shank and one for the threaded section, the new bolts will be partially threaded. The threaded section area is 20.1mm^2 and the shank area is 28.3 mm^2. The length of the threaded section in the grip is 13mm and the shank is 11mm long. To get the total bolt stiffness Kb, you combine the stiffnesses like this : Kshank*Kthread/(Kshank+Kthread)

RE: Aluminum Weldment Design Help

I don't think the thread length should be consider in calculating the stiffness. Any change in thread length is resisted by engagement in the helicoil/aluminum.

Ted

RE: Aluminum Weldment Design Help

(OP)
Hydtools
Please see the section on bolt stiffness here : https://mechanicalc.com/reference/bolted-joint-ana...

The thread length I am referring to above is the length of the thread within the grip. The bolt is 35mm long, 11mm shank and 24mm thread, but only 13mm of threaded length is in the grip and contributes to the bolt stiffness.

RE: Aluminum Weldment Design Help

Hi Andre3

Okay I checked the stiffness and I think the calculation you did for tightening the bolt using the angle method is correct.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox

Thank you, I have presented the concept and it has been well received.

I plan to make a test fixture for our Instron with a scaled down bolt pattern of 4 or 8 bolts and test the separation force. I think it will be a nice way to prove out the torque method and wrap up the solution. Does anyone have a suggestion on identifying the separation load during testing? I am hoping the slope change of the load curve at separation will be clear enough for me to identify when the bolts start taking the full load but it will be a first for me.

RE: Aluminum Weldment Design Help

Hi Andre3

You’re very welcome, it’s been a very interesting problem and I think we’ve hopefully solved the problem and it’s always worth trying to understand why a failure as occurred because in doing so it usually leads to a better solution😀.
I am pleased that your design as been well received and all I would ask is that you keep us posted when you have tested the product, it’s always good to hear what happens in the end.

I will have a think about how you might get indication of joint separation, it’s not something I have any experience of.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Hi Andre3

Not sure whether you could use these bolts that change colour when the reach a certain tension?
You could contact them and talk to their technical people.

http://www.smartbolts.com/dti/

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

(OP)
desertfox
Yes I will post updates as we test and implement the solution, I am very grateful for everyone’s time and consideration.

That is an interesting idea I have never seen a product like that. At a quick glance it appears that they do not have any M6 A4-80 options, I am still tied to that bolt spec.

RE: Aluminum Weldment Design Help

Hi Andre3

Thanks for the response, shame they don’t go down to the bolt size you are looking for but I will have another look to see what I can find👍.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

Hi Andre3

Is there any news yet on this project?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: Aluminum Weldment Design Help

No survivors.

RE: Aluminum Weldment Design Help

(OP)
Yes! I have a layout for a 6 bolt test fixture to be used in our Instron and will be ordering the hardware shortly. There have been some programmatic challenges that put this problem in the background but we are still moving forward!

RE: Aluminum Weldment Design Help

Hi Andre3

Thanks for the update 👍

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

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