Bolt holding strength? 1

[nocam1334]

I have two flanges bolted together and their is a problem with the way the holes have been drilled. Normally, the assembly is held together by 10 bolts. Because of the error in drilling, if we were to try bolting then together only 6 of the holes could be used, but I believe they are equally spaced apart.

Basically, I am trying to figure out if the 6 bolts will hold the flanges that (I assume) were designed for 10. How would I go about determining this? Any advice would be appreciated.

 

[rb1957]

what sort of load is applied to the bolts ? tension, shear ?

what sort of margin has been determined ? (if it is at least 0.67 you might be ok)

is the flange still ok ? with the attachment further apart, the stresses in the flange are going to increase. with the attachment further apart, are the flanges going to leak ? can you off-set this by increasing the preload in the fasteners ??

what's the impact of the redundant holes ?

 

[nocam1334]

Not sure about the load or margin, as this is an old design done by someone else and I have no information in that regard. I apologize for the lack of information and wish I had more info to work with.

The flanges are fine. It might be possible to increase the preload, I will have to find out what it is supposed to be.

 

[rb1957]

pls post more of the stroy ...

what do these flanges do ? reinforce an I-beam, connect a pressure vessel, ...

 

[Twoballcane]

IMO if the design called for 10 bolts then I would go back and drill in the other four holes (or move the other four holes). I would not risk any danger because of schedule.

Tobalcane
"If you avoid failure, you also avoid success."

 

[nocam1334]

I have since talked to the machine shop lead and now I have a little more info.

The flanges are for a cast volute for a pump to which a hydraulic motor is connected. The original drawing calls for 12 bolts to attach the motor flange to the volute flange. We have a few of the volute castings on hand that do not have any holes drilled in them yet and our machine shop supervisor wants to put them to use. The problem is there is an extension jutting out and up on the side of the volute that prevents him from creating the 12 bolt hole pattern. However, he believes he could do an 8 hole equally spaced pattern, or possibly drill 10 unequally spaced (basically drilling the 12 hole pattern but leaving out the 2 holes he cannot drill.

I'm honestly not sure how they did it before, but I can only trust what our machine shop lead is telling me about not being able to create the pattern as shown on the drawing.

 

[drawoh]

nocam1334,

I would say that the quick and dirty answer to your question is "No". The joint with six bolts is not as strong. Whatever standard the old joint was designed to, you are only achieving 60% of the strength.

If you want to get the new joint approved, you will have to get a qualified engineer to review the requirements and the design. There is not enough information online here for us to make a call like that.

JHG

 

[MintJulep]

Sounds like a tooling problem, not an engineering problem.

If your shop doesn't have the right tool to make the part to print then go find a shop that does.

 

[MiketheEngineer]

If they built it with 12 bolts before - then why not 12 bolts now?? Sounds like somebody doesn't want to work too hard.....

 

[CoryPad]

One possible way to do this is to use fewer fasteners, but ones with higher strength. What were the previous ones (material, property class, preload)?

 

[hydtools]

If you have the space, use 6 larger bolts; 1.7 times larger.

I would prefer the 'make it likes the print' response, if it was made to the print before.

Ted

 

[desertfox]

Hi nocam1334

Your first post says there should be 10 bolts, but later ones say 12, I suspect the motor flange will drive the hole pattern, quantity and bolt size.
I wonder if it is possible you can establish any data regarding the loads on the bolts ie:- are they to take shear set up from the motor torque or tension generated by system pressure,if so you can establish the preload for the bolts and determine if 6 or better still 10 is enough for your requirements. Does the motor manufacturer give a recommended installation torque figure for motor fastenings as that would be a good starting point from which you could estimate the individual bolt loads.
The downside to not using the correct bolt number is whether your motor supplier will waive his gurantee if anything goes wrong with the motor, the worst side is, if there is an accident and someone is hurt theres only place the buck will stop then.
If you decide that you do actually need 12 bolts, then instead of drilling through the flange is it possible to drill and tap the flange with blind holes for the bolts to screw into, you don't say what the cast material is, but if its cast iron you would probably need inserts and a check on the tapped thread depth to ensure adequate strength.

regards

desertfox

 

[rb1957]

a question back to the OP ...
how are you going to accept this new design ? are you going to take our word for it ? (for all you know we could be a bunch of loons sitting 'round in our bathrobes) or are you going to do some calcs ? if you're going to do calcs, then post some of the info you're uncovering ... is the joint a pressure seal ? or maybe "just" a sump ??

if you accept this design, who do you think is going to have to answer the difficult questions if/when it goes "tit's up" ?? hint, it won't be the tool shop supervisor ... "the engineer said it was ok".

good luck, i think you're in a difficult position.

 

[Adalius]

My experience as a millwright working on pumps is that if you have X number of holes and can only use Y, you're running a much higher chance of a gasket failure (if used) in the area you don't have the bolts, or if no gasket or other sealing means is used, you might just have it flat out leaking.

If this joint is as I picture a normal joint on a volute, then the bolts are, aside from structurally holding the flanges together, are located to apply their clamping force equally around the entire flange so that it seals properly.

Your best bet is to do whatever needs to be done to drill those holes where they need to be. I *have* however seen it done before where if you can't drill a hole in one of the flanges because of a protrusion you instead weld a stud on that location, making sure that it's been properly grooved and ground after welding so the final diameter at the faying faces is the same as the bolt shank. This provents a weld bead from 'holding' the flanges apart. Then you just nut the one side. It's not a pretty solution, and depending on your application might not even be a structurally sound solution, but it's an option.

 

[GregLocock]

The old rule of thumb was that the gasket had to be pressurised for the straight line connecting each adjacent bolt hole (sorry that is badly put).



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[chicopee]

It seems to me that the original bolt hole pattern was threaded for screws between pump and drive.

 

[racookpe1978]

I have no sympathy for a foreman who wants YOU to accept the blame when something fails that he claims he could not put together/drill out.

I agree with the idea of "use two studs and weld the back" or "threaded inserts and use studs."

Pressure? Temperature changes? Vibration? If the things are moving = vibration; if they are carrying pressure you have got to accept the requirement to clamp uniformly the gasket material.

Just drilling 6 (or 8 ?) holes where 12 were originally needed? Dumb idea. Where did the "old" holes go? You need to account for that missing material if you drill out holes from an 8x pattern to a 10x pattern to a 12x pattern to be sure the flanges won't crack.

 

[trainguy]

Hi nocam1334.

I feel your pain.

If the foreman is requesting a design change, you need to take that to your supervisor and ask for his/her input. I find that usually, an experienced engineer will take that request pretty seriously, and then will give his/her opinion about the list of issues (and scheduling impacts) that this request will create. This information can then be fed back to the foreman, who will promptly do what he needs to make the original design work.

tg

 

[ProgressiveRacing]

If the problem is accessibility, fixture the part so it can be drilled from the opposite side?

 

[trickydic]

IF THE FLANGE WAS DESIGNED PRIOR TO PUTTING A NEW FLANGE IN PLACE, IT WAS DONE BEFORE!!! SOME PEOPLE WILL NOT IMPROVISE WITH THE MACHINERY THAT THEY HAVE TO COMPLETE THE TASK. SO THAT BEING SAID THE EASY WAY OUT IS NOT ALWAYS THE RIGHT WAY OUT. WOULDN'T EVEN THINK OF DOING IT DIFFERENTLY UNLESS THIER WAS A FLAW WITH THE PRIOR DESIGN. IN ASSOCIATION WITH THE APPLICATION, I'M SURE ALL THINGS WERE TAKEN INTO CONSIDERATION IN THE FLANGE. GOOD LUCK !!!