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Minimum Web Thickness
3

Minimum Web Thickness

Minimum Web Thickness

(OP)
Ok, this should be simple, but I am having some difficulty.

I am trying to dictate to my designers the minimum amout of material below the head of a bolt based on the torque applied to the bolt.

T = (0.2)(Dia)(Force) <- Gives the Force due to the Torque

Then I model the plate as a simply supported beam, but when it is all said and done, I get a safety factor of 0.2.

Any suggestions?

RE: Minimum Web Thickness

(OP)
A bit of follow up, the plate is not supported directly under the bolt.

RE: Minimum Web Thickness

JpPhysics

Can you provide a sketch of what you are describing because I cannot visualise what your saying. I am not sure modelling the plate as a simple supported without more information.

regards
desertfox

RE: Minimum Web Thickness

You want to prevent bending (yielding) the plate?

What is torquing the bolt accomplishing, and what does the bolt do?

RE: Minimum Web Thickness

JpPhysics,

   We are tying to understand your model.

   It sounds like you have a plate with a hole in it.  The plate is supported some distance from the hole.  A bolt is inserted into the hole and tightened down hard.

   Is this right?

               JHG

RE: Minimum Web Thickness

if the flange under the bolt-head isn't clamped (butting up against something else), like in a conventional bolt, then i don't think the conventional bolt preload equation applies, 'cause there's so much more flexibility in the joint.  

in a convention joint the bolt torques clamps the flanges together.  in your case it sounds like the structure reacting the bolt preload is a flange in bending.  your case, if i have it right, sounds a bit "funky" ... i can see that you might have an unsupported flange but i would either tighten the bolt to the flange (using an anchor nut) or let the bolt pass though the flange an tighten it up with a nut on a supported flange.  that being said, you situation could be that the loads are known to be very low and you don't want the complexity of these other designs.

RE: Minimum Web Thickness

(OP)
rb1957,

We are using the bolt to apply a clamping load between componenets.  We use grade 8 bolts to protect against fatigue failure because we cannot tighten the bolts up to proof load.

The design is "Funky" which is why I am having trouble.

Thanks for the reply,

John

RE: Minimum Web Thickness

why do you need 6tons of preload ?

in any case, i don't think the conventional preload equation holds ...  

RE: Minimum Web Thickness

(OP)
Because this is a poor design.

This is an antiquated system that I have to support and modify.

The goal is to get 2000 lb/inch at the left side of the drawing.  Since my lever is backwards, I loose a lot of force going from the bolt to the target location.

RE: Minimum Web Thickness

For sure you would need enough material to prevent shearing of the piece under the bolt head.  You might also want to take a look at Roark's equations for flat plates with center loads.

RE: Minimum Web Thickness

if you need the load at the LH reaction, and you're relying on the preload, i'd check your assumption ... maybe use a strain-gauged bolt to verify (and replace with a standard bolt for service).  personally, i wouldn't bother with Roark ... this thing looks like a beam, use something like the minimum thickness to calc I ... should be near enough.

RE: Minimum Web Thickness

BobM3,

   Roarks's equations for flat plates are for flat plates.

   The part does not look like a flat plate to me.  I think the OP needs to tell his boss to choose between FEA and a new part.

   Would this thing fail in shear around the head of the bolt?

                        JHG

               JHG

RE: Minimum Web Thickness

(OP)
We have had some systems fail around the head in shear.  Others would deform around the bolt head which causes a loss of clamping force.

The problem with modeling this as a simply supported beam is that I get very large bending moments on the order of 300 ksi which I know is not correct.  I guess the problem is I need to use a simply supported beam, but I cannot model the bolt as a Point Load, it needs to be a distributed load.  Any suggestions?

Like I said, FEA is an option, but what I am trying to come up with is some way to easily say:  If you use a 5/8 bolt then you need this much material below the bolt head.  If you step up to a 3/4 bolt then you need this much material...etc.

RE: Minimum Web Thickness

how would the guy tightening the bolt know when he has applied 1 ton/in at the reaction edge ?  maybe they're over-torquing the bolt ... maybe you specify a torque, but it gets loose so they tighten it up more ??

analytically its a pretty easy problem ... a single load, a simple reaction, ok a little fussy about the section in bending.  distributing the load under the head of the bolt probably won't change things by more than a few %age points.

RE: Minimum Web Thickness

JpPhysics,

   You cannot model your structure as a simply supported beam unless it is a beam.  The beam equations in the handbooks are based on double integration method.  Read up on it and make sure you understand all the assumptions.

               JHG

RE: Minimum Web Thickness

JpPhysics

Thanks for posting the drawing.
I do not believe you can use simple beam theory because the length to depth ratio of the component looks to small. Secondly I am not sure how your calculating your bending stress but you would need to know the position of the neutral axis for that component shape shown on the drawing assuming beam theory was applicable.
Thirdly your supporting reactions would depend on the stiffness of the supports and those of the component shown within the supporting region.
Finally the central bolt, you don't say by what method your pre-loading the bolt but if I assume torque wrench then this method as a error of +/- 25%, so on one clamp it could be 25% higher than you need and on another 25% less, not including thread tolerances, lubrication, and surface finish on bolt and mating thread, embedding of bolt head etc.
I think if you want an accurate answer then do some FEA with the parts modelled correctly so that you can obtain deflections of the clamp and its supporting components.
If the clamping force needs to be accurate then you need to do some practical tests and come up with a method of assembly that reduces the error in your bolt pre-load rather than just relying on the torque wrench.
If you have had failure's of the clamp component around the head of the bolt, I fail to understand why you use a grade 8 bolt to protect against fatigue failure when you make no mention of bolts failing or have I missed something?
What is the cyclic load on the clamp? maybe you should be looking at a fatigue analysis on the clamp instead of a static analysis.

regards

desertfox

RE: Minimum Web Thickness

(OP)
desertfox,

Thanks for the reply.

This is what I get for posting half a question. thumbsdown

The drawing is simply a sample of one of the many products we offer that use this type of clamping configuration.  All of these products have been used for more than 20 years.  In the early days, these products were not "Engineered" they were simply "beefed" up until they held together.  The old trial and error method.

Enter 2009... how can we save some money while maintaining system stability.

These bolts are torqued to 120 ft-lbf.  We have done enough tests to determine that the clamping force we get at the tip of the clamp is sufficient.

These components spin at anywhere from 1100 to 2500 rpm and take an impact load equivalent to 1000 lb per inch all day long.  (2 to 3 times per day the system is shut down for exactly 1 hour.)  Under these conditions, the system shown will last for a couple of years.  In fact, the clamp will wear out before it fails.  (Now you can see why we use Grade 8 Bolts.  Additionally, these components are sharp so having one go flying around and hitting someone is not generally a good idea.  Better safe than sorry.)

The bolt is removed one to two times per day then re-torqued depending on the severity of the work being done.  The bolt is not replaced on a regular basis, only if it breaks or is visibley worn on the head.

Moving forward, we are designing new systems that do not allow for a lot of material under the head of the bolt.  I am the only person capable of running the FEA software at my work, so I become a major bottleneck when we have 10 to 20 new projects running at a time.

My thought was, what if I could get my designers (not equal to engineers) a chart or equation that they could use to get close to the correct amount of material below the head of the bolt thereby reducing the number of simulations that I would have to run.

Does anyone have any suggestions or rules of thumb that we could use or am I simpy doomed to run simulations all day long?

RE: Minimum Web Thickness

Is this a machine tool hold down clamp?
What is the mode of failure you are using to calculate your FS or design margin?
Were the bolt failures really shear failures due to overtorque or in fact tension failures?
The resistance to bending will depend on the overall clamp thickness as well as the thickness under the head of the bolt.  How have you figured the overall clamp thickness into your calculations?
You can use the bolt torque-tension equation to estimate the pull down force.

Ted

RE: Minimum Web Thickness

If shear is the problem then it's simple.  Because the part is bending you would be safe to assume that most of the bolt head force is applied near the OD of the head.  The shear area is:

As =  pi x Bolt Head OD x thickness under bolt

Given your bolt load and the allowable shear stress, tau, the thickness  =

t = Bolt Force/(tau x pi x Bolt Head OD)

The part material under the bolt head will also bend.  That's were you can use Roarke or Blodget's simple plate models for calculating bending stresses.  Model the annular ring under the bolt head as a plate with a thickness equal to the thickness under the bolt head.  The ID of the ring is the bolt hole diameter.  The OD of the ring is the counterbore diameter.  The load is a concentric line load at the bolt head OD.  The trick is the edge restraint.  It lies between simply supported and fixed.  To be conservative use simply supported.  If that yields too large a required thickness then you really have your work cut out for you to take it further.

My guess is that shear stresses will control the thickness of the material.


 

RE: Minimum Web Thickness

Hi jpPhysics

Thanks for the reply I suppose the short answer is no I don't have any rough rules of thumb at this point in time that might help you on the basis of the information you have given.
Firstly you need to establish the cause of the clamp failure, was it due to Fatigue,or the bolt not being retightening properly after removal or some other mechanism.
I have never heard of an impact load being given as a force per inch and how as this impact figure been established?
I assume from your last post that the clamp rotates with whatever it is holding in position and the bolt axis is parallel to the axis of rotation. This being the case the bolt will need to resist being sheared by any centrifugal force generated by the supported component if its centre of gravity is not on the axis of rotation, in addition if there is insufficient clamping load on the bolt for whatever reason then other forces may well come into play particularly when the impact force rears its head.
How is this impact force applied is it through intermittent cutting action? if so you might be able to measure it using strain gauges.
As regards saving money on the design and to minimise the amount of material under the bolt head then surely the best way forward is a more refined analysis ie FEA, if you use a rule of thumb then the chances are it will err on the safe side and therefore will not achieve your aim.
I think the real problem is you need more manpower on the stress analysis side,whether someone else in house is trained in the use of FEA or your company hire or sub contract some of the stress work is a management decision
 

RE: Minimum Web Thickness

is it worth controlling the preload with PLI washers ?

what sort of steel are we talking about 75ksi, 125ksi, 160ksi ? (ie, maybe there are material choices).

there are going to be limits to how you can apply any design.  you guys clearly know what you're doing (with all the "research" you've done).  but you can't do everything with anything.  your design works, with maintenance and an acceptable failure rate.  it's difficult to test the set-up, for some of the reasons already posted ... preload variability due to torque, temperature?.  what makes this "litte" detail part so critical to the commercial success of everything that people want to mess with it.

on the other hand, if you're the only FE guy there, well then, that's job security !

RE: Minimum Web Thickness

(OP)
Thanks to everyone for all the information.  I think I have enough to justify a third seat of Cosmos!

John

RE: Minimum Web Thickness

Hi John

You welcome

desertfox

RE: Minimum Web Thickness

A bit late--Check out Kent's Design and Production which I have or possibly Marks Enginnering Handbook which I dont have for stresses and deflection of circular plates.  Granted your part is not circular so use the largest dimension and be conservative.  One of the formulas which are referenced from Roark deals with outer edge fixed and supported with a uniform load around the hole.

RE: Minimum Web Thickness

Before you fork over the back 40 for an extra seat of Cosmos, refer back to general principals. (Pardon my typing,I put in my wife's contacts this morning instead of mine) There is an excellent discussion on this topic in Shigley and Mischke's "Mechanical engineering Design." I have the 5th edition, p. 338. Basically the pressure distribution has been shown theoretically and through analysis to be in the shape of a cone of close to a 60 degree included angle. Trig out the thickness of your pressure distribution so that when the cone exits the web it is as close to, but not larger in diameter, then the inscribed circular diameter of the fastener. I can get deeper into this after lunch when I have close up vision once again (he distance resolution is much better then mine. time for a new eye test!)

RE: Minimum Web Thickness

Oh, and one other thing: Don't forget that the tolerance on torque when stacked up against the wrench, fastener and fastened member elasticity and lubricity is conservatively 15%. Be sure to consider that when you perform your calculations. I have years of rocket engine turbopump design experience, and the last thing you want to see is fasteners falling off!

So, pop quiz time: define the difference between a screw and a bolt. Too many mechanicals don't know!

RE: Minimum Web Thickness

(OP)
A bolt has a nut on it.  A screw does not.

Thanks for the information, I'll have to pull out my Shigley book.  I think Juvinall has that same picture you are talking about, it referes to clamping pressure.

Thanks for the help.

RE: Minimum Web Thickness

Actually Jp hit the screw on the head with his first shot. It's pretty simple: A nut goes in and a bolt goes through. A bolt needs a nut. Other then that or it's being used to hold a fence shut!

RE: Minimum Web Thickness

"Then I model the plate as a simply supported beam, but when it is all said and done, I get a safety factor of 0.2."

But think about it: You've got a very, very short; very, very wide "beam" - then you remove 85% of the middle of the beam with a big hole.  No, that's not really a simply supported beam with a single load in the middle (the bolt) and two end supports (the clamped pieces.)

Load points aren't exactly at the the two points of where the two arrows are either: there is a little "spread" in the ontact points that will matter in the FEA analysis at those small a distances.  Remember the "areas" of contact as well.  Small differences, but you have a small part as well.

FEA is - frankly - required, but is it justified?:  "Thumb rules" work from experience, and your 20 years of experience with this part IS the experience you want.  

Now, you just have to figure out how "overbuilt" the original part is, and how expensive it will be to "rebuild it" down towards a more economical part.
 
Fabrication: Can you make it cheaper?  Can your FEA (time and money and "proofreading" the results) find results that might make a cheaper (easier to make/simpler) part economical?   

Tough calls.  In my humble opinion: phrase it like that.  Don't promise money back - higher sales by advertising a simpler or more economical or "better" part right away - but "maybe this can make a better/cheaper/prettier/more attractive clamp" - that we can sell in a tight market "better".  

One FEA (at a high  expense on an already produced "overbuilt" item) might, just might, point the way to re-fabricating tens of thousands of clamps - at a savings of a few pennies each ......

Dollar/yen/euros/canadian .... That's your bottom line - and an "investigation" might be (probably will be!) warranted.


 

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