Minimum Web Thickness 3

[JpPhysics]

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?

 

[BobM3]

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.

 

[desertfox]

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

 

[rb1957]

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 !

 

[JpPhysics]

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

John

 

[desertfox]

Hi John

You welcome

desertfox

 

[chicopee]

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.

 

[GuyFromDenver]

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!)

 

[GuyFromDenver]

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!

 

[JpPhysics]

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.

 

[drawoh]

So, pop quiz time: define the difference between a screw and a bolt.

It depends on your definition of screws and bolts. Check out thread301-166708, thread1103-224665, and thread1010-92939. I did not go past page[&nbsp;]1 of of my search.

JHG

 

[GuyFromDenver]

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!

 

[racookpe1978]

"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.