Torque required to bring mating parts in contact

[NaikD]

Hello,

I am asked to estimate torque required to bring mating parts together that could have an initial gap.

A simple example could be an assembly similar to a shoe box with cover. If I were to provide fasteners on longer sides of the cover to assemble the cover to box, and if initially there was say 0.005” of gap due to manufacturing tolerances, what would be torque required to bring the mating surfaces to contact.

One approach could be to run finite element analysis to estimate force to bring the mating surfaces in contact and then split the force to number of fasteners and then estimate the torque.

Hand calculations in place of FEM analysis could be little involved……I think.
Is there is any other approach or has anyone done this analysis before?

Thanks for your help!
- Dipak

 

[r13]

How much torque is required to make a full turn of a screw? What is the common gap in between threads (assume terminology correct) of a screw?

 

[NaikD]

Hi,

Thanks for the quick response.

For the example of the "Shoe Box and Cover", One can install fasteners on one side easily and as you torque the fasteners, parts will make contact but the opposite side of the assembly will have a gap due to clearance/tolerances.

I was referring to that gap that could exist between mating surfaces of the parts and force that is required to pull together the two mating surfaces even before preloading those. This is not a hermetically sealed assembly and gaskets/shims are not used to fill the gap.

I never had to do these calculations and assumed that the mating surfaces were in contact to begin with or any gaps were negligible for a not hermetically sealed assembly.

Regards,

- Dipak

 

[Ron]

Don't worry about the torque required....just bring the surfaces in contact. You are not likely to overstress the bolts in doing so. If you do, the fit up tolerances are way out of whack and need to be corrected.

 

[IRstuff]

If this is a recurring problem, then you need to change the manufacturing tolerances, or change your assembly procedure, by simply installing fasteners on all sides in sequence. This is how car wheels are mounted on their hubs; the installer only finger tightens the lug nuts and then wrench tightens in a skipping pattern

TTFN (ta ta for now)
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[XR250]

No easy way to calculate that if it is a complex part without FEA.
Depends on the shape, stiffness and MOE of the assembly.

 

[jayrod12]

We've also indicated many times in other threads that torque values are a bit of a red-herring as they are affected by many things including but not limited to condition of the threads, tolerances of the holes, calibration of the wrench.

 

[JLNJ]

Leaving the AISC requirements aside, you might be able to model it as a stiff spring which compresses as the bolt elongates until equilibrium is reached. This would tell you how much turn of the nut you would theoretically need to apply to lengthen the bolt and compress the spring.

But, as others have said, it's pretty dicey to relate that theoretical bolt tension to torque.

 

[r13]

Right or wrong, let's try this.

To close the gap between two objects, is liking pulling one towards the other, therefore work done = F*d, d represents the gap distance, and F is the friction force of one object, F = µW. Now set T = F*d = µW*d. Make sense?

Note, you really need to provide more details on your application. Otherwise, everybody is only guessing when the sun and the moon to meet

 

[IRstuff]

It's not always friction, or even friction related.

Sometimes, it's either poor parallelism or warpage, so it should be more like bending a beam force/work. I've had the occasion to have had optical assemblies that misaligned when the cover was installed, because the cover was stiffer than the base, so the base wound up conforming to the cover, rather than the other way around. We wound up having to completely disassemble the laser, machine lap both sides of the flange, and then re-building everything, which contributed to the cost overrun, but that's another story.

TTFN (ta ta for now)
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https://www.youtube.com/watch?v=BKorP55Aqvg

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[IRstuff]

Please refrain from double posting, as that contributes to confusion, since I wound up thinking that I hadn't responded to your OP because of thread404-462083.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[r13]

Sure, it can be drag force if we are trying to pull two airplanes together in the air

 

[rb1957]

Other comments are IMO right … don't focus on torque (to drive the screw), the important thing to the load imposed by closing the gap. So that with the "lid" on the "shoebox", how much force is required to close the gap ? I'd also be concerned about the gap in the first place … does something need to be controlled in the drawing to create a consistent and/or small gap ?

a typical "rule of thumb" would be 20 lbs pressure (to close the gap). Anything you can close with "finger pressure" is fine.

another day in paradise, or is paradise one day closer ?

 

[r13]

NaikD,

This website may have something that may assist your study.

Link

 

[NaikD]

All,

Thanks for all the helpful suggestions.
___________________________________________________________________________________
IRstuff
I made a mistake in posting the same question on other forum. Sorry about that. I appreciate your help!
___________________________________________________________________________________
My observation was that if the lid is too stiff then the bolt torque could be too high for the size of the bolt to bring the parts together with little left for preloading the joint. However, I have not yet looked at that yet.

Looks like as suggested here, I need to look at tolerance stack up/manufacturing process to reduce bolt force to mate the parts.

Amazing to see so many helping me in such a short time!
I wish to thank all for helping me on this topic.

With Best Regards
- Dipak

 

[rb1957]

the lid may be stiff, but won't it then "just" drag the box side to close the gap ?

Again, quit focusing on bolt torque. Bolt torque will be excessive if bolt load is excessive. Load is the thing to focus on. If the load to close the gap is significant then you have to either account for it in the analysis (along with the worst gapping condition) or you redesign to reduce the gap or you fill the gap with a shim (and add this into your analysis).

another day in paradise, or is paradise one day closer ?

 

[Guest090822]

If the parts have been made already then why not use a torque wrench and tighten it small increments to see when the gap closes? The value on your torque wrench will be the torque you need.....