Torque required to bring mating parts in contact

[NaikD]

Hello,

I am asked to estimate torque required to bring mating parts in contact 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

 

[hydtools]

Why is there a gap? Part distortion? Gravity?

Ted

 

[IRstuff]

Why would you even need to go to FEA? The lid can be treated as a beam and the force required to deflect it can be calculated. From there, given the screw pitch, etc., you should be able to calculate the torque.

A small amount of non-parallelism is expected in most machined components, but one should be cautioned if either side of the interface contains alignment sensitive parts, such as a laser cavity, since the side that is the stiffest will win the battle, and if the side that loses is the side that has the alignment critical components, they won't be aligned afterwards.

All that said, I've yet to come across any component that was so stiff that you couldn't screw it to mate, unless there were actual burrs or deformities that prevented mating. If the latter case, then there might never be sufficient torque, and the correct solution is to correct the manufacturing process.

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

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[MintJulep]

then split the force to number of fasteners

If we imagine you mean "split equally" then no, that's wrong.

IRStuff is correct in stating that this is all about stiffness.

 

[dvd]

Beam theory only goes so far for the simulation of plates. The lifting of corners is a possibility, depending on bolt hole layout -

https://journals.sagepub.com/doi/pdf/10.1243/03093247v044306

 

[NaikD]

All,

Thanks for all the helpful suggestions.
___________________________________________________________________________________
The lid can be treated as a beam and the force required to deflect it can be calculated. From there, given the screw pitch, etc., you should be able to calculate the torque.
___________________________________________________________________________________
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.

I wish to thank all for helping me on this topic.

With Best Regards
- Dipak

 

[GregLocock]

The way I'd kick off the analysis is to consider the simplest case of interest, say a rectangular lid with a bolt at each corner, being screwed onto a rigid box. If one of the mating faces causes 1mm extra clearance when the other three are done up, what bolt force is necessary to close the gap? Mr Roark is still on holiday, but I bet the deflection of a flat plate supported at 3 corners is in there.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[electricpete]

A possibility that comes to my mind is that a curved part may cause a gap to remain between bolts even with firm flat metal to metal contact at the bolt location, such that no reasonable amount of torque is going to close that gap. This is more likely when bolts/holes are far apart. More bolts/holes reduces this (along with better tolerances).

Of course, some applications like fluid-retaining joints may use gaskets to help cope with the situation of imperfect contact of the mating parts.

=====================================
(2B)+(2B)' ?

 

[IRstuff]

Conversely, you need to decide if 0.005 gap is tolerable, because if it's not, then you need to change the machining tolerance if you indeed have uber stiff assemblies that can't be mated otherwise.

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

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Tmoose]

https://www.eng-tips.com/viewthread.cfm?qid=461013

Without anything to go on I'm guessing E-pete's bowed cover could tax some of the fasteners more than un-twisting a cover twisted the same amount.

I'd solve for a single fastener being able to un-twist a severely rogue cover, plus still being capable of applying the PSI the gasket needs.

 

[SnTMan]

Meanwhile, in the structural forum for those keeping score

The problem with sloppy work is that the supply FAR EXCEEDS the demand