Installation torques for instrument threads

[Tunalover]

Folks-
We are using NAS721 fillister head instrument screws in the 0.50 UNM and 1.00 UNM sizes. The product size constraints have forced us to use these in several places!

These are darn small screws (.5mm and 1mm major diameters, respectively) and we have been totally UNABLE to find anyone who has tested these to come up with installation torques. The screw matls are 303 SST and the internal thread materials are AL6061-T6 and AL7075-T6. The internal threads are blind and are in material much thicker than the depth of the threads. The thread penetrations are currently .19inches or less.

The project we're working on is has a very tight schedule so we simply can't afford to deal with any stripped internal threads or broken screws (besides they don't make an EZ out for these sizes!)

HELP!
TIA!



Tunalover

 

[CoryPad]

This is an application fraught with problems: tiny screws with low strength, and screw and nut with high friction. I recommend using lubricant to minimize galling.

I made a few assumptions:

screw yield stress = 240 MPa (303 annealed, since NAS 721 doesn't specify)
friction coefficient = 0.2 (lubricated stainless)
hole size = 1.1 * screw size

With those values, I calculated tightening torques of 3 N mm for the 0.5 UNM screw and 30 N mm for the 1.0 UNM screw. I don't know how you plan to accurately measure that torque during tightening - please share your plan.


Regards,

Cory

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

 

[Screwman]

Yuo may want to run a quick reality check by building up a couple dummy assemblies using the same bearing surfaaces and screws as the actual application and taking the parts to failure. This will give you an upper bound to look out for.
I agree with Cory that torque control in these sizes is going to be a challenge, there aren't many people making accurate torque wrenches and not many ways to calibrate drive tools in this torque range.
This reminds me of driving M1.2 screws into hard drives and having them sieze in place before the heads reached the bearing surface. That was difficult to solve (Nitronic 60).

 

[kirstant]

You can download a demo or full bolt tightening software called TORKSense from

http://www.boltscience.com/

I use it to check my own calcs. Doing a quick check it comes up with 48Nmm for the 1mm screw. I think this is definitely one for experiment.
Regards
Anton