Installation torques for instrument threads
Installation torques for instrument threads
(OP)
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!
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





RE: Installation torques for instrument threads
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.
RE: Installation torques for instrument threads
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).
RE: Installation torques for instrument threads
Regards
Anton