Torque values for small fastners

[duc916]

I am trying to find a general torque "rule of thumb" spec for the following fastener sizes:
6-32
6-40
8-32
8-36
10-24
10-32

The fasteners are holding cover plates and are being threaded into Aluminum, due to the nature of the application the only stress is what is induced by the applied bolt torque. The only reason I need the spec is the “gorillas” that are installing the plates find the largest tool possible and strip the threads. With a torque spec I can hopefully stop the abuse. Thanks for the help.

Dan

 

[Screwman]

Rules of thumb always seem to have many exceptions that end up applying to the case at hand.
One of the best ways that we have found over the years to deal with this type of situation (where the fasteners and joint don't fall into any handy tables) is to take a valid sample size of parts and torque them to failure. Then take 65 -75% of the mean failure torque as your nominal seating torque. If fastener breakage is the failure mode, this will end up with fasteners that are tightened in the range of about 75% of yield (making a lot of assumptions about material properties).
This is the most accurate quick and dirty method that I know to get realistic seating torque number.

 

[diamondjim]

6-32 16 INCH LBS
6-40 18
8-32 30
8-36 31
10-24 43
10-32 49
these are dry torque values
per grade 5 fasteners.
A good place to start.
If they still break try
5/8 times the above values.

 

[desertfox]

Hi duc916

You need to calculate the stress imposed on the aluminium
threads and check the length of thread engagement between the aluminium and the fastener (steel I assume).
The "macinery's handbook" will give you formula for calculating the above. Once you have been through those calculations a rough approximation for the tensile load generated within the fastner post torque is:-


F=T/(0.2*d)


where F = tensile force
T = Torque
0.2= friction factor (dry)
d = nominal bolt diameter



regards desertfox