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Stainless screw to Aluminum torque values

Stainless screw to Aluminum torque values

Stainless screw to Aluminum torque values

(OP)
Motor bracket is 6061, screw is 8-32 SS SHCS, threaded into 6061, torque spec is 8 ftlbs but totally insufficient, recomend 18?? Any input out there??

RE: Stainless screw to Aluminum torque values

What is the problem under the current loading of the screw?
What is your thread engagement length?
What temper is the 6061?
Normally a screw torque is given in in/lbs.

RE: Stainless screw to Aluminum torque values

The site below says it is 19. There is also a formula in Machinery Handbook for length of engagement. Al has a problem of threads pulling out if thickness is too thin, especially if screw is to be removed after. If material is thin it is better to use a nut plate or rivnut.

www.engineersedge.com/torque.htm

RE: Stainless screw to Aluminum torque values

drgauf,

   You said your units were ft.lb.  I am pretty certain you meant in.lb.  

   Recently, I did a screw torque chart mostly for stainless steel screws.  I did not worry about the material of the tapped holes.  I made a whole bunch of assumptions and I got 17 lb.in for 8-32UNC.

   These assumptions are as follows...

-  The holes are deep enough that the screw will fail in tension rather than shear.  
-  The threads will be lubricated by something, probably Loctite 242.
-  The yield stress for 18-8 stainless steel is 55ksi.  

   That last assumption is complicated.  All sorts of stress values are quoted for this material.  I used 55ksi because a local vendor quoted it for his stainless steel socket head cap screws, and this was the lowest value I found.  Metric cap screws in stainless are usually supposed to be A2 stainless with a tensile strength (ultimate?) of 800N/mm^2 (~115ksi).  When I torque down metric screws as per my torque charts, they feel much stronger than 55ksi.

   The effect of using Aluminium 6061-T6 for your tapped hole that you need to exceed some minimum thread engagement length to ensure a tensile failure of the screw.  As noted above, this is worked out in the Machinery's Handbook.

                            JHG
    

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