Backing force of bolt based on torque and thread pitch

[sawells56]

Hey everyone,

I have a threaded pin with a 4.5" diameter that is torqued into place and as a secondary retention method has set screws that stop the pin from backing out. I know the relation of Torque to Clamp load: T=K*D*Fc, but i cant find any relationship of thread pitch to backing force. I know the T applied to the pin is 180000 in-lbs. see attached pdf for my set-up (threads not shown). Any info on the relationship of threads to axial force would be appreciated.

Thanks

 

[hydtools]

Do the basic inclined plane analysis of the forces on the thread.

Ted

 

[MikeHalloran]

I don't see a setscrew.
I see the tip of a machine screw, projecting part way into an annular groove in the big pin.
Since the head of the machine screw seats on the outer member, the machine screw does not prevent the big pin from starting to rotate.
After the big pin has rotated a bit, one flank of the annular groove will contact, and eventually deform, the threads at the tip of the machine screw, making the machine screw non-removable.
Whatever torque is applied to the big pin will have the mechanical advantage of the big pin's thread/inclined plane in deforming the machine screw tip.

I may be misinterpreting your illustration.





Mike Halloran
Pembroke Pines, FL, USA

 

[sawells56]

the machine screw is what I meant as a set screw, and the deformation of the threads is expected, however the machine screws are simply there as a safety measure.

As far as the mechanical advantage of the plane:
Is the axial force then just: F*(P/D)?
where F is the tangent force from the torque F=T/(D/2), P is pitch, and D is pin/screw diameter.

 

[hydtools]

Thread pitch is in the equation used to calculate K, the torque coefficient which combines the effects of helix angle, thread form, and friction.
See here:

http://euler9.tripod.com/fasteners/preload.html

The result is the equation relating torque to clamp load which you used to calculate torque to create a preload.

Ted