Fastener preload.

[mcaskey]

Can any one tell me an easy way to measure the preload on a small screw. I have a #10 screw used to assemble 2 housing halves. I am looking to switch to a thread forming screw to elliminate the secondary opperation of tapping. The dillima comes from the driving torque of the thread rolling screw. It takes 16in/lbs to drive the thread rolling screw and my #10 has an assembly torque called out at 20in/lbs. The thinking around our office is that this takes away from the end clamping force by using most of the torque while driving. The screw manufacturer however disagrees with this and says that the torque will not effect the clamp force.
So I need a quick, cheap way to measure this to decide which school of thought to go with.
Any suggestions??

 

[hydrae]

You can verify preload by placing a split ring washer(s) between the clamped pieces as a test, get the right number and size of washers so that they clamp to 1/2 their width with the maching screws at the 20 in-lb, then see how much torque is needed with the self tapping screws to produce the same clamping force.
Hydrae

 

[MikeHalloran]

For a through hole (or a very thin wall around the bottom of the hole), the drive torque for a thread- forming screw falls off to ~nothing, once it has formed a perfect mating thread. In that instance, there would be no reason to change the final seating torque spec.

In this instance, a blind hole with no relief at the bottom, the torques must add.

If you're working fast, add a little vigorish for whatever air is being compressed in the hole, and be stingy with the Loctite.

Similar to the danger of hydraulic locking, some of the point forms on thread rolling screws will force a few chips ahead of the screw, so be sure the cored/drilled hole has a little extra depth, or better, make it a through hole.



Mike Halloran
Ft. Lauderdale, FL, USA

 

[IFRs]

If you are thread forming in both parts, there is no clamping force after the threads are formed and the screw head bottoms out.

 

[dvd]

If it isn't worth spending money on the proper testing it must not be all that important. I'd recommend that you look at the skidmore-wilhelm torque tension tester. Call and talk to them about your particular needs. See more at:

http://www.skidmore-wilhelm.com/sw_industrial_torque.asp