stick slip torque

[PhilMcC]

Hi,

We seat a number of screws in our plant dynamically and then audit check the seating torque statically. Sometimes these torque readings don't jive. I attribute this to the differences between static and dymanic friction or stick slip affects. Does anyone know of an accepted approximate difference between static and dynamic torque readings? Is it 10% or more? I know the friction numbers are material dependent but I am only trying to find an estimated range. Is anyone aware of a published document that I can reference?
Thanks.

 

[MintJulep]

There is an SAE standard or recommended practice for auditing torque. I don't have immediate access to the number.

Given the variability with torque control and the initial preload achieved, and the additional variability that you note in the check, I believe that the value of such a check is limited to only identifying fasteners that were only hind tight to begin with.

 

[zekeman]

You should be able to determine this easily using a torque wrench.
You simply measure the torque at the point of near maximum seating dynamically; then stop and watch as you restart the torqueing (forward and reverse about the same point)sufficient to just make the nut turn. The ratio of the two torques would be the ratio of the static to dynamic coefficients.

 

[CoryPad]

It can be more than 10%. The are so many variables, that you just have to make your own correlations with your own joints.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

Bekker - An Introduction to the design and behavior of bolted joints.

A half hour with a torque wrench and a clip board on the floor would answer a lot of questions.

The underhead friction usually can make the breakaway torque higher. If gaskets, or composite materials, the torque at checking can be extremely low. If evil (by definition) lockwashers are involved the research will resemble alchemy