Thread engage/disengage torque

[GAllen1]

I am seeking information to understand the relationship between tightening and losening a threaded connection. Does it take more or less torque to lossen a bolted connection than it does to tighten it? The level of detail I am seeking is that which will allow me to calculate fastener combinations for product design comparisons. Thanks

 

[nashjp]

In my experience there is not a relationship between tightening and loosening torque. Tightening torque is dynamic torque, while loosening torque is determined by the static torque. Static torque has very large variability and is affected by everything including materials, coating, friction, lubricant, clamp load, humidity... The thread pitch is actually helping during loosening - the load is trying to run down the ramp of the threads.

On the same joint, I have seen breakaway torques of 120%-60% of the original assembly torque (single bolt - no interaction between bolts).

Are you trying to determine joint relaxation? A return to mark check can provide a good estimate of remaining preload. (Mark the fastener to abutment orientation after tightening, loosen the retighten to the mark). For better results you will have to use ultrasonic or strain measurement techniques.

 

[Metalguy]

If you don't have a corrosion or galling problem, the breakloose torque is usually lower than the tightening torque, at least with well-lubed fasteners.

 

[CoryPad]

I agree with the comments from nashjp and Metalguy.

In Handbook of Bolts and Bolted Joints edited by Bickford and Nassar, equations are shown for the tightening torque and loosening torque. They are functions of the bolt tension, thread flank angle, thread helix angle, friction coefficient for threads, friction coefficient for bearing surface, mean bearing diameter, mean thread contact diameter. Assuming identical variables for tightening and loosening (not necessarily true for friction coefficients), I got a tightening/loosening ratio of 1.3.



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