Ruland Shaft Collars and Couplings

Stick-Slip

Tightening a screw is a deceptively simple task.  When it’s done to attach collars to shafts, any number of difficulties can arise.  One common problem is stick-slip.

Stick-slip is the condition where the screw, instead of rotating uniformly as torquing continues, begins to show a stop-start pattern. Stick-slip can create a false impression that a screw has been tightened to its appropriate stress level. During tightening, a screw rotates uniformly as it’s torqued down, then it reaches a point where its rotation gets sticky.  The screw begins to turn in a choppy manner, stopping and starting even though tightening torque is constantly applied.  The lost torsioning effort during stick-slip is typically absorbed as excess friction between the threads or underside of the head and the mating parts of the clamp body, instead of contributing to the stress in the joint elements.  If these stresses in the joint are too low, the collar will not hold well.

Black Oxide:

The best way to avoid the effects of stick-slip is by using specially coated elements.  For example, black oxide helps to smooth the torqueing of screws without diminishing the frictional characteristics and holding power of the bore.  The black oxide on our carbon steel collars and couplings is formulated as part of the total performance of the product. It affects the holding ability of the collar/coupling, the anti-stick-slip characteristic and helps to keep the torque rating of the screw within its designed parameters. It also provides corrosion protection, although it is not intended to be used as an outside weather protective finish.  Clamping screws that operate smoothly during torqueing are the best assurance that stick-slip is not present.