Prevailing torque is the turning resistance encountered when driving in a fastener such as a self-tapping screw. The initial resistance required to cut the thread can sometimes be higher than the torque required to clamp two mating parts together. The challenge here is to apply enough torque to cut the thread but not enough to strip the thread out prior to reaching the final tightening torque. This problem is often encountered with the joining of thin materials such as sheet metal. As industries search for lighter and thinner materials, engineers are faced with new fastening challenges. Fasteners that were effective on thicker materials are now stripping out on the thinner materials.
Traditional clutch-type screw drivers do not cope well in these situations and programmable DC screwdrivers with multi step torque programs are far too expensive for non critical joints.
One method to overcome this issue is with Kolver‘s range of low-cost DC electric screwdrivers which feature a prevailing torque function. By simply programming a time in which the screwdriver operates at maximum torque before switching to the final torque as set by the user. You can set the timer in 0.1 second increments from 0.1 to 10 seconds.
The graphs to the right show what will happen when faced with a self tapping screw driving into sheet metal where the initial thread cutting torque is higher than the final rundown torque.