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Using Prevailing Talk 

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 material, 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 screw drivers 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 fom 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.

Graph 1 is the Kolver Pluto 3D programmed to achieve a final tightening torque of 1,5 Nm. The prevailing torque function is set at 0,3 seconds.

Graph 2 is the Pluto 3D programmed to achieve a final tightening torque of 1,5 Nm with the prevailing torque function turned off.

Graph 3 shows what happens with a standard clutch screw driver set to overcome the initial thread cutting torque.

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Graph 1

15MnGraph2

Graph 2

ScrewNotSeatedGraph

Graph 3

ThreadStrippedGraph