WHQuiz Answer #8 - Nipped Driven Rollers

Published: March 07, 2012, By Timothy J. Walker

WebHandlersQuiz 8: Traction – When is a nipped roller needed on a driven roller separating two tension zones?

The capacity of an unnipped drive roller is an exponential function of web-to-roller coefficient of friction and wrap angle (in radians). For example, a COF of 0.3 and wrap angle of 180-degree (3.14 radians) has the capacity to separate a 2.56 to 1 tension change without slipping.

Therefore, if the tension change between two zones is less than 2.56:1 (plus a safety factor), a nip should not be required.

Care should be taken to ensure the tension change across the unnipped roller not greater than anticipated. Tension measurements taken several rollers away from a driven roller may not represent tension changes due to roller drag or inertia. A nip might be required for machine threading when tension in a zone may be near or at zero, not set point.

Also, care should be taken to ensure that coefficient of friction measured in low speed testing is not lost at high speed and low tension due to air lubrication.   

More on this topic was covered in this archived Web Lines column: 

Get a Grip: Driving Your Web Jan. 2008 

If you missed it, here are all 20 questions in my Web Handler's Quiz.