DWH Technical Topic: Web slippage
By Clarence Klassen, P. Eng.
Web slippage is quite a common problem for drives. Traction rollers are prone to losing traction, and the drive can introduce slippage. The chart above shows torque for a rubber and chrome nip roller with evidence of slippage. Torque is grabbing and releasing at a very fast rate. In-running tension was 1.7 PLI and controlled with load cells. Outgoing tension was 2.25 PLI and also controlled with a load cell. The slippage problem was solved simply by reducing the outgoing tension to match the in-running tension.
A slipping drive controlled by a load cell can be indicated by the tension regulator running to its output limit and the tension being not quite right. Most drive controls do not monitor the condition of a tension regulator running at its minimum or maximum output limit.
A slipping drive running in draw or ratio control is quite difficult to detect. The speed setpoint and feedback will match. The torque will often be steady. Hints are the ratio is incorrect based on previous experience or the torque is different than when the roller is not slipping.
When a machine section is controlled by several motors (helpers or followers), the load sharing between the motors will not function as expected when the drive is slipping.