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How to Minimize the Impact of a Motor Coupling Failure

by Bassam Dib,, Senior Analyst




When several motors are used to drive a paper machine section, loosing a motor will cause the total load to be transferred to the other motors of the section. This can become an issue if the necessary load exceeds the remaining motor capacity. Speed mismatch can occur resulting in premature wear of the wire, felt or gears which act as a mechanical link between the rolls driven by these motors.

Furthermore, even if the other motors can handle the additional load during operation, their power might be insufficient if the section is stopped and needs to be restarted.


On one of our clients’ paper machine, the shaft between the gear box and the center press roll sheared. The other motors driving the press section had to share the load resulting in higher torque levels reaching up to 90% of rated torque on one motor.

While this did not have an impact on the operation of the machine, the additional load on the remaining motors was a concern.

motor electrical signal image

Our continuous monitoring of the motor’s electrical signals alerted our engineers to the abnormal change in the operation of the press motors.

The armature current of the motor with the broken shaft had dropped while motor speed had increased.

Our engineers alerted the mill of the situation. It was then confirmed by comparing with a strobe the speeds at both ends of the shaft that it was sheared. The mill also started to secure all the parts and manpower to correct the situation.


Meanwhile, the other motor temperatures were monitored on a regular basis.

Motors driving heavy loads are subject to significant torques on their shafts and couplings during speed changes. Different scenarios can cause these to fail when exceeding their mechanical limits. These could result from modifications in the controls such an increased acceleration rate or from significant mechanical wear.

Our engineers rapidly detected the issue, allowing the mill to take the appropriate measures to avoid significant downtime and the consequential failure of other equipment.

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