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How to detect unwind brake issues affecting tension control

by Bassam Dib,, Senior Analyst




Failure of a braking system where brakes no longer engage would be detected by the operator as it will lengthen the deceleration time of the parent roll following a sheet break or e-stop.

On the other hand, partially applied brakes will affect the unwind motor torque level required to maintain the sheet tension on its reference. This can alter the dynamics of the tension control loop, resulting in increased tension deviations. This is harder to detect since the motor torque is constantly being modulated by the control to maintain tension feedback on its reference.

Fortunately, the technique described hereafter enables the detection of such situations. By estimating the sheet tension from unwind motor measurements and comparing it with the tension set point, it becomes possible to detect developing discrepancies.

For example, defective brakes will cause the unwind motor torque to decrease since the additional friction from the brake helps build part of the required sheet tension. Therefore, the force ratio between estimated tension and tension set point will also decrease enabling the detection of an abnormal situation.

In order to detect intermittent or permanent failure of the mechanical components of the unwind brake, the tension force ratio should be monitored on a continuous basis using an automated analysis system which will generate an alert if values fall beyond predefined limits.

Using this technique, we detected a decrease in the force ratio on one of the winders we monitor. Analysis of the data collected on these sets revealed a significant increase in tension deviations during speed changes.

We reported this to the mill with a recommendation to inspect the unwind brake and its hydraulic supply components.


An inspection by the mill revealed a sticking spool inside of the brake’s solenoid valve. This was leaving the brakes partially engaged throughout winding.

Following the replacement of the valve, our continuous monitoring enabled us to confirm that the force ratio was back to its normal level and that the reliability of the unwind brake system was restored.

Our intervention allowed the mill to preserve roll quality avoiding major losses and challenging roll traceability in case of customer complaints.

The continuous monitoring provided the mill assurance that we keep a close eye on the tension control and will report any anomaly which can affect their operation or product quality.

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On winders, an unwind brake malfunction can interfere with the tension control leading to issues with sheet stability and internal tension variations within wound rolls.

Defective operation of the unwind brake system can cause the brake pads to remain partially applied during winding. This situation does not provide obvious signs and therefore can remain undetected until it reaches a critical state.

With the proper tools, it is possible to detect such a situation early on, preventing or limiting damage to roll quality or mechanical components