DWH Technical Topics: Driving a bowed roller

By Clarence Klassen, P. Eng. 

Dr. David Roisum discusses the requirements of the drive for a bowed roller in his blog, To Drive or Not to Drive. An undriven bowed roller must be driven by the web – reducing traction for spreading. A driven bowed roll may be driven at the wrong speed – also reducing traction for spreading. For maximum spreading action, we want tension into and out of the bowed roller to be the same.

If the speed of the bowed roller is not precisely matched to the web speed, a tension variation results, and available traction is reduced. The variation is dependent on the modulus of elasticity and the span between rollers. The Abbot App TSpan may be used to estimate the variation in tension for a small speed error. Enter the parameters for your speed, span, and web. Then select a ∆V (error in speed) of 0.1% of the running speed. The final value (steady state) of tension will show a change in tension due to a speed error.

Your drive specifications may show that your drive is capable of speed accuracies to within 0.01% or even better, but I assure you that the drive does not match the web speed to better than 0.1% speed for a bowed roller.

Since it is so difficult to get the speed correct, torque, the other drive output, may be measured and controlled. Torque for the bowed roller can be measured at various speeds without the web. Torque during acceleration and deceleration should also be measured. Then the torque can be adjusted to these torque values while running with the web. Easier said than done.

Drive techs rarely spend a lot of time setting up drives for bowed rollers. A simple method of getting past the problem is to implement “speed droop with torque” in the drive. A value of 5% is often used. 5% droop means that if the motor runs at 100% (rated) load, the speed will droop by 5% - thus reducing the load. This is simple, but far from precise, especially during speed changes. Droop keeps the load on the motor low, which is more or less what we need at a steady speed.

Dr. Roisum has a good many other blog entries on bowed rollers. Use the Search function on the AIMCAL Blog website.

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