DWH Technical Topic: Unwind speed match

By Clarence Klassen, P. Eng.

A flying splice at the unwind requires excellent speed matching. The speed match we are talking about is the surface speed of the incoming roll in MPM or FPM. Several factors are required to get the speed correct. Accurate speed matching allows tension variations and wrinkling at the splice to be minimized.

The first thing we need to know is the speed of the web at the other unwind roll. This is normally measured at a different downstream roller with good traction and with no other traction points in between it and the unwind. Line speed is normally an easy signal to read accurately. A good rule would be to splice at a fixed speed so effects of acceleration are eliminated.

Next we need to know the diameter of the incoming roller. Any error in diameter measurement produces a proportional error in speed matching. The diameter may be measured with a tape or scanned with a laser or ultrasonic sensor. Use metric tapes or tapes with decimal inches for best accuracy. If the diameter is scanned, the incoming roll should be rotated for at least two revolutions in which the diameter measurement is averaged.

The incoming roller may have a speed match adjustment provided to give a little extra speed for splicing. This will have to be set while running such that the impact on tension and wrinkling are reduced to a minimum.

Next the speed of the incoming roll must be well regulated. The unwind drive should have its speed regulator tuned or auto-tuned for the most common diameter to be run. The time constant of the speed regulator will determine how early the incoming unwind roll must be accelerated such that it is running steadily at the correct speed at the instant of splicing.

Please allow enough time for the incoming roll to accelerate to line speed and settle out before the instant of splicing. The drive often accelerates at torque limit thus guaranteeing a speed overshoot which must be allowed to settle out. The tuning of the unwind drives affects how long it takes for the speed to settle. A drive with at least 5 radians per second will provide satisfactory speed regulator response. Servo drives may be capable of much faster speed response.

The cost of running the incoming unwind drive for a long time prior to the splice is that the roll may entrap air in the outer layers. The splice tape line may even come undone resulting in a missed splice.