I am getting variation in sheet length. What should I do?
Posted on | by Scott

I am getting variation in sheet length. What should I do?

The most often asked troubleshooting question concerns sheet length variation. Troubleshooting these problems can be frustrating but the following are some of the basic things to check:

If the sheets are always short: 

  1. Check for slippage at the nip roll. This can be due to a hard or glazed rubber roll, too little nip pressure, or too much roll stand brake pressure.
  2. Check to see if the knife drive belt is riding up on the flange of the lower pulley.
  3. Check to see if the draw drive belt is tracking properly on the paper pulley.

If the sheets are always long: 

  1. Check for improper pinch at the mouth rolls of the tape system.
  2. Check to see if the draw drive belt is riding up to the flange of the lower pulley.
  3. Check to see if the knife drive belt is tracking properly on the upper pulley. If the machine is equipped with a Change Pulley arrangement using a Poly-V belt, be sure it is not slipping. This can be due to belt wear or insufficient tension.
  4. Check the take-ups to be sure they are functioning properly.

If the sheets are both long and short: 

  1. Check couplings for the signs of misalignment, damage to flex packs, and loose keys.
  2. In most cases, timing belts are used. If these are damaged (for example broken cords), they will not transmit the necessary power to maintain sheet length. As mentioned above, improper tracking can also be a problem. All belts should be checked since belts which ride against the pulley flanges can ride up on the flange and have the affect of changing the ratio slightly. This, in turn, affects sheet length.
  3. Backlash can be found in a number of places depending on what components make up the drive. Any gearbox in the system can have backlash. As mentioned above, keys and keyway throughout the drive system must be checked for looseness. On many machines, backlash can be located by locking the draw roll pulley in place and trying to rotate the knife pulley. If there is movement, have someone look at the drive train while this pulley is rocked back and forth. Different types of variable speed transmission are used. Some use belts and others use chain. Check the condition of the belt or chain. Frequently, this is the problem. Be sure the belt or chain tension is properly set. The adjusting screw/nut mechanism that expands and contracts the cones must not have any play. For specific information, refer to the manufacturer’s instructions.
  4. Check the material flow as it passes through the knife section. Unstable flow causing the web position to fluctuate can cause sheet length variation. Adjustment of the air doctor boards can help stabilize the flow.
  5. Check the condition of the paper rolls. If they are out of round, it may be necessary to run slower to obtain better sheet length accuracy.