When designing the line, the load cell roller should be inside a defined tension zone. That is the tension in this zone should be identical into and out of the load cell roller. This is easier if a few rules are followed. The load cell roll should not be driven. That is it should not form a traction point and create separate zones at the entry and exit. The roller should have enough grip to run exactly at line speed. Bearings should be good enough as to not introduce tension variations into and out of the roller.
Monday, October 05, 2015 1:29 PM
But what if all rollers in web line are driven, ie. load cell must be mounted to the driven roller? In that case, what steps needs to be taken to display tension on control sistem?
Monday, October 05, 2015 11:54 PM
If at all possible, the tension rollers should be undriven. This is not always possible on paper machines where tension rollers may be 800 mm diameter or larger. In those cases, load cells were added to driven rollers.
When doing this, you can expect several problems and must learn to accommodate them.
1) The load cells will have to be able to support he roller and may have very little signal to tare ratio.
2) The load cell will see resonances in the system to a very large extent.
3) The load cell will read incorrectly when the line is accelerated. This can be seen when accelerating the roller with out web and watching the output from the load cell.
Tuesday, October 06, 2015 11:17 AM
So, if load cell is added to driven roller, what force is measured by load cell? Because of the drive, tensions at the entry and exit of roller are different? If so, is it possible to use this signal from load cell to form closed loop tension control system?
Tuesday, October 06, 2015 7:39 PM
A load cell measures force due to tare and tension at a "point". The tare is known and can be subtracted. The tension force vector can be converted to tension.
The load cell measures the average of tension into and out of the roller. If the load cell is undriven and in the middle of a tension zone (between two traction points), tension in and out of the load cell roller is about equal.
If the roller is driven, the tension in and out will almost certainly be different. We do not know the entry and exit tension, only the average.
One more reason not to drive the load cell roller.