Understanding Motor Driven Rollers & Tension Zones

[tomizzo]

So I'm having a very difficult time understanding how motor driven rollers are able to create different tension zone of a moving web. Look at this for example.



So is any zone between two driven rollers a tension zone? And how do these driven rollers change tension while also being able to moving the material at the same speed throughout different tension zones? What effects how much tension is between two driven rollers? Torque?

 

[Chestermiller]

The driven rollers are not running at exactly the same speed. Tension is increasing through the system if the downstream rollers are running faster than the upstream rollers. There is also some slippage that occurs by the web on the leading edge of the downstream rollers.

 

[tomizzo]

The driven rollers are not running at exactly the same speed. Tension is increasing through the system if the downstream rollers are running faster than the upstream rollers. There is also some slippage that occurs by the web on the leading edge of the downstream rollers.

So you're saying that the driven rollers are not running at the same speed, however there is some slippage that occurs which would suggest that the web is running at the same speed? I keep thinking that the web must be moving at the same speed of the driven rollers speed. That's wrong then?

 

[Chestermiller]

So you're saying that the driven rollers are not running at the same speed, however there is some slippage that occurs which would suggest that the web is running at the same speed? I keep thinking that the web must be moving at the same speed of the driven rollers speed. That's wrong then?

The web tension can only increase if the web stretches. This stretching can't happen if the web is going the same speed as the roller. The stretching happens by the web slipping relative to the roller during part of its contact (e.g., in the region near where it departs from the roller). Such slippage can be caused by the advance roller running a little faster than the previous roller.

 

[tomizzo]

The web tension can only increase if the web stretches. This stretching can't happen if the web is going the same speed as the roller. The stretching happens by the web slipping relative to the roller during part of its contact (e.g., in the region near where it departs from the roller). Such slippage can be caused by the advance roller running a little faster than the previous roller.

Say for example then the web was a very strong metal that absolutely could not stretch. Would that mean the tension could not increase?

Also, can the web decrease tension near the end of the line? All of the examples I have seen just show increasing tension until a rewind section. Would it just take the downstream roller running slower than the roller in front of it?

 

[Chestermiller]

Say for example then the web was a very strong metal that absolutely could not stretch. Would that mean the tension could not increase?

For this to happen, the web would have to be slipping over all the rolls except the last one. The static friction would allow the tension to rise. But, it might damage the web.

Also, can the web decrease tension near the end of the line? All of the examples I have seen just show increasing tension until a rewind section. Would it just take the downstream roller running slower than the roller in front of it?

Yes.

Kinematically, the axial tensile strain in the web is proportional to the web velocity minus the initial web velocity.

 

[tomizzo]

For this to happen, the web would have to be slipping over all the rolls except the last one. The static friction would allow the tension to rise. But, it might damage the web.

Yes.

Kinematically, the axial tensile strain in the web is proportional to the web velocity minus the initial web velocity.

Now imagine if these motors are torque controlled. The tension relies directly on how much torque is provided by the motor and also on the diameter/radius of the roll. How can the web speed still be controlled? Say I have web running at 100 fpm at a certain tension, what would be necessary to run the web at 200 fpm? The torque would remain the same in order to keep the same tension correct? Can motors hold a constant torque while also being set to move at a certain speed? Or if you change speed, that also means you changed torque?

 

[Chestermiller]

Now imagine if these motors are torque controlled. The tension relies directly on how much torque is provided by the motor and also on the diameter/radius of the roll. How can the web speed still be controlled? Say I have web running at 100 fpm at a certain tension, what would be necessary to run the web at 200 fpm? The torque would remain the same in order to keep the same tension correct? Can motors hold a constant torque while also being set to move at a certain speed? Or if you change speed, that also means you changed torque?

I don't know the answer to the question about the motors. But, if you double the line speed and the web behaves elastically, I think that the tension profile through the line should remain about the same. (This, of course, assumes that the inlet line speed is also doubled, meaning that the mass throughput rate of the line doubles).