Web PLI

PLI though a gearbox without a loadcell on a web application.
I know I’m being kind of vague. Has anyone tried to read back drive torque and control PLI ?
I know diameter of winder, gearbox, drive actual torque ....

Thoughts?

In this case vague is an understatement. What are you trying to actually measure or do?

pounds per linear inch

The weight of the open web, the total length of the material on the winder roll or something else.
I presume when you say you know the drive torque you mean the winder drive. Not the web drive if there is one. Using motor torque / load is a very poor way to get that information there are just too many variables in play to get an accurate measurement that way. If you need to know the torque then Install a torque gage on the final gearbox output shaft, there are many different manufactures that can supply them to you.
If you are trying to determine the weight of the material on the take-up roll then the diameter of the roll may be close enough for you, ultrasonic sensor or dancer position may work for that.
If you could give us more details then somebody here may be able to better help you.

You may have a tough time with this depending on the specs you are trying to hit. Tension control based on torque feedback only works reasonably well if:

1) The gear reduction is relatively low
2) The tension variation spec if relatively loose
3) the build ratio is relatively small OR the user is planning on natural tension taper.

As GaryS alluded to, your control scheme assumes that all of the torque produced by the motor is used to generate tension and that no necessarily the case. The more additional losses you have the less accurate your control will be.

Keith