Help balancing a tension system

[Mopar_Mudder]

I have built a horizontal masking system that covers a projector screen. Basically amount to faric that is rolled onto a 1-1/2" shaft and is kept in tension to hold it tight by a haning weight attached to a pulley at the end of the shaft.

With the system as it is it takes X amount for force to pull the fabric out and you have to physically lock it in place so way to keep if from retracting. I would like to add an equal constant force pulling the oppisite way on the fabric so that it would be balanced. Another words it would take almost no force to move the fabric and it would stay put when you stop.

If I am correct the problem I see is that the force needed to keep is balnced is going to change with the amount of fabric that is rolled up on the shaft, chaging the diameter of the shaft. Also I could have the same problem with the cable rolled up on the pulley at the end of the shaft if I have more them one wrap of cable.

Looking for thoughts or idea on how to get it to work and how to figure out how much weight it takes to offset what I have.

Attached a drawing of what I have that might make it easier to understand.

 

[Navin A S]

One possibility is to use a spring to provide the counterbalancing force. The spring force would reduce as the fabric is pulled out and increase as it retracts, providing a constant counterbalancing force. You could also use a counterweight system in which the weight of the counterweight would increase as the fabric is pulled out, providing a constant counterbalancing force. The drawback with this method is that you would need to adjust the counterweight for different lengths of fabric.