I believe I've finally hit on the correct solution for this existing machine. I wouldn't do this if I were designing from scratch, but I ended up with a whole lot less room inside the spool-up area that I first thought I'd have.
First, the issue of constant tension: The process will work OK over a fairly narrow range of tensions, but not over the range the machine provided. Part of the problem was hysteresis in the friction clutch, but part of it was due to the large differential between "empty" and "full" spool sizes.
It turns out that a "full" spool holds thousands of feet of product, when a standard run is in the low (3-4) hundreds of feet. So the solution here is to bulk up the spool hub until the difference between empty and full is only about 10% of the spool diameter. That means a 10% difference in empty-to-full tension if constant torque is provided -- and that's well within the range that will work properly for the process.
The other problem - hysteresis in the clutch - turned out to be a lot simpler to solve than I first thought. The spool rests on a driving hub that has pins which engage holes in the spool end, and is supported by a cantelevered bearing in the driving hub. The other end of the spool is supported by a bearing on a swing arm so the spool can be removed/re- inserted in the machine.
The driving hub is on a simple two-row bearing, so when the clutch is not slipping, the spool takes up a lot of tension in the form of stretch in the product. When the clutch finally slips, some of that tension is released by the spool by rotating backwards against the friction clutch until the tension is at/below the slipping torque of the clutch.
Simple fix. Replace the drive hub bearing with a pair of sprag bearing/clutches. VBX (and others) make some nice combination deep groove ball bearing/sprag clutch combinations in standard 6200 bearing profiles. In the 25mm i.d. sizes, they all withstand several times the required back-off torque, and I'm using two in the same hub.
Now when the clutch slips, the spool cannot reverse. Within a half- second or so of starting, the clutch reverts to a constant-slip mode, and never gets back to the stiction mode. Backlash is virtually zero on the VBX sprags.
And hey! It WORKS!
LLoyd