The Tormek I have does not have provision for this and relies on gravity ( the motor is free to swing on an arc) I think the Scheppach is a pretty blatant copy although a Jet clone that I had in for repair a few weeks back has a threaded knob to adjust the tension as well as some other nice features.
You should be able to see how your works by looking inside - maybe the end of the shaft and rubber wheel need a clean up?
Bob