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Old 02-24-2013, 07:13 PM   #3
Join Date: Sep 2004
Posts: 779


I am not sure how one would control tension with an electric motor.
I saw an old prince machine which used a pneumatic cylinder. It seems like it would be much simpler design, and better amenable to controlling tension:

motor pumps the air --> the air reaches required pressure --> pulls the string. There are pressure relief valves. They will let excessive pressure off the cylinder. The ones I've seen were pre-set at manufacturing. I am not sure if adjustable pressure valves exist.

Alternatively, you can buy a tire pressure guage, overpump a bit, then open the valve until the pressue is just right -- similar to what you'll do when pumping a tire on a car. You can call overpumping a "pre-stretch" ( a sofisticated feature that only 3K+ machines provide).


Another alternative: get a reasonably tight spring. Calibrate its elongation under various tensions, say 50 lbs = + 1 in, 60 lbs = + 2 in.
Then attach string -> spring -> motor, and a ruler underneath. The elongation of the string will tell you the tension. If your motor is slow enough, so you can move in tiny steps.
tball is offline   Reply With Quote