A while ago, I had this Eagnas electric tension head come in – together with its twin. I repaired the other one quickly but this one wasn’t acceptable because the tension that it would pull was much higher than the display reading. I.e. when set to 20lbs, it would pull at about 30lbs.
I did look into this further and eventually worked out how to adjust it. To adjust the tension that is required, we rotate a knob on the side of the tension head. By doing this, a spring is loaded up or unloaded as the case may be. Also at the same time, by rotating this knob, it moves the sliding contact of a potentiometer. The electronics then reads the potentiometer resistance that is set by the sliding contact, and then displays the result. To tension the string, a button is pressed, then the motor turns until the tension of the string exceeds the compression force of the spring, allowing a microswitch to be activated. If the string stretches, and it does, it will relax the tension allowing the microswitch to be deactivated, then the motor pulls and so on.
The problem with this machine is that the display tension and the resulting tension are way out. I had a look at the spring and yes, there was a knob that is on a bolt that could be turned to reduce the spring tension. The problem was that the knob was smooth and could not be turned by hand or even with pliers. The only way to adjust this was to remove the motor mounting, to unload the spring, then turn it with vice grips and then reassemble it. After doing this, I was able to get the resulting tension to be lower. Ok, so I need to adjust it again – then I had a bright idea – this happens from time to time. What if I take that little knob out and knurl the sides so that I can adjust it in the machine using pliers.
Since today, I was out in my workshop drilling holes in the stringing machine turntable base, I thought it was an opportune time to do this job. I took the knob out and decided to put a straight knurl pattern on it, instead of a diamond pattern. I mounted the knob onto a M8 bolt and put it in my lathe, then using the knurling tool to cut the pattern. The photo above – shows the spring with that knob on it. Now I can properly adjust the tension easily without disassembling the whole thing. Maybe I should patent this!