BMatthews

As promised here is a couple of pics of my own thrust meter. So far I've used it only for electric motor testing.

In the first picture you see a close up of the test stand with the built in spring scale. I wound the springs from music wire myself as I could not find commercial ones that would work for my arrangement.... well, I actually built myself into a corner and HAD to make my own springs... but that's another story. The arm is some oak I had handy but I would have used a hardwood in any case as I wanted to push fit the ball bearing into the arm and it takes hardwood to hold the bearings without slippling around or enlargning the hole. My hand is pulling on the arm slightly so the tune of about 4 oz of simulated motor thrust. There's actually 3 springs in total. One for small thrust readings that goes from 0 to 16 oz and a second one that starts at 7 and goes up to 40 oz. The third spring is the calibrator so I can set the wingnut for each spring to a known setting so the calibrated scale on the board is accurate. The scale calibration was originally set using a very free pulley and a bucket of weight that I would measure and then hang on the line and mark the spot. This was checked a couple of times to ensure decent accuracy and repeatability. It's easily good and consistent to within 1/4 of an oz and that's close enough for what I'm doing.

The second picture shows the whole setup with a motor in place for testing. The big black box to the left is my new toy, a 25 amp 13.8 volt power supply. $59 US. If anyone is interested I can post the details. Before this I just used a tired but still decent car battery that I'd charge up for a day before the tests. The second box is my self made speed control that has a current meter in it and will handle up to 30 amps or so for long enough at a time to take readings. Note the test rig is clamped to the bench so the arm is over the side for prop clearance.