Saturday, July 6, 2013

Spring gun prototype is done!

The spring gun is fully built. But it functions at ~12% efficiency, which is lower than I hoped for. I suspect this is because of losses in friction between the spring and the buffer tube that it's around. I tried oiling it but this produced no better results.

The pictures here are it shooting through my cronometer. As part of this, I learned that the crono requires it's backdrop/shield to be back-lit and expects this to be from the sun. Indoor it doesn't work with neon lights because the crono can see the 60Hz of those things. It turns out finding incandescent lights needed for the crono was harder than expected since they've become banned. So I just bought then from the manufacturer for a million dollars. Not sure where they get them.

In terms of performance we're expecting, we have to find the energy the spring will release: I measured the displacement of the spring with a 10lb (44N) weight and found it had about 2" (0.0508 m), which results in a K (spring constant) of 44/0.0508 = 886. The current setting is for it to release when it gets to about 3.75" (0.09525 m), which should yield 0.5 * 886 * 0.09525^2 = 3.929J

When fired, it launched a 0.5" ball bearing at ~35ft/s (10.67m/s). The ball weighed in at 8.9g (0.0084kg). It then must have had 0.5 * 0.0084 * 10.67^2 = 0.4782J. This means the machine is operating at 12% efficiency.

The next step is to check the efficiency at different release levels. If efficiency goes down as power increases that would be a bad sign. After that, it's time to try other springs.

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