Using a Stroboscope

Has anyone used a stroboscope to measure the speed of rotation of a shaft?

It seems to me that it will strobe at various frequencies.

For example, say a shaft is rotating at 1000 RPM.

As far as I can see it will strobe at light flashing rates equivalent to 250 RPM, 500 RPM, 1000 RPM, 2000 RPM, etc.

So, if you don't know the shaft speed (as you normally won't), how would you know which one is the correct speed?

With a mark on the shaft, you will get multiple marks for mutliple speeds - but a strobe isn't really used for measuring unknown speeds.

If you know the frequency and duty cycle then it's pretty easy.

Suppose there's one mark on the wheel and the strobe flashes 50ms on and 50ms and there appear to be two marks opposite each other it means that it's moved half a revolution in 100ms.

Strobe's are really for monitoring repeating physical systems under operation where normally you'd have to spin the whole thing down. It can also be good if the strobe is connected to a high precision oscilator so you can see minor changes in the rotation speed with the human eye. Having a hand held non-contact tach is probably a good companion to a strobe, better yet for inspection purposes finding a tach that has an output that can be used to trigger the strobe is perfect. I think there was a thread here last month about someone that used a micro controller and a serires of timed LED strobes to play with 'time' using some sensors to detect falling water droplets and tim ethe strobe to it.