Thread: Strobing

ebay - search for "digital optical tachometer" will give you a zillion hits.

I have this one and it seems to work OK

What sort of work do you do?

The common way used to be to use a strobe light shining on the workpiece or preferably a dot or multiple dots on the spindle, and adjust a calibrated knob on the strobe control unit until the workpiece appears to be stationary (non rotating). Then you read the RPM from the scale associated with the control knob you were adjusting, and apply any obvious corrections. For example you might place three dots around the spindle, or focus the strobe on four slots in a face plate from behind, in this case the indicated speed would either 3 or four times the actual speed.

Issues with this method;

The strobe light has to be brighter than the ambient light in the work area to eliminate any strobing effects associated with the ambient light, eg flouro lighting or discharge lighting.

You need to have a rough idea of the speed range, as you can get strobe effects at multiples of the RPM suggested by the strobe. For example an item that appears to stop at exactly 1000RPM will also do so at at 2000RPM and 3000RPM strobe rates. If you know that the particular belt setup will give a speed range from 800 to 1500RPM, then you could be sure that the actual speed would be 1000RPM, as the other possibilities are outside the realistic capabilities of the machine. Not so if you have a VSD with a range from say 50 to 2500 RPM.

The equipment tends to be large and bulky, and being analogue based, is not highly accurate.

Personally, if I needed to frequently determine the speed of a rotating spindle, I would spend a few dollars and buy an RC model optical tachometer. Small, lightweight, device. Shine a light on the spindle permanently, put two dots on the spindle and aim the unit at the dots, set the unit for a two bladed prop, press the button to take a reading and read directly from the digital display, accuracy beter than .5% typically. All up cost under $50, and the unit would fit in a shirt pocket.

Edit Or go with the unit Bob suggested which incorporates the light source in the unit but is a bit bigger.