Inspecting machinery with a strobe

We all know that maintenance engineering involves inspecting machinery to ensure that it is operating correctly. However stopping a machine in order to inspect it is costly so there is a strong emphasis on running inspections to save this cost. One of the more common tools to this end is the strobe light.

Unfortunately, there isn’t much in the way of resources out there that explain how to properly use these. What exactly are you looking for when using a strobe? While some applications are pretty simple and common sense- such as “freezing” a universal joint so that you can visually inspect it for signs of lack of lubrication or play- how to you check for a worn keyway or a failing bearing?

To simulate a bad bearing, I put a sprocket in a lathe. I first set it up nice and concentric. I started the lathe up at high speed- about 1800RPM and set the strobe to the point where the sprocket froze nicely. About 30Hz, see image below.

You can use a single repeating feature like a stamped part number or a keyway to confirm that you have the correct frequency if you are unsure of the rotational speed. I find it’s easiest to start the strobe too fast and slow it down until the part “freezes.” Below, You can see the strobe at double speed- it looks like it has two keyways.

The below image is with the strobe in sync again, but this time, I deliberately set the sprocket incorrectly in the chuck so that it wobbles as if it has a bad bearing. You can’t see this with the strobe at single speed but at double speed, you can see that on this picture, there is now a slight ghosting. Looking closely around the outline of the sprocket, you can see a double outline. This is the indicator of eccentric movement – an indicator of a failed bearing or play on the shaft.

Below, I have two images back to back to show more clearly the difference that you can see when the sprocket is turning eccentrically.