Machine Parts – Checking The Small Stuff

Quite often it is the little things in life that rare up and bite us when least expected. We were working in a factory that makes large grinding machines. The price of the machine depends on the size and the one we were looking at sold for USD$250,000.

After changing the spindle, the pulleys and the belts, the quality control engineer was at his wits end trying to locate the source of a spindle vibration using a vibration pen.  The amplitude on the spindle exceeded what was allowed before shipping.  We were there for another project but he asked if we could take and look and see what we could find.

First we took a vibration measurement FFT on the spindle housing. This told us that the vibration was at 1780 CPM with an amplitude of 3.1 mm/s. Because the spindle rotated at 3800 RPM we knew that the vibration did not originate with the spindle but from another source that was vibrating at 1780. Of course the only thing that could be operating at that frequency was the drive motor. There was some confusion since we were in Europe, which operates on 50 Hz power, and what we were seeing didn’t match with any 50 Hz sources. That was finally cleared up when they  told us this machine was shipping to the U.S. and they were operating it on 60 Hz.  Some days get frustrating!

Since the vibration was at 1X of the motor, we said the motor or pulley was out of balance. They had the standard answer, “Couldn’t be, it’s expensive and brand new”. So we suggested they remove the belts and run the motor solo. Same results, the unit was out of balance. Next step was to remove the pulley and run the motor again.

We observed as the pulley was removed that it clamped onto the shaft with a set of screws around the perimeter of the pulley and had no keyway. However, the motor shaft had a keyway cut for installation of a key even though in this type of installation, no key was needed.

BINGO!

The amount of metal removed from the shaft for the key, which wasn’t there, was enough to induce an out of balance condition. Solution, install the bottom half of a key in the shaft slot and reinstall the pulley.

Problem solved, the machine was well within vibration tolerances and was shipped that afternoon. Moral of the story, when trouble shooting, you’ve got to watch everything that goes on, even the little parts!