January 16, 2013
In Shaft Alignment, Low Can Sometimes Mean High!
By Stan Riddle
A customer in the northeast US was concerned that his laser wasn’t working properly. He called into our office stating “the motor shaft is a ¼ inch low at the coupling, but the laser shows I need to remove a lot of shims from the motor feet – both front and rear”. His thought was something must be wrong with his laser.
I asked him to rough in the machines with a straightedge until it was pretty close then re-measure with his Fixturlaser Shaft Alignment System. He did, and called back to tell me the offset at the coupling was fairly close (slightly low), but the vertical foot values showed the motor front feet were +191 mils (too high) while the rear feet were +346 mils (too high)!
I asked if he had a smart phone (which he did), and if he would take a photo of the screen, and send it to me. Sorry for the glare, but hopefully you can make out the position of the front and rear motor feet outlined in yellow.
- The motor was too high, but much higher at the rear feet then the front feet.
- The slope, or difference between the front and rear feet, was 155 mils over 16.50″, or 9.4mils/inch – exactly what the angular coupling value shows (outlined in red in the first photo).
- The length of the motor shaft from the front feet to the coupling center was 22 inches. So as the slope of the motor shaft keeps moving towards the coupling center, it’s getting closer and closer to the stationary machine’s reference rotational centerline (zero), until it finally goes to the negative side. In this case 16 mils low at the coupling center.
Happy customer – good learning experience.