April 27, 2017
Precision Shaft Alignment of Machines With Small Diameter Shafts
By Stan Riddle
A VibrAlign customer recently called, stating they were having repeatability problems with aligning a small motor to gearbox to pump. After some phone conversation, I asked if they could send me photos of the machine, and the discrepancy in the measurements. After seeing them, I knew what the problem was.
Machines with small diameter shafts can present a unique challenge. The coupling used is generally flexible, but not when it’s mounted on small diameter shafts. If misalignment is present, the coupling can cause the shafts to deflect, or bend. This means that the lasers (or dials) do not read true rotation centerlines. The effect is that you measure, shim and move as directed, and the alignment result is not as expected.
Here’s how we determined the problem. The first measurement is with the coupling insert in place.
The coupling insert was then removed by sliding the hubs back far enough to remove it. No changes were made to the positions of either the gearbox or pump.
The second image was taken with the insert removed. While the change is not large, it is enough to make the aligner doubt the tool.
The alignment was done with the insert out, and was completed quickly, with values that made sense.
If you encounter a similar situation, taking the insert out means you are truly aligning shaft centerlines.
Save Time. Save Money. Save the Machine.
A quick question, with the coupling insert removed, how were you able to rotate both shafts in sync with one another? I normally install the coupling insert between the coupling halves and use tape to prevent any backlash between the alignment equipment heads when rotating the shafts together. If the shafts do not move in sync, will this not throw off your readings also?
VibrAlign/Fixturlaser’s alignment tools measure when not rotating, similar to how alignment has always been done with dial indicators. Most of VibrAlign alignment tools have built-in inclinometers which measure each detector’s relative angular position. These are displayed in the upper right and left hand corners of the screen.
So, as long as the angles of each transmitter/detectors are at the kept the same angular position (within .5 degrees or so) each time the measurements are taken, it doesn’t matter if the shafts are coupled or not.
PS: the shafts don’t have to be rotated together to each measurement position.