ALIGNMENT

Motors must always be accurately aligned, and this applies especially be where they are directly coupled. Incorrect alignment can lead to bearing failure, vibration and even shaft fracture. As soon as bearing failure or vibration is detected, the aligment should be checked.

Couplings

To determine whether the shafts are parallel, measure first with a feeler gauge the distance x between the outer edges of coupling halves at a point on the periphery  : Fig 

 Then run both halves togheter through 90°, without changing the relative positions of shafts, and measure again at exactly the same point. Measure the distance again after 180 ° and 270 ° rotation. For typical coupling sizes, the difference between the highest and lowest readings must no exceed 0.05 mm.

To check that  the shaft centres are directly opposite each other, place a steel rule parallel with the shafts on the turned periphery of one coupling half and then measure the clearance between the periphery of the other half and rule in four positions as a parallelism check. The difference between the highest and lowest readings must not exceed 0.05 mm.

The best way of achieving proper alignment is to mount a pair of dial gauges as shown in fig

Each gauge is on coupling half  they indicate difference between the coupling halves both axially  and radially. By slowly rotating the shafts while observing the gauge readings it possible to obtain an idea of the adjustments that need to be made. The coupling halves must be loosely bolted together so that they can easily follow each other when they are turned.

When aligning with a machine, the frame of which reaches a different temperature from the motor in normal service, allowance must be made for the difference in shaft height due to differences in the thermal expansion. For the motor, the increase in height is about 0.03 % from ambient temperature to oprating temperature at full output. Mounting instructions from manufacturers of pump, gear units etc. often state the vertical and lateral displacement of the shaft at oprating temperature. It is important to bear in mind this information to avoid vibration and other problems in serfice.