When performing an alignment on a machine train with a motor fitted with a sleeve bearing, it is important to account for the magnetic center of the motor. Failure to do so can cause excessive vibration and premature failure of motor components and the shaft coupling.
If the motor has recently been rebuilt is should come from the motor shop with a magnetic centerline scribed on the shaft. To properly set the shaft coupling gap do the following:
• Determine the correct coupling gap based on the manufacturer’s recommendation. (Note we refer here to the proper installation gap size and its tolerance, not the alignment gap tolerances for angularity.)
• Identify the correct scribe mark on the shaft that represents magnetic center.
• Measure the distance between the scribed mark and the outside bearing housing lip. In the case that the magnetic center scribe mark falls inside the motor housing while at rest, scribe a mark in the rest position. While the machine is un-coupled run the motor and estimate the difference between the newly scribed mark and the magnetic center mark. This is the distance that will need to be compensated for when setting the coupling gap.
• Set the coupling gap according to the manufacturer’s recommendation minus the distance measured for the magnet center correction if the mark is outside the bearing housing. Add the difference if the mark is inside the bearing housing. This will provide the proper coupling gap under the normal running condition.