The Importance of Motor Shaft Alignment

Proper motor shaft alignment increases the operating life span of rotating machinery. To achieve this goal, components that are the most likely to fail must be made to operate within their acceptable design limits.

The Advanced Manufacturing Office of the U.S. Department of Energy has released a white paper with information and tips regarding misalignment and tips for alignment. (Access it here.) The following information comes from the paper.

While misalignment has no measurable effect on motor efficiency, correct shaft alignment ensures the smooth, efficient transmission of power from the motor to the driven equipment. Incorrect alignment occurs when the centerlines of the motor and the driven equipment shafts are not in line with each other. Misalignment produces excessive vibration, noise, coupling- and bearing-temperature increases, and premature failure of bearings, couplings or shafts.

There are three types of motor misalignment:

Angular misalignment occurs when the motor is set at an angle to the driven equipment. If the centerlines of the motor and the driven equipment shafts were to be extended, they would cross each other, rather than superimpose or run along a common centerline. The “gap” (gap difference between coupling faces) or difference in slope of the motor shaft when compared with the slope of the stationary machine shaft can occur in the horizontal direction, vertical direction or both. Angular misalignment, in particular, can cause severe damage to the driven equipment and the motor.

Parallel misalignment occurs when the two shaft centerlines are parallel, but not in the same line. There are two planes of parallel misalignment as shafts may be offset horizontally (displaced to the left or right), vertically (positioned at different elevations) or both.

Combination misalignment occurs when the motor shaft suffers from both angular misalignment and parallel misalignment simultaneously. This is the most common misalignment situation encountered in the field.

Couplings

Larger motors are usually directly coupled to their loads with rigid or flexible couplings. Rigid couplings do not compensate for motor-to-driven-equipment misalignment, while flexible couplings tolerate small amounts of misalignment. Flexible couplings also can reduce vibration transmitted from one piece of equipment to another, and some can insulate the driven-equipment shaft against stray electrical currents. Even flexible couplings have alignment requirements, defined in the instruction sheet for the coupling.

It is a mistake to rely on coupling flexibility for excessive misalignment. This is because flexing of the coupling and of the shaft will exert forces on the motor and driven-equipment bearings. These forces may result in premature bearing, seal or coupling failure, shaft breaking or cracking, and excessive radial and axial vibrations. Secondary effects include loosening of foundation bolts, and loose or broken coupling bolts. Operating life is shortened when shafts are misaligned.

In practice, proper alignment is difficult to achieve without using alignment equipment such as dial indicators or laser alignment tools. The proper shaft-alignment procedure is to secure the driven equipment first because moving a pump, for example, would stress the connected piping. Next, install the coupling to the driven equipment. The motor should then be moved into proper alignment and joined to the coupling.

After the equipment has operated long enough to become temperature-stabilized, shut it down and immediately recheck alignment. Alternatively, monitor the relative positional changes of the machines with a laser system equipped to do so. Due to thermal growth, machines aligned in the “cold” preoperating condition are almost always out of alignment when operating temperatures are attained. Many equipment manufacturers publish thermal offset values so the alignment technician can correct for thermal growth during the initial alignment process.

Suggested actions

• Check newly installed equipment for alignment changes due to foundation settling after 3 to 6 months of operation.

• Check shaft alignment of all production-critical equipment annually.

• Monitor for vibrations and for increasing vibration trends as an indication of misalignment. Misalignment might be caused by foundation settling, insufficient bolt tightening or output shaft faults.

• Apply predictive maintenance techniques, including vibration tests and frequency spectrum analysis, to distinguish between bearing wear, shaft misalignment or electrically caused vibrations.