Motor Vibration Analysis: Keeping it Simple

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When a problem occurs, it is critical to quickly identify the source of the trouble and take corrective action. Using present-day diagnostic techniques can save you money and downtime.

Vibration levels shown in the Table below are considered excessive. If you find these levels during inspection, quickly resolve the situation to ensure you don't sacrifice motor reliability. Levels that are half as high as those in the table indicate excellent machine operation.

Shaft vibration vs. housing vibration

The necessity of obtaining housing vibration data vs. shaft vibration data depends on the problem. It is often desirable to obtain both, especially on a machine with sleeve bearings. If the problem originates in the housing or motor frame (for example, twice line frequency vibration), gather housing vibration data with magnetically mounted accelerometers. If the problem originates in the rotor (unbalance or oil whirl, for instance), obtain shaft vibration data with either a shaft stick or a proximity probe.

Proximity probes yield vibration data of the shaft movement relative to the housing, whereas shaft stick measurements yield vibration data based on an absolute reference location. Obtain shaft stick measurements by placing a handheld accelero-meter attached to a wood fishtail stick against a smooth rotating surface on the motor shaft. If the motor comes with proximity probes, use them. If it does not and relative shaft vibration is required, set up proximity probes with magnetic mounts. Always obtain housing vibration data with an absolute reference.

Take the correct direction of measurement

You should take housing vibration measurements in three planes (vertical, horizontal, and axial) on both bearing housings, as shown in the Figure on the right. Take shaft vibration measurements in two directions (90° apart) on one drive end of the motor. If the motor has proximity probes, you typically mount them at a 45° offset from both sides of the upper vertical centerline.

You can measure vibration in units of displacement (peak-to-peak movement in mils or thousands of an inch), units of velocity (zero-to-peak in in./sec), or units of acceleration (zero-to-peak in gs). Acceleration emphasizes high frequencies, displacement emphasizes low frequencies, and velocity gives equal emphasis to all frequencies. Troubleshooters commonly use displacement units for shaft vibration measurement and velocity units for housing vibration measurement.

Recommended troubleshooting procedure

If you suspect high vibration may be the source of your troubles, various tests can pinpoint the trouble spot. But first, you should perform some basic maintenance checks to answer these questions:

• Are all the bolts tight?

• Have you eliminated “soft foot”? Check for it by loosening the four hold-down bolts that pass through the motor mounting feet one at a time. A dial indicator should show no more than .001 in. of movement in the vertical direction of the mounting foot. Install shims to eliminate the situation.

• Is hot (or operating) alignment good? If it's not possible to verify hot alignment, have you verified cold motor (or not-operating) alignment with appropriate thermal compensation checks?

• Is any part of the system (such as the junction box top cover or piping) vibrating excessively? Are any of the parts attached to the motor resonant?

• Is the foundation or the motor frame vibrating at a higher level (more than 25%) than the motor? If so, there could be a weak motor base.

• Are any parts of the motor or shaft loose?

• Have any fan blades eroded or broken off? Is coupling lubrication satisfactory?

  If the above items are satisfactory, but vibration remains high, perform a thorough vibration analysis.

  You can diagnose a vibration problem in two steps. First, obtain vibration data, but remember factors like noise, combined signals, and amplitude modulation can influence the data. Then, determine what operating conditions increase, decrease, or have no effect on vibration. Obtain vibration measurements with the motor operating under the following conditions:

• Loaded, coupled, full voltage, all conditions stabilized. The first measurements you take should represent the machine under normal operation and may indicate the tests you should perform next.

• Unloaded, coupled, full voltage. This condition removes some, but not all, load-related vibration.

• Unloaded, uncoupled, full voltage. This condition removes all effects of the coupling and mechanical load driven by the motor. It isolates the motor/base system from the measurement.

• Unloaded, uncoupled, reduced voltage (25% if possible). This test minimizes the effect of magnetic pullover. A 25% voltage reduction is usually only possible at a motor service shop or manufacturer's facility. If the motor is a Y-Delta connected machine, the Y connection is effectively 57% voltage as compared to the D connection at the same terminal voltage. A comparison of vibration under both connections reveals the voltage sensitivity of the motor.

• Unloaded, uncoupled, coast down. This test reveals any resonance/critical-speed problem for the entire motor/base/driven equipment system.

More stringent specifications regarding motor vibration call for maximum velocity levels of 0.1 in./sec on the housing and 1.5 mils of displacement vibration on the shaft. The limits apply to a motor mounted on a seismic mass, and uncoupled or coupled to a piece of equipment in a way that eliminates any vibration in the driven equipment.

Vibration problems can vary from a mere nuisance to an indication of imminent motor failure. With a solid knowledge of motor fundamentals and vibration analysis, you can usually identify areas of concern well before they lead to a failure.

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