Because the causes of machine vibration are so numerous, the underlying problem is often difficult to isolate. Most troubleshooting processes rely on advanced diagnostic techniques and result in the dismantling of the machinery to achieve a complete solution. However, you can perform the following basic checks without expensive equipment to determine natural frequency, active foundations, and gross unbalance issues — all of which can cause excessive vibration and should be avoided whenever possible.
The machine foundation plays a vital role in the vibration level of the installed equipment. A rigid and massive foundation (Photo 1) is ideal, because it minimizes background vibration observed at the motor or other equipment. A massive foundation is less active, meaning it will limit the total movement of the system and thus the vibration level of the machine due to its own mass and stiffness. Although rigid and massive foundations are preferred, they are regrettably not the rule. In most instances, it’s not possible to use an ideal foundation due to space, cost, or some other variable. More often than not, the installation will have a less-than-ideal foundation and correspondingly more-than-ideal vibration (Photo 2). If you suspect foundation is an issue, there are additional checks you can perform to confirm if this is the case. Checking the background vibration and foot vibration levels, as well as impact testing, can all aid in deciding whether a foundation is the root cause of your problem.
Per NEMA Standard MG-1 (Motors and Generators), an active foundation allows excessive movement and transmission of external vibration to the machine while a rigid (non-flexible) foundation limits the vibration at the feet — and does not allow excessive transmission of machine vibration into the surrounding structure. Use the following two tests to determine if an active and/or flexible foundation exists.
Active foundation check — Take a measurement of the background vibration level at the same location used to measure the machinery bearing housing vibration during operation. Then, perform a vibration test on the machinery bearing housings with all of the train components shut down. If the overall vibration levels you measure at the bearing housings while the equipment is not operating are 25% or higher than the respective operating vibration levels, then you’re dealing with an active foundation. This check can be somewhat subjective because excessive operating vibration can mask a bad foundation — because the background vibration level may not meet the 25% criteria but may still be excessive. In these instances, follow this general rule: If the overall vibration level at the bearing housings exceed 0.030 in. per sec peak overall (with the equipment not operating), further investigation may be warranted.
Flexible foundation check — With the machine operating in any configuration, measure the overall vibration levels at the bearing housings as well as at the motor feet in as many axes as practical. Compare the overall vibration measured at each axis of the bearing housing to the overall vibration measured at each respective axis of the adjacent foot. If the overall levels measured at the foot are 25% or higher than those measured in the same axis on the adjacent bearing housing, then the foundation is considered to be flexible. As with background vibration, the 25% criteria may not be accurate, depending on the levels observed during testing. Once again, the 0.030 in. per sec peak overall is a good rule of thumb to follow.
How the machine is secured to the foundation is just as important as the foundation itself. Induction motors are typically provided with instruction books to help ensure the machine is secured in accordance with the manufacturer’s recommendations. Failure to follow these recommendations can result in vibration problems.
Check static soft foot by first tightening all of the foot mounting bolts to the required torque level specified by the manufacturer. Then, starting with any foot, place a thousandths (0.001 in.) dial indicator vertically on the foot surface near the bolt location, leaving adequate space to loosen the bolt with a properly sized wrench (Fig. 1). Zero the indicator and begin to loosen the bolt. If the value on the dial indicator increases from zero, make note of the amount of the change — this is the amount of soft foot present.
Each manufacturer may have its own criteria for soft foot, so follow those specific values. If practical, keep all levels of soft foot at less than one thousandth (0.001 in.). If soft foot is present, correct it by appropriate shimming. While the bolt is loose (and prior to shimming), it is recommended that you take a feeler gauge and check the full perimeter of the foot to whatever extent practical. This helps you verify the foot is flat and not angled or distorted. If the foot is not flat, then complicated shim arrangements may be required. Once the foot is properly shimmed, repeat the dial indicator check to verify the amount of soft foot present after correction. If the correction was successful, torque the foot mounting bolt appropriately. Check the next foot, repeating the process until you’ve checked and adjusted all of the mounting feet, as necessary.
Testing for rotor critical speeds and system natural frequencies can be complicated. Once you verify they exist, the following issues are some of the most difficult to correct.
Note that you may also use coast down testing to aid in determining if an induction motor vibration problem is electrical or mechanical in nature. Monitor the vibration levels prior, during, and immediately after the power has been removed from the motor. If the vibration ceases or sharply declines almost immediately, then the problem typically stems from an electrical issue.
You can test horizontal machines in a number of locations (click here to see Fig. 2). These locations may be modified, depending upon need by focusing upon locations where a problem is known to exist or by increasing locations to include other measurements, such as frame skin, bearing brackets, equipment supports, or installation skids. Horizontal motors equipped with sleeve bearings must have any bearing housing or bracket testing performed while in a slow roll condition (200 to 300 RPM) so that the two brackets are not coupled by the shaft. If left stationary, the resultant artificial stiffening will result in erroneous results. Horizontal motors equipped with anti-friction bearings are not affected by this and may have their bearing housings or brackets impact tested while at rest.
Vertical machines have two principal locations you should target with impact testing, which correspond to the reed or natural rocking frequencies. These locations are both at the uppermost bracket, flange, or frame member of the machine with the two measurements being orthogonal (90°) to one another. For ease, this may be with the main lead box and 90° from the main lead box or with the discharge piping and 90° to the discharge piping (click here to see Fig. 3). The 90° separation is important for this test, because external equipment, boxes, piping, etc., can have a large effect on the natural frequency. The rotor does not need to be rotating for reed frequency testing.
As a general rule of thumb, there should be no natural frequencies within ±15% of half times operating speed, one times operating speed, two times operating speed, one times line frequency, and two times line frequency — although some users may have different requirements. Adjustable-speed drive operation of machines may make avoidance of natural frequencies impractical. Natural frequency problems are typically difficult to correct, if even practical, because changing the reed frequency of the system could require rework of the foundation.
A frequent source of mechanical unbalance with motor startups stems from improper key trimming in coupling hubs. The ideal key installation uses a full key for the part of the shaft where the coupling hub is located and then steps down to a half key (Photo 5). Using a full key that does not step down to a half key will create a large mechanical unbalance (Photo 6). Always follow the manufacturer’s recommendations for best results.
Even though the causes of motor vibration seem almost endless, with a few straightforward checks and an inquisitive attitude, you can minimize risk during motor and train startup and maximize overall reliability of the equipment.
Hamm is a service engineer and Evans is a senior test engineer with TECO Westinghouse, Round Rock, Texas. They can be reached at (512) 255-4141.