Vibration Analysis Pinpoints Inadequate Motor Bearing Lubrication

David Stevens, Shell
Tags: bearing lubrication, electric motor lubrication

Rolling element bearings are among the most important components in the vast majority of machines, and exacting demands are made upon their carrying capacity and reliability. The continued research and development of rolling bearing technology has enabled engineers to calculate the life of a bearing with considerable accuracy, thus enabling bearing life and machine service life to be accurately matched.

Unfortunately, it sometimes happens that a bearing does not attain its calculated rating life. There are many reasons for this - heavier loading than had been anticipated, inadequate or unsuitable lubrication, careless handling, ineffective sealing or fits that are too tight causing insufficient internal bearing clearances. Each of these factors produces its own type of damage and leaves its own special imprint on the bearing.

Vibration Analysis Detects Inadequate Lubrication in Fan Bearing
In some cases, it is possible to detect the effect of inadequate lubrication within a rolling element bearing by utilizing vibration analysis as a preventive maintenance tool. This ability to detect lubrication deficiencies with vibration analysis was proven at one facility when a noise was heard in an electric fan motor.

The electric motor is one of six fan motors installed on a cooling tower at a large refinery in England. All six are lubricated with Shell Nerita Grease HV semisynthetic high-speed bearing grease, and are included in the production unit lubrication program.

During the routine scheduled vibration monitoring activity, this electric motor was observed by the condition monitoring operative to be emitting an intermittent high-pitched noise from the nondrive-end (NDE) bearing location, albeit very low in amplitude. Routine overall vibration readings taken throughout the motor exhibited readings below 1 mm/sec rms and were consistent with previous values.

This prompted further “in-field” investigation to determine the location and/or cause of this noise. This investigation was carried out by setting the portable vibration data collector into “analyzer” mode. With the required collection set and frequency max selected, real-time data was obtained from both the NDE and drive-end (DE) motor bearing locations while the motor was at full speed; this data was taken in all three axis (where access permitted).

Examination of the vibration spectrum from the NDE bearing revealed, in the vertical direction, a “haystack” effect in a frequency band between 2 kHz and 3.5 kHz (Figure 1).

Figure 1. Single Spectrum Plot

Much has been written and many projects have been undertaken to determine where a haystack would be evident in terms of frequency band. While it is largely dependent on bearing type, speed, load, etc., this determination seems to be hit and miss, and experience in detection and resolution plays an important role.

Based on the author’s nearly 20 years of experience, this type of spectrum is normally attributed to a reduction in lubrication quality or lubrication effectiveness, resulting in some degree of metal-to-metal contact within the rolling element bearing. If this condition is not resolved, accelerated bearing wear occurs, leading to an increase in operating temperature and ultimately bearing failure.

Once the condition was determined, a given amount of lubricating grease was applied to this bearing. With the vibration data collector still set in analyzer mode, the analysts were able to visually monitor the immediate effect the lubricating grease had on the vibration and this haystack effect.

After the grease was added and had been distributed within the bearing, the live spectra indicated a very slight recurrence of the haystack. It was then decided that a few shots of grease should be applied. The effect was immediately noticeable with a considerable reduction in the vibration haystack (Figure 2), which, after some time, did not return.

Figure 2. Single Spectrum Plot with Reduced High Frequency Vibration

The reason this particular motor showed this symptom is unknown, and the actual severity of the bearing wear/damage is also unknown. Maintenance records show, however, that this motor continued in full service for another five years without the need for bearing replacement. Therefore, it is clear that by locating and correcting this lubrication deficiency, a motor bearing failure due to this condition was prevented.

About the Author
Mr. Stevens started out in the condition monitoring field in the 1980s and has been fully involved in it ever since. He is a qualified mechanical engineer and a fellow member of the Institution of Diagnostic Engineers (FIDiagE). His past employment has included specialist companies such as AV Technology Ltd, Manchester, England. Mr. Stevens now works as the equipment condition monitoring (ECM) solutions manager for Shell Services, responsible for solution development and support to the European arm of the services business.