Lubricating Electric Motors

Jeremy Wright, Noria Corporation
Tags: bearing lubrication, electric motor lubrication

In a previous article published in the November 2007 issue of Machinery Lubrication, titled "Electric Motor Lubrication", I mentioned the use of acoustic monitoring as a feedback tool for refining proper relube intervals and quantity. I have since received several requests to explain this method in further detail. This process is known as the hybrid method, and is the topic of this article.

Determining when to lubricate bearings and how much lubrication to apply are two issues that face technicians responsible for maintaining bearings. Underlubrication can cause a bearing to wear out prematurely. On the other hand, applying too much lubricant can often lead to catastrophic results to the bearing (grease churning and overheating) or long-term damage to motor coils and windings.

The Regrease Interval
There are several methods for determining a regrease time cycle. Multiple calculators, tables and charts provide a good starting point. Most calculators have common factors such as load, operation time, bearing type, temperature, environment and speed. Pick a method you're comfortable with or use the average value calculated from several of the methods and use it throughout your plant to build a database of lubrication intervals.


Figure 1. When to Regrease

Figure 1 illustrates how to use the new database in conjunction with the acoustic monitoring equipment. As time passes (following the trend line from left to right), you will see two possible outcomes. The trend line will cross the alarm threshold or the calculated regrease interval. When either of these two lines is crossed, it is time to relube. After several cycles, a pattern should emerge.

If the trend line never crosses the acoustic monitoring alarm threshold, then the calculated interval is set too short. Likewise, if the calculated interval is never reached, then the calculated interval is too long. Using this pattern or trend, the regrease interval can be adjusted. The interval will be optimized when the trend line crosses both limits at approximately the same moment in time.

As with the interval, there are multiple calculators, tables and charts to determine the correct volume of grease to apply at the determined interval. A simple equation takes a logical approach to determining the volume of grease to be added. The formula is:

G = 0.114 x D x B

Where G = the amount of grease in ounces, D = the outside diameter in inches and B = the bearing width in inches.

Apply this formula to all the greased lube points in the plant and add them to the interval database.

Figure 2 illustrates the significance of this value in the hybrid approach. As the initial charge of grease is added to the lube point, there should be a drop in the acoustic emission. Adding each additional shot will temporarily increase the emission. This presents two possible outcomes. The first result is that grease continues to be added until the calculated regrease volume is reached.

In this instance, no more lubrication should be added to this lube point. If the acoustic signal does not come back down during the addition of grease, discontinue greasing. The database should track when a calculated volume is not reached. This could be a sign of an inherent problem in the bearing.


Figure 2. Regreasing a Bearing Using a Grease Gun

Abridged Procedure Steps for the Hybrid Method

  1. Locate the grease fill ports at the top half of the bearing. Locate the grease relief port or plug at the bottom half of the bearing at either 180- or 120-degree offset.

  2. Remove the drain plug and make sure the grease opening is free of solidified grease.

  3. Attach the magnetic mounted transducer to the bearing housing at the prescribed location and tune the unit to 30 kilohertz (or according to original equipment manufacturer recommendations).

  4. If the decibel (dB) reading is at or below baseline value, record the dB reading. Do not add grease. If the dB reading is 50 percent over the baseline value, the bearing may need lubricating. Record this initial dB reading along with sound quality and proceed to grease the bearing as below.

  5. Wipe the grease fitting and make sure the end of the grease gun connector is free of contamination. Press the grease gun hose end connection onto the grease fitting. Gradually apply the grease while carefully observing the discharge port for old grease.

  6. After each shot, listen to the sound of the bearing and watch the dB meter. If the bearing surface receives lubrication, there should be a noticeable drop in audible and dB sound levels upon application of the grease.

  7. After the sound level has dropped, slowly add more grease and continue to monitor audible and dB levels. Discontinue greasing when either the audible or dB levels rise and remain high, or when the recommended volume of grease has been added.

  8. Replace the dust cap on the zerk (or other) grease fitting. If no cap is available, then leave a thin covering of grease on the fitting.

  9. Allow excess grease to exit the drain, then replace the drain plug.

Hybrid Method
For the hybrid method to be successful, there needs to be a direct communication between all parties involved, from the lube tech to the maintenance planner. The transfer of data can be handled in several ways. It could be through a wireless system that updates in real time via a handheld PDA the technician carries, or as simple as a face-to-face meeting after the lube/inspection route to discuss the nonconformist assets.

Either way, feedback is needed to make adjustments to intervals and volumes. The database that was created should be evolving and continually improving. The only way to accomplish this is through communication.