Improving your motor system’s reliability can be as simple as a good lubrication schedule, which may be a critical maintenance practice victim to a significant amount of misinformation within the industry.
As an example, frequently purging motors of grease is promoted by those lacking the training and knowledge of how motors work. Unfortunately, this procedure reduces the life of the motor, increases the chance for both bearing failure and winding contamination.
Grease purging is the practice of forcing grease completely through the bearing housing and bearing until old grease is removed and new grease shows at the grease relief plug. If your motor is operating in a difficult environment, purging is a common practice. Despite the concept of “clean lubricant,” purging opens the door for more problems. Some grease additives react with winding insulation or provides a thermal “blanket,” reducing the life of the winding, when purged grease leaks through the bearing or bearing cap and onto the winding, even if the grease relief is open.
Bearing lubrication, on the other hand, is the practice of adding enough grease to allow for the lubrication of the bearing friction surfaces and the eventual removal of contaminants from the grease housing.
Writing at Reliability Web, Howard W Penrose provides an overview and direction for the proper lubrication of bearings.
How a Bearing Works
The most common type of bearing is the AFBMA-7 C-3 rated bearing. C-3 relates to the internal clearances of the surfaces of the bearing. In most motor rated bearings, there is a clearance of between 3-5 mils (thousandths of an inch) in which lubrication flows to reduce friction and wear of the machined surfaces. The bearing, itself, consists of an inner race, an outer race, balls and a cage which evenly distributes the balls. Common bearings are designed to allow for a radial load with some limited axial loading. ALL BEARINGS ARE LUBRICATED WITH OIL.
Grease, itself, is an oil sponge. The base (spongy) part of the grease varies depending on the manufacturer, temperature, and environment and user preference. The grease holds the oil in suspension and allows the oil to flow during operation. The oil compresses between the bearing balls, inner and outer races and the cage, reducing friction. Ball bearings have small, microscopically rough surfaces on the balls, these surfaces move the oil, holding it to the ball during operation.
When too much grease is added, the grease is compressed between the bearing surfaces, increasing pressure and resulting with heat. Too little grease causes the surface friction to increase, resulting with heat. In any case, once bearing noise is audible, it has failed. Reducing noise by lubrication requires excessive grease, endangering the motor, and giving the technician the false security of extending the motor life when, in reality, additional damage is occurring to machined surfaces.
Shields or seals may also be mounted on the bearings. Bearing shields are metal fittings that have small gaps between the bearing’s inner race and touch the outer race on either side of the balls and cage. That small gap near the inner race allows some oil and grease to move into the moving parts of the bearing, but the shield prevents large particles from entering the bearing, which would potentially cause some damages.
Sealed bearings have seal surfaces touching the inner race while “non-contact” sealed bearings have extremely close tolerances between the seal surface and the inner race preventing particles under several thousandths of an inch. Sealed, some shielded, bearings are referred to as non-greasable bearings.
Precautions In Motor Greasing
- When greasing electric motors, there are a number of precautions that must be considered:
- When electric motors are manufactured, or repaired, grease fittings may be put in place on motors that are not grease able. Your supplier should be able to provide confirmation that the motor may be greased.
- Electric motors must be de-energized and locked/tagged out (LOTO) before greasing.
- There should be no paint on grease fittings.
- The average grease gun will introduce 1 ounce for every 23 strokes.
- Grease compatibility. The additives in some greases do not mix well and can cause the grease to solidify or liquefy.
- Following is the standard procedure for greasing ball bearings:
- Wipe grease from the pressure fitting, clean dirt, debris and paint around the grease relief plug. This prevents foreign objects from entering the grease cavity.
- Remove the grease relief plug and insert a brush into the grease relief as possible. This will remove any hardened grease. Remove the brush and wipe off any grease.
- Add grease per manufacturer’s requirements.
- Allow the motor to operate for approximately 30 to 40 minutes before replacing the grease relief plug. This reduces the chance that bearing housing pressure will develop.
It is recommended that the type of grease used on each motor is recorded in order to avoid premature bearing failure. In many cases, you may be able to standardize the type of grease used in a majority of your motors. It is also good practice to let your motor repair center know the type of grease in case the standard grease used by the repair center conflicts with your standard grease.
Visiting the Technical Reference section of the L&S Electric website, you’ll find several links regarding lubrication:
Tomorrow, Greasing Electric Motors, Part 2