Table 1: Amount of Grease to Use
The general procedure for greasing is as follows:
- Lock and tag out the electric motor
- 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 Table 1.
- 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.
Bearings should be lubricated at an average frequency as found in Table 2. Operational environment and type of grease may require more frequent lubrication.
Table 2: Bearing Lubrication Frequency
One concept that has been presented is that grease will eventually fill the bearing housing, causing the same problem as an overgreased bearing. We will be addressing this particular issue, as well as a discussion of why the motor should be de-energized during greasing, through this paper. We are limiting this paper to a standard deep-groove ball bearing without shields or seals.
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, 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.
Bearings may also have shields or seals mounted on them. Bearing shields are metal fittings that have small clearances between the inner race of the bearing and contact the outer race on either side of the balls and cage. The small clearances near the inner race allows some oil and grease to move into the moving parts of the bearing, but prevents particles of large size from passing into the bearing potentially damaging machined surfaces. 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, and some shielded, bearings are referred to as non-grease able bearings.
What Happens When The Bearing Is Greased With The Motor Running?
Oil is an ‘incompressible' fluid, which is important when considering the developing issues within the bearing housing (Figure 1) while greasing an operating motor. The ‘soap,' or grease medium, acts as a suspension in the oil, although grease is normally represented as a base with an oil suspension. This becomes an important issue in the physical world of hydrodynamics.
With the bearing housing partially filled with grease, grease is added to the housing. Some of the grease flows through the operating surfaces of the bearing, causing stress. The reduction of clearances causes an increase in friction within the bearings. This will cause the bearing temperature to increase as the bearing surfaces reject the grease medium. Once the temperature drops, the grease is no longer within the bearing surfaces and oil from the grease provides lubrication. The increase in temperature causes a reduction in grease viscosity, allowing it to flow freely, albeit slowly, and excess grease is rejected through the grease plug (grease out). The change in viscosity ensures that enough flow should occur, when the grease plug is removed, and the maintainer does not count on ‘grease relief plugs,' the housing should remain less than full, regardless of the number of greasing operations.
Grease that comes into contact with the shaft, bearing cap opening or housing opening (usually less than 0.010 inches) becomes pumped through the openings due to Couetti Flow. This process is the result of a turning cylinder (motor shaft) with a close, stationary, cyclinder (shaft openings) and an incompressible fluid. The excess grease is literally pumped into the motor housing.
What Happens When The Motor Is Not Running?
In the type of bearing that we are discussing, the grease enters the bearing housing. Some grease comes into contact with the bearing surfaces. When the motor is restarted, this excess grease is ejected from the bearing. The temperature may briefly rise, then fall, once grease has passed through the bearing. The shear stresses and temperature reduce the viscosity of the grease, allowing it to flow.
While some grease is moved into the motor housing, due to Couetti Flow, the amount is considerably less than if the motor is operating.
Conclusion
Electric motor bearing greasing requires the motor to be de-energized during the procedure. The result is reduced risk of excess grease entering the electric motor stator, due to Couetti Flow, and reduced viscosity, due to heat. Combined with safety issues, proper lubrication can maintain the electric motor reliability. Therefore, a limited amount of grease should be added to the bearing housing periodically with the grease plug removed.