Introduction

One of the most important components of any electro-mechanical maintenance program

is the lubrication of bearings. Yet, this vital aspect of preventive maintenance remains
one of the least understood functions of maintenance. There is constant debate
concerning whether a bearing should be ‘flushed,’ a limited amount of grease added, how
often or if the motor should be operating or tagged-out. Many motor manufactures
outline the preferred, and safest, method for lubricating electric motor bearings. There
are specific physical properties for this process in the motor bearing housing and in order
to protect motor windings from contamination.

Table 1: Amount of Grease to Use

Bearing Number Amount in Cubic Inches Approximate Equivalent

Teaspoons
203 0.15 0.5
205 0.27 0.9
206 0.34 1.1
207 0.43 1.4
208 0.52 1.7
209 0.61 2
210 0.72 2.4
212 0.95 3.1
213 1.07 3.6
216 1.49 4.9
219 2.8 7.2
222 3 10
307 0.53 1.8
308 0.66 2.2
309 0.81 2.7
310 0.97 3.2
311 1.14 3.8
312 1.33 4.4

The general procedure for greasing is as follows:

1. Lock and tag out the electric motor

2. 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.

3. 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.

4. Add grease per Table 1.

5. 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

Motor RPM Motor Frame 8 hours per day 24 hours per day
3600 284T-286T 6 months 2 months
324T-587U 4 months 2 months
1800 284T-326T 4 years 18 months
364T-365T 1 year 4 months
404T-449T 9 months 3 months
505U-587U 6 months 2 months
1200 and below 284T-326T 4 years 18 months
364T-449T 1 years 4 months
505U-587U 9 months 3 months

One concept that has been presented is that grease will eventually fill the bearing
housing, causing the same problem as an over greased 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.