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
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
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.
About the Author
Dr. Penrose is the President of SUCCESS by DESIGN Reliability
Services, based in Old Saybrook, CT. He also serves as the Executive
Director of the Institute of Electrical Motor Diagnostics (IEMD).
Starting as an electric motor repair journeyman in the US Navy, Dr.
Penrose lead and developed motor system maintenance and management
programs within industry for service companies, the US Department of
Energy, utilities, states, military, and many others. Most recently he
led the development of Motor Diagnostic technologies within industry as
the General Manager of the leading manufacturer of Motor Circuit
Analysis and Electrical Signature Analysis instruments and training. Dr.
Penrose taught engineering at the University of Illinois at Chicago as
an Adjunct Professor of Mechanical and Industrial Engineering as well as
serving as a Senior Research Engineer at the UIC Energy Resources
Center performing energy, reliability, waste stream and production
industrial surveys. Dr Penrose has coordinated US DOE and Utility
projects including the industry-funded modifications to the US
Department of Energy’s MotorMaster Plus software in 2000 and the
development of the Pacific Gas and Electric Motor System Performance
Analysis Tool (PAT) project. Dr. Penrose is a Past Vice-Chair of the
Connecticut Section IEEE (Institute of Electrical and Electronics
Engineers), a Past-Chair of the Chicago Section IEEE, Past Chair of the
Chicago Section Chapters of the Dielectric and Electrical Insulation
Society and Power Electronics Society of IEEE, is a member of the
Vibration Institute, Electrical Manufacturing and Coil Winding
Association, the International Maintenance Institute, NETA and MENSA. He
has numerous articles, books and professional papers published in a
number of industrial topics and is a US Department of Energy (US DOE)
MotorMaster Certified Professional, a US DOE Pump System Specialist,
NAVSEA RCM Level 2 certified, as well as a trained vibration analyst,
infrared analyst and motor circuit analyst.
