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Electric Motor Bearing Greasing Basics

by Howard Penrose PhD, Success by Design

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

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


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.


Figure 1 - Bearing Greasing




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.




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.



Comments (8)

  • Having over 25 years in maintenance my experience with greasing bearings including motors I prefer the ultrasound listening system during injection since it seems to be the best overall system. You can hear the bearing and watch the noise level drop or not change thus eliminating much of the over greasing that is prevalent. As for running or stopped with ultrasound running is preferred however there is always the safety factor on some machines that are operating. Of course if you prefer the stopped method it is best to trend your bearing before and after but to get the correct amount of grease according to sound it must be operating. It is amazing how so called management people are so ignorant of lubrication. I worked in an Oil&Gas;plant in Northern Alberta and the work order specified 93 hand pumps on one end and 134 pumps on the shaft end. We were pulling bearings all the time and spending 3 hours wiping the grease out of the windings but no one would listen.

    1) Posted 8:42 am, 04 March 2009 by Royce Hamer

  • Kindly advice us the best practice in greasing motor without drain point(grease relief plug).

    Thanks a lot in advance.

    2) Posted 9:34 pm, 13 October 2009 by Mohd Riduan

  • we were told that we should not use a pnuematic grease pump to grease motors. we would like to know why is this bad or have we been told something wrong.

    3) Posted 6:49 am, 18 May 2011 by jeff frederick

  • kindly advice for motor working no-load and the temp. go up to 90 degree centigrade at 30 min what are the reasons for that abnormal temp.

    4) Posted 9:31 am, 19 May 2011 by khaled

  • Dear Prof.

    My inquiry is whether there is a study done, to educate non-electrical colleagues about the impact of running MV motors (>350kW) at vibrations >6,5mm/s (sleeve bearings). There's therefore a request to share such a report so we can share with our counter parts.

    5) Posted 12:43 am, 24 February 2012 by Andrew Sibiya

  • Hello Experts,

    Can anyone share me the guidelines about re-greasing quantity and interval for a motor with double bearing on the drive end side and single bearing on non-drive end side ?
    Please send it to vinaymaithani at gmail dot com


    6) Posted 6:38 am, 21 June 2012 by Vinay Maithani

  • thank you for the information provided,it was most helpful

    7) Posted 4:26 am, 23 July 2012 by raphael ruzvidzo

  • Great advise - Thanks! Can you help me resolve a lube concern? As the recently appointed Reliability Eng to a plant which has never had any structured maintenance management systems or record keeping, I have some 90Kw motors which have not been greased for many years and although they are running well (have not yet established any vibration data) no-one is able to shed any light on when they were last lubricated. The problem is that I am afraid the old grease between the grease nipples and the bearings in the internal capilliary has now gone hard and I do not want to push it into the bearings with the fresh grease. These motors are critical and there is no planned maintenance period long enough that will enable me to pull the motors apart so as to give it a good clean or flush. I am open to suggestions as to how I can avoid disaster without a lengthy plant shut down.

    8) Posted 3:47 am, 10 April 2014 by Graeme Hadden

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