How to choose grease for electric motor bearings

Chris Decker, Americas Products Technical Advisor, ExxonMobil Research and Engineering

The primary functions of an electric motor bearing grease are to:

• Reduce friction and wear
• Protect bearings against corrosion
• Act as a seal to prevent entry of contaminants

Grease is frequently used to lubricate electric motor bearings because of its simplicity of application and unique characteristics. When determining the proper grease for your application, consider:

Viscosity: Oil viscosity should be appropriate for the load and speed of the application at operating temperature. The typical mineral oil viscosity in an electric motor grease is in the range of Kinematic Viscosity 90-120 cSt @40°C.

Consistency: A grease’s consistency or firmness is stated in terms of its National Lubricating Grease Institute (NLGI) grade, which ranges from 000 to 6. Generally, NLGI 2 grade greases are used in electric motor applications involving horizontal shaft configurations, while NLGI 3 grade greases may be more appropriate for motors supporting vertical shaft configurations.

Oxidation resistance: Electric motor greases should have outstanding resistance to oxidation. American Society for Testing Materials (ASTM) high temperature grease life test results give a good indication of oxidation resistance when operating under extreme conditions. Choose a grease with a high ASTM D3336 oxidation life or DIN 51821 FE 9 high temperature bearing life testing.

Anti-wear: Unless a motor is mounted so there is a thrust load on the bearings, it is advisable to choose a grease that does not have extreme pressure additives. These additives can shorten the life of the grease and are not recommended for use where they are not required.

Dropping point: The dropping point is an indication of the temperature at which the grease will melt or the oil will separate from the thickener. A grease with a high dropping point is desirable, given the high temperatures reached during electric motor bearing operations.

Shear stability: The ASTM Cone Penetration of Lubricating Grease test measures the consistency of the grease after it has been worked for 100,000 strokes. An electric motor bearing grease should soften no more than 1 to 1.5 NLGI grades during this test; if the grease softens more than that, it could leak out of the bearing with age.