How to Choose an
Electric Motor Bearing Grease
Anywhere machinery is found, there's an electric motor converting electrical energy to mechanical energy and the machinery around it. Since they are so commonplace and play such an important role in industry, the proper lubrication of electric motors is critical to efficient equipment operation.
The shaft of an electric motor turns on bearings that must be lubricated like any moving part. Grease is used for this, and the primary functions of the grease is to reduce friction and wear, protect the motor bearings from corrosion, and prevent particles and moisture from entering the bearings. Grease is usually the lubricant of choice for this application because it is easy to apply and because of its characteristics.
Grease is a semi-solid lubricant made from a base oil (either mineral or synthetic), a thickening agent, and additives. Usually mineral oils are used, but synthetic oil-based grease is appropriate where higher operating temperatures prevail or if the intervals between re-greasing need to be maximized. Thickeners are used to prevent the lubricating oil from leaking out of the system, forming a colloidal suspension with the oil that delivers it to the moving parts it protects.
The 5 Variables to Consider
Selecting the right grease for electric motor bearings can be tricky because the grease is subjected to different operating conditions, including high and low temperatures, mechanical shear, exposure to corrosive elements, and exposure to moisture and contaminants. Consequently, the grease(s) that works for your applications should be selected based on their physical, chemical and performance characteristics. Generally, select the grease recommended by the motor supplier rather than the bearing supplier. Additionally, in food processing applications only certain non-toxic greases are acceptable.
The most important property of the right grease is its correct viscosity. Check with your motor manufacturer for their recommendations, but be mindful that a mineral oil-based grease has a viscosity of 500-600 SUS (Saybolt universal seconds) in most 'typical' motor bearing applications.
The next variable to consider is how firm the grease is, or its consistency. This property affects the ability to pump the grease where it is needed and its ability to deliver lubricant to the surfaces that require it. The National Lubricating Grease Institute has developed an index of grease firmness ranging from 000 to 6. An NLGI2 grade grease is common in motor bearing applications.
Related to this is shear stability, which is the softening in the consistency of the grease after a period of use. The ASTM D217 Cone Penetration of Lubricating Grease is a test that measures the change in a grease's firmness after 100,000 cycles. If the grease softens too much in service, it could start leaking.
An effective motor grease will have good oxidation resistance across its range of operating temperatures. This property will aid good lubrication and extend motor bearing life. The ASTM D 3336 High Temperature Grease Life test helps measure the grease's oxidation resistance. A grease with a high oxidation life is preferable.
The dropping point of a grease is the temperature at which the grease begins to 'melt.' The grease starts to breakdown and the lubricating oil suspended in the thickener begins to separate from it. All other criteria having been met, select a grease with the highest dropping point.
Do you Need Anti-Wear Agents?
The mounting of some motors is such that there is an applied load on the motor bearings. To lubricate against this load, some greases are charged with anti-wear agents. These are called extreme pressure (EP) additives. These additives shorten the useful life of a grease, so unless you really need them, select a grease without extreme pressure additives.
Speaking of extreme pressure, Acculube knows it applies to the person selecting the greases that keep motors humming. Our knowledgeable staff is available to help relieve that pressure, without a single additive other than exceptional customer service.
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We can help you select the greases - and other critical fluids - best suited for every application in your plant.
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