When should oil be used for bearings?

By Joyce Laird, Contributing Editor

Lubrication: Oil or Grease?

Bearing lubrication methods are typically provided by either an oil or grease. Each lubrication method has specific cases where its use is preferred. Sean P. Kelly, Field Applications Engineer for NMB Technologies Corporation says that Grease is preferred when longer life is desired. “This is due to the grease thickener slowly releasing a base oil providing a lubrication layer between the raceways and rolling elements.”

“Grease may also extend bearing life in circumstances of higher forces through the use of extreme pressure additives. However, in this discussion the focus on the use of oil lubrication in ball bearings, since it is the more prevalent lubricant,” Kelly added.

Oils are the preferred lubricant in open bearing designs or when low torque or high speeds are the priority during bearing operation. “This is due to the relatively lower viscosities as compared to greases. Oil lubrication allows for more efficient movement of the ball through the lubricant, as compared to greases.”

The Difference Between Dry and Lubricated Bearings

Kelly explained that most bearings will require lubrication to some degree; but, occasionally a dry bearing may be specified.

“Dry bearings may be specified when the operating environment and operating conditions require an absence of lubricant and can support the reduced life expectations that result from no lubricant condition. This can be the case for medical applications that do not allow lubricant in the environment (patient bloodstream, as one example).”

However, Kelly added that as there is no lubrication layer between the ball and raceway, caution must be exercised, as a significant reduction in bearing life can be expected. Oil lubrication lowers the operating torque of the bearing and enables high speed operation.

Ensuring Proper Viscosity

The proper viscosity of oil must be selected to match the operating temperature of the bearing to insure the oil film layer functions as designed.

“If the oil viscosity is too high the oil will thicken as temperature decreases, increasing torque and lowering operating speed. If the oil viscosity is too low an adequate lube film layer will not form and metal to metal contact can result leading to shortened bearing life.”

How to Choose the Best Lubrication for Your Application

“Choosing the best oil formulation for your application is based on several factors including speed, lubrication dispersal method, operating temperature, and potential environmental contaminants. We recommend a customer contact our Field Application Engineering team to help determine the most effective lubricant formulation for a specific bearing application,” Kelly said.

Important Closing Considerations

In conclusion, Kelly noted that in a properly designed closed system operating at moderate temperatures, the bearing oil should be replaced once per year. If operating temperatures exceed 80° C the oil may need to be replaced every three months. The best practice is to consult a bearing application engineer for assistance with determining an oil change interval on an application specific basis.

NMB
www.nmbtc.com

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