Lubrication: Grease or Oil

If rolling bearings are to operate reli- ably and realize their full service life they must be adequately lubricated. The function of the lubricant is to form a protective oil film that sepa- rates the bearing components and pre- vent metal-to-metal contact. The lubricant also protects the bearing and related components against corrosion. When grease is used as a lubricant, it can also help protect the bearing against contaminants like dirt, dust, and water. Some important properties of a lubricant include viscosity, film forming ability, and consistency (for grease). The most important determi-

nants of the film thickness are

■ rotational speed

■ bearing temperature

■ load

■ base oil viscosity

■ NLGI grade number or consistency for grease.

The choice between grease lubrication and oil lubrica- tion is chiefly determined by the following factors:

■ grease should be used in applications where the fol- lowing requirements apply:

– simplified maintenance

– improved cleanliness (fewer leaks)

– better protection against contaminants

■ oil lubrication should be used in applications where normal operating temperatures are high as a result of an external heat source or excess heat generated by the machine or its bearings at high speed.

Note that the friction in the bearing is generally lower with grease than with an oil bath, provided that the appropriate type and amount of grease is used and that it is supplied to the bearing in a suitable manner. The oper- ating temperatures will be similar, however, since heat is not removed with grease. Oil lubrication should be used when the relubrication interval for grease is too short.

Using the best and most suitable lubricant related to specific operating conditions is of crucial importance in obtaining appropriate motor performance and reliability. Areas of consideration should include the following:

■ bearing type and size

■ operating temperature

■ load

■ speed range

■ operating conditions e.g., vibration levels, orienta- tion of the shaft (horizontal or vertical)

■ cooling

■ sealing efficiency

■ environment.

Grease Lubrication

Under normal speed and temperature conditions, the bearings in electric motors are usually lubricated  with

grease. Grease has a number of advantages when compared to oil. It allows simpler, more cost-effective housing and sealing designs while offering better adhesion of the lubri- cant to critical surfaces and protec- tion against contaminants.

For small- and medium-sized ball bearings where the grease life is longer than the expected service life of the bearings, one single filling of grease is sufficient. The grease must then be retained in the bearings and prevented from escaping. Polyurea- and urea-based greases are typically furnished by most manufacturers of industrial motors. Lithium-based greases or oil are normally used for roller bearings on larger motors.

The life expectancy of grease depends on several factors including the type of bearing, the type of grease, the ori- entation and speed of the motor, and the operating tem- perature of the bearings. For instance, roller bearings have shorter relubrication intervals than ball bearings. Other factors must be considered, including the sealing arrange- ment, operating environment, and contamination. For small ball bearings in standard motors, since normal grease life usually exceeds the expected service life of the motor, these bearings are usually fitted with seals or shields and lubricated for life. At the end of the bearings’ life, they are simply replaced.