Bearings and Gears

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Lubrication of Rolling Element Bearings and Gears

by Ronald W. Jacobs

Abstract

Proper lubrication of rolling element bearings and gears is critical to long and profitable performance. The basics for successful lubrication of these machine elements are discussed. Procedures for correct selection of oil and grease lubricants are defined. Alternate systems for delivering proper lubrication are presented. Preventive and predictive methods for maintaining lubrication systems at their peak are suggested.

PREVIEW

“Rolling bearing lubricants serve the following primary functions:

Lubricate the sliding contact which exists between the retainer and other parts of the bearing
Lubricate that part of the contact between the races and rolling elements that is not true rolling contact.
Lubricate the sliding contact between the rollers and guiding elements in the bearing.
Lubricate all true rolling contacts between the rollers and other elements.
Protect the highly finished surfaces of the rolling elements and races from corrosion.
Help seal housings against foreign matter.

The engineers from ABEC made several other interesting observations. They recommended the following viscosities for lubricating oils at operating temperature:

Ball Bearings and Cylindrical Roller Bearings 70 Sec. Saybolt
Spherical Roller Bearings 100 Sec. Saybolt
Spherical Roller Thrust Bearings 150 Sec. Saybolt.”

"Oil Lubrication:

The Annular Bearing Engineer’s Committee (ABEC), after extensive research reached the following conclusions:

“The friction torque in a ball bearing lubricated with oil consists essentially of two components. One of these is a function of the bearing design and load imposed on the bearing, and the other is a function of the viscosity and the quantity of the oil and speed of the bearing.”
“It has been found that the friction torque in a bearing is lowest with a very small quantity of oil, just sufficient to form a thin film over the contacting surfaces, and that the friction will increase with greater quantity and with higher viscosity of the oil. With more oil that just enough to make a film, the friction torque will also increase with speed.”
“The energy loss in a bearing is proportional to the product of torque and speed. This energy loss will be dissipated as heat, and cause a rise in the temperature of the bearing and its housing. This temperature rise will be checked by radiation, convection, and conduction of the heat generated to an extent depending upon the construction of the housing and the influence of the surrounding atmosphere. The rise in temperature, due to operation of the bearing, will result in a decrease in viscosity of the oil, and therefore a decrease in friction torque compared with friction of starting, but soon a balanced condition will be reached.”
“With so many factors influencing the friction torque, energy loss, and temperature rise in a bearing lubricated with oil, it is evidently not possible to give definite recommendations for selection of oil for all bearing applications.” (1)

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