9—Lubrication

Up to 50% of the energy used to operate equipment is lost to friction. Lubricants are crucial in combating this energy loss, and a variety of lubrication methods exist to ensure proper lubrication in all kinds of machines.

Purpose of Lubrication

Rolling bearings are lubricated to reduce internal friction, lower wear, and stop components from seizing. The benefits of lubrication are shown below:

Lubrication serves to extend bearing fatigue life, lower friction and wear, transfer heat away from the bearing, and help prevent rust and the entry of foreign matter.

Principles of Lubrication

Lubrication aims to prevent direct contact between the surfaces of two objects in relative motion.

Without lubricant, friction is very high. Under boundary lubrication, a small oil film separates surfaces, but there remains some metal contact that causes wear and friction. Under hydrostatic lubrication, the oil film completely separates metal surfaces; however, the thickness of the film creates pressure on the moving surface and causes friction to increase. The area of least friction lies between boundary and hydrostatic lubrication, a state called "mixed lubrication."

Grease Vs. Oil

In terms of pure lubricating capacity, oil is best. However, grease is a low-cost alternative that allows for a simpler structure around bearings.

Table Key: ◯：Good, ×：Poor

Item	Grease	Oil
Housing structure/ seal configuration	◯：Can be simple.	✕： Somewhat complicated, requires careful maintenance
Rotating speed	✕： Supports 65 to 80% the limiting speeds of oil lubrication	◯：Supports higher speeds than grease
Cooling effect	✕：None	◯：Can effectively dissipate heat (circulating lubrication)
Flowability	✕：Poor	◯：Very good
Full replacement	✕：Somewhat complex	◯：Relatively easy
Contaminant filtration	✕：Somewhat complex	◯：Easy
External contamination from leaks	◯：Surroundings seldom contaminated by leakage.	✕：Often leaks without proper countermeasureres. Not suitable if external contamination must be avoided.