Bearing Lubrication: Oil Versus Grease

Journal Lubrication Both oil and grease are used in the lubrication of journal bearings. The main factor in determining which lubrication to use is the speed of the journal itself. Slower journal speeds have higher viscosity requirements which include greases and heavier gear oils such as ISO-460, 680 or higher, such as AMSOIL Synthetic R & O Gear, Bearing and Hydraulic Oils, where Extreme Pressure additives are not required. Where Extreme Pressure additives are required, AMSOIL Synthetic EP Gear Oil is ideal.

High speed bearings require a lighter oil. Another consideration is that higher speed bearings have a much smaller clearance compared to slower journal bearings. Lubricants should maintain a full fluid film in these types of applications.

Rolling Element Bearings The lubricants in rolling element bearings have three functions. Their primary function is to help reduce sliding and rolling friction which helps reduce wear. A second function is to help extend service life by protecting the finished surfaces of the rolling elements and races from corrosion and pitting. This is an essential element for increased service life. Finally, lubricants used in roller bearings must also act as a sealant. Lubrication requirements differ for rolling element bearings because they are often subjected to high pressures within the bearing contact zone. This increase in pressure results in a unique phenomenon that actually causes a temporary increase in oil viscosity. While journal bearings operate under hydrodynamic full-film film lubrication, the high pressures in a rolling element bearing result in viscosity increases as well as an elastic deformation of the rolling surfaces as they support the loads that are created. This combination results in the formation of a thin elastohydrodynamic or EHD lubricant film which provides lubrication within the contact zone of rolling-element bearings.

Click on graphic to enlarge

Grease is uniquely suited to provide lubrication for rolling element surfaces, bearings and raceways. When grease encounters the high pressures between the rolling elements and their raceways, the pressure actually forces the grease to rupture or shear from its binding agents leaving a thin oil film at the point of contact. The remaining grease maintains its state and serves to function as a seal. When a bearing filled with grease begins to rotate, high friction and high heat situations occur and the grease is thrown throughout the housing, shears and creates an oil film. As the oil film is applied, it actually serves to reduce the temperature, and an equilibrium situation occurs.

Journal Bearing Components
AMSOIL GH Series Extreme Pressure greases are qualified against the highest grease standard, NLGI GC (wheel bearing) and LB (chassis), and are designed for heavy duty applications. These greases are composed of synthetic base oils and lithium-complex thickeners that provide the film strength, shear resistance and mechanical stability critical to high load protection. This combination excels in high and low operating temperature ranges by providing excellent oxidation stability, high temperature dropping points, good low temperature torque values and good low temperature pumping capabilities. The GH Greases are compounded with oxidation and corrosion inhibitors, effective oil soluble EP additives and molybdenum disulfide (moly) solid EP additives for all around protection in severe conditions.

Click on Graphic to Enlarge

A Journal or Sleeve Bearing consists of a cylindrical housing supporting a rotating shaft. The term “journal” refers to the portion of a shaft contained within a bearing. “Sleeve” refers to the bearing configuration. If the bearing is a full-cylindrical (a 360 deg. Design) it is called a bushing. A shaft that is loaded in a single direction can be supported by a journal bearing in the form of a partial cylinder. Such a bearing supports the shaft in the load zone only. For example, cranes, earth-moving equipment and railroad journals use partial-cylinder bearings to support loads directed against the axle.

Ball and Rolling Element Bearings
Ball Bearings are the most familiar type of rolling element bearing. A ball bearing consists of balls, a retainer, races and rotating shaft and bearing housing. Ball bearings are usually made of steel.

Rolling Element Bearings have balls or rollers situated between a stationary housing and a moving journal. Rollers can be cylindrical, spherical, or tapered case hardened steel.

Roller Bearings are part of an assembly that consists of the rollers, a retainer, races, a bearing housing and seals. The most basic type of roller bearing is a Cylindrical Roller Bearing which has cylinders between the bearing races. Cylindrical roller bearings can carry heavy radial loads and operate at high speeds.

Tapered Roller Bearings are commonly used in the automotive industry. Because of its tapered roller/race geometry, it can carry both heavy radial and thrust loads.

Spherical Roller Bearings use convex or barrel shaped rollers to accommodate high radial or shock loads, and provide for a high degree of self-alignment. These bearings do have a speed limit.

Needle Bearings contain cylindrical rollers with a high length to diameter ratio, and provide the highest load capacity for a given radial space of any rolling element bearing. Many of these bearings have no inner race and are able to accommodate oscillating motion.

Grease selection and Lubrication Requirements
Grease for rolling element bearings should have characteristics that allow it to resist oxidation and the formation of deposits which could result in hardening and ultimately result in reduced bearing life.

Grease should also have mechanical stability to resist separation as a result of shearing. Care must also be taken to insure that the greases the proper consistency for the method of application and the operating temperature. This can prevent excessive grease slumping. Greases also need anti-wear properties to resist the wiping action between roller ends and raceway flanges in bearings with thrust loads. Anti-rust protection and water resistance are also frequently required grease features.

The viscosity of the base oil is also an important consideration. Under moderate speeds, the following minimum viscosities at operating temperature are needed:

Ball and Cylindrical Roller Bearing: 12 cSt

Spherical Roller Bearing: 20 cSt

Spherical-Thrust Bearing: 32 cSt

It is important to observe that thrust bearings may need a grease with a heavier oil compared to ball or roller bearings.

In addition to the AMSOIL line of general purpose and heavy-duty moly greases (GL & GH Series), AMSOIL also has a high-viscosity grease (GVC), a water resistant grease (GWR), a food grade grease (GXC) and a high performance racing grease (GRG). All are also superb for a variety of industrial applications.

Re-lubrication
When re-lubricating a grease housing for rolling element bearings, it is recommended that the charge be no more than 1/3 to ˝ the volume of free space in the pillow block base. The formula for determining this amount is:

D = outer diameter (OD)
B = width

If grams are used, the measurement is millimeters. Ounces use the inch measurement. (Note that 1 inch = 25.4 mm)

Grams = 0.005 x D x B
Ounces = 0.114 x D x B

For example: If d = 6 inches and B = 1.5 inches you would multiply as follows:

0.114 x 6 x 1.5 = 1.026

1.026 ounces would be the appropriate amount of grease to install in the pillow block base.

It makes sense to take the time to make this simple calculation. Excess grease can cause too much pressure and could rupture seals. Too much grease can also cause heat buildup which will lead to failure of both the grease and the bearing.

Email to a Friend	Printer Friendly Version