3 Lubricants

3.1 Lubricating Grease

Grease is a semi-solid lubricant consisting of base oil, a thickness and additives. The main types and general properties of grease are shown in Table 2.

It should be remembered that different brands of the same type of grease may have different properties.

(1) Base Oil

Mineral oils or synthetic oils such as silicone or diester oil are mainly used as the base oil for grease.

The lubricating properties of grease depend mainly on the characteristics of its base oil. Therefore, the viscosity of the base oil is just as important when selecting grease as when selecting an oil. Usually, grease made with low viscosity base oil is more suitable for high speeds and low temperatures, while grease made with high viscosity base oil are more suited for high temperatures and heavy loads. However, the thickener also influence the lubricating properties of grease; therefore, the selection criteria for grease is not the same as for lubricating oil.

(2) Thickener

As thickeners for lubricating grease, there are several types of metallic soaps, inorganic thickeners such as silica gel and bentonite, and heat resisting organic thickeners such as polyurea and fluoric compounds. The type of thickeners is closely related to the grease dropping point; generally, grease with a high dropping point also has a high temperature capability during operation. However, this type of grease does not have a high working temperature unless the base oil is heat-resistant. The highest possible working temperature for grease should be determined considering the heat resistance of the base oil.

The water resistance of grease depends upon the type of thickeners. Sodium soap grease or compound grease containing sodium soap emulsifies when exposed to water or high humidity, and therefore, cannot be used where moisture is prevalent.

(3) Additives

Grease often contains various additives such as antioxidants, corrosion inhibitors, and extreme pressure additives to give it special properties. It is recommended that extreme pressure additives be used in heavy load applications. For long use without replenishment, an antioxidant should be added.

(4) Consistency

Consistency indicates the “softness” of grease. Table 3 shows the relation between consistency and working conditions.

Table 3 Consistency and Working Conditions

(5) Mixing Different Types of Grease

In general, different brands of grease must not be mixed. Mixing grease with different types of thickeners may destroy its composition and physical properties.

Even if the thickeners are of the same type, possible differences in the additive may cause detrimental effects.

3.2 Lubricating oil

The lubricating oil used for rolling bearing are usually highly refined mineral oil or synthetic oil that have a high oil film strength and superior oxidation and corrosion resistance.

When selecting lubricating oil, the viscosity at the operating conditions is important. If the viscosity is too low, a proper oil film is not formed and abnormal wear and seizure may occur. On the other hand, if the viscosity is too high, excessive viscous resistance may cause heating or large power loss. What’s more, The rotate speed and load capacity of the bearings also influence the formation of oil film.

In general, low viscosity oils should be used at high speed; however, the viscosity should increase with increasing bearing load and size.

Table 4 gives generally recommended viscosities for bearings under normal operating condition.

Table 4 Bearing types and proper lubricating oils viscosity