How to choose the right bearing grease !

Apr 19, 2022

Grease is made by mixing and dispersing a solid of high oil-affinity (called a thickener) with lubricant oil (as a base), and transforming it into a semi-solid state.

As well, a variety of additives can be added to improve specific performance.

(1) Base oil

Mineral oil is usually used as the base oil for grease. When low temperature fluidity, high temperature stability, or other special performance is required, diester oil, silicon oil, polyglycolic oil, fluorinated oil, or other synthetic oil is often used.

Generally, grease with a low viscosity base oil is suitable for applications at low temperature or high rotation speed; grease with high viscosity base oils are suitable for applications at high temperature or under heavy load.

(2) Thickener

Most greases use a metallic soap base such as lithium, sodium, or calcium as thickeners. For some applications, however, non-soap base thickeners (inorganic substances such as bentone, silica gel, and organic substances such as urea compounds, fluorine compounds) are also used.

In general, the mechanical stability, bearing operating temperature range, water resistance, and other characteristics of grease are determined by the thickener.

(Lithium soap base grease)

Superior in heat resistance, water resistance and mechanical stability.

(Calcium soap base grease)

Superior in water resistance; inferior in heat resistance.

(Sodium soap base grease)

Superior in heat resistance; inferior in water resistance.

(Non-soap base grease)

Superior in heat resistance.

Characteristics of respective greases

(3) Additives

Various additives are selectively used to serve the respective purposes of grease applications.

Extreme pressure agents

When bearings must tolerate heavy or impact loads.

Oxidation inhibitors

When grease is not refilled for a long period. Structure stabilizers, rust preventives, and corrosion inhibitors are also used.

(4) Consistency

Consistency, which indicates grease hardness, is expressed as a figure obtained, in accordance with ASTM, by multiplication by 10 the depth (in mm) to which the coneshaped metallic plunger penetrates into the grease at 25℃ by deadweight in 5 seconds. The softer the grease, the higher the figure. Table 12-4 shows the relationships between the NLGI scales and ASTM penetration indexes, service conditions of grease. (NLGI : National Lubricating Grease Institute)

Table 12-4 Grease consistency

(5) Mixing of different greases

Since mixing of different greases changes their properties, greases of different brands should not be mixed.
If mixing cannot be avoided, greases containing the same thickener should be used. Even if the mixed greases contain the same thickener, however, mixing may still produce adverse effects, due to difference in additives or other factors.
Thus it is necessary to check the effects of a mixture in advance, through testing or other methods.

Table 12-5 Typical examples of standard grease for FV bearings

[Notes]

1）PAO：Polyalphaolefin oil
2）The value is within the range specified by the consistency numbers.