Bearing grease lubricants are relatively easy to handle and require only the simplest sealing devices. For these reason, grease is the most widely used lubricant for rolling bearings. It is used for a bearing that is pre-sealed with grease, or it using an unsealed bearing, fill the bearing and housing with the proper amount of grease, and replenish or change the grease regularly.
1, Bearing grease types
Lubricating grease are composed of either a mineral oil base or a synthetic oil base. To this base a thickener and other additives are added. The properties of all greases are mainly determined by the kind of base oil used and by the combination of thickening agent and various additives. The table below show general grease varieties and characteristics.
| Grease | Lithium grease | Sodium grease (Fiber grease) | Calcium compound base grease | Aluminum grease | Non-soap base grease | |||
|---|---|---|---|---|---|---|---|---|
| Thickener | Li soap | Na soap | Ca+Na soap Ca+Li soap | AI soap | Bentone, silica gel, urea, carbon black, fluorine compounds, etc. | |||
| Base oil | Mineral oil | Diester oil | Silicone oil | Mineral oil | Mineral oil | Mineral oil | Mineral oil | Synthetic oil |
| Dropping point °C | 170 ~ 190 | 170 ~ 190 | 200 ~ 250 | 150 ~ 180 | 150 ~ 180 | 70 ~ 90 | 250 or above | 250 or above |
| Operating temperature range °C | -30 ~ +130 | -50 ~ +130 | -50 ~ +160 | -20 ~ +130 | -20 ~ +120 | -10 ~ +80 | -10 ~ +130 | -50 ~ +200 |
| Mechanical stability | Excellent | Good | Good | Excellent ~ Good | Excellent ~ Good | Good ~ Poor | Good | Good |
| Pressure resistance | Good | Good | Poor | Good | Excellent ~ Good | Good | Good | Good |
| Water resistance | Good | Good | Good | Good ~ Poor | Good ~ Poor | Good | Good | Good |
| Applications | Widest range of applications. Grease used in all types of rolling bearings. | Excellent low temperature and wear characteristics. Suitable for small sized and miniature bearings. | Suitable for high and low temperatures. Unsuitable for heavy load applications due to low oil film strength. | Some emulsification when water is introduced. Excellent characteristics at relatively high temperatures. | Excellent pressure resistance and mechanical stability. Suitable for bearings receiving shock loads. | Excellent adhesion Suitable for bearings receiving vibration | Can be used in a wide range of low to high temperatures. Shows excellent heat resistance, cold resistance, chemical resistance, and other characteristics when matched with a suitable base oil and thickener. Grease used in all types of rolling bearings. | |
As performance characteristics of even the same type of grease will vary widely from brand to brand, it is necessary to check the manufacturers' data when selecting a grease.
1.1, Base oil
Mineral oil or synthetics such as ester or ether oil are used as the base of the grease.
Mainly, the properties of any grease is determined by the properties of the base oil. Generally, greases with low viscosity base oil are best suited for low temperatures and high speeds; Grease using high-viscosity base oil has superior high-temperature and high-load characteristics.
1.2 Thickening agents
Thickening agents are compounded with base oils to maintain the semi-solid state of the grease. Thickening agents consist of two types of bases, metallic soaps and non-soaps. Metallic soap thickeners include: lithium, sodium, calcium, etc.
Non-soap base thickeners are divided into two groups; inorganic (silica gel, bentonite, etc.) and organic (polyurea, fluorocarbon, etc.).
The various special characteristics of a grease, such as limiting temperature range, mechanical stability, water resistance, etc. depend largely on the type of thickening agent used. For example, a sodium based grease is generally poor in water resistance properties, while greases with bentone, poly-urea and other non-metallic soaps as the thickening agent are generally superior in high temperature properties.
1.3, Additives
Various additives are added to greases to improve various properties and efficiency. For example, there are anti-oxidents, high-pressure additives (EP additives), rust preventives, and anti-corrosives.
For bearings subject to heavy loads and/or shock loads, a grease containing high-pressure additives should be used. For comparatively high operating temperatures or in applications where the grease cannot be replenished for long periods, a grease with an oxidation stabilizer is best to use.
1.4, Consistency
Consistency is an index that indicates hardness and fluidity of grease. The higher the number, the softer the grease is. The consistency of a grease is determined by the amount of thickening agent used and the viscosity of the base oil. For the lubrication of rolling bearings, greases with the NLGI consistency numbers of 1, 2, and 3 are used.
General relationships between consistency and application of grease are shown in table below,
| NLGI consistency No. | Consistency 1/10 mm | Applications |
|---|---|---|
| 0 | 355~385 | For centralized greasing use |
| 1 | 310~340 | For centralized greasing use |
| 2 | 265~295 | For general use and sealed bearing use |
| 3 | 220~250 | For general use and high temperature use |
| 4 | 175~205 | For special use |
1.5 Mixing different types of greases
When greases of different kinds are mixed together, the consistency of the greases will change (usually softer), the operating temperature range will be lowered, and other changes in characteristics will occur. As a rule, grease should not be mixed with grease of any other brand.
However, if different greases must be mixed, at least greases with the same base oil and thickening agent should be selected.
2, Bearing grease quantity
The quantity of grease to be packed in a housing depends on the housing design and free space, grease characteristics, and ambient temperature. For example, the bearing for the main shafts of machine tools, where the accuracy may be impaired by a small temperature rise, require only a small amount of grease. The quantity of grease for ordinary bearings is determined as follows.
Sufficient grease must be packed inside the bearing including the cage guide face. The available space inside the housing to be packed with grease depends on the speed as follow:
- 1/2 to 2/3 of the space: When the speed is less than 50% of the limiting speed.
- 1/3 to 1/2 of the space: When the speed is more than 50% of the limiting speed.
3, Replacement of grease
Grease, once packed, usually need to be replenished for a long time; however, for severe operating conditions, grease should be frequently replenished or replaced. In such cases, the bearing housing should designed to facilitate grease replenishment and replacement.
When replenishment intervals are short, provide replenishment and discharge ports at appropriate positions so deteriorated grease is replaced by fresh grease.
4, Grease replenishment
Lubricating grease slowly degrades and therefore has a limited life. Grease life depends on the operating conditions of the bearing and the grease type. Rolling bearings therefore have to be relubricated if:
- The grease life is shorter than the specified bearing life
- The grease becomes contaminated
The figs below show the replenishment time intervals for various bearing types running at different speeds. The figs apply for the condition of high-quality lithium soap-mineral oil grease, bearing temperature of 70°C and normal load (P/C=0.1).
If the bearing temperature exceeds 70°C, the replenishment time interval must be reduced by half for every 15°C temperature rise of the bearings.
In case of ball bearings expecially, the replenishing time interval can be extended depending on used grease type. (For example, high-quality lithium soap-synthetic oil grease may extend about two times of replenishig time interval shown in Fig above. if the temperature of the bearings is less than 70°C, the usage of lithium soap-mineral oil grease or lithium soap-synthetic oil grease is appropriate.)