Rolling bearings enable friction-reduced rotary motion between different machine components.
In engineering, rolling bearings are therefore used for guiding and supporting shafts and axles, among other things. Rolling bearings are based on the principle of rolling friction: They consist of an inner and outer ring, between which rolling elements roll, which are kept separate by a cage to further reduce friction and wear. Depending on the load, different types of rolling elements are used, such as balls or rollers. Due to the rolling friction associated with the rolling element shape, ball bearings are particularly suitable for high speeds, whereas roller bearings are better suited to high loads.

Tasks of the rolling bearing lubricant

The aim of rolling bearing lubrication is to prevent metallic contact between the rolling and sliding surfaces by means of a lubricating film in order to further reduce the sliding friction in the rolling bearing. Furthermore, the lubrication of the rolling bearing leads to an improvement in wear protection. This prevents bearing damage, extends the service life of the bearing and increases operational reliability. Depending on the type of lubricant (oil or grease with appropriate additives), additional tasks of the lubricant in the rolling bearing include corrosion protection, dissipation of heat from the bearing, protection of the bearing against internal and external contamination, damping of the running noise of the bearing and support of the sealing effect of the bearing seals.

Decision criteria for grease or oil lubrication
Grease lubrication is used for approx. 90 % of all rolling bearings. The main advantages of grease lubrication are

• very low design effort
• good support for the sealing of the bearing
• long service life
• low frictional torque
• good emergency running properties with solid lubricants
• good noise damping properties
• If selected correctly, greases provide reliable lubrication for a wide range of speeds and loads for all bearing types except spherical roller thrust bearings.

Structure and characterization of greases
Lubricating greases consist of a base oil that is bound by a thickener. This ensures that the oil remains at the lubrication point. There it ensures effective protection against friction and wear and seals the lubrication point against external influences such as moisture and foreign substances. Greases are therefore ideal for use in rolling bearings. The typical performance characteristics of a grease, such as dropping point, load carrying capacity and water resistance, are determined by the base oil and the thickener. The additives added to the grease improve the corrosion and wear protection, the load-bearing and adhesive capacity and the ageing resistance.

Characteristics for selecting a rolling bearing grease

The selection of a rolling bearing grease depends on the design of the rolling bearing and the material of the rolling bearing cage (metals or plastics), as well as the operating and application conditions such as operating temperature, speed range, pressure load and environmental influences (water, dust or aggressive media). The following characteristics are available for the characterization of a rolling bearing grease.

1. NLGI class

For lubricating greases, consistency is the parameter for the strength of a grease. The classification of greases according to NLGI ranges from very soft (class 000) to very firm (class 6). Greases in NLGI classes 1-4 are suitable for use in rolling bearings.

2. Dropping point (in °C)

The dropping point of a lubricating grease is the temperature at which the grease liquefies. It is significantly higher than the recommended operating temperature, which in the case of a rolling bearing is determined not only by the ambient temperature, but also by the heat that develops in the bearing during operation.

3. VKA value

The four-ball tester is a test device for lubricants that are used at high surface pressures. It consists of a rotating running ball that slides on three stationary balls. During the test for maximum load absorption of the lubricant, a test force acts on the ball, which is gradually increased until the four-ball system welds as a result of the frictional heat.

4. DN value (speed factor)

The DN value specifies the maximum circumferential speeds up to which a lubricating grease can be used in a rolling bearing. It is calculated from the bearing speed, the mean bearing diameter (in mm) and a factor to take into account the sliding friction component of the bearing design.

5. SKF Emcor value

The SKF Emcor method is used to assess the anti-corrosion properties of rolling bearing greases. Water is added to the grease and tested for corrosion in self-aligning ball bearings with a defined running time, speed and certain downtimes in accordance with DIN 51802. If the visual inspection of the test rings reveals no corrosion, the corrosion grade is 0. If the corrosion is very severe, the corrosion grade is 5.

Lubrication of rolling bearings

An important prerequisite for the high operational reliability of a rolling bearing is the continuous supply of lubricant. The bearing manufacturer's instructions must be observed when lubricating the bearing for the first time or re-lubricating it. It must be filled in such a way that all functional surfaces are reliably greased. Slow-running bearings (DN value < 50,000) and their housings can be filled completely, fast-running bearings (DN value > 400,000) up to 1/4 of the free bearing interior. Otherwise, filling up to 1/3 is recommended.

Grease-lubricated bearings are sufficiently reliable if the relubrication intervals are not exceeded. Relubrication is necessary if the grease service life is shorter than the expected bearing service life. Relubrication is carried out using a grease gun or automatic lubrication systems. If possible, relubrication should be carried out during operation. The relubrication quantities are 50-80 % of the initial filling quantity. If it is not possible to remove the used grease, the grease quantity should be limited to prevent overlubrication of the bearing.

If relubrication intervals are long, the grease should be replaced as completely as possible. Before changing to a different type of grease, the rolling bearing must always be cleaned or the grease relubricated until the old grease has been completely pressed out of the bearing. In this case, the miscibility of the lubricants and the material compatibility must be checked in advance.