Bearing grease has the following functions:

1. Reduce the rolling resistance of the rolling element and raceway when separating the surface.

2. Minimize the sliding friction between rolling element, raceway and cage.

3. Anti-corrosion effect.

4. Grease can also act as a sealant to help the seal ring prevent liquid and solid contaminants from entering the bearing interior.

Oil film thickness is important

When the load acts on the bearing, the roller and the raceway in the load zone will have elastic deformation, contact each other in a limited area, and due to the addition of lubricating oil, contact oil film will be formed between each other, and the appropriate oil film thickness will extend the service life of the bearing and slow down the wear of the bearing components. When the bearing temperature rises, the working viscosity of the grease will be reduced, the resulting oil film will be thinned, and the bearing life will be greatly reduced.

λ ratio = oil film thickness/composite roughness of two contact surfaces

Lubricating oil film thickness is an extremely important factor to maintain bearing performance, λ ratio in bearing system analysis used as a lubrication life adjustment factor, in rolling mill bearing applications, it is usually considered that the λ ratio of 1.5 can be separated from the contact surface of the rolling body and raceway, it means that the oil film thickness is 1.5 times the combined roughness of the contact surface. A λ ratio of less than 1.0 will cause rough contact on the contact surface, resulting in peeling damage to the rolling element and raceway surface under extreme application conditions.

Reasonable selection of grease

The thickness of the oil film formed depends on the working conditions of the bearing, the temperature rise will reduce the viscosity of the lubricating oil, but too high, thick oil viscosity will also increase the friction resistance, the working surface roughness of the bearing is too high, will not be able to form a lubricating oil film.

Grease is generally composed of thickeners, base oils, additives, etc. The relevant parameters of these components constitute the main parameters of the grease, mainly drop point, needle penetration, and base oil viscosity.

Dropping point

Grease softens with increasing temperature. When the grease is heated under specified conditions, the temperature at which the first drop of liquid (or oil column) of the grease flows out of the experimental container is called the drop point. The test methods of the drop point are:

1) GB/T270

2) GB/4929, ASTM D566, ISO2167

3) GB3498, ASTM D2665

Generally speaking, the maximum operating temperature of the grease is 30-50 degrees lower than the drop point. For low speed conditions, the maximum service temperature of the grease is 15-30 degrees lower than the drop point.

Needle penetration

Needle penetration, also known as cone penetration, is a measure of the consistency of the oil (or how soft or hard the oil is). Pin penetration refers to the depth of the cone pattern at a given height under the specified load, time and temperature conditions, and its unit is 0.1mm. The greater the penetration, the softer the grease, and the harder the vice versa.

Needle penetration can be measured using GB/T269-91, the equivalent international standard is ISO/DIS2173.

There is also a difference between working and non-working needle penetration. Most are used to test oil load state stability. (Obtained by comparing the injection degree of grease after and before work)

Base oil viscosity of grease

Grease consists of thickening agent (soap base), base oil and additives. Base oil viscosity is an indicator of the internal friction of lubricating oil. In the case of no additives, the greater the viscosity, the higher the strength of the oil film, the worse the flow.

Performance requirements for roll bearing greases

Cold rolling equipment grease must be able to withstand high loads and abnormal impact loads of heavy duty grease. With the following properties:

Soap base: lithium, calcium, sulfonic acid or equivalent products

Consistency: Grade 1 or 2

Additives: Anti-corrosion and antioxidant EP additive

Base oil: solvent refined petroleum or synthetic oil

Base oil viscosity: usually 320 to 460

Viscosity index: minimum 80

Pour point: Max. -10°C

Initial grease usage

Excessive addition of grease will cause the bearing to heat, usually roughly calculate the internal free volume of the bearing, and then according to the application, the roller bearing should be injected to reach the internal free volume of one-third to one-half of the grease, the weight of the grease is calculated according to the density of 0.9g/cm3. For low speed (bearing speed below 10rpm), add 100% grease.

The internal free volume can be calculated as follows: V= (D2-d2)*T*π/4-M/A

(D2-d2)*T*π/4- bearing contour volume

M/A- bearing single weight

Supplementary grease

Adding grease is usually based on the effectiveness of the sealing system and based on experience, and taking into account the temperature, sealing efficiency, pollutants, grease loss, etc., according to convention, grease should be added every time the roll is replaced, when the grease aging and pollutant intrusion is serious, the grease fails, the bearing needs to be cleaned, and the grease needs to be replaced.

For open (non-sealed) bearings, a third of the initial injection of grease is required each time the roll is replaced.

Compatibility of greases

Avoid mixing greases of different grades or additives, which may cause grease failure.