Bearing Lubrication Method

Purpose of lubrication

The purpose of lubrication of rolling bearings is to reduce friction and wear inside the bearings and prevent seizure. Its lubrication effects are as follows:

(1) Reduce friction and wear

Prevent metal contact at the contact parts of the bearing rings, rolling elements and cages, reduce friction and wear.

(2) Extend fatigue life

When the rolling contact surface of the bearing rotates, the rolling fatigue life is extended; on the contrary, if the oil viscosity is low and the lubricating oil film thickness is insufficient, the rolling fatigue life is shortened.

(3) Discharge and cooling of friction heat

The heat generated by friction or heat transmitted from the outside is discharged and cooled by the lubricating oil through the circulation oil supply method, thereby preventing the bearing from overheating and the deterioration of the lubricating oil itself.

(4) Others

Prevent foreign matter from entering the bearing interior, and also prevent rust and corrosion.

Lubrication method

For machine tool spindles that require high processing accuracy, it is not only important to achieve high-speed rotation, but also to suppress temperature rise and reduce thermal deformation.

The factors affecting bearing heating are divided into: load items determined by bearing type and load, and speed items determined by lubrication method and rotation speed. Compared with the load term and the speed term, the speed term has a greater impact. However, if a lubrication method with low heat generation is selected in the speed term, the impact of the load term cannot be ignored. Therefore, it is important to select a low-heat bearing (load term) and a lubrication method (speed term).

The lubrication method and the amount of lubricant have a great impact on the actual heat generation. The lubrication method using a small amount of grease is a low-cost, maintenance-free, and low-heat generation lubrication method that is widely used. In order to maintain a low operating temperature during high-speed rotation, an oil-air lubrication method with a very small amount of oil can be used.

Grease lubrication

(1) Recommended grease

As a grease for machine tool spindle bearings, it is required to have low temperature rise and long life under high-speed rotation conditions. It is suitable to use a grease with a viscosity of 2 to 3 based on synthetic oil (diester oil or diester oil + mineral oil).

The grease life is greatly affected by temperature. Keeping the bearing temperature within a low range, including the ambient temperature, can help extend the grease life.

(2) Grease filling amount for high-speed spindle bearings

When grease lubrication is used for high-speed bearing operation, the recommended grease filling amount is 10-30% of the bearing space volume. Excessive filling amount will generate abnormal heat, especially during the initial operation after grease filling, causing grease deterioration. To avoid this, it is necessary to ensure sufficient time for running-in operation.

(3) Grease filling amount for ball screw support bearings

Generally, under low-speed, high-load, and discontinuous conditions, in order to improve reliability, the recommended grease filling amount is 30-55% of the bearing space volume.

Oil lubrication

The lubricating oil for bearings is a highly refined mineral oil or synthetic oil with high load capacity, good oxidation stability and rust prevention performance.

When selecting a lubricating oil, the most important thing is to select an oil with appropriate viscosity according to the operating temperature. If the viscosity is too low, the oil film cannot be fully formed, which may cause abnormal wear and sticking. On the contrary, if the viscosity is too high, the viscous resistance will cause heat and increase power loss.

The speed and load of the bearing will also affect the formation of the oil film.

Generally, the higher the speed, the lower the viscosity of the oil used. The greater the load and the larger the bearing, the higher the viscosity of the lubricant used. Under normal working conditions and the oil temperature around the operating bearing, please consult MONTON for the required viscosity of the lubricant.

(1) Spray lubrication and oil-air lubrication (micro-lubrication)

The spray lubrication method uses air to atomize the lubricant and then spray it into the bearing. It is also called oil mist lubrication.

The oil-air lubrication method is a lubrication method that continuously supplies oil to the bearing by quantitatively and intermittently spraying a micro-amount of lubricant through a nozzle and slowly drawing out the lubricant in the compressed air through a mixing valve.

(2) Spray lubrication

Spray lubrication is suitable for high-speed bearings with a dmn value exceeding 100×104. Lubricant at a certain pressure is sprayed from one or more nozzles and penetrates the inside of the bearing. When running at high speed, the air near the bearing runs at the same speed as the bearing, forming an air barrier. Therefore, the speed at which the lubricant is sprayed from the nozzle must be more than 20% higher than the linear speed of the inner ring outer diameter surface. In addition, for the same oil supply, in order to reduce the unevenness of cooling, it is hoped to increase the number of nozzles, and to increase the heat dissipation effect, the oil drain hole can be enlarged or forced oil drainage can be performed.