The effect of medium grease lubrication and oil lubrication of rolling bearings is better, see the friction torque test results

January 25, 2024

The effect of medium grease lubrication and oil lubrication of rolling bearings is better, see the friction torque test results

The research shows that the grease can form a thicker EHL oil film on the rolling surface when the rolling bearing is running at low speed, and its function is to prevent the direct contact of the metal under low speed conditions and reduce the friction torque. Although this effect does not have much effect on energy saving, it can effectively improve the operability of using bearing equipment.

The friction torque of rolling bearing is mainly caused by the sliding friction resistance and rolling viscous resistance generated by the rolling surface. When lubricating with grease, all areas are in EHL, so there is no metal contact even in low-speed areas, and friction torque is significantly reduced compared to when lubricating with oil.

Test this claim with the following test. During the test, a device was used to measure the thickness of the EHL oil film in the bearing (figure below). The same axial load is applied to each tested 6204 bearing during operation, the housing mounted on the outer ring of the bearing is contacted with the measuring load unit of the fixed device on the bench to limit the rotation of the outer ring and the rotation of the entire housing, and the friction and torque of the bearing are measured.

The Base Oil of the sample grease used three viscosity levels of PAO base oil, the viscosity was PAO 4-25mm2/s, PAO 8-81 mm2/s, PAO 40-741 mm2/s, and the type of thickening agent was lithium 12 hydroxy-stearate, and the coning grade was No.3. At the same time of testing the grease samples, the lubrication experiments of the corresponding three base oils were also carried out.

The test results are shown in the two curves below. The horizontal axis is the oil film thickness c and oil at the theoretical center, and the number axis is the average frictional torque.

The figure above shows the data of the three base oils. In the right high-speed area, the three base oils are basically on the same curve, and the friction torque increases with the increase of the oil film thickness c,oil. On the other hand, in the left low-speed area, when the oil film thickness c,oil is reduced to below 40nm, the frictional torque reaches the minimum value, and with the occurrence of direct metal contact, the frictional torque increases again, which reflects the difference brought by the viscosity of the base oil. The smaller the viscosity, the greater the frictional torque, the greater the viscosity, the smaller the frictional torque.

The following figure shows the data of the three greases. In the high-speed area with the right oil film thickness c and oil above 40nm, the friction torque is basically the same as that of the base oil. However, in the low-speed region where the film thickness c and oil is lower than 40nm, the frictional torque is significantly reduced, and the viscosity of the base oil does not bring significant difference.

In summary, in the case of low speed, grease lubrication can better reduce friction torque than oil lubrication, improve the efficiency of equipment and reduce friction.

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