Grease is a commonly used lubricant in rolling bearings, and many factors need to be considered in the process of selecting grease, among which the most important one is viscosity.
The main components of grease are base oil and thickener. The base oil viscosity has the greatest impact on the formation of the lubrication film, hence, when we talk about the choice of grease viscosity, it primarily refers to the choice of base oil viscosity.
Of course, the viscosity of the thickener also needs to be chosen, but for general rolling bearings, mainly No. 2 and No. 3 greases are used, and they are selected according to some usage principles, which we will not expand on here.
Regarding the choice of base oil viscosity for grease, there are many influencing factors and the relationship is relatively complex, hence, we will introduce related quantitative computations.
From the basic principles of lubrication, we know that there are three states of bearing lubrication: fluid dynamic lubrication, mixed film lubrication, and boundary lubrication. The following is the famous Stribeck curve:
The goal of selecting grease viscosity is to ensure the bearing operates under fluid dynamic lubrication as much as possible. It’s not difficult to see from the diagram that these states are divided by the value of k, which is the viscosity ratio.
Typically, under specific rotational speeds, temperatures, and loads, a motor requires a minimum base oil viscosity, v1, to achieve lubrication. The base oil of the grease we select also has an actual viscosity, v, under these conditions. Thus, the viscosity ratio is defined as follows.
When k is less than 1, an effective oil film cannot be established between the rolling element and the raceway of the bearing, resulting in direct metal-to-metal contact to bear the load. In this situation, extreme pressure additives are required to prevent poor lubrication. Moreover, when k is less than 0.1, the rated static load should be considered when calculating bearing life.
When k is equal to or greater than 1, an oil film forms between the rolling element and the raceway, establishing a state of mixed-film lubrication. The rolling element and raceway are separated, but occasional metal-to-metal contact may occur.
When k is equal to or greater than 2, a substantial oil film forms between the rolling element and the raceway, leading to a state of fluid film lubrication, with the rolling element and raceway completely separated.
When k is equal to or greater than 4, the rolling element and raceway form a dynamic fluid film, completely separating the two. The bearing load is primarily borne by the oil film. However, an excessively high base oil viscosity can cause the bearing temperature to rise, especially at higher rotational speeds.
Therefore, the calculation of viscosity essentially returns to the calculation of the k value. The viscosity ratio, also known as the kappa coefficient, is the ratio of the actual viscosity to the required viscosity as observed in its formula. The actual viscosity can be obtained from the following table:
The required viscosity can be referenced from the table below:
Typically, lubrication viscosity calculations are verification computations, carried out according to the tables derived from the kappa calculation, aiming to ensure that the chosen lubrication kappa factor falls between 2 and 4.
When factors such as operating temperature and rotational speed change, the kappa factor can be recalibrated to guarantee the reliable formation of the lubricating oil film.