Hazards of Over-Lubrication in Bearings and Their Remedies

　　A common saying among bearing engineers is, “Bearings are the heart of machinery.” If that’s the case, we might liken lubrication to the “blood of machinery.” There’s an intriguing notion that the entire mechanical industry essentially floats atop oil films just a few micrometers thick. All these descriptions underscore the critical importance of lubrication—not only for bearings but for machinery as a whole.

　　Excessive selection of lubricant viscosity and handling

　　According to the lubrication mechanism, the viscosity of the base oil directly affects the formation of the lubricating oil film. Specifically, under the same rotational speed and the same load, a higher viscosity of the base oil is more conducive to forming an effective oil film. As a result, it’s not uncommon for many people to conclude that the higher the viscosity of the base oil, the better. However, in reality, as the lubricating viscosity increases, the rolling elements encounter greater resistance during their motion on the raceway surface, which in turn leads to an increase in the bearing’s running torque.

　　In some applications where there are requirements for bearing resistance torque, the bearings may exhibit inflexible rotation. For example, certain air-conditioning manufacturers specify that the motor’s low-temperature starting torque must fall within a particular range. When the bearing is exposed to low temperatures, the viscosity of the lubricant increases due to the temperature drop, which in turn leads to an increase in the bearing’s starting torque and subsequently causes starting difficulties.

　　On the other hand, the excessive viscous resistance caused by a lubricant with too high a viscosity will eventually dissipate as heat during bearing rotation; thus, an excessively high lubricant viscosity can also lead to an increase in bearing temperature. In engineering practice, there have even been cases where the grease became clumped due to its excessive consistency, preventing it from effectively forming a lubricating film between the rolling elements and the raceways.

　　The indicator for determining whether the viscosity of bearing grease is appropriate is the Kappa coefficient, k. At the operating temperature, if the Kappa coefficient, k, exceeds 4, it indicates that the bearing grease viscosity is significantly too high. Therefore, when selecting a lubricant, it’s essential to verify the Kappa coefficient based on the operating temperature. Additionally, when the bearing experiences heating related to lubrication, the Kappa coefficient should also be rechecked to ensure that the selected bearing grease is appropriate.

　　Excessive lubrication

　　After the bearing is installed, it’s essential to fill it with an adequate amount of grease to ensure that, during operation, the rolling elements and raceways are well lubricated with a uniform distribution of grease, thereby forming a stable oil film. Indeed, there are specific quantitative requirements for grease filling. A proper lubricant distribution means that, as the bearing operates, a continuous layer of lubricant is maintained on both the rolling elements and the raceway surfaces, creating a circulating lubricant flow. Under these conditions, the space between the rolling elements and the raceways isn’t completely filled with grease. This observation aligns with common-sense expectations: when installing a bearing, you might initially find that the interior is packed full of grease. However, after running the bearing for a period of time and then opening it up, you’ll notice that most of the grease has been squeezed out, leaving only a thin layer distributed on the bearing’s surface. This clearly demonstrates that, during operation, the grease between the rolling elements gets “squeezed out.” While “squeezed out” is merely a general descriptive term, this process reveals that, as the bearing runs, the rolling elements actually exert a certain degree of mechanical agitation on the grease. If the grease were continuously and uniformly distributed throughout the entire bearing interior, this constant agitation would lead to increased rotational resistance, ultimately causing the bearing to heat up.

　　The basic requirements for bearing grease filling are as follows: During initial lubrication, the bearing interior must be completely filled with grease; meanwhile, the space within the bearing housing—not occupied by the bearing itself—should be filled with 30% to 50% of grease.

　　After the bearing is put into operation, it’s important to ensure that the grease drainage path remains unobstructed. When there is excessive lubricant inside the bearing, the excess grease will be expelled as the bearing operates.

　　When replenishing lubrication, the appropriate amount is 0.5% of D × B, where D is the outer diameter of the bearing and B is the bearing’s width.

　　Whether it’s the initial lubrication or a replenishment, it’s recommended not to exceed the amounts mentioned above; otherwise, there could be over-lubrication of the bearing.