What factors should be considered in the selection of high-speed rolling bearing grease?

February 25, 2024

What factors should be considered in the selection of high-speed rolling bearing grease?

The speed of most industrial equipment is relatively high, and for grease-lubricated parts, grease can lead to increased bearing heat, increased resistance and premature failure. With the correct choice of grease, these higher speeds can be handled, reducing potential failures.

The operating temperature of the bearing is often inseparable from the operating speed of the bearing. Because the bearings with the hottest operating temperatures are often the ones that rotate the fastest (external heat is also considered). In addition, according to the lubrication theory, we know that the speed is also one of the important factors affecting the thickness of the lubricating oil film.

So what is high speed? How to choose grease for these high speed bearings?

For example, a chemical plant vertical air cooler, bearing speed 2000 r/min. Direct motor drive centrifugal pump, bearing speed of about 3000 r/min.

If a multi-purpose grease is simply applied without considering the actual situation of the bearing, the bearing may be damaged. To understand what bearings need in terms of lubrication, we first need to understand how to determine the speed factor of the bearing.

Calculation of bearing speed factor

The speed factor defines the relationship between the rotating speed of the bearing and the bearing size. The main calculation method is: ndm = (outer diameter + inner diameter) /2 * rotation speed.

Based on the velocity factor, it is possible to determine the properties of various lubricants and then use these properties to select the appropriate lubricant. Among these properties, the main consideration is the Base Oil viscosity and consistency grade of the grease.

Speed classification of bearings (From SKF)

Application examples of speed and grease

High speed bearing grease base oil viscosity selection

The most important physical property of a lubricant is viscosity. In the case of load, speed and surface contact form, the viscosity determines the oil film thickness of the bearing. The base oil viscosity of most general purpose greases is about 100 to 220 cSt. Such greases are generally used for medium speed and load situations, but when the bearing speed increases, the required rated viscosity will be reduced accordingly.

There are many ways to calculate viscosity. We can use the standardized chart provided by the bearing manufacturer to determine the rated viscosity of the bearing at the operating temperature. Taking fan bearings as an example, the ndm value of bearings is about 300 000, and the operating temperature of bearings is about 80 ° C, then the minimum rated base oil viscosity required for bearings is about 8 cSt.

If you choose a mineral oil with a standard viscosity index of 95, it needs to be equivalent to ISO VG 22-32 base oil. If we use a standard multi-purpose grease (base oil viscosity is 220 cSt), it is 10 times the required viscosity. Although high viscosity does not necessarily cause equipment failure, too much is a waste.

Excessive viscosity can result in excessive heat generation and increased energy consumption. The hotter the bearing, the lower the viscosity of the grease. At the same time, it also increases the consumption of oil and requires more frequent replenishment of fresh lubricant. Excessive viscosity increases the resistance to rotation, which causes more energy consumption, resulting in increased costs.

In addition: grease is generally suitable for bearings whose speed factor is not greater than 500,000. Of course, there are also some advertisements on the market that can reach 2 million speed factors of ultra-high speed greases.

Considerations for the selection of high-speed grease for rolling bearings:

Base oil viscosity: Ensure that the viscosity is enough to provide the bearing oil film, but not too thick oil film resulting in excessive resistance and excessive heat.

Grooving characteristics: The grease should have grooving characteristics, and no excess heat can be generated from the agitation of the grease.

High drop point: The drop point of the grease should be well above the operating temperature to avoid excessive oil separation and possible bearing failure.

Mechanical stability: Greases with good mechanical stability can extend the grease refill interval in high-speed applications.

Thickener type: Select a thickener that provides the appropriate drop point, grooving performance, and oil separation characteristics.

Consistency grade: The consistency of the grease will affect the oil separation characteristics and channel characteristics of the grease, and the appropriate consistency should be selected.

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