A few days ago, an engineer asked about selecting lubricants for bearings. Lubricant manufacturers provided a multitude of performance indicators, but the meaning behind these parameters was unclear.
Today we will discuss the basic parameters and their meaning for engine bearing lubricants.
Bearing lubrication
In terms of bearing lubrication, there are solid lubricants, liquid lubricants and lubricating greases. For engines, grease lubrication is most commonly used. Large engines can use oil for lubrication, and engines for some special purposes can use solid lubricants.
In this context, we will mainly focus on parameters related to grease lubrication.
1. Grease lubrication
Generally, lubricating grease consists of a thickening agent, base oil and additives. The parameters associated with these components constitute the main characteristics of the grease.
From a component point of view, grease has several parameters, including physical and chemical properties. Without going into detail about each parameter, here we focus on several commonly used in engine bearing lubrication: dropping point, penetration and base oil viscosity.
In engine design, grease lubrication performance is calculated through the viscosity of the base oil, while grease suitability is evaluated from the dropping point and penetration perspectives.
(1) Dropping point
As the temperature increases, the lubricating grease softens. The dropping point is defined as the temperature at which the first drop (or column) of liquid flows out of the experimental vessel when the lubricating grease is heated under specified conditions. Test methods for drop point include:
1)GB/T270
2) GB/4929, ASTM D566, ISO2167
3) GB3498, ASTM D2665
If you are interested, you can look for these methods specifically. Details will not be expanded upon here. For engine engineers, the practical significance of the dropping point when choosing lubricating grease is as a temperature limit.
Generally, the maximum use temperature of lubricating grease should be 30-50 degrees lower than the dropping point. For low-speed scenarios, the maximum use temperature of lubricating grease should be 15-30 degrees lower than the dropping point. (Definitions of high speed and low speed will be introduced in articles related to rotational speed.)
Applying the dropping point directly as the maximum use temperature of the grease is generally inappropriate.
Some manufacturers directly provide the working temperature range of lubricating grease, which is more helpful for engine engineers when choosing. It is important to note that simply choosing a lubricating grease within the dropping point range is not enough. Final selection should also involve viscosity considerations and calculations.
(2) Penetration
Penetration, also known as cone penetration, is a measure of the viscosity of fats and oils, that is, their degree of hardness or softness. Penetration refers to the depth to which a cone-shaped specimen falls under specified conditions of load, time, and temperature. The unit of measurement is 0.1 mm. The greater the penetration, the softer the fat or oil and vice versa.
Penetration can be measured using GB/T269-91. The equivalent international standard is ISO/DIS2173.
There are distinct differences between functional and non-functional penetration. It is mainly used to test the stability of fats and oils under load. This is achieved by comparing penetration before and after the fats and oils are put into action.
Penetration is divided into nine degrees:
| NGLI viscosity classification | JIS viscosity classification | Penetration range |
| No. 000 | No. 000 | 445~475 |
| No. 00 | No. 00 | 440~430 |
| #0 | #0 | 355~385 |
| #1 | #1 | 310~340 |
| #2 | #2 | 265~295 |
| No. 3 | No. 3 | 220~250 |
| #4 | #4 | 175~205 |
| Number 5 | Number 5 | 130~160 |
| Number 6 | Number 6 | 85~115 |
Typically, the lubricating grease used in electric motor bearings is grade 2 or 3, with other grades being rarely used. Selection of grease grades will be specifically covered in subsequent articles.
(3) Base oil grease viscosity
Grease is made up of a thickener (soap base), base oil and additives. The base oil is particularly important. There are many base oil parameters, among which the most significant for engine engineers is the viscosity of the base oil.
This viscosity is an indicator of the friction within the lubricant. Without additives, the higher the viscosity, the greater the resistance of the oil film and the lower the fluidity. Detailed calculation methods will be introduced later.
The text above describes only the three most critical indicators for engine bearing lubrication. In fact, there are many other indicators for electric motor bearing lubrication, such as viscosity index, flash point, pour point and so on.
Based on my practical experience, mastering the application of the above three indicators can solve more than half of the lubrication selection problems faced by engine engineers.
