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Part I
Lubrication Theory
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Chapter 1
Properties of Lubricants
Many fluids serve as lubricant in industry. Among them, oil and grease are the most commonly used. Air, water and liquid metals are also used as special lubricants. For example, liquid sodium is often used as a lubricant in nuclear reactors. In some situations, solid lubricants, such as graphite, molybdenum disulfide or polytetrafluoroethylene (PTFE) and so on can also be used. In this chapter we will discuss the viscosity and density of lubricants, which are the two important physical properties associated with lubrication.
In lubrication theory, the most important physical property of a lubricant is its viscosity, which is the most important factor to determine the lubrication film thickness. In hydrodynamic lubrication, the lubricant film thickness is proportional to the viscosity, while in elasto-hydrodynamic lubrication it is proportional to the viscosity of 0.7th powers. Although in boundary lubrication the viscosity does not directly influence the film thickness, the oil packages formed between peaks and valleys of roughness will carry part of the load. Therefore lubricant viscosity is closely related to its load carrying capacity.
Furthermore, viscosity is also an important factor influencing the frictional force. A high-viscosity lubricant not only causes a lot of friction loss, but produces a lot of heat that make cooling control difficult. Because temperature rise caused by friction may lead to failure of lubricant film, the surface will be worn increasingly. Therefore, a reasonable viscosity is required for practical lubrication.
The performance of elasto-hydrodynamic lubrication (EHL) also depend on the rheological characteristics of a lubricant. In point or line contacts, an EHL film is very thin, less than one micro-meter, but the pressure is very high, up to one GPa. And, because the contact area is often very small, the shear rate may be higher than 107 s−1 such that the passing time is very short, less than 10−3 s. Therefore, a friction process is always accompanied by high temperature. For such conditions, the properties of a lubricant are quite different from those of a Newtonian fluid. In such cases, therefore, it is necessary to study the rheological properties of lubricants. Experiments show that although the film thickness formula derived from the Newtonian fluid model is usually applied to the elasto-hydrodynamic lubrication, the frictional force and temperature calculated by a Newtonian fluid model will cause a big error. Therefore, in thermo-elasto-hydrodynamic lubrication (TEHL), more realistic non-Newtonian fluid models should be used. These belong to a lubricant rheology study which will not only help us understand the lubrication mechanism more deeply but also has important significance in energy conservation and improvement in the life of mechanical elements.
1.1 Lubrication States
The purpose of lubrication is to form a lubricant film to separate the friction surfaces to carry a load with a low shear stress to reduce friction and wear of materials. A lubricant film can be a liquid, a gas or a solid. According to the mechanisms of lubricant film formation, lubrication states can be divided into the following six basic types: (1) hydrodynamic lubrication; (2) hydrostatic lubrication; (3) elasto-hydrodynamic lubrication; (4) thin film lubrication; (5) boundary lubrication; and (6) dry friction. The features of the lubrication states are listed in Table 1.1.
Table 1.1 Basic features of lubrication states.
A lubrication state has its typical film thickness. However, we cannot determine the lubrication state simply and accurately based on the thickness alone because the surface roughness also needs to be considered. Figure 1.1 lists the thickness orders of different lubricant films and roughnesses. Only whe...
