Fundamentals of Lubrication

Abstract

Lubricants are substances used to minimize the friction and wear of moving parts. Additionally, they can serve to distribute heat, remove contaminants, and improve the efficiency and lifetime of mechanical systems. Lubricants can generally be categorized as liquid, solid, or gaseous. Liquid lubricants consist of base oils such as natural oils, mineral (petroleum) oils, and synthetic oils with combinations of additives that further enhance the properties of the lubricants. Solid or dry lubricants are generally powders or semisolids in the form of a grease or solid–liquid suspension. Gaseous lubricants have a much lower viscosity than liquid or solid lubricants and utilize gasses such as air under pressure. The selection of an appropriate lubricant for a mechanical system requires a thorough understanding of the rheology of lubricants, the effects of additive combinations, and the knowledge of lubrication theory. Lubrication theory is linked to numerous fields of expertise outside of tribology, and without this interdisciplinary aspect, the progression of lubricants and lubrication technologies within the vast array of applications may not have reached the necessary levels of success. The use of liquid lubricants is ubiquitous in most applications, ranging from automotive fluids, to industrial oils, and process oils. Within the lubrication industry, there are over 10,000 different lubricants used around the world. This chapter explores the many aspects of lubricants and lubrication technologies including lubrication fundamentals, rheology of liquid lubricants, liquid lubricant additives, and liquid lubrication theory.

Appendices

Exercises

• 1. Define the term “lubricant” and explain its importance in tribology.

• 2. Define the term “additive.” Why are additives used in lubricants? List various families of additives.

• 3. Explain the various lubrication regimes.

• 4. What is the Stribeck curve? Define Hersey’s number.

• 5. Explain how lubrication regimes can be determined based on oil-film thickness and roughness of the contacting surfaces.

• 6. Explain various temperature characteristics of lubricants.

• 7. Explain:

  • (a) Pour point vs. cloud point

  • (b) Flash point vs. fire point

  • (c) Volatility vs. evaporation

  • (d) Oxidation vs. thermal stability

• 8. Write a note on:

  • (a) Boundary additives

  • (b) Extreme pressure additives

  • (c) ZDDP

  • (d) Oxidation inhibitors

  • (e) Corrosion inhibitors

• 9. What is the underlying difference between thrust bearings and journal bearings in regard to the Reynolds equation? Provide the Reynolds equation for both bearing types.

Answers

• 1. A lubricant is a substance introduced between two moving surfaces to reduce friction, minimize wear, distribute heat, and remove foreign contaminants to ultimately improve efficiency. Lubricants are important because they allow mechanical components to achieve the desired amount of work while minimizing the amount of energy required to perform the work.

• 2. Additives are substances used to improve the performance of lubricants. Additives are selected based on their ability to reduce friction and wear, increase viscosity, improve viscosity index, resist corrosion and oxidation, increase component and lubricant lifetime, and minimize contamination. The main families of additives are antioxidants, antiwear formulations, antifoaming agents, corrosion inhibitors, detergents, dispersants, extreme pressure, friction modifiers, metal deactivators, and viscosity index improvers.

• 3. The three main regimes of lubrication can be referred to as boundary lubrication, mixed/elastohydrodynamic lubrication, and full film hydrodynamic lubrication. Boundary lubrication or boundary friction is the lubrication regime with the most asperity contact between the surfaces occurring due the presence of a thin fluid film. Mixed film lubrication is the combination of full film hydrodynamic lubrication and boundary lubrication. In this lubrication regime, the surfaces are transitioning away from boundary lubrication into hydrodynamic lubrication where there may be frequent asperity contacts, but at least a portion of the bearing surface remains supported partially by a hydrodynamic film. Hydrodynamic lubrication also known as fluid-film or thick-film lubrication involves two nonparallel surfaces in relative motion with a layer of fluid pulled in between the surfaces to develop adequate dynamic pressure to support the load of the opposing surfaces and prevent asperity contact. In this lubrication regime, the surfaces are no longer in contact and the fluid has established itself in significant form to create a thick film.

• 4. The Stribeck curve is a plot of the variation of the coefficient of friction against a nondimensional number, known as Hersey’s number (also referred to as the Stribeck number), which is instrumental in demarcation of the lubrication regimes. Hersey’s number is given as (ηv/p), where “η” is the lubricant viscosity, “v” is sliding velocity, and “p” is load per unit width.

• 5. Lubrication regimes can be determined based on oil-film thickness and roughness of the contacting surface by using the Hamrock and Dowson formula given as

  $$ \lambda =\frac{h}{\sigma } $$

  (10.28a)
  $$ \sigma =\sqrt{\sigma_1^2+{\sigma}_2^2} $$

  (10.28b)

  Here, (10.28a) is the ratio of the fluid-film thickness, h, and the composite surface roughness, σ. In (10.28a), the composite surface roughness is given in (10.28b) where σ₁ and σ₂ are RMS roughness of the two mating surfaces. Equation (10.28a and 10.28b) allows for the calculation of the minimum film thickness in lubricated contacts. The fluid-film thickness parameter, λ, decides the lubrication regime with boundary lubrication characterized by a value of λ less than 1, mixed or elastohydrodynamic lubrication described as 1 ≤ λ ≤ 3, and hydrodynamic lubrication characterized by a value of λ greater than 3.

• 6. The important thermal properties of lubricants are specific heat, thermal conductivity, and thermal diffusivity. These three parameters are important in assessing the heating effects in lubrication, i.e., the cooling properties of the oil and the operating temperature of the surfaces. The temperature characteristics are important in the selection of a lubricant for a specific application.

• 7.

  • (a) The pour point of a liquid is the lowest temperature at which it becomes semisolid and loses its flow characteristics. The cloud point of a fluid is the temperature at which dissolved solids are no longer completely soluble, precipitating as a second phase giving the fluid a cloudy appearance.

  • (b) The flash point of a lubricant is the temperature at which its vapor will flash ignite. The fire point of oil is the temperature at which enough vapor is produced to sustain burning after ignition. The fire point is higher than the flash point.

  • (c) In many applications the loss of lubricant due to evaporation can be significant. At elevated temperatures, in particular, oils may become more viscous and eventually dry out due to evaporation. Volatility of lubricants is expressed as a direct measure of evaporation losses.

  • (d) Oxidation stability is the resistance of a lubricant to molecular breakdown or rearrangement at elevated temperatures in an open-air environment containing oxygen. Thermal stability is the resistance of the lubricant to molecular breakdown or molecular rearrangement at elevated temperatures in the absence of oxygen.

• 8.

  • (a) Adsorption or boundary additives control the adsorption type of lubrication and are also known as “friction modifiers,” since they are often used to prevent stick-slip phenomena. Adsorption or boundary additives are vital in boundary lubrication. Examples of additives in current use are fatty acids, esters, and amines derived from fatty acids.

  • (b) Extreme pressure (EP) compounds are designed to react with metal surfaces under extreme conditions of load and velocity to prevent welding of the moving parts that would otherwise cause severe damage.

  • (c) ZDDP is a very important antiwear additive commonly used in engine oil formulations. It was originally developed as an antioxidant and detergent, but was later revealed to exhibit superior antiwear properties and function as a mild EP additive.

  • (d) Many lubricating oils contain antioxidant additives to delay the onset of severe oxidation of the oil. These are either natural antioxidants or artificially introduced additives that are able to suppress oxidation.

  • (e) Corrosion control additives are classified as corrosion inhibitors and rust inhibitors. Corrosion inhibitors are used for nonferrous metals and are designed to protect their surfaces against any corrosive agents present in the oil. Rust inhibitors are used for ferrous metals to protect ferrous surfaces against corrosion.

• 9. Thrust bearings and the journal bearings differ by the velocities contributing to the pressure generated. In thrust bearings, the difference in tangential velocities creates pressure, whereas in journal bearings the sum of the tangential velocities creates pressure.

  To simulate a conventional thrust bearing where the relative motion between the surfaces in contact is pure translation, the equation of motion is

  $$ \frac{\partial }{\partial x}\left(\frac{h^3}{\mu}\frac{\partial p}{\partial x}\right)+\frac{\partial }{\partial z}\left(\frac{h^3}{\mu}\frac{\partial p}{\partial z}\right)=6\left({U}_1-{U}_2\right)\frac{\partial h}{\partial x}+12\left({V}_2-{V}_1\right) $$

  To simulate a traditional journal bearing where relative motion of the surfaces in contact is not parallel, a component of the rotational velocity augments the relative motion in the tangential direction resulting in an equation of motion as follows:

  $$ \frac{\partial }{\partial x}\left(\frac{h^3}{\mu}\frac{\partial p}{\partial x}\right)+\frac{\partial }{\partial z}\left(\frac{h^3}{\mu}\frac{\partial p}{\partial z}\right)=6\left({U}_1+{U}_2\right)\frac{\partial h}{\partial x}+12\left({V}_2-{V}_1\right) $$