Lubrication

Lubrication is the process of separation of machine components’ tribological interfaces for friction and wear reduction and optimal operation of the machinery. The lubricant goes between two contacting solid bodies in motion to reduce friction and wear. Lubrication can be dry or wet in a tribological system. Tribological interfaces operating in dry air and at elevated temperatures require special lubrication mechanisms. Solid lubricants, like graphite, MoS2, WS2, TiO2, nitrides of boron etc. are used in dry contacts as necessitated by operational requirements and temperature.

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The tribological contact situation and mechanisms of friction and wear in some dry contacts can allow for adoption of self-lubrication methods [82]. Nanomaterials based on one or more of the solid lubricants are currently being considered as additives in liquid lubricants. They are being incorporated in conventional and bio-based liquid lubricants. The lubricating merits of the parent material are being harnessed, at the nano-scale level, for lubrication purposes.

3.6.1 Lubrication Regimes: Stribeck Curve and Lambda Parameter

Generally, fluid lubricated systems employ liquid lubricants to hydraulically separate the surfaces in a tribo-pair to reduce friction and wear. In cases where the contacting surfaces are completely separated by a film of lubricant, it is referred to as hydrodynamic lubrication. Here, the film of lubricant is built within the contact zone through motion. When the contacting surfaces are not completely covered by the lubricant film, there occur asperities interactions. This is the mixed lubrication regime, also called elasto-hydrodynamic lubrication regime. Further reduction in the lubricant film available in the contact zone leads to a dominance of asperity contact. The means of surface separation would be through physically or chemically absorbed boundary lubricant film. This is the boundary lubrication regime [42].

Fluid lubricated systems are conventionally characterised by the Stribeck curve shown in Figure 3-5 as explained by Woydt [83]. The Stribeck curve is a plot of the coefficient of friction against the Hersey number. The Hersey number is a dimensionless number expressed as the product of viscosity, η and the rotational speed, N, RPM, divided by the average pressure, P, N/m2. The average pressure, P = load/projected area.

𝐻𝑒𝑟𝑠𝑒𝑦 𝑁𝑢𝑚𝑏𝑒𝑟 = 𝜂.𝑁_𝑃

Equation 3-12

Although this does not account for the influence of additives in the lubricant, especially NPs, it is still an acceptable basis for lubrication analysis. The three categories of fluid lubrication or friction regimes, based on the Stribeck curve [83] are:

(i) Boundary lubrication (BL), (ii) Mixed lubrication (ML),

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The Stribeck curve stemmed from carefully conducted, wide ranging series of experiments on journal bearings. The graphs clearly showed the minimum value of friction (coefficient) now known as the transition between full fluid-film lubrication and some solid asperity interactions. Functionally speaking, based on the Stribeck curve, the coefficient of friction depends on the dimensionless product of sliding speed and viscosity divided by the contact pressure.

Figure 3-5: Characterisation of friction regimes according to the Stribeck curve in conformal contacts [83]

Figure 3-6: Classification of lubrication regimes based on lambda parameter in non-conformal contacts [84]

The Lambda parameter, λ, shown in the Stribeck curve (Figure 3-5) is further explained in Figure 3-6 by Kalin [84]. The lambda parameter is used to determine the lubrication regimes. This demonstrates the relative magnitude of friction coefficient, and the ability of a lubricant to separate the contacting surfaces. The lambda parameter was designed to determine the effect of the contact conditions on the lifetime of a tribo-pair. Thus, a more precise classification also includes λ ≥ 4 for fully elasto-hydrodynamic lubrication (EHL), with no effects on wear. The mixed-lubrication regime is also divided up, from 1 to 1.5; and from 1.5 to 3. These indicate the different influences of friction on wear. The lubricant presence or otherwise,

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impacts the wear behaviour of a lubricated system. As well, surface roughness impacts highly on lubrication of surfaces if lubricant film thickness is of the same order as the roughness [85]. The relationship between the dimensionless film parameter, λ, and the minimum film thickness,

ho, is expressed as:

Lambda, 𝝀 = 𝒉𝒐

√𝑹𝒒𝑨𝟐 + 𝑹𝒒𝑩𝟐

Figure 3-10

Where, ho minimum fluid film thickness RqA RMS surface roughness of body ‘A’ RqB RMS surface roughness of body ‘B’

Typically, the four lubrication regimes [86], based on lambda are identified and described as follows;

(i) Hydrodynamic lubrication: 5 < λ < 100 (ii) Elasto-hydrodynamic lubrication: 3 < λ < 10 (iii) Partial or mixed lubrication: 1 < λ < 5 (iv) Boundary lubrication: λ < 1

Full separation between asperities in the contact zone would avoid direct metal-to-metal contact. Lubricant film thickness should be above the critical ho, for needed separation of

interfacial peak asperities. The Lambda values are rough estimates, and running-in is known to affect the film parameter. Lambda increases with running-in, as the combined surface roughness will decrease. With running-in, the peaks of the asperities become more spherical (from conical), and flattens.

Automotive engine lubricants should provide adequate lubrication performance in all the regimes of lubrication. This is because some engine components undergo all the regimes of lubrication during operation. Metal-metal contact can take place at low speeds and high loads, especially with low viscosity lubricants. This is the case with the piston ring-cylinder liner sliding in the mixed or boundary lubrication regime. In this condition, surface contact occurs and chemical films or reaction products are essential for surface protection [3].

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