THEORY OF LUBRICATION

Lubrication is concerned with reducing friction by interposing suitable material between the two bearing (rubbing) surfaces.

Dry Friction: Due to the surface irregularities actual contact area A A0 , the surface area. Therefore the stress (pressure) at the contact points are very high (may be order 1010 N/m2). Then the projections merge producing adhesion or welding and a force (friction) which opposes the motion is created. Experiments on solid friction shows that F P or F = P, where - coefficient of friction.

Fluid Friction: Introduction of a lubricant (usually fluid) keeps the surfaces apart, thereby producing a condition of fluid friction, where the friction force is very small.

Friction causes : reduction (loss) of power wear reduction of speed heat generation

Functions of Lubricants : reduce friction flush away contaminants remove heat dampers vibration & noise

Types of Lubricants : (a) Solid Materials : Graphite - for high temperature applications Talc (hydrated magnesium silicate) - for low temperature application. lead iodide, disulphide, borax, molybdenum, mica (b) Liquid: Most common

water oil - mineral, animal or vegetable, compound (5-25% vegetable & Mineral)

(c) Gas: Air is used for air bearings with very low frictional effects (used in gyroscopes, antennas) (d) Semi-solids: eg. greases

Properties of Lubricants : ideal viscosity if the viscosity is too high - excessive resistance (higher power consumption)

- excessive wear due to higher fluid friction - higher operating temperature if the viscosity is too low - excessive wear due to breakdown of oil film(dry fr.)

- increase leakage Good lubricity fire resistance low volatility foam resistance chemical and environmental stability good heat transfer capacity minimum toxicity low pour point high flash point

Note: 1. In general, the fluid lubricants can not be interchanged or mixed directly. 2. In vehicle engines, heavily loaded components are lubricated by forced oil circulation (eg.

crank shaft main bearings, connecting rod big-end bearings, valve mechanism, cam shaft bushings, timing gear bushings)

Lub oil additives : anti-oxidants corrosion inhibitors detergents (dispersant additive) pour point depressants extreme pressure additives foam inhibitors viscosity index improvers oiliness agents

Regular tests carried out on lub oil samples : specific gravity viscosity water content acidity flash point sediments ash contents

Lubrication Regimes:

(A) Hydrostatic Lubrication The lubricant is pumped (from an external source) at a high pressure to a pocket(s) in the bearing which lifts the required component (shaft, flat pad or plate). Generally used in slow moving heavily loaded mechanisms.

(B) Hydrodynamic Lubrication (Fluid Film Lubrication) A continuous unbroken film of oil, which completely separates the surfaces (due to pressure), is maintained by the relative motion of the two surfaces. The operation is due to hydrodynamic action. Journal bearings and thrust bearings (conformal surfaces) operate on this principle. In this situation the physical properties of the lubricant, especially the dynamic viscosity, dictate the behaviour. The magnitude of the pressure developed is not generally large enough (usually less than 5 MPa) to cause significant elastic deformation of the surfaces. The minimum film thickness normally exceeds 1 m.

(C) Elastohydrodynamic Lubrication Elastohydrodynamic lubrication is a form of hydrodynamic lubrication where elastic deformation of the lubricated surface becomes significant. This occurs between non-conformal surfaces (point or line contact) where the pressures are high enough to deform the surfaces elastically. In this form of lubrication the elastic deformation and the pressure-viscosity effects are equally important. The maximum pressure is typically 0.5 to 3 GPa; the minimum film thickness normally exceeds 0.1 m. The elastic deformation are several orders of magnitude larger than the minimum film thickness and the viscosity can vary by as much as 10 orders of magnitude within the lubricating conjunction. The rise of viscosity assists the formation of an effective fluid film.

(D) Boundary Lubrication This occurs when the oil film is not thick enough to separate the surfaces completely. Some degree of metal to metal contact usually occurs. The physical and chemical properties of thin surface film of molecular proportions and the solid at the common interface determine the frictional characteristics. Viscosity of the lubricant is not an influential parameter. The surface film vary in thickness from 1 to 10 nm depending on the molecular size. Boundary lubrication is used for heavy loads and low running speeds, where fluid film lubrication is difficult to attain.

(E) Partial Fluid Film Lubrication (Mixed Lubrication) If the pressure in elastohydrodynamically lubricated machine elements are too high or the running speeds are too low, the lubricant film will be penetrated and some contact will take place between the asperities, and mixed lubrication will occur. The friction characteristics is governed by a combination of boundary and fluid film effects. The average film thickness is between 0.01 - 1 m. Load Carrying Mechanisms :

(a) external pressure (b) wedge action (c) squeeze action