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 :
Functions of Lubricants :
reduction (loss) of power
reduction of speed
reduce friction
remove heat
wear
heat generation
flush away contaminants
dampers vibration & noise
Types of Lubricants :
(a) Solid Materials :
(b)
(c)
(d)
Graphite - for high temperature applications
Talc (hydrated magnesium silicate) - for low temperature application.
lead iodide, disulphide, borax, molybdenum, mica
Liquid: Most common
water
oil - mineral, animal or vegetable, compound (5-25% vegetable & Mineral)
Gas: Air is used for air bearings with very low frictional effects (used in gyroscopes, antennas)
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
