AN INTRODUCTION TO LUBRICANTS
Basic Lubrication Principles
This article serves to be a practical guide to lubricants and lubrication.
The proper selection and application of lubricants will hopefully be made
simpler if a little of the basic theory is understood.
When one surface moves over another, there is always some resistance
to movement, and the resisting force is called friction. If the friction is
low and steady, there will be smooth, easy sliding. At the other extreme,
the friction may be so great that movement becomes impossible,
resulting in the surfaces to overheat and be seriously damaged. A study
of the science of friction is called Tribology.
Lubrication is simply the use of a material to improve the smoothness of
movement of one surface over another, and the material which is used in
this way, is called a lubricant. Lubricants are usually liquids or semiliquids, but may also be solids or gases or any combination thereof.
Generally speaking, the smoothness of movement is improved by
reducing friction but this is not always the case and there may be
instances in which it is more important to maintain steady friction than
to obtain the lowest possible friction. Examples of such scenarios are
found in the control of chatter in a machine tool slideway or grinding
operation, or strip movement in metal rolling, etc. In addition to reducing
or controlling friction, lubricants are usually expected to reduce wear, to
prevent overheating and corrosion, and will be considered later.
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Friction
In most cases dry friction between two bodies closely follows two laws
which have been described by Leonardo da Vinci is about 1500.
The first of these laws states that the friction between two solids is
independent of the area of contact. If a brick slides over a flat surface,
then the friction will be the same whether it is sliding on its base, or on
its end.
The second law is that the friction is proportional to the load exerted by
one surface on the other. To continue the example of the brick, if a
second brick is placed on the first the friction will be doubled, with a third
brick, trebled, and so on. It is this second law which makes it possible to
define the coefficient of friction. If the friction is proportional to the load,
then the friction is equal to a constant multiplied by the load.
F=constant x W
or
F/W = constant
And this constant is the coefficient of friction, usually written as µ. The
coefficient of friction depends mainly on the materials which are in
sliding contact.
The coefficient of friction between two bodies is, in fact not quite
constant. It often varies slightly with change in load, and usually varies
with sliding speed. The static friction, which is the force required to start
one body sliding over another, is almost always greater than the dynamic
friction, which is the force needed to keep it moving at the speed once
sliding has started.
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The friction between two dry surfaces arises from two main sources,
adhesion and deformation. Remember that even the smoothest
engineering surfaces are rough when viewed under a high-powered
microscope. Two solid surfaces touching one another will in fact only be
in contact at the peaks of their asperities as shown in the exaggerated
form in Fig 1.
Fig. 1 Contact of Two Solid Surfaces
If two surfaces touch, even with the load of a few grams, the load will be
carried entirely on a small number of these asperities, and the actual
contact pressure may be as high as 1400 MN/m² (200000 psi). Such
enormous pressures can squeeze the asperities on hardened steel or cast
iron surfaces out of shape, or even cause them to weld together. Once
this happens, force is required to separate them and is determined by
how great the adhesive friction is.
The other main component of friction, deformation friction, is usually
small and it is obvious why this must be so. The deformation which takes
place when two surfaces rub together must be either elastic (temporary)
or plastic (permanent). If it is the former, the energy which is used to
produce the deformation will be recovered when the surface resumes its
original shape and there will be no net friction. If it is plastic, there will be
a permanent change in shape and the amount of change which can be
tolerated in any engineering component is limited. There is one
complication when some substances deform elastically though, in that
there is a delay in their return to their original shape.
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When this happens, the energy released may be too late to be returned
to the sliding system and there will be a net frictional loss. In addition
elastic deformation can cause some loss of energy in the form of heat
and can contribute towards fatigue damage.
It should be noted that when two asperities adhere, they will often
separate by a rupture, or tearing inside one of the surfaces. This results
in a very small amount of material being transferred from one surface to
the other so that there is in fact, some deformation taking place. The
primary influence however, is adhesion and if it is reduced or eliminated,
the resulting deformation will also be reduced or eliminated.
The first requirement of all lubrication is thus the elimination or
reduction of force required to shear the adhesive junction formed
between asperities. This can be done either by interposing a material
which is more easily sheared, or by using a chemical which will alter the
shear stress of the asperities. As mentioned earlier, the material
imposed, may be a gas, a liquid or a solid. If it is any of the first two types,
then a third type of friction is introduced, notably, viscous friction (or
viscous drag).This is the force required to shear a viscous fluid.
The Mechanism of Lubrication
The way in which liquids lubricate can be explained by considering the
example of a bearing as shown in Fig. 2
Fig. 2 Lubrication of a bearing
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As the shaft rotates in the bearing, lubricating oil is dragged into the
loaded zone and the pressure and the volume of the oil in the loaded
zone, both increase. The pressure rise, and therefore the thickness of the
oil film, will depend on the shaft speed and the lubricant viscosity. The
relationship between speed, viscosity, load, oil film thickness and friction
can be understood by considering a graph such as shown in Fig 3.
Fig. 3 Stribeck Curve
There are distinct zone in the graph. The coefficient of friction is at its
lowest in the Hydrodynamic and Thin Film zones. It is at this point when
the oil film is just thick enough to ensure that there is no contact
between asperities on the shaft and bearing surfaces. As we move to the
right of the graph in the hydrodynamic zone, the oil film thickness is
increasing due to increasing viscosity of the oil, increasing speed or
decreasing load and the coefficient of friction increases.
Moving towards the left of the curve, the oil film thickness decreases so
that the asperities rub against each other. The amount of rubbing, and
thus the friction, increases as the oil film thickness decreases to the point
described as mixed film zone. In this zone the oil film thickness has been
reduced virtually to nil and the load between shaft and bearing is being
carried on asperity contacts.
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As we move further left down the curve, the coefficient of friction is
almost independent of load, viscosity and shaft speed. This zone is
known as the boundary lubrication zone. The oil film thickness is too
small to give full fluid separation of the surfaces and the asperities start
to contact one another. Lubricant properties, other than the bulk
viscosity, start to become important.
In most normal situations asperities are initially coated with a film of
oxide. When the asperities rub together, their tendency to adhere is
relatively mild. However, once the oxide film is removed, the exposed
metal surfaces have a very powerful tendency to adhere. To reduce
friction and where under these conditions, require a better engineered
lubricant with enhanced properties to reduce friction and wear. This can
be done iin several ways.
Adsorption – all solid surfaces tend to attract a thin film of some
substance from their environment. Such films may be only a few
molecules thick and are said to be “adsorbed” onto the surface. The
strength of adsorption depends upon the electronic structure and “polar”
molecules (those in which there is a variation in electronic charge) tend
to be adsorbed with their molecules perpendicular to the surface.
Thicker and more strongly adsorbed films will give greater protection to
the metal surfaces, so the preferred boundary lubricants for such
conditions, are long chain polar organic chemicals. Adsorption is a
reversible process and an absorbed substance can be “de-adsorbed” if
heated to a critical temperature, or displaced by a substance which is
more strongly absorbed. This is valuable in boundary lubrication as the
most strongly adsorbed substances present in the lubricant can be
preferentially adsorbed.
Chemisorption – After adsorption onto a metal surface, some substances
will react with the metal or oxide surface to produce a new chemical
compound. Such substances are said to be “chemisorbed”. These
materials are more strongly bound to metal surfaces than are adsorbed
materials, and the chemisorptions process is not reversible.
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Chemical reaction – Adsorbed and chemisorbed films are very effective in
reducing friction and mild wear under light or moderate rubbing. They
are however, fairly easily removed mechanically under severe rubbing
conditions. To handle such situations, more reactive chemicals can be
added to the lubricant to react with the metal surfaces and to produce
protective films. Suitable films include organic compounds of chlorine,
phosphorus and sulphur. Such materials are said to have Extreme
Pressure (EP) properties. And are commonly used in gearbox and metal
working fluids.
The different lubrication zones have an important influence on wear but
generally speaking the amount of wear which takes place depends on the
severity with which surfaces rub against each other. As the oil film
becomes thinner moving from right to left on the curve, there is
increasingly severe contact between the surfaces and therefore, a
greater tendency to wear.
Choice of Lubricant Type
If there were only a few lubricants available, the problem of lubricant
selection would hardly arise. One would use the available oil or grease
and put up with its shortfalls and disadvantages, and whatever
equipment life it gave. This was generally the case before the midnineteenth century when things like animal fat, crude oil which seeped
from the ground and natural graphite were used.
Nowadays the variety of lubricants available is enormous. Most lubricant
manufacturers can supply scores of different mineral or synthetic oils and
greases. To add to the complexity of the matter, equipment is often
designed and built before any thought is given to their lubrication or the
lubricant is chosen as an afterthought. It is not uncommon for large and
expensive equipment to fail because the wrong, or sub-standard,
lubricant was used.
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In choosing a lubricant for a particular application, the objective should
always be to obtain the best long-tem performance at the best possible
cost… and this does not mean using the cheapest available lubricant. It is
no use buying a cheap oil, if either the oil, or the equipment in which it is
used, breaks down or maintenance cost soars as a direct result of the
lubricant. Reliability is therefore far more important that price.
There are four basic types of lubricants (oils, greases, dry lubricants and
gases) and Fig. 4 summarizes their broad properties. It is important to
select the lubricant type best suited for the design of the equipment it is
to lubricate.
Lubricant
Property
Hydrodynamic
lubrication
Boundary
lubrication
Cooling
Low friction
Ease of
application
Ability to stay in
place
Ability to seal
Oil
Grease
Excellent
Fair
Dry
Gas
Lubricant
Nil
Good
Poor to
excellent
Very good
Fair to good
Good
Good to
excellent
Poor
Fair
Fair
Good to
excellent
Nil
Poor
Poor
Usually
poor
Fair
Excellent
Good
Poor
Good
Poor
Poor
Very good
Protect against
corrosion
Temperature
range
Volatility
Fair to
excellent
Fair to
excellent
Very high to
low
Good to
excellent
Good
Very
good
Fair to
good
Poor to
fair
Very
good
Low
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Generally
low
Very poor
Poor
Excellent
Very high
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Flammability
Compatibility
Cost
Complexity of
design
Life determined
by
Very high to
very low
Fair
Generally
low
Fair
Generally Depends
low
on gas
Excellent Generaaly
good
Low to high
Fairly high
Fairly
Fairly
high
high
Fairly low
Fairly high
Fairly
Generally
high
low
Deterioration Deterioration Wear
Ability to
&
maintain
contamination
supply
Fig 4 Main lubricant types and their properties
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