Types Of Friction And Lubrication Regimes - Know about Lubricants, Oils and Greases

Types Of Friction And Lubrication
Regimes

Ever wonder, why it is difficult to move flat objects while it’s easy to move round
objects?
The answer is Friction.
The force generated by two surfaces that touch and slide against each other is called
Frictional Force. It acts between the surfaces in the same or in opposite directions.
Friction is dependent on
 Surface texture
 Magnitude of Force
 Angle and Position of Object
A frictional force is related with the motion of the body and can hardly get rid of.
However, lubricating fluids can help in decreasing the frictional force between the
bodies but not completely.
Large frictional forces can result in abrasion and wear and tear of the surfaces that can
be harmful in the long run. It deforms the surfaces gradually, leading to reduction in the

mechanical stability of the body. Also, a high amount of friction generates greater heat
that can be detrimental.
There are generally four types of friction
 Static friction
 Kinetic friction
 Rolling friction
 Fluid friction
 Static friction
The frictional force that keeps a body at rest is called static friction. It is seen when a
stationary body is tried to move on a surface without initiating any relative motion
between the body and surface. Static friction depends on the coefficient of static friction
and is always equal when the body is at rest. It is a self-regulating force and occurs in
the opposite direction to the applied force.

 Kinetic friction
The force that acts between moving surfaces is referred to as kinetic friction. It is
experienced in the opposite direction of the moving body. Kinetic friction is dependent
on the coefficient of kinetic friction. The coefficient of kinetic friction is generally lesser

than the static friction. As it is the force that opposes sliding motion and hence is also
called sliding friction.

 Rolling friction

When a body rolls over another surface, it has only a single point of contact. This gives
rise to a resistance known as rolling friction. It is generally lower than the sliding friction.
Due to only a single point of contact, surfaces don’t rub against each other. Rolling
friction is dependent on coefficient of rolling friction, which in turn dependent on the
following:

 Radius of the rolling body
 Density
 Toughness of the Surface

 Fluid friction

The force that curbs the movement of solid, liquid, or gas through the liquid medium is
known as fluid friction. Any movement inside the fluid can cause the internal resistance
to arise. This is also known as viscous drag or fluid friction.

Lubrication is needed to lessen the effect of friction on any surface. Specific operating
conditions ask for application of varied lubrication film on the surfaces. This is defined
as Lubrication regimes. It is dependent on the degree of contact between the surfaces.
Generally, there are four types of lubrication regimes.

Boundary Lubrication

Metal to metal contact between two sliding surfaces of the machine is generally linked
with the boundary lubrication. The metal surfaces that are lubricated may come in
contact with each other during heavy load, start-up or shutdown. Hence, it is advised to
apply thick lubrication to overcome the surface friction during heavy load conditions.
Correct lubricant viscosity can reduce boundary lubrication.

Mixed Lubrication

When the sliding speed between the two surfaces of the machine increases, boundary
lubrication gets reduced. It creates a lubricant film between the surfaces that drops the
friction impressively. This condition is known as Mixed Lubrication. It is the lubrication
regime between the hydrodynamic lubrication and boundary lubrication.

EHD Lubrication

Whenever there is a rolling motion between the two surfaces of a machine, a condition
known as Electrohydrodynamics (EHD) lubrication occurs. As rolling motion has a small
area of contact between the surfaces, it develops a high-contact pressure when the
lubricant fluid enters the contact zone. This condition enhances the fluid pressure that
ultimately results in high viscosity and high loading capacity of the fluid. The metal
surfaces may slightly deform due to high load at the contact zone but returns to normal
form as the rotation continues.

Hydrodynamic Lubrication

Hydrodynamic lubrication regime is seen in sliding surfaces, mostly between a rotating
shaft and journal bearing. This lubrication regime has low-contact pressure between the
surfaces of similar geometry in relative motion. It is perfect to avoid friction and wear as
oil film lifts the two surfaces away from each other. Optimum oil viscosity is very
necessary for the effective hydrodynamic lubrication to work efficiently in any operating
condition.

Molygraph has decades of industry experience in manufacturing engineered lubricants
that help enhance output for various industries. We have successfully assisted 100+
customers with our engineered lubricants over the years.

Get in touch with us if you are looking to discuss lubrication requirements for your
company. Our experts will be glad to satisfy your needs by ensuring you pick the right
product for the right job.