Machine Lubrication is very much required to get useful work from the machine. It also prolongs the life of the machine elements. Lubrication plays a similar role in the life of living beings as well. Every joint of the skeletal system of a living being is lubricated with a certain kind of fluid, which keeps our joint bones moving smoothly without any trouble. Similarly, in machine lubrication, a lubricant film is formed between the moving parts of a machine to avoid rubbing. It reduces the friction among them and makes their movement smooth and trouble-free.
In machine lubrication, certain substances like oil, grease, and water are fed into the moving parts of a machine. It minimizes the heat generated due to the rubbing of the machine parts against each other during movement. It smoothens the movement of the machine part. The first use of lubricant as water in front of sledge to transport a statue of Ti is recorded in history, in 2400 BC.
FUNCTION OF LUBRICANT
(1)Reduces friction and wear in moving parts of a rotating machine.
(2)It protects the metal surface of the machine part against dust and corrosion.
(3)It dissipates heat generated, between rubbing machine parts and controls machine part temperature, to prevent failure in them due to thermal expansion.
(4)It flushes out the contaminants, from machine elements.
(5)It transmits hydraulic power.
(6)It absorbs and dampens the shock loads.
(7)It forms a seal and does not allow foreign matter like dust and water to enter the machine components.
PROPERTIES OF LUBRICANT
Good lubricant properties:
(2)Good thermal stability.
(3)Good resistance to oxidation.
(4)Good resistance to corrosion.
(5)Must have a high boiling point.
(6)Must have a low freezing point.
(7)Good hydraulic stability.
(8)Ability to demulsify.
TYPE OF LUBRICANTS
Lubricants are classified into three main categories:
(3)Solids like Graphite and PTFE
Oil lubrication is mostly considered for high-speed and high-temperature applications. It is very effective in application to remove heat generated by bearings in machine parts. Oil dissipates the heat in the atmosphere. Under normal operating conditions, spindle oil, machine oil, turbine oil, and other mineral oils are used in lubrication. For temperatures above 150-degree centigrade and below 30 degrees centigrade, synthetic oil silicone is used.
The viscosity of the oil determines its lubrication efficiency. If the viscosity is too low, the oil film will not be sufficiently formed. When the viscosity is too high, the viscosity resistance also will be too high, which causes the temperature to rise. For higher speed, low viscosity oil should be used, and for heavy loads, high viscosity oil should be used.
ADVANTAGE OF OIL LUBRICATION:
(1)Oil is easy to drain and refill in the system.
(2)The amount of lubricant filled in the system is easy to control.
(3)Oil flows through the system easily and therefore reaches most areas of machine elements.
(4)Oil is more stable at a higher temperature, hence used for high-speed machines.
Grease lubricants are easier to handle than oil-based lubricants. Because of their viscosity, they are longer-lasting and require simple sealing devices. Grease lubricant is easy to change because the usual method is to pump the grease into a grease fitting and let the new grease push out the old grease.
ADVANTAGE OF GREASE LUBRICATION:
(1)Grease lubrication system is easy to maintain.
(2)Do not require much visual inspection.
(3)Frequency of greasing is not much.
(4)Required quantity of grease is easy to maintain in the system, hence system design is simpler.
(5)Excessive leakage from the system is rare, so the greasing is effective in applications where contamination must be avoided.
Solid lubrication is more effective in high load and high-speed applications. Commonly used solid lubricants are graphite, PTFE, Molybdenum disulfide, and Boron Nitride.
VISCOSITY OF LUBRICANT
Viscosity is the resistance between the layers of a fluid lubricant when it in motion. It is the measure of friction between different layers when a fluid is set in motion. This is measured in mm 2/sec, CST.
Where CST is Cent i-Stoke, 1 cent i-stoke = mm 2/sec
MACHINE COMPONENT AND LUBRICANT VISCOSITY:
The following table indicates the required viscosity of a lubricant for various components of the machine for its smooth running.
REPLACEMENT OF OIL LUBRICANT:
The following table indicates the maximum operating temperature of the lubricant, and its frequency of replacement for the smooth and efficient running of the machine.
CAUSES OF BEARING FAILURE:
Failure of bearings is caused due to many reasons. As per the tribological study, reasons for bearing failure along with their percentage of occurring are listed below,
Reason for bearing failure and their Percentage
(2)Contamination ————————- 14%
(3)Poor Fitting ————————-30%
(5)Achieve specified life —————————6%
KIND OF LUBRICATION:
Lubrication is categorized into three main categories, depending on their film thickness, between the moving parts of a machine. They are:
(1)Thick-film lubrication, known as Hydrodynamic lubrication.
(2)Thin film lubrication, known as Boundary layer lubrication.
(3)Thin-film formation due to machine part pressure, known as Elastohydrodynamic lubrication.