A lubricant (also sometimes called as “lube”) is a substance (usually a liquid) introduced between two moving surfaces to reduce the friction between them which in turn improves the efficiency and reduces the wear. It also has the function of transporting foreign particles. A good lubricant possesses the following characteristics:
- High boiling point
- Low freezing point
- High viscosity index
- Thermal stability
- Hydraulic Stability
- Corrosion prevention
- High resistance to oxidation
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each other, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (e.g. graphite and MoS2 etc.) a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (a grease) or, exceptionally, a gas. Types of lubrication are shown in Fig 1.
Fig 1 Types of lubrication
Besides doing the function of lubricating, lubricants also carries out some or all of the following functions:
- Act as a coolant to remove the frictional heat which may be sometimes considerable.
- Keeps moving parts apart.
- Reduces friction between moving parts
- It dissolves and transports contamitants and debris arising both from internal and external sources.
- Acts as a hydraulic medium in some applications
- It protects against wear of highly loaded machine parts
- It prevents corrosion and rusting of machine parts
- It helps in transmitting power
- It offers protection against the accumulation of deposits (sludges and varnish) in lubrication system.
- It acts as a seal for gases
- It resists aeration and foaming, which can cause mal-functioning
- It resists or aid emulsion formation in wet systems.
- It stops the risk of smoke and fire of objects
- It has ability to separate water coming from air breathing or external sources.
Some of the above functions are catered to by any properly refined mineral oil of suitable viscosity while for some other functions are provided by the lubricants with the help of additives. The types and amount of additive needed depend upon the performance features which a lubricant is reqired to meet. Typically lubricants contain more than 90% base oil (Mineral oil) and less than 10% additives.
A large number of additives are used to impart performance characteristics to the lubricants. The main families of additives are:
- Antioxidants to control oil oxidation
- Detergents and dispersants to control deposit formation throughout the system
- Anti wear to provide the load carrying capacity and prevent scuffing of moving parts.
- Metal deactivators
- Corrosion inhibtors and rust inhibitors to control rust and corrosion
- Friction modifiers to provide oiliness and reduce friction
- Extreme pressure to provide the load carrying capacity and prevent scuffing of moving parts
- Anti foaming agents to control foam formation
- Viscosity index improvers to improve viscosity temperature relationship
- Demulsifiers to reduce emulsion formation or for easy separation of water.
- Emulsifying agents to reduce surface tension
- Stickiness improver which provides adhesive property
- Pour point depressents to have gravity flow properties at low temperatures.
- Mist suppressors to reduce oil mist formation, to reduce loss of lubricant and environmental pollution
- Biocides to control bactirial and fungus growth
- Complexing agent (in case of greases)
A lubricant has a life. With time its properties deplete and as the properties deplete lubricant fail to lubricate. The following are the basic reasons for the failure of a lubricant.
- Oxidation of lubricating oil which is the result of the chemically combining of the oil molecules with oxygen of the air. The rate of oxidation increases with the increase in the working tempeartures. Metal and their salts catalyze the oxidation process. Antioxidants additives used for controlling this are of sacrificial type and need replishment with time.
- Additives in general during use/service get depleted and lubricants lose their functional properties because of this depletion a failure can happen.
- Lubricant getting contaminated from internal and external sources.