A lubricant has several valuable functions within machinery. These are:
Lubrication and Friction Control – The key lubricant property generally accepted is to separate two contacting and moving surfaces thus preventing metal to metal contact. Failure of this function will lead to abnormal wear.
Corrosion Control – air and moisture can lead to rusting, whilst acids generated by combustion can lead to strong acidic corrosion. Lubricants must therefore reduce the potential of corrosion through the use of anti-rust additives, particularly the alkaline additives – often referred to as Base Number). Failure of this function will lead to corrosion and rusting of key components and eventually wear within the system.
Sealing – Lubricants form a Liquid seal between components, such as rings and liners to help prevent combustion gases entering the crankcase. Oils are also designed to control the amount of seal swelling to prevent oil leaks, while grease forms a more viscous seal barrier to help prevent dirt and water ingress. Failure of any form of sealing can lead to leaks or contamination ingress both of which can ultimately lead to abnormal wear.
Cooling and temperature control – Oils circulate through working areas of a machine operating under high loads, high temperatures or extreme environmental conditions, transferring this generated heat to the oil; in turn the oil is cooled by the coolers/radiators. Inefficient cooling leads to oxidation, visibly evident when an oil is heavily oxidised, creating varnish deposits and sludge, which may collect in the coolers, leading to undesirable insulation of the system and even more inadequate cooling, and overheating of the system. The increased temperature will initially lower the lubricant’s viscosity (as oils become thinner when heated) and potentially allow metal to metal contact. The increased temperature will eventually lead to thickening (increase in viscosity) of the oil, as it oxidises to form sticky polymers.
Clean – Oils possess the ability through additives, to remove contamination from the component surfaces and suspend it in solution (i.e. if using detergent and dispersant based formulations), or simply transport this insoluble material to the filters, which is designed to remove these contaminants. The inability of the lubricant to suspend particulates, dirt, sludge and oily deposits, within the lubricant will lead to abnormal wear and inadequate cooling of the system.
Transmission of power – The purpose of the fluid in hydraulic systems is to transmit force (power) and therefore motion from one part of the system to another. Failures of the fluid to transmit power by any type of particulate contamination and varnish will block valves. Ingress of entrained air will lead to loss of power or uncontrolled and unsafe hydraulic movements, which must be avoided at all times.
Protect (wear, but also see corrosion control above) – Lubricants are compounded with numerous additives including those designed to protect bearing metal surfaces by forming a barrier layer, termed the shear mix layer or boundary lubrication layer. These coatings also help protect against corrosion by acting as metal deactivators, simply by providing a surface coat, to isolate the metal surface from contact with corrosive acids, air and water.