Typically the lubricant to surface friction is much less than surface to surface friction in a system without any lubrication. Thus use of a lubricant reduces the overall system friction. Reduced friction has the benefit of reducing heat generation and reduced formation of wear particles as well as improved efficiency. Lubricants may contain additives known as friction modifiers that chemically bind to metal surfaces to reduce surface friction even when there is insufficient bulk lubricant present for hydrodynamic lubrication e.g. protecting the valve train in a car engine at startup.
Keep moving parts apart
Lubricans are typically used to separate moving parts in a system. This has the benefit of reducing friction and surface fatigue together with reduced heat generation, operating noise and vibrations. Lubricants achieve this by several ways. The most common is by forming a physical barrier ie a thick layer of lubricant separates the moving parts. This is termed hydrodynamic lubrication. In cases of high surface pressures or temperatures the fluid film is much thinner and some of the forces are transmitted between the surfaces through the lubricant. This is termed elasto-hydrodynamic lubrication.
Lubricants perform the following key functions.
- Keep moving parts apart
- Reduce friction
- Transfer heat
- Carry away contaminants & debris
- Prevent corrosion
Both gas and liquid lubricants can transfer heat however liquid lubricants are much more effective on account of their high specific heat capacity. Typically the liquid lubricant is constantly circulated to and from a cooler part of the system. This circulatory flow also determines the amount of heat that is carried away in any given unit of time. High flow systems can carry away a lot of heat and have the additional benefit of reducing the thermal stress on the lubricant. Thus lower cost liquid lubricants may be used. The primary drawback is that high flows typically require larger sumps and bigger cooling units. A secondary drawback is that a high flow system that relies on the flow rate to protect the lubricant from thermal stress is susceptile to catastrophic faillure during sudden system shut downs. Automotive oil-cooled turbocharger is a typical example. Turbochargers get red hot during operation and the oil that is cooling them only survives as its residence time in the system is very short i.e. high flow rate. If the system is shut down suddenly (pulling into a service area after a high speed drive and stopping the engine) the oil that is in the turbo charger immidiately oxidizes and will clog the oil ways with deposits. Over time these deposits can completely block the oil ways, reducing the cooling with the result that the turbo charger experiences total failure typically with seized bearings. Non-flowing lubricants such as greases & pastes are not effective at heat transfer although they do contribute by reducing the generation of heat in the first place.
Carry away contaminants & debris
Lubricant circulatory systems have the benefit of carrying away internally generated debris and external contaminants that get introduced into the system to a filter where they can be removed. Lubricants for machines that regularly generate debris or contaminants such as automotive engines typically contain detergent and dispersant additives to assist in debris and contaminant removal and transport to the filter. Over time the filter will get clogged and require replacement, hence the recommendation to change a car’s oil filter at the same time as changing the oil. In closed systems such as gear boxes the filter may be supplemented by a magnet to attract any iron fines that get created. It is apparent that In a circulatory system the oil will only be as clean as the filter will allow it to be thus it is unfortunate that there are no industry standards for consumers to readily assess the filtering ability of automotive filters. Poor filtration significantly reduces the life of the machine (engine) as well as making the system inefficient.
Quality lubricants are typically formulated with additives that form chemical bonds with surfaces to prevent corrosion and rust.