Desirable properties of lubricant and oil additives to enhance the properties
Lubricant A lubricant is a substance introduced to reduce friction friction between between moving surfaces. It may also have the function of transporting foreign particles. The property of reducing fricti on is known as lubricity lubricity.. A good lubricant possesses the following characteristics:
High boiling point. Low freezing point. High viscosity index. index. Thermal stability. Corrosion prevention.
High resistance to oxidation
Keep moving parts apart Lubricants are typically used to separate moving parts in a system. This has the benefit of reducing friction and surface fatigue, together with reduced heat generati on, operating noise and vibrations. Lubricants achieve this by several ways. The most common is by forming a physical barrier i.e., a thin layer of lubricant separates the moving parts. This is analogous to hydroplaning, the loss of friction observed when a car tire is separ ated from the road surface by moving through through standing water. This is termed hydrodynamic 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.
Reduce friction 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 wea r 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.
Transfer heat
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, although lubricants may be used to warm as well as to cool when a regulated temperature is required. This circulating 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 t hermal stress on the lubricant. Thus lower cost liquid lubricants m ay 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 susceptible to catastrophic failure during sudden system shut downs. An 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 immediately 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 and debris Lubricant circulation systems have the benefit of c arrying away internally generated debris and external contaminants that get introduced into the system to a fi lter 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 transport to the filter and removal. Over t ime the filter will get clogged and require cleaning or replacement, hence the recommendation to change a ca r's oil filter at the same time as changing the oil. In closed s ystems 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 can make it, thus it is unfortunate that there are no industry standards by which consumers can readily assess the filtering ability of various automotive filters. Poor filtration significantly reduces the life of the machine (engine) as well as making the system inefficient.
Transmit power Main article: Hydraulics Lubricants known as hydraulic fluid are used as the working fluid in hydrostatic power transmission. Hydraulic fluids comprise a large portion of all lubricants produced in the world. The automatic transmission's torque converter is another important application for power transmission with lubricants.
Protect against wear Lubricants prevent wear by keeping the moving parts apart. Lubricants may also contain antiwear or extreme pressure additives to boost their performance against wear and fati gue.
Prevent corrosion Good quality lubricants are typically formulated with additives that form chemical bonds with surfaces, or exclude moisture, to prevent corrosion and rust....
Seal for gases Lubricants will occupy the clearance between moving parts through the capillar y force, thus sealing the clearance. This effect can be used to seal pistons and shafts.
An additive is an extraneous substance that is added to a substance in very small quantities to enhance the existing properties of the substance, to impart some desirable properties to the substance, or to supress certain properties.
Lubricant Properties and the Role of Additives: Some of the most important properties ne cessary for satisfactory lubricant performance are: 1. Low volatility under operating conditions. Volatility characteristics are essentially inherent in the choice of base oil for a particular type of service and cannot be improved by the use of additive materials. 2. Satisfactory flow characteristics in the temperature range of use. Flow characteristics largely depend on the choice of base oil; however, they can be improved modifiers. The former improve low-temperature flow properties, while the latter enhance high-temperature viscosity characteristics. 3. Superior stability or ability to maintain desirable characteristics for a reasonable period of use. While these characteristics depend to some extent on the base materials, which enhance base fluid properties in this area. Lubricant stability is affected by the environment in which it operates. Such factors as temperature, oxidation potential and contamination with water, unburned fuel fragments, and corrosive acids limit the useful life of a lubricant. This is the area where additives have made a major contribution in improving the performance characteristics and extending the useful life of lubricants. 4. Compatibility with other materials in the system. Compatibility of l ubricants with seals, bearings, clutch plates, etc., may also be partially associated with the base oil. However, additive chemistry can have a major i nfluence on such characteristics. Additives can be classified as materials that impart new properti es to or enhance existing properties of the lubricant or fuel into which they are incorporated. (Lubrizol)
are substances formulated for improvement of the anti-friction, chemical and physical properties of base oils (mineral, synthetic,vegetable or animal), which results in enhancing the lubricant performance and extending the equipment life Additives
Friction modifiers Friction modifiers reduce coefficient of friction, resulting in less fuel consumption. Crystal structure of most of friction modifiers consists of molecular platelets (layers), which may easily slide over each other. The following Solid lubricants are used as friction modifiers:
Graphite;
Molybdenum disulfide;
Boron nitride (BN);
Tungsten disulfide (WS2);
Polytetrafluoroethylene (PTFE). to top
Anti-wear additives Anti-wear additives prevent direct metal-to-metal contact between the machine parts when the oil film is broken down. Use of anti-wear additives results in longer machine life due to higher wear and score resistance of the components. The mechanism of anti-wear additives: the additive reacts with the metal on the part surface and forms a film, which may slide over thefriction surface. The following materials are used as anti-wear additives:
Zinc dithiophosphate (ZDP);
Zinc dialkyldithiophosphate (ZDDP);
Tricresylphosphate (TCP). to top
Extreme pressure (EP) additives Extreme pressure (EP) additives prevent seizure conditions caused by direct metal-tometal contact between the parts under high loads. The mechanism of EP additives is similar to that of anti-wear additive: the additive substance form a coating on the part surface. This coating protects the part surface fr om a direct contact with other part, decreasing wear and scoring. The following materials are used as extra p ressure (EP) additives:
Chlorinated paraffins;
Sulphurized fats;
Esters;
Zinc dialkyldithiophosphate (ZDDP);
Molybdenum disulfide;
to top
Rust and corrosion inhibitors Rust and Corrosion inhibitors, which form a barrier film on the substrate surface reducing the corrosion rate. The inhibitors also absorb on themetal surface forming a film protecting the part from the attack of oxygen, water and other chemically active substances. The following materials are used as rust and corrosion inhibitors:
Alkaline compounds;
Organic acids;
Esters;
Amino-acid derivatives. to top
Anti-oxidants Mineral oils react with oxygen of air forming organic acids. The oxidation reaction products cause increase of the oil viscosity, formation ofsludge and varnish, corrosion of metallic parts and foaming. Anti-oxidants inhibit the oxidation process of oils. Most of lubricants contain anti-oxidants. The following materials are used as anti-oxidants:
Zinc dithiophosphate (ZDP);
Alkyl sulfides;
Aromatic sulfides;
Aromatic amines;
Hindered phenols. to top
Detergents Detergents neutralize strong acids present in the lubricant (for example sulfuric and nitric acid produced in internal combustion engines as a result of combustion process) and remove the neutralization products from the metal surface. Detergents also form a film on the part surface preventing high temperature deposition of sludge and varnish. Detergents are commonly added to Engine oils. Phenolates, sulphonates and phosphonates of alkaline and alkaline-earth elements, such as calcium (Ca), magnesium (Mg), sodium (Na) or Ba (barium), are used as detergents in lubricants. to top
Dispersants Dispersants keep the foreign particles present in a lubricant in a dispersed form (finely divided and uniformly dispersed throughout the oil). The foreign particles are sludge and varnish, dirt, products of oxidation, water etc. Long chain hydrocarbons succinimides, such as polyisobutylene succinimides are used as
dispersants in lubricants. to top
Pour point depressants Pour point is the lowest temperature, at which the oil may flow. Wax crystals formed in mineral oils at low temperatures reduce their fluidity. Pour point depressant inhibit formation and agglomeration of wax particles keeping the lubricant fluid at low temperatures. Co-polymers of polyalkyl methacrylates are used as pour point depressant in lubricants. to top
Viscosity index improvers Viscosity of oils sharply decreases at high temperatures. Low viscosity causes decrease of the oil lubrication ability. Viscosity index improvers keep the viscosity at acceptable levels, which provide stable oil film even at increased temperatures. Viscosity improvers are widely used in multigrade oils, viscosity of which is specified at both high and low temperature. Acrylate polymers are used as viscosity index improvers in lubricants. to top
Anti-foaming agents Agitation and aeration of a lubricating oil occurring at certain applications (Engine oils, Gear oils, Compressor oils) may result in formation ofair bubbles in the oil foaming. Foaming not only enhances oil oxidation but also decreases lubrication effect causing oil starvation. Dimethylsilicones (dimethylsiloxanes) is commonly used as anti-foaming agent in lubricants.
Properties of Engine Oil Additive 1. Reduces abrasion and wear The nanoparticles in the additive form a self-renewing protection layer on the piston ring and reduce
frictional wear on the contact surfaces 2. Excellent detergent-dispersal properties
Ultrafine nanoparticles (100nm to 200nm) will not clog up the filters as shown in
Utilization Increase margins of operational safety Increase availability by decreasing downtime Increase overall component lifespan Control standby equipment and replacement part requirements Decrease fuel and oil consumption
