What are the main lubricants and what are their characteristics?
Lubrication is a process which aims at reducing friction between two moving pieces. When two surfaces come in contact with one another, a fluid must be injected to separate them. The word « greasing » applies when grease is used to lubricate.
What are the main purposes of lubrication?
Lubrication allows to:
- Reduce friction (rubbing or deformation)
- Prevent pieces from wear
- Absorb/reduce shocks
- Protect from corrosion
- Isolate components from contamination
- Clean/get rid of contaminants.
Lubricants aim at reducing friction between moving pieces and at reducing passive resistance of the stationary parts. They are produced by refining heavy fractions of crude oil (remaining crude oil parts after refining hydrocarbons such as gas, fuel oil or kerosene). There are different kinds of lubricants: liquid or fluid (oils), semi-solid (grease, silicon gels) or solid (Teflon, graphite).
Performances and characteristics may differ from one lubricant to another, but they all have in common the same main component called “base oil”. Typically, lubricants contain between 75% and 85% base oil of mineral or synthetic origin.
Base oil groups
There are two main base oil groups:
Mineral base oils are derived from crude oil. They are by far the most common lubricants in the automotive industry as well as for industrial applications. Mineral base oils are based on hydrocarbons that have been refined several times.
Synthetic base oils are derived from the chemical reaction of several components. The two main categories of products used to formulate lubricants are: esters and synthetic hydrocarbons. The viscosity of these products is remarkably stable regardless of the temperature. This characteristic is a major advantage compared to mineral oil bases, which require additives in order to improve viscosity in higher quantities. They are also more resistant to oxidation, from where an increased longevity which requires less-frequent oil changes. Also, there are semi-synthetic base oils derived from a mixture of mineral (from 70% to 80%) and synthetic oils (from 20% to 30%).
Oils are a mixture of lubricating base oil and additives.
There are between 15% and 25% additives in finished oils for two reasons:
- Either to reinforce some properties of the base oil
- Or to give new properties to the base oil
Viscosity improvers: they are added to make oil more fluid at high and low temperature, so that moving pieces do not touch one another. These improvers are polymers added to lubricating oil bases. Such oil is said to be multi-grade. At lower temperatures, long polymer chains contract and do not offer sufficient resistance to the movement of oil molecules, but at higher temperatures, the chains uncoil and prevent the mixture from fluidizing.
Antifatigue additives: they reinforce the antifatigue action of the lubricant. A protective film is formed by immediate reaction, or when the reaction product comes in contact with the metal surfaces.
Antioxidant additives: they retard the oxidation phenomenon of the lubricant and contribute to space out the oil change intervals by offering a better resistance to higher temperatures.
Detergent additives: they prevent the formation of deposits or coating in the hottest parts of the fuel system, such as the piston rings. They enhance the detergent action, particularly inside the engines where they prevent combustion char or oxidized compounds to form deposits or gums on metal surfaces. The most recent additives are basic nitrogen compound polymers, which do not generate ashes. Detergent oils have to be used with caution on old fuel systems since their capacity to get rid of sedimented deposits (calamine for example) may obstruct the lubricating system.
Dispersant additives: they suspend solid impurities formed while the engine is working: unburnt residues, gums, sludge, ashes, deposits cleaned by detergents. They prevent solid deposits from agglomerating, and they prevent sludge from forming in the cold parts of the engine (carter).
Basicity additives: they neutralize acid deposits from the fuel combustion as they form, mostly on diesel engines.
Anti-corrosion additives: they protect ferrous metals against combined attacks of water, oxygen, air, and some oxides formed during the combustion. A protective film or a passivating layer is formed on the surface to be protected.
Antifreeze additives: they allow the lubricant to remain fluid enough at lower temperatures (from -15°C to -45°C).
Anti-moss additives: oil foaming can be due to other additives (detergent additives react to oil the same way as soap reacts to water: they clean the engine but they tend to foam). It can also be due to the greasing system design, which may provoke turbulences when the lubricant is poured into the engine, which makes the air/oil mixing easier. The role of these additives is to avoid dispersing a large air volume into the oil.
Extreme pressure additives: they aim at reducing friction torques and as a result to save energy and to protect surfaces exposed to very high pressure. These additives add specific sliding properties to the lubricant, in particular to organs equipped with gears or friction lining immersed in an oil bath (limited-slip differentials, automatic or manual transmissions, oil immersed brakes, etc.).
Greases consist of:
- 70 to 95 % base oil (mineral, synthetic or vegetable)
- 0 to 10 % additive as previously mentioned
- 3 to 20 % thickening or gelling agent which increases the viscosity of the lubricant (semi-fluid, fluid, soft or hard) and to trap base oil and additives and to avoid leaking.
Greases stand out in particular for their excellent adhesion to the surfaces to be lubricated; also, they are insoluble in water, they resist to shearing and last longer. Generally speaking, grease cannot be heated above 300°C (temperature at which the base oil separates from the thickener). Beyond this temperature, copper or aluminum-based thermal pastes or coatings are more suitable.
In addition to its lubricating role (reduction of mechanical fatigue and energy losses due to friction), grease creates a waterproof barrier against external elements (dust, water, solvents, heat, etc.).
- Silicon greases: they are polymers based on silicon organic compounds that are thermally stable, chemically inert and electrically insulating. Silicon is very resistant to high temperatures, to oxidation and to ultraviolet light. There are three types of silicon: oil, elastomer and resin.
- Food-grade lubricants: these greases are safe to come in accidental contact with food. In France, these additive and gelling lubricants must conform to the CNERNA prescription (National Center for Study and Research on Nutrition and Feeding). To this day, the CNERNA is the only European organization referencing raw material that may be used to come in contact with food.
Graphite and molybdenum disulfide are the two main lubricants usually used in extreme conditions (high vacuum, too high or too low temperatures). Graphite can be used up to 400°C in the presence of air and up to 1900°C in an inert atmosphere. Molybdenum disulfide can be used up to 450°C and beyond that temperature, it becomes abrasive. In lower temperatures, it is recommended to use Teflon, nylon or diverse polyamide.
Some liquids are used to cool down and to lubricate the equipment during cutting operations. The cooling process increases the tool lifetime and makes it easier to obtain standard cuttings on finished pieces. Lubrication enables to reduce frictions, which avoids excessive heat releases and reduces the energy necessary for a particular cutting. These cutting oils are aqueous solutions, chemically inactive oils or synthetic liquids. With the CMMS Mobility Work, you can save your lubrication data uploading the safety record with the document management tool.
The process of choosing a lubricant must take into account the functional conditions of the lubricating mechanism and in particular the working temperature, the pressure force, the relative velocity and the environment conditions. Oil and grease manufacturers are the most qualified to determine which lubricant to use according to the mechanism to be lubricated.
However, manufacturers and machine designers now also offer lubricants for their own equipment, as well as a timing regarding the lubricant change. At this stage, the CMMS Mobility Work gives you the possibility to define your maintenance schedules (regarding oil changing and lubricant changing) using the maintenance plan feature.