Lubricants: design features that save energy & protect against wear

High-performance products of tribology

Lubricants to reduce friction in sliding and rolling elements

Lubricants are by far not an invention of modern industry, - even if it might seem so now with the addition of numerous additives - but were already used by our ancestors to reduce friction. Also, the term "greasing" has historical origins and derives from the Middle High German word "Smer", i.e. of raw animal fat.
In this day and age, the designation "lubricants" covers all products that are used for lubricating and reducing friction in sliding and rolling elements. But even products related in composition, preparation and properties are classed as lubricants, even if, as insulation oils, corrosion inhibitors or process oils, they act as auxiliaries for industrial processes. The lubricant share of total oil consumption worldwide is an average of about 0.8%, in industrialised countries, around 1%. However, from an economic point of view, lubricants are of far greater importance than their relatively small proportion in the oil market would suggest. 30% of all energy produced in the world is consumed by friction; wear causes annual losses in the billions. For designers then, lubricants are not only necessary supplies, but design elements that help to boost energy efficiency, avoid machine downtime and reduce the cost of spare parts and maintenance. The development of lubricants is therefore permanently driven forward by intensive research.

Which lubricants exist?

Lubricants can be subdivided into lubricating oils and greases, with the lubricating oils derived from petroleum being used much more than greases that are used only for specific purposes in industry. Generally lubricants are used for a wide range of tasks in the automotive sector or in industry. A distinction is made in:
Automotive lubricants:
  • Engine oils
  • Gear oils
  • Brake fluids
The tasks of automotive lubricants are varied and are used to lubricate parts sliding on to each other to reduce friction and wear, to cool the motor, the seal, corrosion protection and the pressure transmission.
Industrial lubricants:
  • Hydraulic fluids
  • Gear oils
  • Compressor oils
  • Turbine oils
  • Corrosion protection products
  • Cooling lubricants
  • Insulating oils and white oils
  • Lubricating greases
The main tasks of industrial lubricants include a broad reduction in friction and wear on gear flanks, the prevention of pitting and micro-pitting and removing emerging friction heat. A particularly aggravating factor is that the lubricants need to ensure the reliability of the machine even under extreme temperature conditions.

Base fluids for lubricants

The starting product of all lubricants - whether mineral oil based or synthetic - is crude oil, which is fragmented in an atmospheric distillation into different products and subsequently in further steps cleared of excess impurities. Depending on the processing, the base oil may be either mineral oil of different viscosities or so-called hydrocrack mineral oil, although significantly more mineral oil raffinates are manufactured and used than hydrocrack mineral oils.

Hydrocrack oils

Hydrocrack oils can be slack wax or vacuum gas oil-based. They are also called HC synthetic oils and compared to mineral oils they have a significantly higher viscosity index (120 to 150) and a better low-temperature properties (Pour Point up to -21 degrees C).

Synthetic base fluids (Polyalphaolefins, Esters)

Synthetic oils are produced in a multi-step chemical process by linking specific hydrocarbon molecules. In synthetic oils mainly polyalphaolefins (PAO), also often called synthetic hydrocarbons or esters, are used. The production of synthetic base fluids is more complex than the extraction of mineral oil based fluids, with the result that synthetic oils are much more expensive than mineral oils, but due to their manufacturing process offer many advantages.

Benefits of synthetic lubricants

Synthetic oils have a higher film thickness than mineral oils, i.e. even at high temperatures their viscosity is very high: The wear protection is significantly higher, the addition of viscosity index improving additive is reduced and at the same time the shear stability is improved. But even at extremely low temperatures, synthetic oils, with their very good low-temperature behaviour, are superior to mineral oils, thus allowing for an improved cold start in internal combustion engines and prompt lubrication of engines. The absence of unstable components ensures better oxidation and thermal stability. With their low volatility and low evaporation loss, oil and fuel consumption decrease, a higher thermal resistance ensures better engine cleanliness and extended oil change intervals.

Important indicators

  • Viscosity

    The most important property of lubricating oils is their so-called viscosity, which is the measure of the internal friction during the flow of oil. It is dependent on temperature: at low oil temperature, the internal friction is large and the viscosity is high. The hotter the oil temperature, the lower the internal friction and viscosity. A distinction is made between kinematic and dynamic viscosity, with the kinematic viscosity being derived from the dynamic viscosity by dividing this by the density.

    All lubricants are classified according to the ISO viscosity classification. The only exception is the engine and gear oils of the automotive sector, which are divided into SAE grades. In the ISO VG (International Organisation for Standardisation- Viscosity Grade) 18 viscosity grades of 2 mm2 / s to 1500 mm2 / s are standardised and a mean viscosity of 40 ° C set, from which inside of a viscosity grade, there may be a deviation of 10% upward or downward.
  • Viscosity-Temperature-Behaviour (Viscosity index)

    Depending on the oil, viscosity changes may differ completely due to rising or falling temperature. The viscosity-temperature behaviour is denoted by a dimensionless parameter, the so-called viscosity index (VI). The higher the viscosity index, the lower the viscosity drop when the temperature increases.
  • Viscosity-Pressure-Behaviour

    The viscosity of oil is dependent not only on the temperature but also on the pressure exerted on the lubricant film. This feature is particularly important for tribological calculations when oils of different base fluids are available
  • Shear rate/shear stability

    In multi-grade oils containing viscosity index improver, the viscosity is also influenced by the shear rate. In mono-grade oils without viscosity index improvers, known as Newtonian fluids, the velocity gradient has no effect on the viscosity
  • Pour Point

    The Pour Point indicates the so-called flow limits of the lubricating oil, which was previously described by the solidification point. To determine the Pour Point, the solidification point is ascertained and then 3°C added to this.
  • Evaporation loss

    The evaporation loss is dependent on the viscosity and the degree of refining of mineral oils. Hydrocrack oils have a significantly lower evaporation loss than conventional mineral oils.

Lubricant additives (H3)

Additives are added to lubricants to confer special properties. Type and quantity should be precisely matched to the particular application and the additive percentage can be between 1% and 30%. Depending on their mode of action, these additives can be divided into three types:

  • Surface-protecting additives

    These include detergents, dispersants, high-pressure and wear protection additives, corrosion and rust protection additives, friction modifiers and surface-sealing additives such DuraGear® and PowerShot®, which use the friction occurring in aggregates to form a protective surface layer of silicon. (Link to landing page)
  • Oil-improving additives

    Viscosity improvers reduce a potential drop in viscosity when the temperature rises. Pour point improvers work more in the low temperature range and delay the formation of wax crystals so that the oil can flow longer even at low temperatures.
  • Oil-protecting additives

    Anti-ageing additives, or antioxidants, slowing the ageing of the oil caused by the entry of oxygen, high temperatures and catalytic metals. Oil-ageing products are decomposed and adverse reactions ended. Antifoaming agents modify the surface tension of the oil and thus prevent the formation of stable surface foam.

Lubricating greases

Lubricating greases are solid lubricants usually containing mineral oil, which in a wide consistency or penetration range are manufactured solidly from liquid to tallow-like. They are used when liquid lubricants are not suitable due to their liquid nature and for example, run away from the lubricating point on roller and friction bearings, open gearboxes, wire ropes or chain drives. Lubricating greases consist up to 70-95% of a base oil, 3-30% of thickeners and 0 - 5% of additives.

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