Lubrication

The act of lubricating; the act of making slippery.

The use of lubricants to reduce friction and wear. Whenever two bodies in contact are made to slide relative to one another, a resistance to the motion is experienced. This resistance, called friction, is present in all machinery. Approximately 30% of the power of an automobile engine is consumed by friction. Friction and wear can be significantly reduced, and thus relative motion of machine parts made possible, by interposing a lubricant at the interface of the contacting surfaces; the machine elements designed to accomplish this are called bearings. Bearings can be lubricated by solids such as graphite or, more commonly, by liquids and gases. See also Antifriction bearing; Friction; Graphite; Lubricant; Surface and interfacial chemistry; Wear.

Conventionally, lubrication has been divided into (1) fluid-film lubrication (hydrostatic, hydrodynamic, and elastohydrodynamic), where the sliding surfaces are separated by a relatively thick, continuous film of lubricant; and (2) boundary lubrication, where contact surface separation is but a few molecular layers and asperity contact is unavoidable.

Hydrostatic bearings

Hydrostatic films are created when a high-pressure lubricant is injected between opposing (parallel) surfaces (pad and runner), thereby separating them and preventing their coming into direct contact. Hydrostatic bearings require external pressurization. The film is 5–50 micrometers thick, depending on application. Though hydrostatic lubrication does not rely on relative motion of the surfaces, relative motion is permitted and can even be discontinuous. Figure 1 is a schematic of a hydrostatic bearing pad. To handle asymmetric loads, hydrostatic systems generally employ several evenly spaced pads. Hydrostatic bearings find application where relative positioning is of extreme importance. They are also applied where a low coefficient of friction at vanishing relative velocity is required.

Hydrostatic bearing pad.

Hydrodynamic bearings

Hydrodynamic bearings are self-acting. To create and maintain a load-carrying hydrodynamic film, it is necessary only that the bearing surfaces move relative to one another and ample lubricant is available. The surfaces must be inclined to form a clearance space in the shape of a wedge, which converges in the direction of relative motion. The lubricant film is then created as the lubricant is dragged into the clearance by the relative motion. This viscous action results in a pressure build-up within the film (Fig. 2). The fact that hydrodynamic bearings are self-generating and do not rely on auxiliary equipment makes these bearings very reliable. Hydrodynamic journal bearings and thrust bearings are designed to support radial and axial loads, respectively, on a rotating shaft.

Hydrodynamic film formation.

Rolling contact bearings

Journal and thrust bearings are conformal bearings; that is, the opposing bearing surfaces conform in shape. Ball and roller bearings, also known as rolling contact bearings, are counterformal. Counterformal bearings always operate in the hydrodynamic mode, but because the contact area in these bearings is small the pressure attains high values, in the range of 1–3 gigapascals (10,000–30,000 atm). In consequence, the surfaces deform elastically and the lubricant viscosity increases by several orders of magnitude.

Lubricants

Today, mineral oils manufactured from petroleum are the most common liquid lubricants. The manufacturer of petroleum lubricants can choose from a wide variety of crude oils, and the choice is of great importance because the lubricating oil fraction of crude oils varies widely. See also Petroleum; Viscosity.

The noun has 2 meanings:

Meaning #1: the condition of having been made smooth or slippery by the application of a lubricant

Meaning #2: an application of a lubricant to something

Lubrication of the ship steam engine crankshaft. The two bottles with lubricant are attached to the piston and move while the engine is operating

Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (eg graphite, MoS₂)^[1] a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (greases) or exceptionally a gas.

In the most common case the applied load is carried by pressure generated within the fluid due to the frictional viscous resistance to motion of the lubricating fluid between the surfaces.

Lubrication can also describe the phenomenon such reduction of wear occurs without human intervention (aquaplaning on a road).

The science of friction, lubrication and wear is called tribology.

Adequate lubrication allows smooth continuous operation of equipment, with only mild wear, and without excessive stresses or seizures at bearings. When lubrication breaks down, metal or other components can rub destructively over each other, causing destructive damage, heat, and failure.

The regimes of lubrication

As the load increases on the contacting surfaces three distinct situations can be observed with respect to the mode of lubrication, which are called regimes of lubrication:

• Fluid film lubrication is the lubrication regime in which through viscous forces the load is fully supported by the lubricant within the space or gap between the parts in motion relative to one another (the lubricated conjunction) and solid-solid contact is avoided.^[2]
  • Hydrostatic lubrication is when an external pressure is applied to the lubricant in the bearing, to maintain the fluid lubricant film where it would otherwise be squeezed out.
  • Hydrodynamic lubrication is where the motion of the contacting surfaces, and the exact design of the bearing is used to pump lubricant around the bearing to maintain the lubricating film. This design of bearing may wear when started or stopped, as the lubricant film breaks down.

• Elastohydrodynamic lubrication: The opposing surfaces are separated but there occurs some interaction between the raised solid features called asperities, and there is an elastic deformation on the contacting surface enlarging the load bearing area whereby the viscous resistance of the lubricant becomes capable of supporting the load.

• Boundary lubrication (also called boundary film lubrication): The bodies come into closer contact at their asperities; the heat developed by the local pressures causes a condition which is called stick-slip and some asperities break off. At the elevated temperature and pressure conditions chemically reactive constituents of the lubricant react with the contact surface forming a highly resistant tenacious layer, or film on the moving solid surfaces (boundary film) which is capable of supporting the load and major wear or breakdown is avoided. Boundary lubrication is also defined as that regime in which the load is carried by the surface asperities rather than by the lubricant.^[3]

Beside supporting the load the lubricant may have to perform other functions as well, for instance it may cool the contact areas and remove wear products. While carrying out these functions the lubricant is constantly replaced from the contact areas either by the relative movement (hydrodynamics) or by externally induced forces.

Lubrication is required for correct operation of mechanical systems pistons, pumps, cams, bearings, turbines, cutting tools etc where without lubrication the pressure between the surfaces in close proximity would generate enough heat for rapid surface damage which in a coarsened condition may literally weld the surfaces together, causing seizure.

References

1. ^ www.engineersedge.com/lubrication/applications_solid_lubrication.htm - 14k
2. ^ San Andrés. L. "Introduction to pump rotordynamics, Part i. Introduction to hydrodynamic lubrication". ("MEEN626 Lubrication Theory Class:Syllabus FALL2006"). [1] (11 Dec 2007)
3. ^ Bosman R. and Schipper D.J.. Microscopic Mild Wear in the Boundary Lubrication regime. Laboratory for Surface Technology and Tribology, Faculty of Engineering Technology, University of Twente, P.O. Box 217, NL 7500 AE Enschede, The Netherlands. 

See also

Look up lubrication in Wiktionary, the free dictionary.

• Society of Tribologists and Lubrication Engineers
• Machinery Lubrication magazine
• International Council for Machinery Lubrication
• Lubricant
• Lubrication theory
• Bearing (mechanical)
• Grease (lubricant)
• Oils
• Penetrant

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