Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of the processes involved is called tribology, and has a history of more than 2000 years.
A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.
Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings. Thus, it is possible for some fluid bearings to have near-zero wear if operated correctly.
A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.
Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical or chemical. The study of wear and related processes is referred to as tribology.
Tribology is the science and engineering of understanding friction, lubrication and wear phenomena for interacting surfaces in relative motion. It is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, materials science, mathematics, biology and engineering. The fundamental objects of study in tribology are tribosystems, which are physical systems of contacting surfaces. Subfields of tribology include biotribology, nanotribology and space tribology. It is also related to other areas such as the coupling of corrosion and tribology in tribocorrosion and the contact mechanics of how surfaces in contact deform. Approximately 20% of the total energy expenditure of the world is due to the impact of friction and wear in the transportation, manufacturing, power generation, and residential sectors.
A plain bearing, or more commonly sliding contact bearing and slide bearing, is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the part of the shaft in contact with the bearing slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.
In mechanical engineering, a rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing rolling elements between two concentric, grooved rings called races. The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding.
False brinelling is a bearing damage caused by fretting, with or without corrosion, that causes imprints that look similar to brinelling, but are caused by a different mechanism. False brinelling may occur in bearings which act under small oscillations or vibrations.
Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface. This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface.
Fretting refers to wear and sometimes corrosion damage of loaded surfaces in contact while they encounter small oscillatory movements tangential to the surface. Fretting is caused by adhesion of contact surface asperities, which are subsequently broken again by the small movement. This breaking causes wear debris to be formed.
The stick–slip phenomenon, also known as the slip–stick phenomenon or simply stick–slip, is a type of motion exhibited by objects in contact sliding over one another. The motion of these objects is usually not perfectly smooth, but rather irregular, with brief accelerations (slips) interrupted by stops (sticks). Stick–slip motion is normally connected to friction, and may generate vibration (noise) or be associated with mechanical wear of the moving objects, and is thus often undesirable in mechanical devices. On the other hand, stick–slip motion can be useful in some situations, such as the movement of a bow across a string to create musical tones in a bowed string instrument.
Friction modifiers are added to lubricants in order to reduce friction and wear in machine components. They are particularly important in the boundary lubrication regime, where they can prevent solid surfaces from coming into direct contact, substantially reducing friction and wear.
The Archard wear equation is a simple model used to describe sliding wear and is based on the theory of asperity contact. The Archard equation was developed much later than Reye's hypothesis, though both came to the same physical conclusions, that the volume of the removed debris due to wear is proportional to the work done by friction forces. Theodor Reye's model became popular in Europe and it is still taught in university courses of applied mechanics. Until recently, Reye's theory of 1860 has, however, been totally ignored in English and American literature where subsequent works by Ragnar Holm and John Frederick Archard are usually cited. In 1960, Mikhail Mikhailovich Khrushchov and Mikhail Alekseevich Babichev published a similar model as well. In modern literature, the relation is therefore also known as Reye–Archard–Khrushchov wear law. In 2022, the steady-state Archard wear equation was extended into the running-in regime using the bearing ratio curve representing the initial surface topography.
The Stribeck curve is a fundamental concept in the field of tribology. It shows that friction in fluid-lubricated contacts is a non-linear function of the contact load, the lubricant viscosity and the lubricant entrainment speed. The discovery and underlying research is usually attributed to Richard Stribeck and Mayo D. Hersey, who studied friction in journal bearings for railway wagon applications during the first half of the 20th century; however, other researchers have arrived at similar conclusions before. The mechanisms along the Stribeck curve have been in parts also understood today on the atomistic level.
Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium.
Albert Kingsbury was an American engineer, inventor and entrepreneur. He was responsible for over fifty patents obtained between the years 1902 to 1930. Kingsbury is most famous for his hydrodynamic thrust bearing which uses a thin film of oil to support weights of up to 220 tons. This bearing extended the service life of many types of machinery during the early 20th century. It was primarily outfitted on Navy ships during World War I and World War II.
Spiral groove bearings are self-acting, or hydrodynamic bearings used to reduce friction and wear without the use of pressurized lubricants. They have this ability due to special patterns of grooves. Spiral groove bearings are self-acting because their own rotation builds up the pressure needed to separate the bearing surfaces. For this reason, they are also contactless bearings.
In fluid mechanics, the Reynolds equation is a partial differential equation governing the pressure distribution of thin viscous fluid films. It was first derived by Osborne Reynolds in 1886. The classical Reynolds Equation can be used to describe the pressure distribution in nearly any type of fluid film bearing; a bearing type in which the bounding bodies are fully separated by a thin layer of liquid or gas.
Extreme tribology refers to tribological situations under extreme operating conditions which can be related to high loads and/or temperatures, or severe environments. Also, they may be related to high transitory contact conditions, or to situations with near-impossible monitoring and maintenance opportunities. In general, extreme conditions can typically be categorized as involving abnormally high or excessive exposure to e.g. cold, heat, pressure, vacuum, voltage, corrosive chemicals, vibration, or dust. The extreme conditions should include any device or system requiring a lubricant operating under any of the following conditions:
