Bearing (mechanical)

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 to the directions of the loads (forces) applied to the parts.

Rotary bearings hold rotating components such as shafts or axles within mechanical systems, and transfer axial and radial loads from the source of the load to the structure supporting it. The simplest form of bearing, the '' plain bearing'', consists of a shaft rotating in a hole. Lubrication is used to reduce friction. In the ''ball bearing'' and ''roller bearing'', to reduce sliding friction, rolling elements such as rollers or balls with a circular cross-section are located between the races or journals of the bearing assembly. A wide variety of bearing designs exists to allow the demands of the application to be correctly met for maximum efficiency, reliability, durability and performance.

The term "bearing" is derived from the verb " to bear"; a bearing being a machine element that allows one part to bear (i.e., to support) another. The simplest bearings are bearing surfaces, cut or formed into a part, with varying degrees of control over the form, size, roughness, and location of the surface. Other bearings are separate devices installed into a machine or machine part. The most sophisticated bearings for the most demanding applications are very precise components; their manufacture requires some of the highest standards of current technology.

History

The invention of the rolling bearing, in the form of wooden rollers supporting, or bearing, an object being moved is of great antiquity. It may predate the invention of a wheel rotating on a plain bearing.

Though it is often claimed that the Egyptians used roller bearings in the form of tree trunks under sleds, this is modern speculation. The Egyptians' own drawings in the tomb of Djehutihotep show the process of moving massive stone blocks on sledges as using liquid-lubricated runners which would constitute plain bearings. There are also Egyptian drawings of plain bearings used with hand drills.

Wheeled vehicles using plain bearings emerged between about 5000 BC and 3000 BC.

The earliest recovered example of a rolling element bearing is a wooden ball bearing supporting a rotating table from the remains of the Roman Nemi ships in Lake Nemi, Italy. The wrecks were dated to 40 BC.

Leonardo da Vinci incorporated drawings of ball bearings in his design for a helicopter around the year 1500; this is the first recorded use of bearings in an aerospace design. However, Agostino Ramelli is the first to have published sketches of roller and thrust bearings. An issue with ball and roller bearings is that the balls or rollers rub against each other, causing additional friction. This can be reduced by enclosing each individual ball or roller within a cage. The captured, or caged, ball bearing was originally described by Galileo in the 17th century.

The first practical caged-roller bearing was invented in the mid-1740s by horologist John Harrison for his H3 marine timekeeper. In this timepiece the caged bearing was only used for a very limited oscillating motion, but later on Harrison applied a similar bearing design with a true rotational movement in a contemporaneous regulator clock.

Industrial era

The first patent on ball bearings was awarded to Philip Vaughan, a British inventor and ironmaster in Carmarthen in 1794. His was the first modern ball-bearing design, with the ball running along a groove in the axle assembly.

Bearings played a pivotal role in the nascent Industrial Revolution, allowing the new industrial machinery to operate efficiently. For example, they were used for holding wheel and axle assemblies to greatly reduce friction compared to prior non-bearing designs.

The first plain and rolling-element bearings were wood, closely followed by bronze. Over their history bearings have been made of many materials, including ceramic, sapphire, glass, steel, bronze, and other metals. More recently, plastic bearings made of nylon, polyoxymethylene, polytetrafluoroethylene, and UHMWPE, among other materials, are also in use today.

Watch makers produce "jeweled" watches using sapphire plain bearings to reduce friction, thus allowing more precise time keeping.

Even basic materials can have impressive durability. Wooden bearings, for instance, can still be seen today in old clocks or in water mills where the water provides cooling and lubrication.

The first patent for a radial style ball bearing was awarded to Jules Suriray, a Parisian bicycle mechanic, on 3 August 1869. The bearings were then fitted to the winning bicycle ridden by James Moore in the world's first bicycle road race, Paris-Rouen, in November 1869.

In 1883, Friedrich Fischer, founder of FAG, developed an approach for milling and grinding balls of equal size and exact roundness by means of a suitable production machine, which set the stage for creation of an independent bearing industry. His hometown Schweinfurt later became a world leading center for ball bearing production.

The modern, self-aligning design of ball bearing is attributed to Sven Wingquist of the SKF ball-bearing manufacturer in 1907, when he was awarded Swedish patent No. 25406 on its design.

Henry Timken, a 19th-century visionary and innovator in carriage manufacturing, patented the tapered roller bearing in 1898. The following year he formed a company to produce his innovation. Over a century the company grew to make bearings of all types, including specialty steel bearings and an array of related products and services.

Erich Franke invented and patented the wire race bearing in 1934. His focus was on a bearing design with a cross section as small as possible and which could be integrated into the enclosing design. After World War II he founded together with Gerhard Heydrich the company Franke & Heydrich KG (today Franke GmbH) to push the development and production of wire race bearings.

Richard Stribeck's extensive research on ball bearing steels identified the metallurgy of the commonly used 100Cr6 (AISI 52100), showing coefficient of friction as a function of pressure.

Designed in 1968 and later patented in 1972, Bishop-Wisecarver's co-founder Bud Wisecarver created vee groove bearing guide wheels, a type of linear motion bearing consisting of both an external and internal 90-degree vee angle.

In the early 1980s, Pacific Bearing's founder, Robert Schroeder, invented the first bi-material plain bearing that was interchangeable with linear ball bearings. This bearing had a metal shell (aluminum, steel or stainless steel) and a layer of Teflon-based material connected by a thin adhesive layer.

Today's ball and roller bearings are used in many applications which include a rotating component. Examples include ultra high speed bearings in dental drills, aerospace bearings in the Mars Rover, gearbox and wheel bearings on automobiles, flexure bearings in optical alignment systems, and air bearings used in Coordinate-measuring machines.

Common

By far, the most common bearing is the plain bearing, a bearing which uses surfaces in rubbing contact, often with a lubricant such as oil or graphite. A plain bearing may or may not be a discrete device. It may be nothing more than the bearing surface of a hole with a shaft passing through it, or of a planar surface that bears another (in these cases, not a discrete device); or it may be a layer of bearing metal either fused to the substrate (semi-discrete) or in the form of a separable sleeve (discrete). With suitable lubrication, plain bearings often give entirely acceptable accuracy, life, and friction at minimal cost. Therefore, they are very widely used.

However, there are many applications where a more suitable bearing can improve efficiency, accuracy, service intervals, reliability, speed of operation, size, weight, and costs of purchasing and operating machinery.

Thus, there are many types of bearings, with varying shape, material, lubrication, principle of operation, and so on.

Types

There are at least 6 common types of bearing, each of which operates on a different principle:
* Plain bearing, consisting of a shaft rotating in a hole. There are several specific styles: bushing, journal bearing, sleeve bearing, rifle bearing, composite bearing;
*Rolling-element bearings, whose performance does not depend on avoiding or reducing friction between two surfaces but employ a different principle to achieve low external friction: the rolling motion of an intermediate element in between the surfaces which bears the axial or radial load. Classified as either:
**Ball bearing, in which the rolling elements are spherical balls;
**Roller bearing, in which the rolling elements are cylindrical rollers, linearly-tapered (conical) rollers, or rollers with a curved taper (so-called spherical rollers);
*Jewel bearing, a plain bearing in which one of the bearing surfaces is made of an ultrahard glassy jewel material such as sapphire to reduce friction and wear;
*Fluid bearing, a noncontact bearing in which the load is supported by a gas or liquid (i.e. air bearing);
*Magnetic bearing, in which the load is supported by a magnetic field;
* Flexure bearing, in which the motion is supported by a load element which bends.

Notable characteristics of each of these types of bearing are summarized in the following table.

Motions

Common motions permitted by bearings are:

*Radial rotation e.g. shaft rotation;
*linear motion e.g. drawer;
*spherical rotation e.g. ball and socket joint;
*hinge motion e.g. door, elbow, knee.

Friction

Reducing friction in bearings is often important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and premature failure of the bearing. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.

*By shape, gains advantage usually by using spheres or rollers, or by forming flexure bearings.
*By material, exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
*By fluid, exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.
*By fields, exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.
*Air pressure exploits air pressure to keep solid parts from touching.

Combinations of these can even be employed within the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.

Loads

Bearing design varies depending on the size and directions of the forces that they are required to support. Forces can be predominately radial, axial ( thrust bearings), or bending moments perpendicular to the main axis.

Speeds

Different bearing types have different operating speed limits. Speed is typically specified as maximum relative surface speeds, often specified ft/s or m/s. Rotational bearings typically describe performance in terms of the product ''DN'' where ''D'' is the mean diameter (often in mm) of the bearing and ''N'' is the rotation rate in revolutions per minute.

Generally, there is considerable speed range overlap between bearing types. Plain bearings typically handle only lower speeds, rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which are limited ultimately by centripetal force overcoming material strength.

Play

Some applications apply bearing loads from varying directions and accept only limited play or "slop" as the applied load changes. One source of motion is gaps or "play" in the bearing. For example, a 10 mm shaft in a 12 mm hole has 2 mm play.

Allowable play varies greatly depending on the use. As an example, a wheelbarrow wheel supports radial and axial loads. Axial loads may be hundreds of newtons force left or right, and it is typically acceptable for the wheel to wobble by as much as 10 mm under the varying load. In contrast, a lathe may position a cutting tool to ±0.002 mm using a ball lead screw held by rotating bearings. The bearings support axial loads of thousands of newtons in either direction and must hold the ball lead screw to ±0.002 mm across that range of loads

Stiffness

A second source of motion is elasticity in the bearing itself. For example, the balls in a ball bearing are like stiff rubber, and under load deform from round to a slightly flattened shape. The race is also elastic and develops a slight dent where the ball presses on it.

The stiffness of a bearing is how the distance between the parts which are separated by the bearing varies with applied load. With rolling element bearings this is due to the strain of the ball and race. With fluid bearings it is due to how the pressure of the fluid varies with the gap (when correctly loaded, fluid bearings are typically stiffer than rolling element bearings).

Service life

;Fluid and magnetic bearings

Fluid and magnetic bearings can have practically indefinite service lives. In practice, there are fluid bearings supporting high loads in hydroelectric plants that have been in nearly continuous service since about 1900 and which show no signs of wear.

;Rolling element bearings
Rolling element bearing life is determined by load, temperature, maintenance, lubrication, material defects, contamination, handling, installation and other factors. These factors can all have a significant effect on bearing life. For example, the service life of bearings in one application was extended dramatically by changing how the bearings were stored before installation and use, as vibrations during storage caused lubricant failure even when the only load on the bearing was its own weight; the resulting damage is often false brinelling. Bearing life is statistical: several samples of a given bearing will often exhibit a bell curve of service life, with a few samples showing significantly better or worse life. Bearing life varies because microscopic structure and contamination vary greatly even where macroscopically they seem identical.

L10 life

Bearings are often specified to give an "L10" life (outside the US, it may be referred to as "B10" life.) This is the life at which ten percent of the bearings in that application can be expected to have failed due to classical fatigue failure (and not any other mode of failure like lubrication starvation, wrong mounting etc.), or, alternatively, the life at which ninety percent will still be operating. The L10 life of the bearing is theoretical life and may not represent service life of the bearing. Bearings are also rated using C₀ (static loading) value. This is the basic load rating as a reference, and not an actual load value.

;Plain bearings
For plain bearings, some materials give much longer life than others. Some of the John Harrison clocks still operate after hundreds of years because of the ''lignum vitae'' wood employed in their construction, whereas his metal clocks are seldom run due to potential wear.

;Flexure bearings
Flexure bearings rely on elastic properties of a material. Flexure bearings bend a piece of material repeatedly. Some materials fail after repeated bending, even at low loads, but careful material selection and bearing design can make flexure bearing life indefinite.

;Short-life bearings
Although long bearing life is often desirable, it is sometimes not necessary. describes a bearing for a rocket motor oxygen pump that gave several hours life, far in excess of the several tens of minutes life needed.

Composite bearings

Depending on the customized specifications (backing material and PTFE compounds), composite bearings can operate up to 30 years without maintenance.

Oscillating bearings

For bearings which are used in oscillating applications, customized approaches to calculate L10 are used.

External factors

The service life of the bearing is affected by many parameters that are not controlled by the bearing manufacturers. For example, bearing mounting, temperature, exposure to external environment, lubricant cleanliness and electrical currents through bearings etc. High frequency PWM inverters can induce currents in a bearing, which can be suppressed by the use of ferrite chokes.

The temperature and terrain of the micro-surface will determine the amount of friction by the touching of solid parts.

Certain elements and fields reduce friction while increasing speeds.

Strength and mobility help determine the amount of load the bearing type can carry.

Alignment factors can play a damaging role in wear and tear, yet overcome by computer aid signaling and non-rubbing bearing types, such as magnetic levitation or air field pressure.

Mounting

There are many methods of mounting bearings, usually involving an interference fit. When press fitting or shrink fitting a bearing into a bore or onto a shaft, it's important to keep the housing bore and shaft outer diameter to very close limits, which can involve one or more counterboring operations, several facing operations, and drilling, tapping, and threading operations. Alternatively, an interference fit can also be achieved with the addition of a tolerance ring.

Maintenance and lubrication

Many bearings require periodic maintenance to prevent premature failure, but many others require little maintenance. The latter include various kinds of polymer, fluid and magnetic bearings, as well as rolling-element bearings that are described with terms including ''sealed bearing'' and ''sealed for life''. These contain seals to keep the dirt out and the grease in. They work successfully in many applications, providing maintenance-free operation. Some applications cannot use them effectively.

Nonsealed bearings often have a grease fitting, for periodic lubrication with a grease gun, or an oil cup for periodic filling with oil. Before the 1970s, sealed bearings were not encountered on most machinery, and oiling and greasing were a more common activity than they are today. For example, automotive chassis used to require "lube jobs" nearly as often as engine oil changes, but today's car chassis are mostly sealed for life. From the late 1700s through the mid-1900s, industry relied on many workers called oilers to lubricate machinery frequently with oil can

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