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. 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 devices; their manufacture requires some of the highest standards of current technology. The invention of the rolling bearing, in the form of wooden rollers supporting, or bearing, an object being moved is of great antiquity, and may predate the invention of the wheel. Though it is often claimed that the Egyptians used roller bearings in the form of tree trunks under sleds,  this is modern speculation.
There are also Egyptian drawings of bearings used with hand drills.
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.
Types of Bearings
Leonardo da Vinci incorporated drawings of ball bearings in his design for a helicopter around the year 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. 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 mids by horologist John Harrison for his H3 marine timekeeper.
This uses the bearing for a very limited oscillating motion but Harrison also used a similar bearing in a truly rotary application in a contemporaneous regulator clock. The first modern recorded patent on ball bearings was awarded to Philip Vaughan , a British inventor and ironmaster who created the first design for a ball bearing in Carmarthen in His was the first modern ball-bearing design, with the ball running along a groove in the axle assembly.
Bearings have played a pivotal role in the nascent Industrial Revolution , allowing the new industrial machinery to operate efficiently. For example, they saw use for holding wheel and axle to greatly reduce friction over that of dragging an object by making the friction act over a shorter distance as the wheel turned.
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 , other metals and plastic e.
Watch makers produce "jeweled" watches using sapphire plain bearings to reduce friction thus allowing more precise time keeping. Even basic materials can have good durability. As examples, wooden bearings 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 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 In , 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 and formed the foundation for creation of an independent bearing industry.
The modern, self-aligning design of ball bearing is attributed to Sven Wingquist of the SKF ball-bearing manufacturer in , when he was awarded Swedish patent No. Henry Timken , a 19th-century visionary and innovator in carriage manufacturing, patented the tapered roller bearing in 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 and an array of related products and services.
Erich Franke invented and patented the wire race bearing in His focus was on a bearing design with a cross section as small as possible and which could be integrated into the enclosing design. Designed in and later patented in , 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 degree vee angle.
In the early s, Pacific Bearing's founder, Robert Schroeder, invented the first bi-material plain bearing which was size 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 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, bicycle wheel hubs, and air bearings used in Coordinate-measuring machines.
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. 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.
Types of Bearings - Different Types of Bearings
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.
Combinations of these can even be employed within the same bearing. 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.
Applications and Industries
Different bearing types have different operating speed limits. 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. 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. Allowable play varies greatly depending on the use. As example, a wheelbarrow wheel supports radial and axial loads. 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. 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 and which show no signs of wear.
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 varies because microscopic structure and contamination vary greatly even where macroscopically they seem identical. Bearings are often specified to give an "L10" life outside the USA, 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.
Type of bearing and its applications pdf to word
The L10 life of the bearing is theoretical life and may not represent service life of the bearing. Bearings are also rated using C 0 static loading value. This is the basic load rating as a reference, and not an actual load value. 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 rely on elastic properties of material. Flexure bearings bend a piece of material repeatedly.
How Bearings Work
Some materials fail after repeated bending, even at low loads, but careful material selection and bearing design can make flexure bearing life indefinite. Although long bearing life is often desirable, it is sometimes not necessary. Harris 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. Depending on the customized specifications backing material and PTFE compounds , composite bearings can operate up to 30 years without maintenance.
For bearings which are used in oscillating applications, customized approaches to calculate L10 are used. 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 use of ferrite chokes.
The temperature and terrain of the micro-surface will determine the amount of friction by the touching of solid parts. 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. Many bearings require periodic maintenance to prevent premature failure, but many others require little maintenance.