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 the directions of the loads applied to the parts.
The term "bearing" is derived from the verb "to bear"; a bearing being a machine element that allows one part to bear 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.
Types of bearings
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. Lubricants come in different forms, including liquids, solids, and gases. The choice of lubricant depends on the specific application and factors such as temperature, load, and speed. 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.History
It is sometimes assumed that the invention of the rolling bearing, in the form of wooden rollers supporting– or bearing –an object being moved, predates the invention of a wheel rotating on a plain bearing; this underlies speculation that cultures such as the Ancient Egyptians used roller bearings in the form of tree trunks under sleds. There is no archaeological evidence for this sequence of technological development. The Egyptian drawings in the tomb of Djehutihotep are believed to show the process of moving massive stone blocks not with rollers but upon sledges pulled over sand which was wetted to increase its cohesion and reduce friction. There are also Egyptian drawings of plain bearings used to support the far end of hand drills.Wheeled vehicles using plain bearings emerged between about 5000 BC and 3000 BC.
A recovered example of an early 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 roller and thrust bearings sketches. An issue with the 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.
The first patent on ball bearings was awarded to Philip Vaughan, a Welsh 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.
file:Early Timken roller bearing.jpg|thumb|Early Timken tapered roller bearing with notched rollers
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 the 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 with Gerhard Heydrich the company Franke & Heydrich KG 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, 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 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.
Design
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.
Materials
| Bearing Material | Attributes | Trade-offs |
| Chrome Steel SAE 52100 Case Hardening Steel SAE 4118 |
| |
| Stainless Steel AISI 440C | ||
| High Alloy Steel AISI M-50High Alloy Steel M50NiL |
| |
| Stainless Steel DD400 | ||
| Ceramics: Silicon Nitride, Zirconia, Silicon Carbine | ||
| White Metals or Babbitt Metal | ||
| Copper-Lead Alloys | ||
| Bronze | ||
| Aluminum Alloys | ||
| Silver | ||
| Plastics | ||
| Carbon Graphite |
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.