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An example of a four-point contact ball bearing


A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.

Bearing friction

Plain bearings are simply a hole of the correct shape containing the relatively moving part, and use surfaces in rubbing contact, often with a lubricant such as oil or graphite. They are very widely used. Particularly with lubrication they often give entirely acceptable life and friction.

However, 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.


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.

Principles of operation

Animation of ball bearing
There are at least six common principles of operation:

Motions

Common motions permitted by bearings are:

  • Axial 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


Loads

Bearings vary greatly over the size and directions of forces that they can support.

Forces can be predominately radial, axial (thrust bearings) or 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 Some applications apply bearing loads from varying directions and accept only limited play or "slop" as the applied load changes of the product DN where D is the 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 and elasticity

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. As example, a 10 mm shaft in a 12 mm hole has 2 mm play. A second source of motion is elasticity in the bearing itself. As 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.

Allowable play varies greatly depending on the use. As example, a wheelbarrow wheel supports radial and axial loads. Axial loads may be hundreds of newton 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 ±0.02 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.

Life

Fluid and magnetic bearings can potentially give indefinite life.

Rolling element bearing life is statistical, but is determined by load, temperature, maintenance, vibration, lubrication and other factors.

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.

Maintenance

Many bearings require periodic maintenance to prevent premature failure, although some such as fluid or magnetic bearings may require little maintenance.

Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimise the effects of wear.

History

Tapered bearings
Early Timken tapered roller bearing with notched rollers


The oldest instance of the bearing principle dates to the Egyptians when they used tree trunks under sleds. There are also Egyptian drawings of bearings used with hand drills.

The earliest recovered example of a bearing is a wooden ball bearing supporting a rotating table from the remains of the Roman Nemi shipsmarker in Lake Nemimarker, Italymarker. The wrecks were dated to 40 AD.

Leonardo da Vinci is said to have described a type of ball bearing around the year 1500. An issue with ball bearings is the balls rub against each other, causing additional friction, but rubbing can be prevented by enclosing the balls in a cage. The captured, or caged, ball bearing was originally described by Galileo in the 1600s. The mounting of bearings into a set was not accomplished for many years after that. The first patent for a ball race was by Philip Vaughan of Carmarthenmarker in 1794.

Bearings saw use for holding wheel and axles. The bearings used there were plain bearings that were used 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, but ceramic, sapphire, or glass were also used, and steel, bronze, other metals, ceramics, and plastic (e.g., nylon, polyoxymethylene, teflon, and UHMWPE) are all common today. A "jeweled" pocket watch uses stones to reduce friction, and allow more precise time keeping. Even old materials can have good durability. As examples, wood bearings can still be seen today in old water mills where the water provides cooling and lubrication.

The first practical caged-roller bearing was invented in the mid-1740s 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.

Friedrich Fischer's idea from the year 1883 for milling and grinding balls of equal size and exact roundness by means of a suitable production machine formed the foundation for creation of an independent bearing industry.

A patent on ball bearings, reportedly the first, 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.

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. Through a century, the company grew to make bearings of all types, specialty steel 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.

Today, bearings are used in a variety of applications. Ultra high speed bearings are used in dental hand pieces, aerospace bearings are used in the Mars Rover, and flexure bearings are used in optical alignment systems.

Types

There are many different types of bearings.

Type Description Friction Stiffness Speed Life Notes
Plain bearing Rubbing surfaces, usually with lubricant Depends on materials and construction, PTFE has coefficient of friction ~0.05 Good, provided wear is low, but some slack is normally present Low to very high Moderate (depends on lubrication) The simplest type of bearing, widely used, relatively high friction, suffers from stiction in some applications. Some bearings use pumped lubrication and behave similarly to fluid bearings. At high speeds life can be very short.
Rolling element bearing Ball or rollers are used to prevent or minimise rubbing Rolling coefficient of friction with steel can be ~0.005 Good, but some slack is usually present Moderate to high (often requires cooling) Moderate to high (depends on lubrication, often requires maintenance) Used for higher loads than plain bearings with lower friction
Jewel bearing Off-center bearing rolls in seating Low Low due to flexing Low Adequate (requires maintenance) Mainly used in low-load, high precision work such as clocks. Jewel bearings may be very small.
Fluid bearing Fluid is forced between two faces and held in by edge seal Zero friction at zero speed, low Very high Very high (usually limited to a few hundred feet per second at/by seal) Virtually infinite in some applications, may wear at startup/shutdown in some cases Can fail quickly due to grit or dust or other contaminants. Maintenance free in continuous use. Can handle very large loads with low friction and negligible maintenance.
Magnetic bearings Faces of bearing are kept separate by magnets (electromagnets or eddy currents) Zero friction at zero speed, but constant power for levitation, eddy currents are often induced when movement occurs, but may be negligible if magnetic field is quasi-static Low No practical limit Indefinite Often needs considerable power. Maintenance free.
Flexure bearing Material flexes to give and constrain movement Very low Low Very high Very high or low depending on materials and strain in application Limited range of movement, no backlash, extremely smooth motion
Stiffness is the amount that the gap varies when the load on the bearing changes, it is distinct from the friction of the bearing.


See also



References

  1. Purtell, John (1999/2001). Project Diana, chapter 10: http://nemiship.multiservers.com/nemi.htm
  2. Bicycle History, Chronology of the Growth of Bicycling and the Development of Bicycle Technology by David Mozer


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