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Bearing (mechanical)
Encyclopedia
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.
The invention of the rolling bearing, in the form of an object being moved on wooden rollers, 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 trunk
s under sleds this is modern speculation. They are depicted in their own drawings in the tomb of Djehutihotep
as moving massive stone blocks on sledges with the runners lubricated with a liquid which would constitute a plain bearing.
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 AD.
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 which can be prevented by enclosing the balls or rollers in a cage. The captured, or caged, ball bearing was originally described by Galileo
in the 17th century. 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 Carmarthen
in 1794.
Bearings saw use for holding wheel and axle
s. 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
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.g., nylon
, polyoxymethylene
, polytetrafluoroethylene
, and UHMWPE) which are all used today.
Watch makers produced "jeweled" pocket 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 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.
A patent
on ball bearing
s, 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 idea of Friedrich Fischer
, founder of FAG
, 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.
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.
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 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 and bicycle wheel hubs.
, 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.
There are at least six common principles of operation:
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.
Forces can be predominately radial, axial (thrust bearing
s) or bending moment
s perpendicular to the main axis.
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.
Allowable play varies greatly depending on the use. As 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 ±0.02 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.
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 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.
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 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. 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.
etc.
Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimise the effects of wear.
Bearing life is often much better when the bearing is kept clean and well-lubricated. However, many applications make good maintenance difficult. For example bearings in the conveyor of a rock crusher are exposed continually to hard abrasive particles. Cleaning is of little use because cleaning is expensive, yet the bearing is contaminated again as soon as the conveyor resumes operation. Thus, a good maintenance program might lubricate the bearings frequently but never clean them.
Moving parts
The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....
, 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.
History
Wheel
A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...
.
Though it is often claimed that the Egyptians used roller bearings in the form of tree trunk
Tree Trunk
"Tree Trunk" is a song recorded and released in 1972 by The Doors."Tree Trunk" was recorded by the surviving members of The Doors as they assembled material for their second and final album, Full Circle, following the death of Jim Morrison...
s under sleds this is modern speculation. They are depicted in their own drawings in the tomb of Djehutihotep
Egyptian pyramid construction techniques
There have been many hypotheses about the Egyptian pyramid construction techniques. These techniques seem to have developed over time; later pyramids were not built the same way as earlier ones. Most of the construction hypotheses are based on the idea that huge stones were carved with copper...
as moving massive stone blocks on sledges with the runners lubricated with a liquid which would constitute a plain bearing.
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
Roman Republic
The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...
Nemi ships
Nemi ships
The Nemi Ships were ships built by the Roman emperor Caligula in the 1st century AD at Lake Nemi. Although the purpose of the ships is only speculated on, the larger ship was essentially an elaborate floating palace, which contained quantities of marble, mosaic floors, heating and plumbing such as...
in Lake Nemi
Lake Nemi
Lake Nemi is a small circular volcanic lake in the Lazio region of Italy south of Rome, taking its name from Nemi, the largest town in the area, that overlooks it from a height.-Archaeology and history:The lake is most famous for its sunken Roman ships...
, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
. The wrecks were dated to 40 AD.
Leonardo da Vinci
Leonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...
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
Agostino Ramelli
Agostino Ramelli was an engineer who designed the "book wheel" or "reading wheel".During the Siege of La Rochelle , Agostino successfully engineered a mine under a bastion and breached the fortification, making him popular with his commander, Henri d'Anjou, who later became Henri III of France.In...
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 which can be prevented by enclosing the balls or rollers in a cage. The captured, or caged, ball bearing was originally described by Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...
in the 17th century. 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
Philip Vaughan
Philip Vaughan was a Welsh inventor and ironmaster who patented the first design for a ball bearing in Carmarthen in 1794. His design ran along a track in an axle assembly, known as a ball race, thus originating the modern ball bearing design.-References:...
of Carmarthen
Carmarthen
Carmarthen is a community in, and the county town of, Carmarthenshire, Wales. It is sited on the River Towy north of its mouth at Carmarthen Bay. In 2001, the population was 14,648....
in 1794.
Bearings saw use for holding wheel and axle
Wheel and axle
The wheel and axle is one of six simple machines developed in ancient times and is in the category of a second- or third-class lever. In its simplest form it consists of a rod attached to a wheel so that their movements are coupled when one of the parts is turned...
s. 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
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
closely followed by bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...
. Over their history bearings have been made of many materials including ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
, sapphire
Sapphire
Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...
, glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
, bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...
, other metals and plastic (e.g., nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
, polyoxymethylene
Polyoxymethylene
Polyoxymethylene , also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts that require high stiffness, low friction and excellent dimensional stability....
, polytetrafluoroethylene
Polytetrafluoroethylene
Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
, and UHMWPE) which are all used today.
Watch makers produced "jeweled" pocket 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 practical caged-roller bearing was invented in the mid-1740s by horologist
Horology
Horology is the art or science of measuring time. Clocks, watches, clockwork, sundials, clepsydras, timers, time recorders and marine chronometers are all examples of instruments used to measure time.People interested in horology are called horologists...
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.
A patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
on ball bearing
Ball bearing
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit...
s, reportedly the first, was awarded to Jules Suriray
Jules Suriray
Jules Pierre Suriray was a Parisian bicycle mechanic, who patented ball bearings in 1869.Suriray was awarded the first patent 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...
, a Parisian bicycle mechanic, on 3 August 1869. The bearings were then fitted to the winning bicycle ridden by James Moore
James Moore (cyclist)
James Moore was a bicycle racer. He is popularly regarded as the winner of the first official cycle race in the world in 1868 at St-Cloud, Paris, although there appears to be no verifiable contemporary evidence for this...
in the world's first bicycle road race, Paris-Rouen
Paris-Rouen
Paris–Rouen was the first cycle race covering a distance between two cities. It was held between the cities of Paris and Rouen on November 7, 1869...
, in November 1869.
The idea of Friedrich Fischer
Friedrich Fischer
Friedrich Fischer from Schweinfurt, Germany, is considered the father of the modern ball bearing, having invented the process for milling standard bearings in 1883....
, founder of FAG
Fag
Fag may refer to:* Fag, a colloquialism for cigarette* Fag, a junior boy who acts or acted as servant to a senior boy at a British independent school* Fag, or faggot , an American English slur for a homosexual or effeminate man....
, 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.
The modern, self-aligning design of ball bearing is attributed to Sven Wingquist
Sven Wingquist
Sven Gustaf Wingqvist was a Swedish engineer, inventor and industrialist, and one of the founders of Svenska Kullagerfabriken , one of the world's leading ball- and roller bearing makers. Sven Wingqvist invented the multi-row self-aligning radial ball bearing in 1907.-Chronology:*1876: Born...
of the SKF
SKF
SKF, Svenska Kullagerfabriken AB , later AB SKF, is a Swedish bearing company founded in 1907, supplying bearings, seals, lubrication and lubrication systems, maintenance products, mechatronics products, power transmission products and related services globally.-History:The company was founded on...
ball-bearing manufacturer in 1907, when he was awarded Swedish patent No. 25406 on its design.
Henry Timken
Henry Timken
Henry Timken was an inventor who was born in Bremen, Germany. He founded the Timken Company in 1899, which is located in Canton, Ohio. Timken was inducted into the National Inventors Hall of Fame on September 19, 1998...
, 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
Wire race bearing
A wire-race bearing is a rolling-element bearing, where the balls or rollers run on races resembling a loop of wire. Roller bearings may use just two races, but ball bearings typically use three or four races. Wire race bearings can be large yet lightweight and with small profile and good precision...
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 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 and bicycle wheel hubs.
Common
By far, the most common bearing is the plain bearingPlain bearing
A plain bearing, also known as a plane bearing or a friction bearing is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole...
, a bearing which uses surfaces in rubbing contact, often with a lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
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
Bearing surface
A bearing surface is a mechanical engineering term that refers to the area of contact between two objects. It usually is used in reference to bolted joints and bearings, but can be applied to a wide variety of engineering applications....
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.
Principles of operation
- plain bearingPlain bearingA plain bearing, also known as a plane bearing or a friction bearing is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole...
, also known by the specific styles: bushings, journal bearings, sleeve bearings, rifle bearings - rolling-element bearingRolling-element bearingA rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing round elements between the two pieces...
s such as ball bearings and roller bearings - jewel bearingJewel bearingA jewel bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewel material is usually synthetic sapphire...
s, in which the load is carried by rolling the axle slightly off-center - fluid bearingFluid bearingFluid bearings are bearings which support the bearing's loads solely on a thin layer of liquid or gas.They can be broadly classified as fluid dynamic bearings or hydrostatic bearings. Hydrostatic bearings are externally pressurized fluid bearings, where the fluid is usually oil, water or air, and...
s, in which the load is carried by a gas or liquid - magnetic bearingMagnetic bearingA magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact; for example, they can levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear...
s, in which the load is carried by a magnetic fieldMagnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;... - flexure bearingFlexure bearingA flexure bearing is a bearing which allows motion by bending a load element.A typical flexure bearing is just one part, joining two other parts. For example, a hinge may be made by attaching a long strip of a flexible element to a door and to the door frame...
s, in which the motion is supported by a load element which bends.
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
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.
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
Bearings vary greatly over the size and directions of forces that they can support.Forces can be predominately radial, axial (thrust bearing
Thrust bearing
A thrust bearing is a particular type of rotary bearing. Like other bearings they permit rotation between parts, but they are designed to support a high axial load while doing this.Thrust bearings come in several varieties....
s) or bending moment
Bending Moment
A bending moment exists in a structural element when a moment is applied to the element so that the element bends. Moments and torques are measured as a force multiplied by a distance so they have as unit newton-metres , or pound-foot or foot-pound...
s 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 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 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 ±0.02 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.
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 bearingsFluid 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
False brinelling
False brinelling is damage caused by fretting, with or without corrosion, that causes imprints that look similar to brinelling, but are caused by a different mechanism...
. Bearing life is statistical: several samples of a given bearing will often exhibit a bell curve
Bell curve
Bell curve can refer to:* A Gaussian function, a specific kind of function whose graph is a bell-shaped curve* Normal distribution, whose density function is a Gaussian function...
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.
Plain bearings
For plain bearings some materials give much longer life than others. Some of the John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...
clocks still operate after hundreds of years because of the lignum vitae
Lignum vitae
Lignum vitae is a trade wood, also called guayacan or guaiacum, and in parts of Europe known as pockenholz, from trees of the genus Guaiacum. This wood was once very important for applications requiring a material with its extraordinary combination of strength, toughness and density...
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 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. 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.
L10 life
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.), 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.External factors
The service life of the bearing is affected by many parameters that are not controlled by the bearing manufactures. For example, bearing mounting, temperature, exposure to external environment, lubricant cleanliness and electrical currents through bearingsShaft voltage
Shaft-Voltage occurs in motors powered by variable-frequency drives. Variable Frequency Drives induce shaft voltages onto the shaft of the driven motor because of the extremely high speed switching of the insulated gate bipolar transistors which produce the pulse width modulation used to control...
etc.
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.
Bearing life is often much better when the bearing is kept clean and well-lubricated. However, many applications make good maintenance difficult. For example bearings in the conveyor of a rock crusher are exposed continually to hard abrasive particles. Cleaning is of little use because cleaning is expensive, yet the bearing is contaminated again as soon as the conveyor resumes operation. Thus, a good maintenance program might lubricate the bearings frequently but never clean them.
Types
There are many different types of bearings.| Type | Description | Friction | Stiffness Stiffness Stiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:... † |
Speed | Life | Notes |
|---|---|---|---|---|---|---|
| Plain bearing Plain bearing A plain bearing, also known as a plane bearing or a friction bearing is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole... |
Rubbing surfaces, usually with lubricant; some bearings use pumped lubrication and behave similarly to fluid bearings. | Depends on materials and construction, PTFE has coefficient of friction ~0.05-0.35, depending upon fillers added | Good, provided wear is low, but some slack is normally present | Low to very high | Low to very high - depends upon application and lubrication | Widely used, relatively high friction, suffers from stiction Stiction Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the term "static friction", perhaps also influenced by the verb "stick".... in some applications. Depending upon the application, lifetime can be higher or lower than rolling element bearings. |
| Rolling element bearing | Ball or rollers are used to prevent or minimise rubbing | Rolling coefficient of friction with steel can be ~0.005 (adding resistance due to seals, packed grease, preload and misalignment can increase friction to as much as 0.125) | 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 moment loads than plain bearings with lower friction |
| Jewel bearing Jewel bearing A jewel bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewel material is usually synthetic sapphire... |
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 bearing Fluid bearings are bearings which support the bearing's loads solely on a thin layer of liquid or gas.They can be broadly classified as fluid dynamic bearings or hydrostatic bearings. Hydrostatic bearings are externally pressurized fluid bearings, where the fluid is usually oil, water or air, and... |
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. Often negligible maintenance. | Can fail quickly due to grit or dust or other contaminants. Maintenance free in continuous use. Can handle very large loads with low friction. |
| Magnetic bearings | Faces of bearing are kept separate by magnets (electromagnet Electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off... s or eddy current Eddy current Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of... s) |
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. Maintenance free. (with electromagnet Electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off... s) |
Active magnetic bearings (AMB) need considerable power. Electrodynamic bearing Electrodynamic bearing Electrodynamic bearings are novel, promising systems that can be used to realize contactless electrodynamic suspension of rotating shafts.... s (EDB) do not require external power. |
| Flexure bearing Flexure bearing A flexure bearing is a bearing which allows motion by bending a load element.A typical flexure bearing is just one part, joining two other parts. For example, a hinge may be made by attaching a long strip of a flexible element to a door and to the door frame... |
Material flexes to give and constrain movement | Very low | Low | Very high. | Very high or low depending on materials and strain in application. Usually maintenance free. | 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 Friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:... of the bearing. |
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See also
- Ball splineBall splineBall splines are a special type of linear motion bearing that are used to provide nearly frictionless linear motion while allowing the member to transmit torque simultaneously. There are grooves ground along the length of the shaft for the recirculating ground balls to run inside...
- Combined bearing
- Hertz contact stress
- HingeHingeA hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...
- Main bearingMain bearingIn a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings.All engines have a minimum of two main bearings, one at each end of the crankshaft, and they may have as many as one more than the number of crank pins...
- Needle roller bearingNeedle roller bearingA needle roller bearing is a bearing which uses small cylindrical rollers. They are used to reduce friction of a rotating surface.Needle bearings have a large surface area that is in contact with the bearing outer surfaces compared to ball bearings...
- Pillow block bearingPillow block bearingA pillow block, also known as a plummer block or bearing housing, is a pedestal used to provide support for a rotating shaft with the help of compatible bearings & various accessories...
- Race (bearing)Race (bearing)The rolling-elements of a rolling-element bearing ride on races. The large race that goes into a bore is called the outer race, and the small race that the shaft rides in is called the inner race.-Manufacture:...
- RolamiteRolamiteRolamite is a technology for very low friction bearings developed by Sandia National Laboratories in the 1960s. Invented by Sandia engineer Donald F. Wilkes and patented by him on June 24, 1969 these devices use a stressed metal band and counter rotating rollers within an enclosure to create a...
- ScrollerwheelScrollerwheelA scrollerwheel is a mechanical device composed of a number of rollers and connective bands under tension, which wrap around and weave between the rollers forming a self-supporting cluster possessing a central roller...
- Slewing bearingSlewing bearingTo "slew" means to turn without change of place; a "slewing" bearing is a rotational rolling-element bearing that typically supports a heavy but slow-turning or slow-oscillating load, often a horizontal platform such as a conventional crane, a swing yarder, or the wind-facing platform of a...
- Spherical bearingSpherical bearingA spherical bearing is a bearing that permits angular rotation about a central point in two orthogonal directions...
External links
- Comprehensive review on bearings, University of Cambridge
- How bearings work
- Early bearing failure detection
- How to measure a bearing
- Kinematic Models for Design Digital Library (KMODDL) - Movies and photos of hundreds of working mechanical-systems models at Cornell University. Also includes an e-book library of classic texts on mechanical design and engineering.
- Types of bearings, Cambridge University