Ball bearing
Overview

A ball bearing is a type of rolling-element bearing
Rolling-element bearing
A rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing round elements between the two pieces...

that uses balls
Ball (bearing)
Bearing balls are special highly spherical and smooth balls, most commonly used in ball bearings. The balls come in many different grades, as defined by the American Bearing Manufacturers Association , which defines the precision of the balls...

to maintain the separation between the bearing
Bearing (mechanical)
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...

races
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:...

.

The purpose of a ball bearing is to reduce rotational friction and support radial
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

and axial
Axial
Axial may mean:* Along the same line as an axis of rotation in geometry* A type of modal frame in music* One of several anatomical directions in an animal body* Axial age, the period from 800 to 200 BC in China, India and the western world...

loads. It achieves this by using at least two races to contain the balls and transmit the loads through the balls. In most applications, one race is stationary and the other is attached to the rotating assembly (e.g., a hub or shaft).
Discussions
Encyclopedia
A ball bearing is a type of rolling-element bearing
Rolling-element bearing
A rolling-element bearing, also known as a rolling bearing, is a bearing which carries a load by placing round elements between the two pieces...

that uses balls
Ball (bearing)
Bearing balls are special highly spherical and smooth balls, most commonly used in ball bearings. The balls come in many different grades, as defined by the American Bearing Manufacturers Association , which defines the precision of the balls...

to maintain the separation between the bearing
Bearing (mechanical)
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...

races
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:...

.

The purpose of a ball bearing is to reduce rotational friction and support radial
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

and axial
Axial
Axial may mean:* Along the same line as an axis of rotation in geometry* A type of modal frame in music* One of several anatomical directions in an animal body* Axial age, the period from 800 to 200 BC in China, India and the western world...

loads. It achieves this by using at least two races to contain the balls and transmit the loads through the balls. In most applications, one race is stationary and the other is attached to the rotating assembly (e.g., a hub or shaft). As one of the bearing races rotates it causes the balls to rotate as well. Because the balls are rolling they have a much lower coefficient of friction than if two flat surfaces were rotating on each other.

Ball bearings tend to have lower load capacity
Structural loads or actions are forces, deformations or accelerations applied to a structure or its components.Loads cause stresses, deformations and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis...

for their size than other kinds of rolling-element bearings due to the smaller contact area between the balls and races. However, they can tolerate some misalignment of the inner and outer races.

## Common designs

There are several common designs of ball bearing, each offering various trade-offs. They can be made from many different materials, including: stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

, chrome steel
Chrome steel
Chrome steel is one of a class of non stainless steels such as AISI 52100, En31, SUJ2, 100Cr6, 100C6, DIN 5401 which are used for applications such bearings, tools and drills.-Popular culture:...

, and 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...

(silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...

(Si3N4)). A hybrid ball bearing is a bearing with ceramic balls and races of metal.

### Angular contact

An angular contact ball bearing uses axially asymmetric
Asymmetric
Something which is asymmetric displays asymmetry. Specific uses of the term may include:*Asymmetric relation for information on such relations in mathematics and set theory*Asymmetric warfare for information and theories of modern war...

races. An axial load passes in a straight line through the bearing, whereas a radial load takes an oblique path that tends to want to separate the races axially. So the angle of contact on the inner race is the same as that on the outer race. Angular contact bearings better support "combined loads" (loading in both the radial and axial directions) and the contact angle of the bearing should be matched to the relative proportions of each. The larger the contact angle (typically in the range 10 to 45 degrees), the higher the axial load supported, but the lower the radial load. In high speed applications, such as turbines, jet engines, and dentistry equipment, the centrifugal forces generated by the balls changes the contact angle at the inner and outer race. Ceramics such as silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...

are now regularly used in such applications due to their low density (40% of steel). These materials significantly reduce centrifugal force and function well in high temperature environments. They also tend to wear in a similar way to bearing steel—rather than cracking or shattering like glass or porcelain.

Most bicycles use angular-contact bearings in the headsets because the forces on these bearings are in both the radial and axial direction.

### Axial

An axial ball bearing uses side-by-side races. An axial load is transmitted directly through the bearing, while a radial load is poorly supported and tends to separate the races, so that a larger radial load is likely to damage the bearing.

### Deep-groove

In a deep-groove radial bearing, the race dimensions are close to the dimensions of the balls that run in it. Deep-groove bearings have higher load ratings for their size than shallow-groove, but are also less tolerant of misalignment of the inner and outer races. A misaligned shallow-groove bearing may support a larger load than the same sized deep-groove bearing with similar misalignment.

The Conrad style ball bearing is named after its inventor, Robert Conrad, who was awarded British 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....

12,206 in 1903 and U.S. patent 822,723 in 1906. A Conrad bearing is assembled by placing the inner race into an eccentric position relative to the outer race, with the two races in contact at one point, resulting in a large gap opposite the point of contact. The balls are inserted via this gap and then evenly distributed around the bearing assembly, causing the races to become concentric. Assembly is completed by fitting a cage to the balls to maintain their positions relative to each other. Absent the cage, the balls would eventually drift out of position during operation, causing the bearing to fail. The cage carries no load and serves only to maintain ball position.

Conrad bearings have the advantage that they are able to withstand both radial and axial loads, but have the disadvantage of lower load capacity due to the limited number of balls that can be loaded into the bearing assembly. Probably the most familiar industrial ball bearing is the deep-groove Conrad style. The bearing is used in most of the mechanical industries.

### Slot-fill

In a slot-fill radial bearing, the inner and outer races are notched so that when they are aligned, balls can be slipped in the slot to fill the bearing. A slot-fill bearing has the advantage that the entire groove is filled with balls, called a full complement, resulting in a higher radial load capacity than a Conrad bearing of the same dimensions and material type. However, a slot-fill bearing cannot carry a significant axial load, due to the loading slots. Also, the slots cause a discontinuity in the races that has an adverse effect on strength. Note that an angular contact bearing can be disassembled axially and therefore is a full complement design.

### Split-race

The outer race may be split axially or radially, or a hole drilled in it for filling. These approaches allow a full complement to be used, but also limit the orientation of loads or the amount of misalignment the bearing can tolerate. Thus, these designs find much less use.

### Rows

There are two row designs: single-row bearings and double-row bearings. Most ball bearings are a single-row design, which means there is one row of bearing balls. This design works with radial and thrust loads.

A double-row design has two rows of bearing balls. Their disadvantage is they need better alignment than single-row bearings.

### Flanged

Bearings with a flange on the outer ring simplify axial location. The housing for such bearings can consist of a through-hole of uniform diameter, but the entry face of the housing (which may be either the outer or inner face) must be machined truly normal to the hole axis. However such flanges are very expensive to manufacture.
A more cost effective arrangement of the bearing outer ring, with similar benefits, is a snap ring groove at either or both ends of the outside diameter. The snap ring assumes the function of a flange.

### Caged

Cages are typically used to secure the balls in a Conrad-style ball bearing. In other construction types they may decrease the number of balls depending on the specific cage shape, and thus reduce the load capacity. Without cages the tangential position is stabilized by sliding of two convex surfaces on each other. With a cage the tangential position is stabilized by a sliding of a convex surface in a matched concave surface, which avoids dents in the balls and has lower friction. Caged roller bearings were invented by 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...

in the mid-18th century as part of his work on chronographs. Caged bearings were used more frequently during wartime steel shortages for bicycle wheel bearings married to replaceable cups.

### Ceramic hybrid ball bearings using ceramic balls

Ceramic bearing balls can weigh up to 40% less than steel bearing balls, depending on size and material. This reduces centrifugal loading and skidding, so hybrid ceramic bearings can operate 20% to 40% faster than conventional bearings. This means that the outer race groove exerts less force inward against the ball as the bearing spins. This reduction in force reduces the friction and rolling resistance. The lighter ball allows the bearing to spin faster, and uses less energy to maintain its speed.

Ceramic hybrid ball bearings use these ceramic balls in place of steel balls. They are constructed with steel inner and outer rings, but ceramic balls; hence the hybrid designation.

### Self-aligning

Self-aligning ball bearings are constructed with the inner ring and ball assembly contained within an outer ring that has a spherical raceway. This construction allows the bearing to tolerate a small angular misalignment resulting from deflection or improper mounting.

### Lifespan

The calculated life for a bearing is based on the load it carries and its operating speed. The industry standard usable bearing lifespan is inversely proportional to the bearing load cubed. Nominal maximum load of a bearing (as specified for example in SKF datasheets), is for a lifespan of 1 million rotations, which at 50 Hz (i.e., 3000 RPM) is a lifespan of 5.5 working hours. 90% of bearings of that type have at least that lifespan, and 50% of bearings have a lifespan at least 5 times as long.

The industry standard life calculation is based upon the work of Lundberg and Palmgren performed in 1947. The formula assumes the life to be limited by metal fatigue
Fatigue (material)
'In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.Fatigue occurs...

and that the life distribution can be described by a Weibull distribution. Many variations of the formula exist that include factors for material properties, lubrication, and loading. Factoring for loading may be viewed as a tacit admission that modern materials demonstrate a different relationship between load and life than Lundberg and Palmgren determined .

### Failure modes

If a bearing is not rotating, maximum load is determined by force that causes non-elastic deformation of balls. If the balls are flattened, the bearing does not rotate. Maximum load for not or very slowly rotating bearings is called "static" maximum load.

If that same bearing is rotating, that deformation tends to knead the ball into roughly a ball shape, so the bearing can still rotate, but if this continues for a long time, the ball fails due to metal fatigue. Maximum load for rotating bearing is called "dynamic" maximum load, and is roughly two or three times as high as static maxload.

If a bearing is rotating, but experiences heavy load that lasts shorter than one revolution, static maxload must be used in computations, since the bearing does not rotate during the maximum load.

In general, maximum load on a ball bearing is proportional to outer diameter of bearing times width of bearing (where width is measured in direction of axle).

### Lubrication

For a bearing to operate properly, it needs to be lubricated. In most cases the lubricant is based on elastohydrodynamic
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

effect (by oil or grease) but working at extreme temperatures dry lubricated bearings are also available.

For a bearing to have its nominal lifespan at its nominal maximum load, it must be lubricated with a lubricant (oil or grease) that has at least the minimum dynamic viscosity (usually denoted with the Greek letter ) recommended for that bearing.

The recommended dynamic viscosity is inversely proportional to diameter of bearing.

The recommended dynamic viscosity decreases with rotating frequency. As a rough indication: for less than , recommended viscosity increases with factor 6 for a factor 10 decrease in speed, and for more than , recommended viscosity decreases with factor 3 for a factor 10 increase in speed.

For a bearing where average of outer diameter of bearing and diameter of axle hole is , and that is rotating at , recommended dynamic viscosity is .

Note that dynamic viscosity of oil varies strongly with temperature: a temperature increase of causes the viscosity to decrease by factor 10.

If the viscosity of lubricant is higher than recommended, lifespan of bearing increases, roughly proportional to square root of viscosity. If the viscosity of the lubricant is lower than recommended, the lifespan of the bearing decreases, and by how much depends on which type of oil being used. For oils with EP ('extreme pressure') additives, the lifespan is proportional to the square root of dynamic viscosity, just as it was for too high viscosity, while for ordinary oil's lifespan is proportional to the square of the viscosity if a lower-than-recommended viscosity is used.

Lubrication can be done with a grease, which has advantages that grease sticks to the bearing and protects bearing metal from environment, but has disadvantages that this grease must be replaced periodically, and maximum load of bearing decreases (because if bearing gets too warm, grease melts and runs out of bearing). Time between grease replacements decreases very strongly with diameter of bearing: for a bearing, grease should be replaced every 5000 working hours, while for a bearing it should be replaced every 500 working hours.

Lubrication can also be done with an oil, which has advantage of higher maximum load, but needs some way to keep oil in bearing, as it normally tends to run out of it. For oil lubrication it is recommended that for applications where oil does not become warmer than , oil should be replaced once a year, while for applications where oil does not become warmer than , oil should be replaced 4 times per year. For car engines, oil becomes but the engine has an oil filter to continually improve oil quality; therefore, the oil is usually changed less frequently than the oil in bearings.

Most bearings are meant for supporting loads perpendicular to axle ("radial loads"). Whether they can also bear axial loads, and if so, how much, depends on the type of bearing. Thrust bearings (commonly found on lazy susan
Lazy Susan
A Lazy Susan is a rotating tray, usually circular, placed on top of a table to aid in moving food on a large table or countertop.- Origin :The term "Lazy Susan" made its first written appearance in a Good Housekeeping article in 1906, although their existence dates back to the 18th century...

s) are specifically designed for axial loads.

For single-row deep-groove ball bearings, SKF's documentation says that maximum axial load is circa 50 % of maximum radial load, but it also says that "light" and/or "small" bearings can take axial loads that are 25 % of maximum radial load.

If both axial and radial loads are present, they can be added vectorially, to result in total load on bearing, which in combination with nominal maximum load can be used to predict lifespan.

The part of a bearing that rotates (either axle hole or outer circumference) must be fixed, while for a part that does not rotate this is not necessary (so it can be allowed to slide). If a bearing is loaded axially, both sides must be fixed.

If an axle has two bearings, and temperature varies, axle shrinks or expands, therefore it is not admissible for both bearings to be fixed on both their sides, since expansion of axle would exert axial forces that would destroy these bearings. Therefore, at least one of bearings must be able to slide.

A 'freely sliding fit' is one where there is at least a 4 µm clearance, presumably because surface-roughness of a surface made on a lathe is normally between 1.6 and 3.2 µm.

### Fit

Bearings can withstand their maximum load only if the mating parts are properly sized. Bearing manufacturers supply tolerance
Tolerance (engineering)
Engineering tolerance is the permissible limit or limits of variation in# a physical dimension,# a measured value or physical property of a material, manufactured object, system, or service,# other measured values ....

s for the fit of the shaft and the housing so that this can be achieved. The material and hardness may also be specified.

Fittings that are not allowed to slip are made to diameters that prevent slipping and consequently the mating surfaces cannot be brought into position without force. For small bearings this is best done with a press because tapping with a hammer damages both bearing and shaft, while for large bearings the necessary forces are so great that there is no alternative to heating one part before fitting, so that thermal expansion allows a temporary sliding fit.

If a shaft is supported by two bearings, and center-lines of rotation of these bearings are not same, then large forces are exerted on the bearing that may destroy it. Some very small amount of misalignment is acceptable, and how much depends on type of bearing. For bearings that are specifically made to be 'self-aligning', acceptable misalignment is between 1.5 and 3 degrees of arc. Bearings that are not designed to be self-aligning can accept misalignment of only 2–10 minutes of arc.

## Applications

Today the ball bearing is used in numerous everyday applications. Ball bearings are used for dental and medical instruments. In dental and medical hand pieces, it is necessary for the pieces to withstand sterilization and corrosion. Because of this requirement, dental and medical hand pieces are made from 440C stainless steel, which allows smooth rotations at fast speeds.
• Hard drive bearings used to be highly spherical, and were said to be the best spherical manufactured shapes, but this is no longer true, and more and more are being replaced with 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...

s.
• German ball bearing factories were often a target of allied aerial bombings during World War II
Strategic bombing during World War II
Strategic bombing during World War II is a term which refers to all aerial bombardment of a strategic nature between 1939 and 1945 involving any nations engaged in World War II...

; such was the importance of the ball bearing to the German war industry.
• In horology, the company Jean Lassale
Jean Lassale
Jean Lassale is a Swiss watch company that designed the thinnest mechanical watch movement : 1.2 mm. It is the Calibre 1200.In the years 1970, Pierre Mathys, master watchmaker in La Chaux-de-Fonds, designed and built the prototype of a revolutionary watch caliber, with the goal of making the...

designed a watch movement that used ball bearings to reduce the thickness of the movement. Using 0.20 mm balls, the Calibre 1200 was only 1.2 mm thick, which still is the thinnest mechanical watch movement.
• Aerospace bearings are used in many applications on commercial, private and military aircraft including pulleys, gearboxes and jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

shafts. Materials include M50 tool steel (AMS6491), Carbon chrome steel (AMS6444), the corrosion resistant AMS5930, 440C stainless steel, silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...

(ceramic) and titanium carbide
Titanium carbide
Titanium carbide, TiC, is an extremely hard refractory ceramic material, similar to tungsten carbide.It is commercially used in tool bits. It has the appearance of black powder with NaCl-type face centered cubic crystal structure...

-coated 440C.
• Skateboarding. The wheels in a skateboard contain two bearings in each of the four wheels. Most commonly bearing 608-2Z is used (a deep groove ball bearing from series 60 with 8 mm bore diameter)
• Yo-Yos, there are Ball bearings in the center of high quality Yo-Yos.
• Agricultural Equipment. The many moving parts in a piece of farm machinery depend on several different types of bearings to operate. Under the heavy loads and dusty conditions, these bearings need to be lubricated, repaired, or replaced often.

## History

Although roller bearings had been developing since ancient times, the first recorded 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 bearings 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.

Ball bearings were first produced in Europe so they were standardized to metric dimensions. American manufacturers came along later so they produced ball bearings in metric dimensions prior to the early-1990s.

## Designation

The ball size increases as the series increases, for any given inner diameter or outer diameter (not both). The larger the ball the greater the load carrying capacity. Series 200 and 300 are the most common.

• Ball screw
Ball screw
A ball screw is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with...

• Bearing Specialists Association
Bearing Specialists Association
Bearing Specialists Association is an industry trade group of distributors of bearings. It is based in Glen Ellyn, Illinois and is managed by CM Services, an association management company....

• Brinelling
Brinelling
Brinelling is a material surface failure caused by contact stress that exceeds the material limit. This failure is caused by just one application of a load great enough to exceed the material limit. The result is a permanent dent or "brinell" mark. It is a common cause of roller bearing failures,...

, a common failure mode
• Linear bearing
• 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....