Ball Bearing

A Ball Bearing is a type of rolling-element bearing that uses the ball bearing race to maintain separation. The purpose of ball bearings 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 the load through the balls. In most applications, one race is stationary and the other is attached to a rotating assembly (eg, 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 sliding against each other.

Ball bearings have a lower load capacity for their size than other types of rolling-element bearings , because of the smaller contact area between the balls and the race. However, they can tolerate some misalignment of the inner and outer races.

Ball Bearing


Although bearings had been developed since ancient times, the first modern recorded patent on ball bearings was awarded to Philip Vaughan , a Welsh inventor and ironmaster who created the first design for a ball bearing in Carmarthen in 1794. His first modern ball bearing was Design, with a ball running along a groove in the axle assembly. [1]

Jules Surriere , a Parisian bicycle mechanic , designed the first radial-style ball bearing in 1869, [2] which had been fitted into the winning bicycle ridden by James Moore in the world’s first bicycle road race, Paris Rouen in November 1869,

Common design

There are several common designs of ball bearings, each offering different performance trade-offs. They can be made from many different materials, including: stainless steel , chrome steel , and ceramic ( silicon nitride ( Si3N4 ) ) . A hybrid ball bearing is a bearing with ceramic balls and a metal race.

Angular Contact

An angular contact ball bearing uses an axially asymmetric race. An axial load passes in a straight line through the bearing, while a radial load takes an oblique path which serves to separate the races axially. So the angle of contact on the inner race is the same as 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 must match the relative ratio of each. The greater the contact angle (usually in the range of 10 to 45 degrees), the greater the axial load, but the lower the radial load. In high-speed applications such as turbines, jet engines and dental equipment, the centrifugal force generated by the balls changes the contact angle in the inner and outer races. silicon nitrideAs such ceramics are now routinely used in such applications due to their low density (40% that of steel). These materials significantly reduce centrifugal force and function well in high temperature environments. They wear similar to bearing steel, rather than breaking or shattering like glass or porcelain.

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


An axial or thrust ball bearing uses side-to-side running. An axial load is transmitted directly through the bearing, whereas a radial load is poorly supported and tends to separate the race, so that a large radial load can damage the bearing.

Deep groove

In a deep groove radial bearing, the dimensions of the race are close to the dimensions of the balls moving in it. Deep groove bearings support greater loads than shallow groove bearings. Like angular contact bearings, deep groove bearings support both radial and axial loads, but without the choice of contact angle to allow the choice of the relative proportion of these load capacities.

Preloaded pairs

The above basic types of bearings are usually applied in a method of preloaded pairs , where two separate bearings are rigidly fastened with a rotating shaft facing each other. This is the nominal clearance required between the bearing balls and the race ( preloading) improves axial runout. Pairing also provides an advantage of distributing the load more evenly, nearly doubling the total load capacity compared to a single bearing. Angular contact bearings are almost always used in opposing pairs: the asymmetric design of each bearing supports axial loads in only one direction, so an opposite pair is needed if the application demands support in both directions. . The preloading force must be carefully designed and assembled, as it deducts from the axial force capability of the bearings, and may damage the bearings if applied excessively. The coupling mechanism may be directly facing the bearings, or may separate them from a shim, bushing or shaft feature.

Construction type


The Conrad style ball bearing is named after its inventor Robert Conrad, who was awarded British Patent 12,206 in 1903 and US Patent 822,723 in 1906. These bearings are assembled by placing the inner ring in an eccentric relative position to the outer ring. Two rings in contact at one point, resulting in a large gap opposite the point of contact. The balls are inserted through the gap and then evenly distributed around the bearing assembly, allowing the rings to be centered. Assembly is accomplished by caged the balls to maintain their position relative to each other. Without the cage, the balls would eventually fall out of position during operation, causing the bearing to fail. The cage carries no weight and serves only to maintain the position of the ball.

Conrad bearings have the advantage that they are able to withstand both radial and axial loads, but have the disadvantage of low 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. Bearings are used in most mechanical industries.


In a slot filling radial bearing, the inner and outer races are notched on one face so that when the notches are aligned, the resulting balls can be slipped into the slot to assemble the bearing. A slot-fill bearing has the advantage that more balls can be assembled (even allowing for a full complement design), resulting in higher radials than a Conrad bearing of similar dimensions and material type. load capacity. However, a slot-fill bearing cannot take a significant axial load, and the slots cause an imbalance in the race which can have a small but adverse effect on strength. 

Race for relief

As the name suggests race ball bearings are relieved, either the OD of the inner ring is decreased on one side, or the ID of the outer ring is increased on one side. This allows a greater number of balls to be gathered in the inner or outer race, and then press fit on the relief. Sometimes the outer ring will be heated to facilitate assembly. Like the slot-fill construction, the relief race construction allows a higher number of balls than the Conrad construction, up to full complement and the additional ball count gives additional load capacity. However, a relief race bearing can only support significant axial loads in one direction (away from the ‘free’ race).

Fragmented caste

Another method of fitting more balls into a radial ball bearing is to load the balls, reassemble the fractured part, and then use a steel band to cut it all the way through one of the radial ‘fracturing’ (slicing) rings. Using a pair, hold the segmented ring sections together in alignment. Again, this allows for more balls, including full ball complement, although unlike slot filling or relief race construction, it can support significant axial loading in either direction.


There are two row designs: single-row bearings and double-row bearings. Most ball bearings are a single-row design, meaning that the bearing is a single row of balls. This design works with radial and thrust loads.

double-row design has two rows of bearing balls. The advantages of double-row bearings over single-row bearings include that they can bear radial and axial loads in both directions. Double-row angular contact ball bearings have a fast mounting, which can also bear the bending impact. Other advantages of double-row bearings are their rigidity and compactness. Their disadvantage is that they require better alignment than single-row bearings.

Obtrusive lapel

Flanged bearings on the outer ring simplify axial placement. For such bearings there may be a hole of a similar diameter in the housing, but the entrance face of the housing (which can be either the outer or inner face) must be machined exactly normal to the hole axis. Although the manufacture of such flanges is very expensive. A more cost-effective arrangement of the bearing outer ring, with similar benefits, is a snap ring groove at both ends of the outer diameter. The snap ring assumes the function of a flange.


Cages are commonly used to secure balls in Conrad-style ball bearings. In other construction types they can reduce the number of balls depending on the specific cage size, and thus reduce the load capacity. Sliding two convex surfaces without cages on each other stabilizes the position of the tangents. Sliding the convex surface into a matching concave surface with a cage stabilizes the tangent position, which avoids dents in the balls and reduces friction. Caged roller bearings were invented by John Harrison in the mid-18th century as part of his work on chronology .

Hybrid ball bearings using ceramic balls

Ceramic bearing balls can weigh up to 40% less than steel, depending on size and material. This minimizes 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 as the bearing spins against the ball. This reduction in force reduces friction and rolling resistance. Lighter balls allow the bearing to spin faster, and use less power to maintain its speed.

Ceramic balls are usually harder than race balls. Over time they will make a groove in the race due to wear. This is preferable to wear balls which will leave them with potentially flat spots which will significantly damage performance.

While ceramic hybrid bearings use ceramic balls in place of steel ones, they are constructed with steel inner and outer rings; Hence the hybrid designation. While the ceramic material itself is stronger than steel, it is also harder, resulting in increased stress on the rings, and therefore reduced load capacity. The ceramic balls are electrically insulating, which can prevent ‘arcing’ failures when current flows through the bearing. Ceramic balls can also be effective in environments where lubrication may not be available (such as in space applications).

In some settings only a thin coating of ceramic is used on metal ball bearings.

Fully ceramic bearings

These bearings use both ceramic balls and races. These bearings are impervious to corrosion and rarely require lubrication. These bearings are noisy at high speeds due to the stiffness and rigidity of the balls and races. The hardness of ceramics makes these bearings brittle and liable to crack under load or impact. Since both the ball and the race are of equal hardness, wear can cause chipping of both the ball and the race at high speeds, which can lead to sparking.


Self-aligning ball bearings, such as the Wingquist bearing shown in the figure , are constructed with the inner ring and ball assembly contained within an outer ring that has a circular raceway. This construction allows the bearing to tolerate a small angular misalignment resulting from shaft or housing deflection or improper mounting. Bearings were mainly used in bearing arrangements with very long shafts, such as transmission shafts in textile factories. [6] A drawback of self-aligning ball bearings is a limited load rating, as there is very little oscillation in the outer raceway (the radius is much larger than the ball radius). This led to the invention of spherical roller bearings , which have a similar design but use rollers instead of balls. Spherical Roller Thrust BearingAnother invention that derived from findings by Wingquist.

Operating conditions

Life span

The calculated life for a bearing is based on the load it carries and its operating speed. The industry standard usable bearing life span is inversely proportional to the bearing load cube. citation needed ] The bearing’s nominal maximum load is for a lifetime of 1 million rotations, which is a lifetime of 5.5 working hours at 50 Hz (ie, 3000 rpm). 90% of bearings of that type have at least that lifetime, and 50% of bearings have a lifespan of at least 5 times longer.

The industry standard life calculation is based on the work of Lundberg and Palmgren done in 1947. The formula assumes that life is limited by metal fatigue and that the life distribution can be described by the Weibull distribution . Several variations of the formula exist which include factors of physical properties, lubrication and loading. Factoring for loading can be seen as a tacit acknowledgment that modern materials exhibit a different relationship between load and life than Lundberg and Palmgren.

Fail mode

If no bearing is rotating, the maximum load is determined by the force that causes plastic deformation of the elements or raceways. Indentations caused by the elements can concentrate stress and produce cracks in components. The maximum load for bearings with no or very slow rotation is called the “static” maximum load.

In addition, if no bearing is moving, the oscillating forces on the bearing can damage the bearing race or rolling elements, a process known as brinelling . A second short form called false brinelling occurs when the bearing rotates only in a small arc and pushes the lubricant away from the rolling elements.

For a rotating bearing, the dynamic load capacity refers to the load at which the bearing bears 1,000,000 cycles.

If a bearing is rotating but experiences a heavy load that lasts less than one revolution, the static maximum load must be used in the calculation, as the bearing does not rotate during maximum load. [7]

If a sideways torque is applied to a deep groove radial bearing, an unequal force in the shape of an oval is concentrated by the rolling elements on the outer ring in two regions on opposite sides of the outer ring. If the outer ring is not strong enough, or if it is not sufficiently bound by the supporting structure, the outer ring will deform into an oval shape from sideways torque stress, until the gap is large enough for the rolling elements to escape. The inner ring then wears out and the bearing structurally collapses.

A sideways torque on radial bearings also applies pressure to the cage which keeps the rolling elements equally spaced, as the rolling elements are trying to slide all together at the location of the highest sideways torque. If the cage collapses or breaks, the rolling elements tend to group together, the inner ring loses support, and may drop out of center.

Maximum load

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

Bearings have a static load rating. These are based on not exceeding a certain amount of plastic deformation in the raceway. For some applications these ratings can exceed a large amount.


In order for the bearing to operate properly, it needs to be lubricated. In most cases the lubricant is based on the elastohydrodynamic effect (by oil or grease), but dry lubricated bearings operating at extreme temperatures are also available.

In order for a bearing to last its nominal lifetime at its nominal maximum load, it must be lubricated with a lubricant (oil or grease) that has at least a minimum dynamic viscosity (usually denoted with the Greek letter ) of that bearing . recommended for. V

The recommended dynamic viscosity is inversely proportional to the diameter of the bearing. [7]

The recommended dynamic viscosity decreases with rotation frequency. As a rough indication: for less than 3000 rpm , the recommended viscosity increases with a factor 6 for a factor 10 decrease in speed, and for more than 3000 rpm , the recommended viscosity factor is a factor of 10 for an increase in speed. decreases with 3. [7]

For a bearing where the average of the outer diameter of the bearing and the diameter of the axle hole is 50 mm , and which is rotating at 3000 rpm , the recommended dynamic viscosity is 12 mm²/s . [7]

Note that the dynamic viscosity of oil varies greatly with temperature: an increase in temperature of 50–70 °C causes the viscosity factor to drop below 10. [7]

If the viscosity of the lubricant is higher than recommended, the lifespan of the bearing increases, roughly proportional to the square root of the viscosity. If the viscosity of the lubricant is less than recommended, the life of the bearing is reduced, and how much depends on what type of oil is being used. For oils containing EP (‘extreme pressure’) additives, the lifetime is proportional to the square root of the dynamic viscosity, as if it were for very high viscosity, whereas for normal oils the lifetime is proportional to the square root of the viscosity if lower-recommended. viscosity is used. [7]

Lubrication can be done with a grease, which has the advantages that the grease is usually kept within the bearing releasing the lubricating oil as it is compressed by the balls. This provides a protective barrier for the bearing metal from the environment, but has the disadvantages that this grease must be changed periodically, and the maximum load of the bearing is reduced (because if the bearing becomes too hot, The grease melts and falls out of the bearing). The time between grease replacements is greatly reduced with the diameter of the bearing: for a 40 mm bearing, the grease should be changed every 5000 working hours, while for a 100 mm bearing it should be changed every 500 working hours. [7]

Lubrication can also be done with an oil, which has the advantage of a higher maximum load, but requires some way to keep the oil in the bearing, as it usually leaks out of it. For oil lubrication it is recommended that for applications where the oil is not hotter than 50 °C , the oil should be changed once a year, while for applications where the oil does not warm up to more than 100 °C , the oil should be changed 4 times per year. , For a car engine, the oil gets to 100 °C but the engine has an oil filter to maintain the quality of the oil; Therefore, the oil is usually changed less frequently than the oil in the bearings.[7]

If the bearing is used under oscillation, oil lubrication should be preferred. [8] If grease lubrication is necessary, the structure should be optimized according to the parameters to be taken. Preference should be given to greases with high bleeding rates and low base oil viscosity, if possible. [9]

Load direction

Most bearings are meant to support loads perpendicular to the axle (“radial load”). Whether they can also bear axial load, and if so, how much, depends on the type of bearing. Thrust bearings (commonly found on lazy Susans) are specifically designed for axial loads. [7]

For single-row deep-groove ball bearings, the SKF documentation states that the maximum axial load is approximately 50% of the maximum radial load, but also states that “light” and/or “small” bearings can take axial load which is 25% of the maximum radial load. [7]

For single-row edge-contact ball bearings, the axial load can be approximately 2 times the maximum radial load, and for cone-bearings the maximum axial load is between 1 and 2 times the maximum radial load. [7]

Often Conrad-style ball bearings will exhibit contact ellipsoidal truncation under axial load. This means that either the ID of the outer ring is large enough, or the OD of the inner ring is small enough to reduce the area of ​​contact between the balls and the raceway. When this happens, it can significantly increase the stress in the bearing, often invalidating the general rules of thumb regarding the relationship between radial and axial load capacity. With construction types other than Conrad, the outer ring ID can be further reduced and the inner ring ID can be increased to protect against this.

If both axial and radial loads are present, they can be vectorized, resulting in the total load on the bearing, which in combination with the nominal maximum load can be used to predict the lifetime. [7] However, ISO/TS 16281 must be used with the help of calculation software to accurately estimate the rating life of ball bearings.

Avoiding unwanted axial load

The part of the bearing that rotates (either the axle hole or the outer circumference) must be fixed, while the part that does not rotate is not required (so it can be allowed to slide). If a bearing is loaded axially, both sides must be fixed. [7]

If an axle has two bearings, and as the temperature varies, the axle contracts or expands, so it is not acceptable to fix both bearings on either side, as expansion of the axle will cause axial forces that will destroy these bearings. Therefore, at least one of the bearings must be able to slide. [7]

A ‘freely sliding fit’ is one where there is at least a 4 µm clearance, possibly because the surface roughness of the mandrel made on the lathe is normally between 1.6 and 3.2 µm. [7]


Bearings can withstand their maximum load only if the mating parts are properly sized. Bearing manufacturers supply tolerances for the fit of the shaft and housing to achieve this. The material and hardness can also be specified. [7]

Fittings that are not allowed to slip are made in 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 as tapping with a hammer damages both the bearing and shaft, whereas for larger bearings the forces required are so high that there is a need to heat a part before fitting. There is no substitute, so that thermal expansion can allow a temporary sliding fit. [7]

Avoid torsional loads

If a shaft is supported by two bearings, and the center-lines of rotation of these bearings are not equal, large forces are applied to the bearing which can destroy it. Some very small amounts of misalignment are acceptable, and how much depends on the type of bearing. For bearings that are specifically designed to be ‘self-aligning’, the allowable misalignment is between 1.5 and 3 degrees of arc. Bearings that are not designed for self-alignment may only accept 2–10 minutes of arc misalignment. [7]


In general, ball bearings are used in most applications that involve moving parts. Some of these applications have specific features and requirements:

  • Computer fan and spinning device bearings used to be highly spherical, and were said to be the best spherical manufactured shape, but this is no longer true for hard disk drives, and more and more are being replaced with fluid bearings.
  • In Clock Manufacturing Arts, the Jean Lassale company designed a watch movement that used ball bearings to reduce the thickness of the movement. Using 0.20 mm balls, the Caliber 1200 was only 1.2 mm thick, which is still the thinnest mechanical watch movement ever made. [10]
  • Aerospace bearings are used in many applications on commercial, private and military aircraft, including pulleys, gearboxes and jet engine shafts. Materials include M50 tool steel (AMS6491), carbon chrome steel (AMS6444), corrosion-resistant AMS5930, 440C stainless steel, silicon nitride (ceramic) and titanium carbide-coated 440C.
  • A skateboard wheel consists of two bearings, which are subjected to both axial and radial time-varying loads. The most commonly used bearing is the 608-2Z (a deep groove ball from the Series 60 to 8 mm bore diameter)
  • Many yo-yos, from beginners to professional or competition grades, incorporate ball bearings.
  • Many fidget spinner toys use multiple ball bearings to increase the weight and allow the toy to spin.
  • in centrifugal pumps.
  • Railroad locomotive axle journals. Side rod action of the latest high speed steam locomotives before the railroad was converted to diesel engines.


The size of the ball increases as the chain increases for a 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.