Spherical balls enclosed between two concentric rings permit the rings to rotate relative to each other, whilst supporting a radial load; this is the essence of a ball bearing. Roller bearings use cylinders instead of balls and have a greater load bearing capacity because of the greater contact between the rolling element and the rings. Taper roller bearings take this concept further by making the rings and rollers tapered, to increase the contact area, permitting large radial and thrust loads. In needle roller bearings, the cylinders are long and thin, so that the outer diameter of the bearing is not much greater than that of the inner ring. This makes for a compact design which can be an advantage when space is at a premium. A spherical–roller bearing uses barrelled cylinders as the rolling elements, with two sets of rollers inclosed by the rings. This allows the bearing to accommodate a misaligned load.
The bearings illustrated below represent a small set of the huge variety available, each designed for a specific set of engineering requirements.
Deep groove bearing 
The balls fit well into the deep grooves, enabling the bearing to support axial loads in both directions, in addition to radial loads. The bearing illustrated here has a single row of balls. 
Thrust ball bearing 
A thrust ball bearing such as the one illustrated here can support an axial load in one direction. Not designed to accommodate radial loads. The bearing components can easily be separated. 
Tapered roller bearing 
Both of the rings and the rollers are tapered in order to simultaneously support axial and radial loads. The ratio of the loads supported depends on the angle between the roller and bearing axes. A greater angle helps support a larger axial load. 
Angular contact ball bearing 
This particular design of angular contact ball bearing is able to accommodate a large thrust load in one direction, in addition to radial loads. 
Selfaligning ball bearing 
There are two sets of balls which run on a pair of grooves on the inner ring, with a single outerring concave surface. This allows the bearing to accommodate misalignment of the shaft. 
Needle roller bearing 
This has long and thin rollers  the design is suited for applications where radial space is limited. 
Spherical roller bearing 
Because of the angular contact between the rollers and raceways, the bearing is able to accommodate both axial and radial loads; the double set of rollers also permits the bearing to accommodate shaft misalignment. Notice that the rollers are not cylindrical, and hence the adjective `spherical'. 
Cylindrical roller bearing 
The cylindrical rollers are able to accommodate large radial loads. This is a singlerow bearing. Cylindrical roller bearings played a seminal role in the development of the continuous rolling mill, now used in the manufacture of billions of tonnes of widestrip steel (Aylen, 2010). Prior to this, the rolling process was by repeatedly passing the steel through a single mill, involving many steps of handling and heating. The original bearing design had an outer forged steel ring and a fixed bronzebearing race holding steel rollers in position. Modern bearings of this kind would be made entirely of steel. 
Wheel hub bearing 
Large numbers of these bearings are manufactured annually to satisfy demand particularly from the automotive industries. Such bearings obviously support the radial load due to the weight of the automobile, but also thrust loads arising when the motion of the vehicle is not strictly linear. 
When a large number of measurements are made of the lifetime of a component and a distribution is plotted, the probability of small or long lives is generally small, with the distribution peaking somewhere in between, Fig. 1a. A Weibull probability density function has the form
(1) 
Associated with each such distribution is a failure rate function , where represents the probability of failure during a time interval for a component which has survived to the time . Objects that have survived longer would in general be expected to have a higher probability of failure [1]:
(3) 
The versatility of the Weibull distribution to take on the characteristics of other types of distributions is illustrated by setting , in which case the distribution takes the form of an exponential, with a constant failure rate function, Fig. 1. A constant failure rate is unusual but might arise in special cases where there is regular maintenance.
Equation 2 is sometimes written with an additional parameter as follows:
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Part II 
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Part IV 
Part V 
Bearings for extreme conditions 

The schematic images have kindly been provided for educational purposes by SKF via John Beswick.
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