Bearings dramatically reduce the amount of friction between a part that spins within a part that doesn't, or spins at a different speed.
Types of Bearings
Different types of bearings are used based on what forced will be dealt with, and how strong the bearing needs to be.
It is likely that the image that comes to mind of anyone that has a basic understanding of what a bearing is happens to be a radial bearing. The rolling components within the bearing ride against an outer race, the outermost portion of the bearing.
Some thrust bearing designs allow the load to press along the length of the shaft it is mounted to, rather than upon the surface, while others can support both types of loads. A common location for this type of bearing in a vehicle is in a clutch, where tapered roller thrust bearings are often used.
Thrust bearings are also used in transmissions because the forward gears in most gearboxes use helical gears which, while aiding in smoothness and noise reduction, cause axial forces that need to be supported.
Bearings which ride on metal spheres are considered ball bearings. This type of bearing is a low friction design, but also can't take much load.
Pictured at the right is a ball bearing manufactured by NSK in Japan, part number 6317. This bearing has a visible cage for the eight contained bearing balls.
Parts of a Bearing
There are multiple parts of a bearing, and it is important to know them if you are looking to buy a replacement bearing, as they are sometimes sold as incomplete units.
The typical bearing has two races, an inner and outer, or top and bottom. The outer race is the outer part of the bearing, and is often pressed in to components. An outer race may also be called a cup, and an inner race may be called a cone.
Rollers take the form of cylinders, balls, needles, and conical cylinders. The type used will depend on application, but all other parts of the bearing will be designed to accept them, so the type of roller cannot be changed by itself.
The cage holds rollers in place, usually spacing them out evenly as well. Not every bearing has a cage, but most automotive bearings do. A cage may also be called a retainer.
A bearing shield is used to keep lubrication in and contaminants out. Many bearings won't have a shield because they are sealed off other ways, such as with a seal or being contained within a sealed unit.
Lubricants such as grease or oil are used to help further reduce friction, extend bearing life, and in some cases, flush out contaminants.