Types of Bearing - Shafts and Bearings

               
The most basic type of bearing is the journal bearing, which is little more than a shaft running and sliding in a hole. The greatest strength of the journal bearing  is the high load capacity due to the shaft and bore being so well-fitted to each other. However, all this sliding and rolling means that there is a high amount of friction, and that friction will gradually damage the shaft and the interior of the bearing. 
 
Thankfully, there are several ways to reduce this friction, such as using slick materials like Teflon and graphite-filled nylon, keeping dust and grit out of the bearing, and lubrication. Oil is particularly useful as a lubricant, because it actually gets thicker under high pressure. Inside a bearing, this means that it can thicken right where the contact between the shaft and bearing needs it the most, preventing metal-on-metal contact. This is why oil-lubricating systems are so prevalent in the engines of both automobiles and aircraft, since most of the important bearings in those engines are journal bearings.
 
The other common type of bearing is the rolling-element bearing. While ball bearings are the most common type and use balls as their rolling element (obviously), other types of rolling-element bearings use cones, cylinders, and needles. The common element of the rolling-element bearing is that they all roll without slipping on the races (the tracks that the rolling elements roll on), which means that there is no sliding involved. Compared to the plain journal bearings, this means that rolling-element bearings cause much less wear and tear on their individual elements, and typically require less lubrication than journal bearings  However, because rolling-element bearings have a smaller contact area than journal bearings, they can place more stress on a smaller surface area. This means that rolling-element bearings are typically better suited for lighter-duty work.
 
While plain and rolling-element bearings are the most common, they are not the only types of bearings in use. Less-common bearings include the magnetic bearing, which uses magnetic fields to support the load, and fluid bearings, that prevent contact between the shaft and bearing with a gas or liquid.  Due to the engineering and power requirements that these types of bearings have, however, they are not as widely used as the more conventional plain and rolling-element types.