One of the earliest and best-known rolling-element bearings are sets of bearing load calculation pdf laid on the ground with a large stone block on top. As the stone is pulled, the logs roll along the ground with little sliding friction. As each log comes out the back, it is moved to the front where the block then rolls on to it.
It is possible to imitate such a bearing by placing several pens or pencils on a table and placing an item on top of them. A rolling element rotary bearing uses a shaft in a much larger hole, and cylinders called “rollers” tightly fill the space between the shaft and hole. As the shaft turns, each roller acts as the logs in the above example. However, since the bearing is round, the rollers never fall out from under the load. Rolling-element bearings have the advantage of a good tradeoff between cost, size, weight, carrying capacity, durability, accuracy, friction, and so on.
There are five types of rolling elements that are used in rolling-element bearings: balls, cylindrical rollers, spherical rollers, tapered rollers, and needle rollers. Most rolling-element bearings feature cages. The cages reduce friction, wear, and bind by preventing the elements from rubbing against each other. 18th century as part of his work on chronometers. Typical rolling-element bearings range in size from 10 mm diameter to a few metres diameter, and have load-carrying capacity from a few tens of grams to many thousands of tonnes. Each race features a groove usually shaped so the ball fits slightly loose. Thus, in principle, the ball contacts each race across a very narrow area.
However, a load on an infinitely small point would cause infinitely high contact pressure. The race also yields slightly where each ball presses against it. Thus, the contact between ball and race is of finite size and has finite pressure. Note also that the deformed ball and race do not roll entirely smoothly because different parts of the ball are moving at different speeds as it rolls. Overall, these cause bearing drag.
Common roller bearings use cylinders of slightly greater length than diameter. Roller bearings typically have higher radial load capacity than ball bearings, but a lower capacity and higher friction under axial loads. If the inner and outer races are misaligned, the bearing capacity often drops quickly compared to either a ball bearing or a spherical roller bearing. Spherical roller bearings have an outer ring with an internal spherical shape.