is a bearing which carries a load by placing rolling elements (such as balls or rollers) between two bearing rings called races.The relative motion of the races causes the rolling elements to roll with very little rolling resistance and with little sliding.
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.
Roller bearings are the earliest known type of rolling-element-bearing, dating back to at least 40 BC.
Caged roller bearings were invented by John Harrison in the mid-18th century as part of his work on chronometers.
A particularly common kind of rolling-element bearing is the ball bearing.
The downside to this bearing is manufacturing complexity.
Gear bearings could be used, for example, as efficient rotary suspension, kinematically simplified planetary gear mechanism in measuring instruments and watches.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.In practice, the ball deforms (flattens) slightly where it contacts each race much as a tire flattens where it contacts the road.The race also yields slightly where each ball presses against it.Thus, in principle, the ball contacts each race across a very narrow area.