|Metric ball and metric roller bearings come in many types, each used for different purposes, most bearings are also available in imperial sizes.|
Metric Ball Bearings
Are probably the most common type of bearing. They are found in everything from electric motors to hard drives. They can handle both radial and thrust loads, and are usually found in applications where the load is relatively small.
The load is transmitted from the outer race to the ball, and from the ball to the inner race. Since it is a sphere, it only contacts the inner and outer race at a very small point, which helps it spin very smoothly. But it also means that there is not very much contact area holding that load, so if it is overloaded, the balls can deform or squish, ruining the bearing.
Metric Roller Bearings
Are used in applications like conveyer belt rollers, where they must hold heavy radial loads. In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load out over a larger area, allowing the roller bearing to handle much greater loads than a ball bearing. However, this type of roller bearing is not designed to handle much thrust loading.
A variation of this type of bearing, called a needle bearing, uses cylinders with a very small diameter. This allows the bearing to fit into tight places.