Roller bearings are a type of bearing that use rolling elements to support loads and reduce friction. As opposed to ball bearings, roller bearings have barrel-shaped rolling elements instead of spherical balls. They are capable of supporting heavier loads than similarly sized ball bearings but cannot handle as high of speeds as ball bearings. Roller bearings contain an inner and outer ring that house the rollers and bearing cage. Also known as a retainer or separator, the cage maintains spacing between rollers and holds the bearing together. There are a wide range of roller bearings, each with different designs used in varying applications. Cylindrical, spherical, tapered, and needle are the four primary types of roller bearings. Cylindrical roller bearings handle high radial loads and low thrust loads at high speeds. They also accommodate fast acceleration. Spherical roller bearings contain two rings on the inner raceway to handle a variety of loads and misalignment concerns. The rollers share a single sphered outer ring. With two rows of spherical, barrel-shaped rollers, spherical roller bearings can withstand heavy radial loads and certain axial loads in both directions. They are self-aligning to handle shaft misalignment and mounting problems. Tapered roller bearings contain tapered inner and outer raceways and rollers to accommodate simultaneous radial and thrust loads. Providing true rolling motion and low friction, tapered roller bearings are ideal for supporting heavy combined loads. Needle roller bearings use long, thin cylindrical rollers to support radial loads. Featuring a thinner cross section than other roller bearings, needle roller bearings are ideal in situations that require high load-carrying capacities within a limited space.