Bearings are components that allow parts to rotate or move linearly with respect to each other. One part is usually stationary while the bearing acts as a support for the moving part. Most bearings support rotating shafts against radial or axial loads. The bearing surfaces can be separated by a film of fluid like oil or gas, known as sliding bearings, or by balls or rollers known as rolling bearings. Under certain conditions of load, speed, film thickness and geometry, a liquid film can form in sliding bearings to isolate the surfaces, called a hydrodynamic film. Low-friction bearing materials are used to limit wear when metal-to-metal contact occurs.