In four-stroke engines the movement of the valves is controlled by a series of eccentrics, of which the camshaft is equipped. This mechanical part rotates with half speed compared to the crankshaft, from which it is driven by chains, toothed belts or gears. The camshafts, which normally rest on plain bearings (usually called camshaft bearings), may be made of steel or cast iron. The working surfaces of the eccentrics must have a high hardness and during operation should be conveniently lubricated. The camshaft may be arranged in the crankcase and, in that case, it moves the valves by tappets, rods and rocker arms. In most modern engines, however, the camshaft is housed in the crankcase and controls the valves by acting on cup tappets or rocker arms. In most modern engines, however, the camshaft is housed in the cylinder head and controls the valves acting on cup tappets or rockers (which, depending on the case, can be a finger follower or with two arms).