Cams are machine elements whose function is to control lateral movement through its rotational displacement. One application is the control of valve positions (they control inlet of air-fuel mixture as well as outlet of the exhaust gases e.g CO,CO2 & H2O). They are shaped to be eccentric, so that the axis of rotation is slightly offset, and to enable a follower to trace out the path that the make as they rotate. They need to be made of a material with a hard surface on the face that contacts the follower. The bearing surface is either a bush or a plain bearing drilled and ground in the material itself. They are often lubricated as they turn to try to minimise wear. Rotational speeds need to be quite low to maintain full contact of the follower wheel on the cam surface with out jumping. Cams have been superceeded in some machines by hydraulic or electric actuators that offer more flexibility in control of the displacement graph.