Rail-guiding elements (wheel flanges, guide pulleys) ensure that the crane stays on the rails. Due to uneven distribution of mass, track errors, rails offset and different driving speeds, the rail-guiding elements can mesh. The lateral forces arising from this can lead to noise, stress in the crane construction or wear of the wheels and rails. Machines of this type require considerable maintenance. In order to reduce the latter, so-called tracking controllers are used in order to ensure that the crane wheels are kept central to the rail track. In this case, a number of sensors keep the controller informed about the position and direction of the crane, as well as any deviations that occur.