Abstract: A wear monitor suitable in particular for monitoring the wear and tear of rider rings (7) on the pistons (5) of horizontally operating reciprocating compressors presents a sensor (9) that monitors the thickness of the rider ring (7) at least indirectly, is arranged in the area of the rider ring (7) in such a way as to move together with it and can be interrogated without contact via a motionless interrogation unit (16), which communicates with an evaluation unit (18). The sensor (9) preferably presents a passive surface acoustic wave element (23), whose electromagnetic reflection behavior depends on the thickness of the rider ring (7) consequent upon the degree of wear and tear thereof and which communicates with a sensor antenna (19) moving together with it, while the interrogation unit (16) presents a communicating interrogation antenna (20).

Abstract: A mobile ballast cleaning machine mounted on a track for movement in an operating direction incorporates apparatus for continuously measuring parameters indicating the track position and any changes therein with respect to a reference system caused by ballast excavation, and for recording the measured parameters. This apparatus comprises hydrostatic level measuring devices mounted in the region of the front and rear undercarriages at the laterally extending sides of the machine frame, two further hydrostatic level measuring devices transversely spaced apart and mounted in the laterally outermost regions of the endless ballast excavating chain, an additional hydrostatic level measuring device mounted in the region of a track sensing element arranged on the frame forwardly of the front undercarriage, and a conduit connected to the hydrostatic level measuring devices for delivering a liquid thereto.