A method of measuring one or more motion parameters of a main shaft in a gyratory or cone crusher is disclosed. Signals are directed from a stationary sensor means located apart from the main shaft to one or more signal reflective surfaces situated on a target located on or adjacent to an upper portion of the main shaft. The target rotates and otherwise moves in unison with the movement of the main shaft. From the elapsed time the distances from the sensor the reflective surfaces on the target are determined. Analyzing the distances over time will yield information on the movement of the main shaft.