magnetic detector 14 a is mounted alongside the line of travel of weight 1 and detects the passage of magnet 14 on the downward travel of the weight 1 detector 14 a generates a signal to connect the cylinders of the rams 4 to the supply of high pressure hydraulic fluid to initiate the lifting stroke of the rams when weight 1 is about to strike the chisel 3 a further magnetic detector 13 b can be located at a lower level to detector 13 a so as to detect when the weight 1 enters the over run zone of its travel and to disconnect the feed of high pressure fluid to the ram cylinders initiated by detector 14 the relative positions of the magnets and detectors can be selected according to the requirements of any given case using simple trial and error preferably detector 14 a also triggers a timing sequence for example by way of the timer module 27 in the control box 19 in fig5 which timing sequence would terminate in disconnection of the hydraulic feed to the rams should the weight 1 not first reach the position to actuate detector 13 a as shown in fig6 the flow of hydraulic fluid to and from the cylinders of the rams is controlled by a valve assembly 16 under the influence of a control box 19 in the valve assembly 16 the pipes 15 from the cylinders of the rams connect with a vented pilot to open check.