HYDRAULICALLY OPERATED IMPACT MOTOR

An hydraulically operated impact motor, for example a jack hammer, has a hammer piston (12) with a single land (18). The hammer piston surfaces (19, 20) of the land (18) form two cylinder chambers (21, 22) together with the cylinder of the piston. The rear cylinder chamber (21) is constantly pressurized by the pump pressure and the front cylinder chamber (22) is periodically pressurized and drained by means of a valve (28). The valve (28) is controlled by two control passages (32, 41) that have ports (33, 34 and 47-50 respectively)into the cylinder and conveys the control pressure to valve shifting piston surfaces (30, 39). The larger one (39) of the valve shifting piston surfaces (30, 39) is the end surface of a plunger (36) and an intermediate chamber (35) is formed between the plunger (36) and the valve spool (28). The intermediate chamber (35) is constantly pressurized through a passage (54). Internal leak passages (52, 55) in the valve spool makes the valve spool stable in its two positions.