Hydraulically operated impact motor

Hydraulically operated impact motor comprising:

• a source (23) of high-pressure hydraulic motive fluid;
• a cylinder (11) having first and second port means (33, 34;
• 47) leading into said cylinder;
• a hammer piston (12) reciprocably mounted in said cylinder (11) and arranged to impact upon an anvil means (72),
• said hammer piston (12) defining with said cylinder (11) first and second pressure chambers (21, 22, respectively), said first and second pressure chambers (21, 22) being in selective communication with said first and second port means;
• a single piston land (18) on said hammer piston (12);
• a first piston surface (19) on said land and located in said first pressure chamber (21) to effect a stroke of said hammer piston (12) in one direction;
• means (25, 26) coupled to said first pressure chamber (21) for constantly pressurizing said first pressure chamber (21) in use;
• a second piston surface (20) on said land and located in said second pressure chamber (22) to effect a stroke of said hammer piston in the other direction;
• a valve (28) coupled to connect at least said second pressure chamber (22) alternatively to said source (23) of high-pressure hydraulic motive fluid (23) and to a motive fluid outlet (24);
• said valve (28) comprising:
• means defining a valve cylinder;
• a valving element (28) axially movable in said valve cylinder;
• a first piston means (30, 31) on said valving element (28) for forcing said valving element (28) into a first position when subject to pressure;
• a first control passage (32) leading between said first piston means and said first port means (33, 34) which leads into said cylinder (11);
• a second piston means (36, 39) for forcing said valving element (28) into a second position when subject to pressure, said second piston means (36, 39) being movable away from said valving element (28);
• a second control passage (41) leading between said second port means (47) which leads into said cylinder and said second piston means (36, 39);
• a third piston means (38) for moving said second piston means (36, 39) away from said valving element (28) when said second piston means (36, 39) is relieved of pressure;
• means (54, 56) coupled to said third piston means (38) for constantly subjecting said third piston means to pressure;
• said piston land (18) being arranged to selectively block said first and second port means (33, 34;
• and 47, respectively) in response to its axial position in the cylinder.