Hydraulically driven striking device

In a hydraulically operated percussion apparatus comprising a housing having a bore in which an impact piston is reciprocatingly movable between a retracted end position and an impact end position in contact with an impact tool for transferring impact energy to said impact tool, pressure chambers defined between said housing and said piston and a channel system including a high pressure and a low pressure branch for conducting hydraulic pressure fluid to and from said pressure chambers, a first one of said pressure chambers surrounding said piston and being defined, at the end of said chamber remote from said impact tool, by an annular shoulder on said piston, and at its opposite end, by an annular surface of said housing, a second pressure chamber being open to said low pressure branch, a third pressure chamber being open to said high pressure branch, first and second port means opening into said first chamber and communicating respectively with said high pressure branch and low pressure branch, said second port means being located between said first port means and said annular surface of said housing, a sleeve shaped distribution valve member surrounding said piston in said first pressure chamber and being reciprocatingly movable in response to the reciprocating movement of said piston, a first valve chamber defined between said distribution valve member and a first recess in the surface of said bore between said first and second port means, said valve member having a first shoulder means facing toward said first port means and communicating with said first valve chamber, said valve member being movable between

• (a) a first end position in which said first port means is covered and said second port means is uncovered,
• (b) an intermediate position in which both port means are covered, and in which, during operation of said apparatus, said valve member and said piston are in contact in said impact position of the latter, and,
• (c) a second end position, out of contact with said piston, in which said first port means is uncovered and said second port means is covered,
• the improvement comprising:
• first valve means connecting said first valve chamber to said high pressure branch through said second pressure chamber in response to said piston moving a selected distance from said retracted end position,
• second valve means connecting said first valve chamber to said low pressure branch through said third pressure chamber in response to said piston moving a distance less than said selected distance from said retracted end position,
• a second valve chamber communicating with said high pressure branch and defined by a portion of said distribution valve member and a second recess in the surface of said bore between said first and second port means,
• said distribution valve member having a second shoulder means facing said second port means and communicating with said second valve chamber,
• said first shoulder means having a greater surface area than said second shoulder means,
• said distribution valve member having at its opposite ends pressure actuated surfaces in said first pressure chamber which at one of said ends has a greater surface area facing in the direction of said tool than the surface area of the other end of said valve member facing in the direction of the said retracted end position of said piston,
• the pressure actuated surfaces of said first and second shoulder means of said valve member and the pressure actuated surfaces of said piston being dimensioned such that said piston hits said valve member shortly before said piston impacts against said tool.