Hydraulically operated percussion device

In a hydraulically operated percussion apparatus comprising a housing having a bore in which an impact piston is reciprocatingly movable for transferring impact energy to an impact tool, pressure chambers defined between said housing and said piston, and a channel system including a high pressure branch and a low pressure branch for conducting hydraulic pressure fluid to and from the pressure chambers, a first one of said chambers in a ringlike manner surrounding said piston and being defined, at its end remote from the impact tool, by an annular shoulder on said piston, and at its opposite end, by an annular surface of said housing, 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 chamber being reciprocatingly movable in response to the reciprocating movement of the piston for alternately covering and uncovering said first and second ports, the improvement that said annular surface of said housing has an annular groove into which one end of said valve member slidingly fits when said second port is covered by said valve member, third and fourth port means in the surface of said bore below said annular surface, said third port means communicating with the bottom of said annular groove and said fourth port means communicating with said low pressure branch, said piston having a mantle surface having recess means therein located to connect said third and fourth port means to each other when the piston is in its end position contacting the tool, and to connect said third port means with said annular chamber when the piston is in a position between its extreme end positions.