Abstract: The object of the invention is a method in a pneumatic oscillating device for observing an obstacle within the length of the work stroke and continuing the oscillating movement after this. The oscillating device includes a double-acting operating cylinder (1), a principal operating valve (6) controlling this, and a reversing valve (4), by which the principal operating valve (6) is controlled. A resistance to the flow on whichever return line (1.31, 1.41) of the operating cylinder (1) is in use, is arranged in order to maintain the return side pressure of the operating cylinder (1) at a level that is clearly higher than the ambient pressure during a work stroke in a normal situation, and sensing the pressure of the return side and, if it drops below a set limit, which is indicated by the movement of the pressure slowing or stopping completely, giving a command to the reversing valve (4) to change direction on the basis of this.

Abstract: A hydraulic hammer for driving a pile into soil in which an impact weight is arranged for being driven with reciprocating motion along guideways in a casing by a double-acting hydraulic cylinder. A pump having a drain line and a pressure line is connected to drive a piston in the hydraulic cylinder. Two two-position valves connect a piston cavity of the hydraulic cylinder with the pressure line or the drain line of the pump. Each valve has a valving element whose front axial end bears against a seat along an annular contact surface when the valve is closed. The valves carry at their rear axial ends control pistons, each of which has a diameter smaller than the diameter of the contact surface of the valving element with the seat. The two valves are coaxially arranged in opposition to one another and control communication between the hydraulic cylinder and the pressure or drain lines of the pump.