BACKGROUND OF THE INVENTION

5 This invention relates generally to a hγdraulically operated percussive rock drill and is particulariy concerned with a valve arrangement therefore.

Generally speaking two types of valves are used for controlling oil 10 flow to a reciprocating piston in a rock drill. With a spool type valve the shape of the valve spool, inside the rock drill housing, regulates oil flow. Overlapping surfaces close oil inlet lines or oil outlet lines thereby allowing the piston to move.

15 With a cartridge-type valve a piloted cartridge closes oil flow through the valve by bringing the cartridge into sealing contact with a valve seat.

A common feature of existing valve designs known to the applicants 20 is that when the valve closes oil flow there is only one sealing surface. In a spool-type valve this is provided by the aforementioned overlapping surfaces while in a cartridge-type valve this is provided by a ring surface formed when the cartridge seals against the valve seat. 25

In a hydraulic rock drill volumetric efficiency is important. High oil pressures, high frequencies of operation and relatively small sealing faces on the valve lead to substantial oil leaks particularly as the rock drill ages and when the oil temperature rises. This leakage reduces 30 the volumetric efficiency of operation and decreases the output power. Heat builds up in the hydraulic system and this calls for

1775 PC17GB95/02453

greater cooling as it is necessary to reduce the viscosity of the oil. The life of the oil is also reduced.

SUMMARY OF THE INVENTION

The invention provides a valve arrangement in a hydraulic percussive rock drill wherein a piston, located in a cylinder, is caused to reciprocate by hydraulic fluid flow, the valve being positioned between opposing first and second surfaces of two walls in which are formed at least first and second ports respectively, the valve having at least a third port which is movable into register, and out of register, with the first and second ports, and having first and second hydraulic fluid sealing surfaces at least when the third port is out of register with the first and second ports.

The cylinder may include first and second cylinder sections and the said first and second surfaces may be formed by opposing outer and inner surfaces of the first and second cylinder sections respectively.

The said first and second surfaces may define an annular cylindrical space in which the valve, in the nature of a complementary annular cylindrical sleeve, is located.

The invention also extends to a hydraulic percussive rock drill which includes a piston, first and second cylinder sections which define a chamber in which the piston reciprocates and, between them, an annular cylindrical space with opposing first and second walls, and a reciprocable valve which is located in the annular cylindrical space, the first and second walls having a first pair of respective ports, and a second pair of respective ports, the valve having a first port which is moved into and out of register with the first pair of ports, and a

second port which is moved into and out of register with the second pair of ports, when the valve reciprocates thereby to control the flow of the hydraulic fluid into the chamber and to permit hydraulic fluid to flow from the chamber, the valve having sealing surfaces adjacent each end of the first port and the second port which engage with respective opposing surfaces of the first and second walls.

BRIEF DESCRIPTION OF THE DRAWING

The invention is further described by way of example with reference to the accompanying drawing which is a side view, partly sectioned, of a hydraulic percussive rock drill which includes a valve arrangement according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

The accompanying drawing illustrates from the side and partly cross sectioned a portion of a hydraulic percussive rock drill 10 which includes a valve arrangement according to the invention.

The rock drill includes a cylinder which is made from a first cylinder section 12 and a second cylinder section 14. These sections define a chamber 16 in which a piston 18 is located. A port 20 in the first cylinder section 12 is used to admit hydraulic fluid into the chamber 16 to drive the piston in one direction, ie, to the right in the drawing. The sections 12 and 14 define an annular cylindrical space 22 between them. Mounted in the space is a valve 24 in the nature of an annular cylinder, or sleeve. A wall 26 of the section 12 has two ports 28 and 30 formed in it. A wall 32 of the section 14 has ports 34 and 36 formed in it. The valve in turn has ports 38 and 40.

Outer sealing surfaces 24A and 24B of the valve are respectively in intimate sliding contact with an outer surface 26A of the wall 26 and an opposed inner surface 32A of the wall 32.

The valve is caused, in a known manner, to reciprocate inside the cylindrical space 22. The port 36 is connected to the pressure side of a hydraulic pump and when the port 40 is in register with the ports 30 and 36 fluid flows into the chamber 16 to drive the piston to the left in the drawing. At this stage the port 38 is out of register with the ports 28 and 34. On the return stroke of the piston the valve moves to the right so that the port 40 is out of register with the ports 30 and 36 and the port 38 is in register with the ports 28 and 34. Hydraulic fluid can then be exhausted from the right hand side of the chamber and returned to tank.

The essence of the invention lies in the fact that the valve 24 has sealing surfaces on it outer and inner walls which mate with the opposing inner and outer surfaces of the walls 32 and 26 of the cylinder sections respectively. Thus at each of the ports 38 and 40 there are two sealing surfaces, marked 42 and 44 respectively which act in series to minimise oil leakage. The operating valve oil consumption is thereby reduced and there is less heat build-up in the hydraulic system. A lower cooling capacity is required and the viscosity of the oil stays at an acceptable level. There is a higher volumetric efficiency which means that the power output of the rock drill is correspondingly increased.