FIELD OF THE INVENTION This invention relates to a down-the-hole hammer drill and specifically to such a drill which employs a control rod for supply of working fluid to drive and return chambers within the assembly.

BACKGROUND TO THE INVENTION

Many hammer drills have been made and operated but there is a constant need to improve the structure and efficiency of such drills. These drills usually have a check valve built into the inlet end of the drill. The valve is normally located in the back head or back head end of a control rod assembly.

With the check valve positioned in the assembly as described the flow of compressed air has to open it usually against a spring which biases it to a closed position. The air then has to find its way in a complicated manner passed and around the check valve, into and along the control rod to the porting which leads to the drive and return chambers. Since the check valve is held open only by the flow of air it will shut under spring action as soon as there is no flow. During the piston cycle, ports are opened and closed, and there are short periods of time when the airflow stops and starts. During the operation of the hammer, the check valve closure will usually be exposed to pressurised working fluid at both its opening surface area and other, oppositely facing areas. The latter produces a closing force that acts together with the spring. When flow stops, the check valve oscillates under spring tension and air pressure from both sides, causing a flow restriction through a partially closed check valve. This oscillation also leads to spring failure.

OBJECT OF THE INVENTION

It is an object of the invention to provide a simplified hammer operation to overcome or at least reduce these difficulties.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a control rod for a drill hammer comprising a flow passage extending from an inlet at a back head end to an outlet adjacent an opposite, inner end which is slidably locatable inside a bore of a piston and having a check valve located at the outlet.

The invention further provides for a control rod as defined in which the outlet is provided by a radial opening through a sidewall of the control rod, the check valve including a closure provided in sliding suitably sealed engagement with the sidewall and moveable across the opening with an opening area on a face of the closure exposed to the passage and a spring provided on an opposite side to bias a closure over the opening.

Further features of the invention provide for a control rod as defined in which the closure is provided with a peripheral seal which slidably engages the sidewall; in which an area on the closure opposite to the face is in use substantially isolated from supply fluid pressure at the face; and in which the area on the closure opposite to the face is exposed to an exhaust pressure environment at a free end of the control rod, inside the piston bore. In accordance with one aspect of the invention there is provided a control rod as defined in which a stem extends from the closure and through the spring and an axial opening in an end wall of the control rod; in which the stem has a stop for engagement against the end wall around the axial opening to limit travel of the closure; in which the stop is a head made of resiliently deformable material and having an interference fit through the opening; in which the head is made integral with the stem and closure; and in which the head includes an axial bore formed into a free end of the stem.

In accordance with another aspect of the invention there is provided a control rod as defined in which a stop is located in the passage adjacent the outlet and the valve closure is spring biased against the stop; in which a plug is secured into the free end of the rod; in which the plug includes an axial opening; and in which the plug is secured through screwthreaded engagement. In accordance with the invention there is still further provided a drill hammer including a control rod as defined above.

The invention further provides for a hammer as defined in which the bore of the piston has a first radial port in communication with a longitudinal supply passage that extends rearwardly to a drive chamber and a second port, located in spaced apart relationship between the first port and a rear end of the piston, in communication with a longitudinal supply passage that extends forwardly to a return chamber; and in which the first and second ports extend outwardly from annular recesses in the piston bore. BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will become apparent from the following description, made by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a cross-section of a hammer drill;

Figure 2 shows a cross-section of a portion of a control rod having an alternative embodiment of a check valve.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1 , a hammer drill (1 ) consists of back head (2), a casing in the form of a wear sleeve (3), a piston (4) and a drill bit (5). The back head (2) is secured through screw-threaded engagement into the wear sleeve (3). Into the back head (2) is fitted a control rod (6) through which an air supply for driving the drill (1 ) will be provided in use. A check valve (7) is mounted in an inner end of a control rod (6), which is provided as a hollow air deliver tube. The control rod (6) accordingly provides an inlet flow passage (8) which leads to the check valve (7). The check valve (7) is loaded or biased to the closed position illustrated in Figure 1 by a spring (9). The inner end of the control rod (6) is closed around a stem (14) of the check valve (7). Adjacent that end of the control rod (6) are outlet openings provided as radial ports (18).

The communication of working fluid into the drill assembly is effected when the air supply in the passage (8) acts on a face (34) of the check valve (7) to open it under pressurised air supply.

The stem (14) extends from a closure (26) of the check valve (7) which is provided as an enlarged head. The closure (26) is slidably mounted for movement along a sidewall (12) inside the control rod (6). An annular groove around the closure (26) is fitted with a peripheral seal (28). This provides suitable sealing between the closure (26) and sidewall (12) for operation of the valve (7) to control air supply in this embodiment. The stem (14) extends from the closure (26) through an axial opening in an end wall (32) of the control rod (6) as shown. A free end of the stem (14) is provided with a stop (29) which is provided as an enlarged head. The stop (29) engages against the end wall (32) to limit travel of the closure (26) under bias of the spring (9). In this embodiment, the closure (26) and stem (14) are integral and made from Nylon material. The stop end of the stem (14) is provided with a bore (30). The bore (30) facilitates deformation of the resilient Nylon stop (29). The stop (29) is also formed with a tapered outer surface (31 ). These features allow the stop (29) which is sized as an interference fit to be pressed through the opening in the end wall (32). The stem (14) between the stop (29) and closure (26) is a clearance fit in the opening. A shoulder on the inner side of the stop (29) secures the check valve (7) in the assembly once the stop (29) reverts to its undeformed condition.

Instead of a head providing a stop, the stem may be provided with a transverse passage adjacent its end. A pin can be secured through the passage once the stem is located through the end wall. Such a stop can be used conveniently with valve closures made from steel, Nylon or some other suitable material.

The drill bit (5) is slidably mounted in the end of the wear sleeve (3) opposite to the back head (2). Between the back head (2) and drill bit shank (13) is located the piston (4) with a piston stem (15) engaging in the piston stem bush (10). This piston (4) has an axial bore (1 6) communicating directly with a similar bore (17) through the bit shank (13).

The bore (1 6) of the piston (4) at a back head end is a sliding fit on the inner end of the control rod (6) as required for the operation described below. The outlet ports (18) provide a passage adjacent the inner end of the control rod (6) for the supply of operating air to the working components of the hammer assembly. This is effected firstly through an annular recess (19) formed partway along the bore (1 6) of the piston (4) and radial passage (20) communicating between the recess (19) and slots (21 ) which extend down the side of the piston (4). These slots (21 ) open into a return chamber (22), which is provided between the casing (3), piston stem bush (10), drill piston stem (15) and bit shank (13) where it seals off vents (1 1 ).

Compressed air enters into the back head (2) and opens the check valve (7) to permit flow from the back head (2) through passage (20) down slots (21 ) into the return chamber (22). The air in return chamber (22) thus acts to move the piston (4) upwardly away from the drill bit (5). This results in closing of the radial passage (20) and cut off of the air supply to chamber (22).

Momentum of the piston (4) causes it to continue moving to withdraw the piston stem (15) from the piston stem bush (10). Once this point is reached chamber (22) will exhaust via the bore (17) through the drill bit (5).

A further annular recess (23) is provided in the wall of the bore (16) through the piston (4). This recess (23) connects the bore (1 6) through radial passage (24) to slots (25) in the wall of the piston (4) to effect a second path of fluid supply to the working components of the assembly. These slots (25) extend from the inner back head of the piston (4) partway down its length to enable communication between the recess (23) in the wall of the bore (1 6) and a drive chamber (27) formed between the back head (2), piston (4) and wear sleeve inner wall. The continued upward movement of the piston (4) places the recess (23) in communication with the ports (18) and simultaneously seals the drive chamber (27) as the recess (23) moves over the control rod (6). Further travel of piston (4) opens passage (24) and slots (25) to pressurise the chamber (27). This drives the piston (4) to strike the drill bit (5) and towards the end of the impact stroke, the air is exhausted from chamber (27) via slots (25) through passage (24) and piston bore (1 6), through bit bore (17) to atmosphere. The downward travel of the piston (4) places the recess (19) back into alignment with ports (18), air is again introduced into the return chamber (22) and the cycle repeated. The assembly thus provides for the operation of the hammer drill (1 ) to generate return and drive strokes of the reciprocating piston (4)

When the hammer (1 ) is lifted from bottom of the drill hole, the bit (5) moves out of the housing and rests against a retaining ring (33). In this condition, the bit (5) has uncovered radial outlets at the bottom of longitudinal vents (1 1 ) in the piston stem bush (10) and places the return chamber (22) in direct communication with the bore (17). Piston (4) follows bit (5) and ports (18) are uncovered into chamber (27) from where compressed air will flow via slots (25), through passage (24) and piston bore (1 6), through bit bore (17) and to the atmosphere. The drill hammer (1 ) assumes a flushing mode which brings the piston (4) to a halt until the bit (5) is again pressed into the housing to cover the vents (1 1 ).

A working area for opening of the check valve (7) is provided on the face (34) of the valve closure (26). This area is exposed to the inlet passage (8) of the control rod (6). During operation of the hammer (1 ), an area on the closure (26) opposite to the face (34) is exposed to a low pressure environment at a free end of the control rod (6). The pressure is that of the exhaust fluid, inside the piston bore (1 6). Whereas, the opening area (34) is constantly exposed to supply pressure in the inlet passage (8). The arrangement serves to maintain the check valve (7) in an open condition for as long as there is supply pressure in the inlet passage (8). This prevents valve oscillation due to cessation or fluctuation of fluid flow into the assembly. When air pressure acting on surface (34) is shut off, the hydrostatic pressure caused by the water pushing up in the borehole will act on the surface area opposite to area (34) and will cause the check valve (26) to shut thereby maintaining a pressure in the hammer to prevent water and mud from entering the tool. When there is no water in the borehole, the check valve will shut under spring tension from spring (9).

Referring to Figure 2, a control rod (41 ) with an alternative embodiment of a check valve (42) is shown. A pair of radial ports provide outlet openings (43) from the control rod (41 ). An internal annular rib (44) is located on a sidewall (45) inside the rod (41 ). The rib (44) is provided adjacent the ports (43), inside a supply passage (46) which leads from an inlet to the control rod (41 ). A shoulder on the rib provides a stop (47) for a valve closure (48). The closure (48) has an opening face (49) providing a working area. The periphery of the face (49) locates against the shoulder (47) when the valve (42) is closed.

A spigot (50) extends to the opposite side of the closure (48). The spigot (50) is located in a compression spring (51 ) which acts to close the valve (42). The opposite end of the spring (51 ) is located against a plug (52). The plug (52) is secured into an open, free end of the rod (41 ) through screwthreaded engagement.

A chamber for movement of the valve closure (48) is thus provided between the rib (44) and the plug (52). An axial opening (53) in the plug (52) places the portion of the chamber adjacent the spigot (50) in communication with the exhaust pressure environment of the piston bore.

A person skilled in the art will appreciate that a number of variations can be made to features of the described embodiments without departing from the scope of the invention. Other fluid supply pathways to the drive and return chambers as well as exhaust pathways may be used. The hammer may be provided with a foot valve tube. Also the check valve assembly may be made in accordance with various designs and configurations.