Pneumatic down-the-hole drill

Abstract

A pneumatic down-the-hole (DTH) drill having a frame, and a pneumatic percussion piston that moves in a reciprocating manner as pressurized air is fed into the DTH-drill and strikes a tool in the front end of the frame is mounted movably in the longitudinal direction of the frame. A feed channel for feeding compressed air to the DTH-drill shoulders in the frame and in the percussion piston for controlling compressed air to provide an impact movement. In the rear end of the frame the DTH drill has a combustion chamber and an acceleration piston that is arranged to push the percussion piston during the impact movement for a portion of the percussion piston travel, and means for feeding combustion air and fuel into the combustion chamber, whereby the percussion piston is arranged to push the acceleration piston after the impact into the combustion chamber and to compress the air in the combustion chamber prior to feeding the fuel into the combustion chamber.

1 . A pneumatic down-the-hole drill having a frame and inside the frame a pneumatic percussion piston that moves in a reciprocating manner in a longitudinal direction of the frame when pressurized air is fed into the down-the-hole drill and at the end of its impact movement strikes a tool that is in the front end of the frame and mounted movably in the longitudinal direction of the frame, a feed channel for feeding pressurized air between the frame and the percussion piston, and shoulders in the frame and in the percussion piston to guide the pressurized air to provide the impact movement the down-the-hole drill comprising:
 a combustion chamber at a rear end of the frame;   a separate acceleration piston disposed in the combustion chamber between the frame and the percussion piston, moving in the longitudinal direction of the frame and operating by fuel combustion in the combustion chamber, which acceleration piston is arranged to push the percussion piston during the impact movement only for a portion of the percussion piston travel;   an air channel for feeding combustion air into the combustion chamber;   means for injecting fuel into the combustion chamber; and   an exhaust channel for exhausting combustion gases from the combustion chamber, whereby the percussion piston is arranged to push the acceleration piston by means of pressurized air back into the combustion chamber after each impact movement and thus to compress the air in the combustion chamber prior to feeding fuel into the combustion chamber.   
     
     
         2 . The down-the-hole drill of  claim 1 , wherein the acceleration piston is arranged to close the exhaust channel before penetrating into the combustion chamber and to open the exhaust channel before its forward motion ends. 
     
     
         3 . The down-the-hole drill of  claim 1 , wherein the air channel includes a blocking valve that opens as the pressure in the combustion chamber drops below a predetermined pressure level and that from the air channel, the blocking valve being open, pressurized air is arranged to flow for flushing the combustion chamber and for filling the combustion chamber with fresh combustion air. 
     
     
         4 . The down-the-hole drill of  claim 1 , wherein the acceleration piston includes a stop shoulder and at the same point in the frame, in an axial direction, a counterpart shoulder, so that as the shoulders meet the acceleration piston stops before the percussion piston strikes the tool. 
     
     
         5 . The down-the-hole drill of  claim 4 , wherein the percussion piston and the acceleration piston are for a portion of their length closely nested so that no clearance opens at any stage between them, the percussion piston including in its upper end a blocking shoulder, which as the pistons move in the impact direction, before the stop shoulder of the acceleration piston hits the counterpart shoulder of the frame, together with the counterpart shoulder closes a connection from a space between the stop shoulder and the counterpart shoulder in such a manner that a damping chamber is provided and while the acceleration piston moves onwards its volume decreases and the pressure of the compressed air contained therein increases and decelerates the movement of the acceleration piston, and correspondingly during the reversing movement of the percussion piston pushes the acceleration piston towards the combustion chamber before the blocking shoulder opens a connection from the damping chamber so that the percussion piston may displace air from the damping chamber and reach the acceleration piston for pushing it back into the combustion chamber. 
     
     
         6 . The down-the-hole drill of  claim 1 , further comprising timing equipment for timing fuel feed in relation to the position of the acceleration piston. 
     
     
         7 . The down-the-hole drill of  claim 1 , wherein the percussion piston includes a working shoulder having a tool-side surface area that is larger than a surface facing the acceleration piston, the frame including an auxiliary shoulder so that in the rear position of the percussion piston, with the shoulders being aligned, the pressure of the compressed air only acts on the surface facing the acceleration piston producing a force that pushes the percussion piston towards the tool, and in the front position of the percussion piston, with the shoulders being apart, the pressure of the compressed air acts on both surfaces producing a force that pushes the percussion piston away from the tool. 
     
     
         8 . The down-the-hole drill of of  claim 7 , wherein in the percussion piston the surface area of the surfaces which are facing the front end of the frame and through which the pressurized air pushes the percussion piston and the acceleration piston towards the combustion chamber, is larger than the surface area of the acceleration piston facing the combustion chamber. 
     
     
         9 . A pneumatic down-the-hole drill comprising:
 a frame;   a pneumatic percussion piston movably mounted in a longitudinal direction within the frame, the pneumatic percussion piston reciprocating along the longitudinal direction of the frame to impact a tool that is disposed at a front end of the frame;   a feed channel to guide pressurized air between the frame and the percussion piston and shoulders in the frame and in the percussion piston;   a combustion chamber disposed at a rear end of the frame;   an acceleration piston disposed in the combustion chamber between the frame and the percussion piston and moving in the longitudinal direction of the frame;   an air channel for feeding combustion air into the combustion chamber;   means for injecting fuel into the combustion chamber; and   an exhaust channel to exhaust combustion gases from the combustion chamber.   
     
     
         10 . The down-the-hole drill of  claim 9 , wherein the acceleration piston is arranged to close the exhaust channel before penetrating into the combustion chamber and to open the exhaust channel. 
     
     
         11 . The down-the-hole drill of  claim 9 , wherein the air channel includes a blocking valve that opens as the pressure in the combustion chamber drops below a predetermined pressure allowing pressurized air to flush the combustion chamber and fill the combustion chamber with fresh combustion air. 
     
     
         12 . The down-the-hole drill of  claim 9 , wherein the acceleration piston includes a stop shoulder and at the same point in the frame, in an axial direction, a counterpart shoulder to stop the acceleration piston before the percussion piston strikes the tool. 
     
     
         13 . The down-the-hole drill of  claim 12 , wherein the percussion piston and the acceleration piston are for a portion of their length closely nested so that no clearance exists therebetween, the percussion piston including in its upper end a blocking shoulder, which as the pistons move in the impact direction, before the stop shoulder of the acceleration piston hits the counterpart shoulder of the frame, together with the counterpart shoulder closes a connection from a space between the stop shoulder and the counterpart shoulder to provide a damping chamber. 
     
     
         14 . The down-the-hole drill of  claim 9 , further comprising timing equipment to time fuel feed in relation to the position of the acceleration piston. 
     
     
         15 . The down-the-hole drill of  claim 9 , wherein the percussion piston includes a working shoulder having a tool-side surface area that is larger than a surface facing the acceleration piston. 
     
     
         16 . The down-the-hole drill of  claim 15 , wherein in the percussion piston the surface area of the surfaces which are facing the front end of the frame and through which the pressurized air pushes the percussion piston and the acceleration piston towards the combustion chamber, is larger than the surface area of the acceleration piston facing the combustion chamber.