It is previously known at downward drilling with a compressible fluid driven down-the-hole drilling machine to achieve flushing only of the drill hole through lifting of the drilling machine, see US-A-4446929. Through this the drill bit sinks so that the hammer piston of the drilling machine can pass the normal impact position. The hammer piston is thereby catched in a forward position so that only flushing is achieved until the drilling machine is lowered again and drilling is resumed. In order to avoid"back hammering"the rear surface is loaded with compressible fluid to keep the piston against the drill bit. It is doubtful if this method works at upward drilling without damages to the retaining ring preventing the drill bit from leaving the drilling machine. This prior art machine does not work with a liquid driving medium since it requires a compressible fluid in order to function.

The present invention, which is defined in the subsequent claim, aims at achieving a liquid driven impact device which temporarily makes continuous flushing of the drill hole possible at upward drilling.

An embodiment of the invention is described below with reference to the accompanying drawings in which fig 1 shows the middle part of a liquid driven down-the-hole drilling machine with the hammer piston in impact position. Fig 2 shows the front part of the liquid driven down-the-hole drilling machine with the hammer piston in impact position. Fig 3 shows the middle part of the liquid driven down-the-hole drilling machine with the hammer piston in flushing position. Fig 4 shows the front part of the liquid driven down-the-hole drilling machine with the hammer piston in flushing position.

The embodiment of the invention shown in the drawings is a liquid driven, e. g. water driven, down-the-hole drilling machine comprising a housing in form of a tube 1 and a split lining 11,12,13 positioned therein and in which a hammer piston 2 is movable to-and-fro for exerting a drill tool 3 to impacts. The hammer piston is journalled in bearings 14,15 which can be made as shown in WO 96/19324. The drill tool 3, which on the front surface is provided with hard metal buttons 16, is connected to housing 1 by means of a splined coupling 17 and prevented from falling out of the housing 1 by a ring 18 which for mounting reasons is divided. An elastic sleeve 19 is arranged about the hammer piston 2 in order to prevent drill cuttings from entering into the impact device at upward drilling. At downward drilling the sleeve 19 prevents liquid in the drill hole from entering in to the impact device. In the rear part of the tube) a valve housing 21,22,23 comprising a valve body 9 is arranged. Behind the valve housing there is a usual, not shown, threaded rear piece for connection to a drill tube string. The drill tube string transfers rotation from a rotary motor to the tube 1 and thus via the splined coupling 17 to the drill tool 3. Furthermore, driving liquid is supplied via the drill tube string to the impact device for driving this and for flushing the drill hole. In the drawings this is marked by the pressure source 5. The valve body 9 cooperates alternately with valve seats 24 and 25.

When the valve body is in the position shown in fig 1 pressure liquid is supplied to a first surface 4 on the hammer piston 2 from pressure source 5 and via a conduit 26, here drawn outside the machine, to a second surface 7 on the hammer piston 2. When the valve body 9 is in its opposite end position the first surface 4 is via a connection 29 connected to a reduced pressure in the flushing channel 6. The flushing channel 6 is connected to the flushing channels 31,32 in the drill tool 3. Surfaces 4 and 7 are situated on a part 28 of the hammer piston 2 having a larger diameter than the rest of the hammer piston. The pressure on the second surface 7 during drilling continuously loads the hammer piston 2 in a direction away from the drill tool 3. The hammer piston 2 is around its periphery provided with a number of channels 8 and a number of slots 27. With the position of the hammer piston 2 shown in fig 1 the room 33 at the valve body 9 is connected via conduit 34, channels 8 and connection 35 to flushing channel 6. Through this the pressure in room 33 is lowered. Valve body 9 is displaced to cooperation with valve seat 25 by the pressure in the continuously pressurized room 36. The supply of pressure liquid to the first surface 4 is disconnected. The first surface 4 is instead connected to the reduced pressure in the flushing channel 6. The pressure on the second surface 7 then drives the hammer piston 2 to the right in fig 1. When the hammer piston 2 has reached its right end position slots 27 connect conduit 26 with conduit 34. Through this room 33 is pressurized so that the valve body 9 is displaced to the position shown in fig 1. The first surface 4 is again supplied with pressure liquid through which the hammer piston 2 makes a working stroke to the impact position shown in fig 1.

In figs 3 and 4 the impact device is shown in the position which the hammer piston takes when the feed force has ceased, e. g. through backing of the down-the-hole drilling machine from the drill hole front. Since the drill tool 3 can be pushed out somewhat from the drilling machine by the pressure in chamber 41, which is higher than the pressure in the drill hole, the hammer piston 2 passes the impact position, which is shown in fig 1. At upward drilling the movement past the impact position is assisted by the action of the hammer piston 2. When the hammer piston 2 reaches the position shown in fig 3 pressure liquid is supplied from conduit 26 via the channels 8 to conduit 34. Through this room 33 is pressurized so that the valve body is locked in the position shown in fig 3. Pressure liquid is herewith supplied to the first surface 4 which through this pushes the hammer piston 2 to the left in fig 3 and locks it in this position. Because both the first surface 4 and the room 33 are pressurized the hammer piston remains in the shown position until the down-the-hole drilling machine again is fed toward the hole bottom so that drilling is resumed. In the position shown in fig 3 pressure liquid is supplied besides to the valve body 9 also via connection 35 to flushing channel 6 and from there to flushing channels 31 and 32 in the drill tool 3.