The present invention refers to a down-the-hole drilling machine including a piston case, a pressure medium activated piston being reciprocably movable against a drill bit, said piston being arranged to control the pressure medium flow in- the machine in cooperation with a control valve located up-stream of the the piston, said piston having portions of different diameter and at least one channel for pressure medium. A down-the-hole drilling machine of this type is previously known from WO 89/00638.

Previously down-the-hole drilling machines have chiefly been of pneumatic type. One disadvantage of such machines is that the required power for a certain penetration rate is high. The large amounts of air required give rise to a high air speed and consequently the exterior of the machine is subjected to a heavy blasting due to the cuttings that are transported out of the drilling hole. Further, to achieve dust binding it is often required that water is mixed with the activating air. This also makes the equipment more complicated and more expensive, and in addition results in considerably decreased efficiency.

Consequently the aim is more and more to switch to down-the- hole drilling machines of hydraulic type, the activating medium normally being water, possibly with some additive for lubrication. It has, however, been normal to design the piston with one or more waists; said design has complicated the manufacture, mounting and sealing since parted sleeves have to be used. Furthermore, complicated flow ways are often used resulting in large losses of pressure. In the known down-the-hole drilling machine mentioned above it is an aim to have pressure medium only in the upper part of the machine and to discharge used pressure medium via a central channel in the piston. Pressurizing in connection with a stroke takes place via a complicated flow way and

furthermore parted sleeves around the piston are used as a consequence of waists upon the piston.

The aim of the present invention is to achieve a simpler and more efficient down-the-hole drilling machine than before, said machine having a more favourable design of the piston and an improved flow of the pressure medium.

This aim is achieved according to the invention in that the piston, seen from the control valve, has a first, second and third piston portion with consecutively increasing diameter, and in that a fourth piston portion having smaller diameter than the third piston portion is connected to said third piston, and in that there are corresponding recesses in the piston case, said piston portions being sealingly movable in said recesses and delimiting chambers adapted to be connected in a predetermined order to an inlet and an outlet.

The first piston portion has a first pressure surface that together with a second pressure surface of the second piston portion has a larger area than a third pressure surface of the third piston portion, said third pressure surface facing in the opposite direction and having a larger area than the first pressure surface. In the striking position of the piston all three pressure surfaces are pressurized while in the return position only the first and the third pressure surfaces are pressurized.

By this design a simple piston is achieved, said piston being easy to manufacture and to mount. Furthermore, a simple structure of the piston case is possible as parted sleeves are not necessary. Still a further advantage is that the flow ways for subjecting pressure upon the piston can be made simple, whereby the losses of pressure are reduced and a faster pressurizing is possible.

Further features and benefits of the invention will become

apparent from the accompanying description and claims.

Below the invention is explained in detail with reference to an embodiment disclosed on the accompanying drawings, where

Fig.l shows a principle sketch of a down-the-hole drilling equipment including a down-the-hole drilling machine according to the present invention,

Fig.2 shows a side view of a piston of a down-the-hole drilling machine according to the present invention,

Fig.3 shows a section through the portion III in Fig.l of a down-the-hole drilling machine according to the present invention in striking position,

Fig.4 shows the down-the-hole drilling machine according to Fig.3 in flushing position, and

Fig.5 shows the control valve of the down-the-hole drilling machine according to Figs.3 and 4 in returning position.

A down-the-hole drilling equipment 1 as shown in Fig.l includes a down-the-hole drilling machine 2 designed in accordance with the invention. Said down-the-hole drilling machine 2 is via an accumulator 3 and a number of drill tubes 4 connected to a rotary engine 5. Via a swivel 6 at the rotary engine 5 pressure medium is supplied for activating the down-the-hole drilling machine 2. The couplings between the drilling tubes as well as the rest of the couplings are preferably provided with sealings, e.g. in the shape of O-rings. A hydraulic medium, preferably water, is used as pressure medium, possibly with certain additives, e.g. lubricants. The used pressure is preferably in the interval 120-200 bars but it might possibly be somewhat higher.

The accumulator 3 is located directly above the down-the-

hole drilling machine 2 having a drill bit 7, said accumulator 3 being adapted to produce an extra pressure medium flow during some milliseconds when the piston accelerates downwards. During the return movement of the piston an extra amount of pressure medium is again stored in the accumulator.

The general shape of a piston 8 of a down-the-hole drilling machine according to the invention can be learnt from Fig.2. Seen from the top a first piston portion 9, a second piston portion 10 and a third piston portion 11 has consecutively increasing diameter. Below the third piston portion 11 there is a fourth piston portion 12 having a diameter that is less than the diameter of the third piston portion, the length of said third piston portion preferably being at least equal to the total length of the three other piston portions 9-11. With its lower portion the fourth piston portion is adapted to strike against the shank of the drill bit 7.

The first piston portion 9 has a an upwardly directed first pressure surface 14 that together with an upwardly directed, annular second pressure surface 15 of the second piston portion 10 has a larger area than a downwardly directed, annular pressure surface 16 of the third piston portion 11. The third pressure surface 16 has a larger area than the first pressure surface 14. The importance of this will be evident from below.

According to Fig.3 a down-the-hole drilling machine 2 according to the invention is provided with a piston case 17 having recesses corresponding to the different piston portions 9-12, said piston portions being sealingly movable in said recesses. The first pressure surface 14 delimits a first chamber 18 in the recess for the first piston portion 9. Besides, the second pressure surface 15 delimits a second chamber 19 in the recess for the second piston portion 10, and the third pressure surface delimits a third chamber 20 in the recess for the third piston portion 11. Additionally,

in the recess for the third piston portion 11 there is a fourth chamber 21 surrounding the second piston portion 10.

In the piston 8 there is a first channel 22 adapted for pressure medium. Said channel 22 extends from the free end of the first piston portion 9 through the first, second and third piston portion and is terminated by a number of transverse channels 23 that emerge in the envelope surface of the fourth piston portion 12 close to the third piston portion 11.

At the top of the piston case 17 there is a pressure medium inlet 24 that via a control valve 25 is constantly connected with the first chamber 18. A valve body 27 is included in the control valve 25, said valve body 27 being forced upwardly by a spring 26 and displaceable between an upper and a lower position. An operating channel 28, controlled by the control valve, emerges in the second chamber 19. A flushing channel 29 emanates from the control valve 25, is, via an opening 30, in connection with the fourth chamber 21, and emerges in a discharge channel 31 at the bottom of the fourth piston portion 12 of the piston 8.

A pilot channel 32, adapted to be affected by the control valve 25, emerges via an opening 33 in the recess of the third piston portion 11.

Above only one operating channel 28, one flushing channel 29 and one pilot channel 32 have been mentioned but normally there are several channels of each type distributed circumferentially in a proper way.

The down-the-hole drilling machine according to the invention functions in the following way:

In Fig.3 the piston 8 is at the end of its downward striking movement. Due to the fact that the pilot channel 32 is pressurized the valve body 27 is kept in its upper

position, and hence pressure medium from the inlet 24 can flow not only to the first chamber 18 but also to the second chamber 19 via the operating channel 28, according to the arrow 34. Hereby, the first pressure surface 14 and the second pressure surface 15 are subjected to pressure. Via the channel 22 and the third chamber 20 also the third pressure surface 16 is subjected to pressure. Due to the previously mentioned area ratio between the three pressure surfaces 14, 15 and 16 the piston 8 is now forced downwards.

hen the piston 8 has reached its position as shown in Fig.3 the third piston portion 11 uncovers the opening 33 of the pilot channel 32 that hence is decompressed via the fourth chamber 21, the opening 30 and the flushing channel 29. The result is that the control valve 25 shifts when the valve body 27 through action from the pressure medium is displaced to its lower position, see Fig.5, whereby the pressurizing of the second chamber 19 and hence the pressure upon the second pressure surface 15 cease. Since the first pressure surface 14 and the third pressure surface 16 still are subjected to pressure, and since the third pressure surface 16 has a larger area than the first pressure surface 14 the piston now returns upwards. Thus the second chamber 19 is emptied via the operating channel 28 that now is connected with the flushing channel 29 (see Fig.5).

When the third piston portion 11 on its way upwards has bypassed the opening 33 of the pilot channel 32, said channel 32 is pressurized via the third chamber 20. This results in the valve body 27, counteracting the force of the downwards flowing pressure medium from the inlet 24 and supported by the spring 26, being forced upwards to its upper final position, as shown in Fig.3. Hereby, the piston 8 turns around and starts a new striking movement that results in drilling by repeated impacts upon the drill bit 7.

In the position of the piston 8, shown in Fig.4, the drill

bit 7 is not in contact with the rock. This position is achieved when the whole device is raised in order to connect new drilling tubes. The piston 8 has now been displaced so far downwards that the transverse channels connected to the channel 22 of the piston 8 have been closed. Thus no upwardly directed force is acting upon the piston 8. Via the first chamber 18 and the operating channel 28, pressure medium from the inlet 24 is now allowed to flow into the second chamber 19 and, via the fourth chamber 21 and the opening 30, from there out into the flushing channel 29 and further on to the discharge channel 31 for flushing at the drill bit 7.

It is possible that the first piston portion 9 might reach the recess constituting the first chamber 18, whereby all flushing takes place via the operating channel 28 and continues into the second chamber 19 and the fourth chamber 21. Since the pressure medium in this case is not able to generate any pressure the valve body 27 will only be subjected to the force from the spring 26 and is thereby kept in its shown upper final position although the pilot channel 32 all the time is without pressure.

The flushing position shown in Fig.4 results in a simple and effective flushing without complicated flushing ways for the flushing medium. Also in the operating position of the piston 8 a simple flow of the pressure medium is achieved, this being beneficial with respect to efficiency.

Due to the chosen "waistless" piston design it is possible to keep the piston case very simple as there is no need for parted details to engage any piston waists.

The piston case 17 suitably consists of a tube 35 into which a lower part 36 is inserted with the fourth portion 12 sliding in said lower part 36. Connected with the lower part 36 is an intermediate part 37 with recesses for the third piston portion 11 and the second piston portion 10.

Connected with the intermediate part 37 is an upper part 38 that holds the control valve 25 and a recess for the first piston portion 9. These three parts 36, 37, 38 are mutually sealed and are kept in place in the tube 35 via a lower holder 39 and an upper holder 40.

In the wall of the first chamber 18 there is a first sealing 41 and in the wall of the second chamber 19 there is a second sealing 42 intended for the first piston portion 9 and the second piston portion 10, resp.. These two sealings are so designed that they are not moved out of position when the piston 8 returns to operating position from the position according to Fig.4. In order to further improve the sealing between the piston 8 and the piston case 17 the envelope surface of the piston portion 11 can be provided with a number of circumferential grooves.

The end positions of the piston 8 are determined by the location of the opening 33, said opening 33 pressurizing or depressurizing the pilot channel 32. Besides, in order to achieve the aimed impact rate the control valve 25 must be so dimensioned as to effect a change in the flow at the right moment.

According to the invention it has proved possible with a machine diameter of three inches and an operating pressure of 150-200 bars to achieve a drilling rate of about 0,6-1,0 m/min having an impact frequence of about 3000 impacts/min. The power required was about 45 kW at a pressure medium flow of about 1201/min. Corresponding drilling rates with a pneumatic machine require a considerably higher power.