The present invention concerns a damping device for a fluid-driven down-the-hole drill according to the introduction to claim 1 . The invention concerns also a fluid-driven down- the-hole drill comprising a damping device according to the introduction to claim 12.

The drilling of holes in rock takes place using percussion drilling. During down-the- hole drilling using what is known as a "fluid-driven DTH" (where "DTH" is an abbreviation for "down-the-hole") drill, not only rotation but also axial blows onto a tool in the form of what is known as a "drill bit" take place by means of a combined impact machine and drill that is located in the borehole. The down-the-hole drill is attached at its rear end at a drill support or drill string that may be constituted by a flexible unit known as a "coil" or by a number of connected rigid sections of drill rod. A machine housing that is a component of the down-the- hole drill includes an impact mechanism that is driven by a fluid under pressure, normally consisting of water or a bentonite emulsion, that is supplied through an internal channel in the drill string from a source of power in the form of what is known as a "drilling rig" located outside of the borehole, at the surface. A hydraulically driven hammer piston produces an impact onto the rear part or neck of the drill bit. The impact generates a shock wave (energy) that generates through the drill bit such a large force that the rock in front of the drill bit is crushed. The crushed rock, known as "drilling cuttings", is transported out of the borehole with consumed driving fluid for the drill.

It is important when drilling a hole that the drill bit has as good a contact with the rock as possible, and the drill bit is for this reason continuously pressed forwards against the rock by means of the drilling rig with a pre-determined axial force of feeding. Through the influence of the force of feeding, the drill bit insert, which is manufactured from, for example, cemented carbide and is mounted on the part of the drill bit located at its front, what is known as the "drill head", comes into forceful contact with the rock. The force of feeding is transferred from the drill string to the machine housing of the down-the-hole drill and onwards to the drill bit. When the drill bit is pushed forwards by the impact mechanism, the rock is crushed through the production of impact energy, after which drilling of the hole is carried out through rotation of the drill bit in combination with the forwards force of feeding. The drill bit is rotated with the aid of a rotation motor and is mounted in a chuck at the forward end of the machine housing in a manner that allows rotation. The drill bit that is mounted in the said chuck is controlled in its reciprocating motion in the axial direction with the aid of a spline mounting or a splined connector.

As a consequence of the impact energy that is transferred from the drill bit to the rock when it is crushed, a force of recoil is created, i.e. a backwards-directed pressure reflection from the rock back to the drill bit. The pressure reflection contains harmful reactive energy that is led from the drill bit and further backwards through the down-the-hole drill. In order to reduce as far as is possible the harmful effects of the said reactive energy, drills of this type are normally equipped with a damping device. The pressure reflections that are led back may, under certain working conditions, be so powerful that they cause damage to the drilling rig. Such damage can be avoided through reducing the power supplied to the down- the-hole drill, which means that the capacity of the drill is reduced.

In order to absorb or hydraulically dampen as much as possible the pressure reflections that arise, modern down-the-hole drills are equipped with damping devices that include one or several damping pistons that can be displaced in the axial direction, where each damping piston operates in a pressure chamber with hydraulic fluid.

Also arrangements are known in which the function of the damping piston is not only to transfer the force of feeding from the machine housing, the drill bit, and onwards to the rock, but also to reduce the reflecting pressure forces from the rock towards the drill bit, the machine housing and further backwards. The compartment behind the damping piston functions in such arrangements not only as damping chamber but also as pressure chamber in order to achieve the force of feeding. The damping chamber may be connected to a shock-absorbing hydraulic circuit that provides a damping facility that protects the drill. In what is known as a "double damping device", the damping piston has two pressurised areas that are connected with each other through a constriction gap. One of these areas is located inside an enclosed volume, which functions as a constriction damper, and which converts through a "tandem" effect reflected energy to heat in the hydraulic fluid during its passage through the constriction gap.

One disadvantage of prior art damping devices for rock drilling machines is that they result in, as a consequence of the relative location of the damping chamber far to the rear in the drill, harmful forces of recoil being allowed to pass through sensitive equipment such as the impact mechanism and rotation motor for the tool. One further disadvantage is the limited ability of prior art arrangements to absorb axial loads, which is a result principally of the limited shock-absorbing cross-sectional area in a radial plane transverse to the main shaft of the drill bit of the pressurised fluid cushion that is formed. It should in this part should be realised that the ability of the pressurised fluid cushion to absorb the forces of recoil that arise, to function as axial bearings for the rotation of the tool and to achieve the required force of feeding against the rock, depends on its possibilities to form and use in an efficient manner the cross-sectional area of the pressurised fluid cushion.

The drilling capacity and the maximum power at which prior art rock drills can work are, consequently, limited to a level that depends on the risk for damage from shaking when forces of recoil arise. By reducing the said forces as far forwards in the rock drill as possible, and in this way also as close as possible to the working area in front of the drill bit, where the harmful pressure reflections are generated, it is possible to avoid the forces being passed backwards and reaching sensitive equipment such as the impact mechanism and a rotation motor.

A damping device for a rock drill is known from EP 0856637 A1 . In order to reduce the recoil, a ring-shaped damping chamber is arranged in a part of the drill that is located relatively far to the rear. To be more precise, the damping chamber is located behind not only the rotation motor but also a rear end section or neck of the drill bit against which a hammer piston that is a component of an impact mechanism acts. The damping chamber forms a pressurised fluid cushion that is limited in the axial direction between a pair of separate damping pistons arranged at the rear part of the drill, of which a forwards, when considered in the axial direction, damping piston that can be displaced is located in front of a second damping piston, which is stationary. The damping chamber is limited in the radial direction by the machine housing of the impact mechanism for the hammer piston.

A damping device is known from SE 392 830 that corresponds essentially to the arrangement described above with respect to the location of the ring-shaped damping chamber relatively far to the rear in the drill. In this prior art arrangement, the pressurised fluid cushion that is enclosed within the damping chamber is used as axial bearings for the rotation of the drill bit.

A first purpose of the present invention, therefore, is to achieve a damping device for a down-the-hole drill that is simpler and more efficient than prior art damping devices. A second purpose is to achieve a damping device that can be used also in order to achieve a forwardly directed driving force for the drill bit from the machine housing twoards the rock. A third purpose of the invention is to achieve a down-the-hole drill that is efficient and that demonstrates high capacity through it being possible to operate the drill at higher power than prior art down-the-hole drills.

These purposes of the invention are achieved through a damping device for a fluid- driven down-the-hole drill that demonstrates the distinctive features and characteristics specified in claim 1 , and a fluid-driven down-the-hole drill that demonstrates the distinctive features and characteristics that are specified in claim 12. Further advantages of the invention are made clear by the non-independent claims.

The use of a directly driven drill bit, i.e. a drill bit that is rotated through the intermittent pressurisation with driving fluid by a rotation motor, where the said rotation motor is of the type known as a "wing motor" or "vane motor", and whose rotor body is arranged for the direct operation of the neck of the drill bit, in combination with a damping device with a damping piston that is located in front of not only the rotation motor but also the rear end or neck of the drill bit, offers a number of advantages. The first is that the damping piston and its associated ring-shaped damping chamber can be placed relatively far forwards in the drill, i.e. in front of not only the rotation motor but also the impact mechanism and the neck at the drill bit against which the piston of the impact mechanism acts.

The second is that a ring-shaped pressure chamber with a relatively large effective driving area can be obtained through the use of the radial extent between the inner jacket of a surrounding element of an outer tube that is a component of the down-the-hole drill and that is located at the front and the peripheral outer surface of a surrounding element of the neck of the drill bit.

The third is that the damping piston and the pressure chamber can be limited in existing parts in the machine housing, between the spline connection of the chuck to the drill bit and a rear end piece of the outer tube that, attached at the forward end of the machine housing, surrounds the chuck, whereby the force that acts on axial bearings that are a component of the chuck can be reduced by balancing not only the force of feeding from the machine housing towards the rock that the pressurised medium in the damping chamber offers, but also the force of feeding at the drill bit that is led through the machine housing and is generated by the drilling rig.

The fourth is that, due to the fact that the outer tube that is located at the front is fixed to be stationary in the machine housing, i.e. the drill bit is mounted in bearings in the outer tube in a manner that allows rotation by direct action, feed lines, sensors and other components that are required to control flows of fluid to and from the pressure chamber behind the damping piston can be easily integrated not only into the tubular jacket of the machine housing but also into the outer tube that is located at the front of the machine housing.

The invention will be described below in more detail with reference to an embodiment that is shown in the attached drawings, of which:

Figure 1 shows a longitudinal section through a forward part of a down-the-hole drill according to the invention, whereby a drill bit that is a component of the down-the-hole drill is shown not only in a withdrawn condition, but also in an alternative protruding condition of a working stroke (crushing stroke) forwards against a rock surface in front of the drill bit,

Figure 2 shows a longitudinal section through a rear part of the down-the-hole drill according to Figure 1 ,

Figure 3 shows a cross-section through a rotation motor that is a component of the down-the-hole drill viewed along the line Ill-Ill in Figure 1 , and

Figure 4 shows an enlarged longitudinal section of a damping device according to the invention that is a component of the down-the-hole drill viewed within the marked region IV in Figure 1 , and in which drawing a hydraulic circuit diagram is schematically shown that explains in more detail the principle of the function of the damping device according to the invention.

The down-the-hole drill driven by pressurised fluid that is described below is based on the self-rotating type that is described in more detail in the Swedish patent application number 1450845-1 and to which reference is made below. The term "self-rotating" is here used to denote a down-the-hole drill of the type in which stepwise advance of the drill bit takes place through the influence of intermittent pressurisation of a rotation motor by driving fluid, preferably a motor of the type known as a "wing motor" or "vane motor", whose rotor body is arranged for the direct operation of the drill bit.

The fluid-driven down-the-hole drill 1 that is shown in Figures 1 and 2 includes a tubular machine housing 2 with an impact mechanism 3, a rotation motor 4 and a drill bit 5A that has a neck 5B at its rear end. The term "neck" is here used to generally denote the rear part of the drill that receives impact energy and that transfers rotation and force of feeding.

The part of the machine housing 2 that is located at the rear has a backing piece 6 that is intended to be connected to a drill support in the form of a drill string (not shown in the drawings) formed of connected rigid drill sections or a flexible unit known as a "coil". A driving fluid under pressure, such as water or a suspension of bentonite and water, is led from a source of power (not shown in the drawings) in the form of a drilling rig connected at the second end of the drill string through a central channel 7 in the drill string in order to drive the down-the-hole drill.

When a hole is drilled in the rock R a forwards-directed axial force of feeding F2, which is normally generated hydraulically, is applied to the drill bit 5A by the said drilling rig. The force of feeding F2 is led through the drill string and machine housing 2 forwards to the inserts of the drill bit 5A, which inserts are brought into contact with the rock R. When the drill bit 5A is pushed forwards by the impact mechanism 3 the rock R is crushed through the production of impact energy, after which drilling of the hole is carried out through the drill bit 5A being rotated by a certain amount under the influence of the forwards force of feeding F2. The work of drilling a hole in the rock is carried out through the steps described above being repeated. The two parallel force arrows F2 in Figure 1 denote the force of feeding of the drill bit 5A in the axial direction from the machine housing 2 towards the rock R, and the force arrow Er shows in a corresponding manner a subsequent force of recoil, i.e. the rearwards- directed pressure reflection from the drill bit towards the machine housing that arises after each working stroke of the drill bit.

The impact mechanism 3 is located at the rear end of the machine housing 2 and comprises a hammer piston 8 that is driven by the flow of fluid. The hammer piston 8 is designed as a shaft with an impact surface A and a penetrating passage 9. The flow of liquid is led through a valve 10 that alternately opens and closes the flow when the hammer piston 8 is displaced between a rear initial position and a forward end position in which the piston makes an impact against a rear end section 5C of the neck 5B of the drill bit. A forward and a rear pressure chamber 13A, 13B are placed under pressure and relaxed in an alternating manner during the axial motion of the hammer piston 8. The lower end position of the hammer piston 8 demonstrates a seating S against the bottom of which an outer impact surface A that is a component of the hammer piston 8 makes contact.

The drill bit 5A is attached in a chuck in the forward end of the machine housing 2 through its extended rear drill neck 5B, which has a lower diameter. The drill neck 5B demonstrates a penetrating axial channel 17. The hammer piston 8 demonstrates a piston surface, and is arranged to make an impact on the rear end section 5C of the drill neck 5B. The hammer piston 8 is connected to the flow of fluid, which through the valve 10 alternately fills and empties the said pressure chambers 13A, 13B with driving fluid under pressure. When the hammer piston 8 has completed its impact against the end section 5C of the drill neck 5B, the forward pressure chamber 13A is placed under low pressure, whereby the hammer piston is caused to carry out its return motion and return to its rear initial position. The drill neck 5B has a splined connection that fits into a chuck fastening 37, which gives the drill bit 5A a controlled reciprocating axial motion and makes it possible for it to rotate inside the machine housing 2, accompanying the rotation of the rotation motor 4.

Figure 3 shows in more detail the rotation motor 4, which is arranged for direct operation of the drill neck 5B. The rotation motor 4 is in this case of wing type or vane type, of the type that comprises a stator housing 22 and a cylindrical rotor body 23 surrounded by this stator housing, and a number of displaceable valved pistons 25 in radial guide slots 24 that run in ring-shaped grooves 26 in the stator housing, which grooves communicate with an inlet 27 and an outlet 28 for driving fluid. The rotation motor 4 is placed for direct operation of the drill neck 5B, between the end section 5C of the neck and the impact mechanism 3 in such a manner that the drill neck 5B runs axially through an opening in the centre of the rotor body of the rotation motor. The drill neck 5B is in this manner connected through splines or a splined connector 29 in order to allow the drill bit 5A to move in the axial direction relative to the rotation motor. When the rotor body 23 of the rotation motor 4 rotates, the drill neck 5B and thus also the drill bit 5A accompany it in its rotation.

The flow of fluid to the rotation motor 4 is started when the hammer piston 8 is located at its rear initial position, the position at which the hammer piston is located closest to the connection 6. When the flow of fluid under pressure reaches the hammer piston 8, the valve 10 is opened, whereby a part of the flow is led to the rotation motor 4, which rotates and takes with it the drill bit 5A into the rotation. A remaining part of the flow of liquid is led to the hammer piston 8, which impacts onto the drill neck 5B. When the hammer piston 8 leaves its rear initial position and moves towards its forward end position, the valve 10 closes a channel 25 that leads to a pressure chamber that is a component of the rotation motor 4, whereby the rotation motor stops. When the rotation motor 4 is stationary, the hammer piston 8 carries out its working stroke and impacts against the drill neck 5B, whereby the drill bit 5A impacts against the rock R. When the hammer piston 8 moves backwards in the direction towards its rear initial position, and reaches its rear initial position, the valve 10 is again opened and the flow is led to the rotation motor 4. Through alternate pressurisation of the rotation motor 4 through the valve 10 in this manner, coupled with the reciprocating impact motion of the hammer piston 8, an intermittent pressurisation of the rotation motor 4 is obtained, and in this way also stepwise advance or rotation of the drill neck 5B, and thus also the drill bit 5A, in pre-determined steps for the drilling of a hole in the rock R.

In order to, according to the present invention, reduce the harmful reactive energy denoted Er in Figure 1 that arises from the said pressure reflection, the down-the-hole drill is equipped with a damping device. This damping device has the task also to generate a forwards-directed axial driving force F1 , i.e. a force of feeding from the machine housing 2 towards the rock R. The damping device demonstrates also a pressurised fluid cushion that functions as axial bearings and that is enclosed between the drill bit and the machine housing, whereby the drill bit constitutes the rotatable part while the machine housing is stationary. The function of the said damping device will be described in more detail below.

With renewed consideration of Figure 1 , an outer tube 32 is attached at the forward part of the tubular machine housing 2, which, since it is stationary, can be said to form a forward part of the tubular machine housing 2, i.e. the hammer casing. An end sheath 35 is screwed onto the forward end of the outer tube 32 by a threaded connection. A chuck fastening 37 that is mounted in bearings in the outer tube 32 in a manner that allows rotation is equipped with a spline connection 38 that acts against the drill neck 5B and forms, together with a ring-shaped damping piston 40 that can be displaced in the axial direction, part of first axial bearings 41 A that comprise a hydraulically operating damping cushion with enclosed pressurised fluid. The said axial bearings 41 obtain support, through a surrounding impact surface 42 that is arranged in the damping piston 40, against a similar surrounding internal shoulder 43 of a ring-shaped rear end piece 44 of the outer tube 32, which end piece has an opening 45, through which the drill neck 5B extends. As is made clear by Figure 4, the damping piston 40 is connected in a manner that allows axial displacement to the chuck fastening 37 through a coupling 46 and is "floating", by which is meant that it can move forwards and backwards between a forward end position L1 and a rear end position L2. It should be realised that the said driver chuck fastening 37 rotates together with the drill bit 5A relative to the outer tube 32.

As is made clear by the enlargement of detail in Figure 1 , the chuck fastening 37 forms in this way part of a chuck that is surrounded by the outer tube 32 and that is mounted through second axial bearings 41 B in the outer tube in a manner that allows rotation together with the first axial bearings 41 A. Reference number 47 denotes a spiral spring that acts in the axial direction between the end sheath 35 and the second axial bearings 41 B. The term "chuck" is used below to generally denote the part or parts that constitute fixtures for a rotatable tool.

Figure 4 and the enlargement of detail in Figure 1 show in more detail how the axial bearings 41 are designed to form a double damping arrangement with concentrically located radial outer and inner ring-shaped chambers 48, 49 with pressurised piston surfaces A1 and A2, respectively, that are connected with each other through a constriction gap 50, whereby the radially outer pressure chamber 48 has a considerably larger operating piston surface than the radially inner pressure chamber 49, i.e. A1 > A2.

The outer and inner ring-shaped pressure chambers 48, 49 are limited between two parts that can be telescopically displaced one inside the other consisting of the facing ends of the said damping piston 40 and a rear end piece 44 of the outer tube 32 formed with a reduced internal diameter. In the radial direction, both the outer and inner pressure chambers 48, 49 are limited over the constriction gap 50 by the inner jacket of the outer tube 32 and the peripheral outer surface of the neck 5B of the drill bit 5A. As a consequence of the essentially radial combined extension of the outer and inner pressure chambers 48, 49, piston surfaces A1 , A2 are obtained that together offer an effective driving area not only in order to achieve a forwards-directed driving force, i.e. a force of feeding F1 towards the rock, but also in order to be able to function as axial bearings during the rotation of the drill bit 5A and to damp the rearwards-directed pressure reflection Er from the rock R that arises after each working stroke of the drill bit.

As is made most clear by Figure 4, the damping piston 40 has a forward end position denoted L1 and a rear end position denoted L2. When the damping piston 40 is located at the said forward end position L1 , the outer and inner pressure chambers 48, 49 demonstrate their maximum volume. The forward end position L1 of the damping piston 40 is limited through interaction between a ring-shaped protrusion 52 directed radially inwards from the inner jacket of the damping piston and a shoulder 53 formed at a transition to a part of the chuck fastening 37 with a greater diameter. The rear end position L2 of the damping piston 40 is limited through interaction between an axially protruding part with lower diameter in the form of a ring-shaped collar 54 at the damping piston 40 that is displaced telescopically into a ring-shaped shoulder 55 of the rear end piece 44 of the outer tube 32, which ring- shaped shoulder forms a seating, whereby the internal diameter of the shoulder is somewhat larger than the external diameter of the collar such that the ring-shaped constriction gap 50 described above is limited between the opposing sections of the parts, through which constriction gap the outer and inner ring-shaped pressure chambers 48, 49 communicate. An enclosed volume of hydraulic fluid is present in the radially inner pressure chamber 49, which fluid functions as constriction damper and converts energy from the rearwards-directed pressure reflection Er to heat in the hydraulic fluid during its passage through the constriction gap 50 to the outer pressure chamber 48. As a consequence of the differences in area between A1 and A2, the inner pressure chamber 49 with its smaller area A2 will be drained of its fluid first to the larger outer pressure chamber 48 with its area A1 , through the constriction gap 50.

In a similar manner, the two concentrically located radial outer and inner ring- shaped chambers 48, 49 with piston surfaces A1 and A2, respectively, offer a hydraulically generated forwards-directed force of feeding F1 that, through its pressurisation by hydraulic fluid, drives the drill bit 5A to contact with the rock R when a hole is being drilled.

Figure 4 illustrates further the function of the damping device with the aid of a hydraulic circuit diagram. It should be understood that the hydraulic components that are described below are integrated into appropriately designed channels and indentations in the machine housing 2 of the down-the-hole hammer drill and in the outer tube 32. The source 60 generally denotes a driving fluid under pressure intended to be used as hydraulic medium in the pressure chambers 48, 49 of the damping device. The driving fluid is intended to be led from the central channel 7 in which it should be realised that driving fluid can be obtained from any appropriate feed channel for driving fluid under pressure that is located close to the forward outer tube 32 in which the axially displaceable damping piston 40 is integrated.

The damping device is designed to control and monitor a damping pressure D1 pr, i.e. the pressure that is to act on the damping piston 40 in order for it to function as damping piston; and a pressure F1 pr in order for it to function as feed piston and to feed the drill bit forwards to its contact with the rock R. In this embodiment of the invention, the feed pressure F1 pr and the damping pressure D1 pr are equal.

An accumulator 61 that has a pre-determined charging pressure is connected to the source 60 through a pressure-reduction valve 62 or similar valve device that can maintain a constant pressure in the outlet and break the connection with the inlet when the pressure in the outlet exceeds a pre-determined value. The accumulator 61 and the chambers 48, 49 of the damping device are charged with driving fluid through the said pressure-reduction valve 62. The accumulator 61 functions as a spring and, depending on the selected choice of properties of the accumulator, a pre-determined constant pressure is maintained in the two pressure chambers 48, 49 of the damping device. As alternatives, the supply flow may take place through a non-return valve instead of the pressure-reduction valve 62, and the output flow through an adjustable throttle valve instead of the accumulator 61 .

The present damping device functions in the following manner:

Feed function: Driving fluid under pressure from the source 60, i.e. driving fluid that has been obtained from the central channel 7, flows through a channel in the machine housing 2 and the outer tube 32 (not shown in the drawings) of appropriate design into the two hydraulic pressure chambers 48, 49 behind the damping piston 40. A pre-determined pressure F1 pr is in this way created in the chambers 48, 49, which in turn, through the influence of the two piston surfaces A1 , A2 of the damping piston 40, generates the required forwards-direct driving force, i.e. the force of feeding F1 for the drill bit 5A against the rock R. The force of feeding F1 is a product of the pressure-accepting area A1 of the radially hydraulic outer pressure chamber 48 and the pressure-accepting area A2 of the radially hydraulic inner pressure chamber 49. Consequently, the resulting driving area A1 + A2 on the chuck fastening 37 will press this out against the second axial bearings 41 B in the chuck, with the force F1 . The force of feeding F2, which arises from the force of feeding of the rig, presses the drill bit 5A against the rock R. By allowing F1 to be greater than F2, i.e. by allowing the force of feeding on the drill bit 5A that arises from the pressure force of the damping piston 40 to be greater than the force of feeding on the drill bit from the rig, the parts will be in the condition shown in Figure 1 . Load on the second axial bearings 41 B in the chuck is then reduced to F2 - F1 , i.e. as the difference between the forces of feeding of the rig and the damping piston.

Damping function:

A corresponding pressure is present in the outer and inner pressure chambers 48,

49, namely D1 pr, which is the pressure that acts on both the outer and the inner piston areas A1 and A2 of the damping piston 40 when it is to function as a damping piston, whereby a force of damping DP is generated that is to receive and in a flexible manner retard the force of recoil, i.e. the rearwards-directed pressure reflection Er that arises after each working stroke of the drill bit 5A. As has been described above, the damping piston 40 is connected in a manner that allows axial displacement to the chuck fastening 37 through a spline connection 46 and is "floating", by which is meant that it can move forwards and backwards between the said forward end position L1 and rear end position L2. The impact energy is transferred exactly as normal, through the kinetic energy of the hammer piston 8 through the drill bit 5A to the rock R. Reflections from the drill bit pass backwards to the chuck fastening 37, and to the hydraulic driving area A1 , A2 and to the pressurised volume in the damping chamber. It can be conceived that the supply flow to the driving surfaces A1 , A2 takes place through a non-return valve, such that the flow into this volume takes place freely with a large area of passage, while the output flow takes place through a constriction or under damping. The reflection is in this case hydraulically damped.

One considerable advantage of the present invention is that there is no metallic contact between the chuck fastening 37 and the hammer casing 2, in the direction in which the reflection travels, which significantly limits the propagation of the vibration that arises backwards in the drill string.

When a pressure reflection Er from the rock R makes impact backwards against the drill bit 5A, the chuck fastening 37 and in this way also the damping piston 40 that is connected to it are driven in the backwards direction, towards the rear end position L2. Driving fluid is in this way pressed out from the said inner pressure chamber 49 through the piston surface A2 that is formed between the collar 54 of the damping piston 40 that protrudes telescopically into the ring-shaped shoulder 55 of the rear end piece 44 of the outer tube 32, which ring-shaped shoulder forms a seating. As a consequence of driving fluid being pressed out through the constriction gap 50 and filling the outer pressure chamber 48, the harmful energy of the pressure reflection Er is reduced, or the recoil is damped by being converted to heat in the hydraulic fluid during its passage through the constriction gap 50.

As the outer pressure chamber 48 is subsequently refilled with driving fluid from the inner pressure chamber 49, driving fluid is pressed into the accumulator 61 for temporary storage. As soon as the pressure reflection Er from the rock R has ceased, driving fluid is forced, through the action of the temporary increase in pressure inside the accumulator 61 , to flow back into the outer and the inner pressure chambers 48, 49 such that the original pressure condition is established. The pressure F1 pr is once again prevalent in both of the pressure chambers 48 and 49, whereby the damping piston 40 is driven forwards towards its forward end position L1.

The invention is not limited to what has been described above and shown in the drawings: it can be changed and modified in several different ways within the scope of the innovative concept defined by the attached patent claims.