MOVEMENT SPEED OF A PERCUSSIVE ELEMENT OF A DRILLING MACHINE

TECHNICAL AREA

The present invention relates to mining industry. More in particular, the invention relates to an arrangement to control the movement speed of a percussive element arranged to be moved in a housing of a drilling machine. The invention further relates to a drilling machine comprising an arrangement to control the movement speed of a percussive element which is arranged to be moved in the drilling machine, as well as a method to control the movement speed of a percussive element which is arranged to be moved in a housing of a drilling machine.

BACKGROUND

In mines or other work sites, rock drills, usually mounted on a rig to drill holes in the bedrock are often employed. Percussive rock drilling constitutes a common method to drill rock. During percussive rock drilling, the rock is crushed when a drill bit strikes the rock at a high frequency, whereby buttons arranged on the drill bit crush the rock. The drill bit is rotated in order for the buttons to hit new parts of the rock with each impact. Since drilling is often performed in deep holes, the drill bit is often arranged on a drill rod, which is in turn arranged on an adapter mounted in a drilling machine. The adapter, the drill rod and the drill bit together form an assembly called a drill string. In a percussive drilling machine, a percussive element, such as a percussive piston, impacts the adapter, whereby the impulse is distributed along the drill string, down the hole and finally into the rock via the drill bit. One or more drill rods may be joined to extend the drill string.

Examples of common drilling methods are top hammer drilling, Down-the-Hole (DTH) and COPROD.

To ensure contact between the drill bit and the rock and that the adapter is in a preferred position in the drilling machine during drilling, a feed force, directed towards the rock, is applied to the drilling machine, e.g. by a hydraulic piston. Hence, the feed force acts on the drill string and on the rock through the drilling machine.

Occasionally, the feed force is not great enough and the adapter may

consequently be located, within certain limits, in an arbitrary position in the drilling machine. This may for instance happen when the drilling machine is freehammering during scaling. Another occasion when it may happen is when the drill string is stuck in the hole. A pulling force then has to be applied to the machine while the machine is at the same time striking and rotating, so called backhammering. The adapter position may also be displaced as a result of wear of the adapter and of components in the housing of the drilling machine. At these occasions, there is a great risk that the drilling machine and the drill string are damaged, since a large part of the percussion energy is absorbed by the machine and by the string.

In order to reduce the risk of damage to the machine and to the string, some manufacturers have chosen to let the percussion mechanism stop at a certain

displacement of the adapter in relation to its position during normal operation. It is also known to use a hydraulic damping chamber to reduce the percussion speed.

The American patent US 5056606 A shows a drilling machine having a damping chamber where the percussive piston is made to stop when the adapter has been displaced from its normal operating position in a direction towards the rock. In a first step, hydraulic oil is pressed from a chamber through a bypass conduit arranged on the percussive piston. The hydraulic oil displaces a damping piston and thereafter brakes the percussive piston in a second step. In a final step, the hydraulic oil is drained from the chamber and the piston stops. To not allow the piston to strike when the adapter is displaced, such as described in US5056606, solves the problem of risk of damage to the machine and to the string. However, an effect of this is that scaling is made impossible which is a great disadvantage since special equipment has to be provided for that purpose, which requires both resources and money. Additionally, it is more difficult to loosen a drill string which is stuck in the hole, which may lead to cutting the whole drill string and leaving it in the hole, which is obviously disadvantageous from economical, practical and energy viewpoints. By hydraulically reducing the piston velocity requires meticulously dimensioned force characteristics which provide a desired velocity at the crucial position when the adapter is in mechanical contact with the drilling machine.

There is therefore a need to achieve an arrangement which at drilling allows the drilling machine to operate while at the same time reducing the risk of damage to the machine and to the string at a displaced adapter position. SUMMARY OF THE INVENTION An object of the present invention is to provide an arrangement for drilling at a displaced adapter position, also called percussion position, which compared to known art allows the drilling machine to work in a better way while at the same time reducing the risk of damage to the machine and to the string. Alternatively to provide an alternative to the known art.

The object is achieved according to a first aspect by an arrangement to control the movement speed of a percussive element which is arranged to move in a housing of a drilling machine. The percussive element comprises a brake surface. The housing further comprises a damping chamber configured to contain a fluid provided to dampen the movement speed of the percussive element in the housing. The housing further comprises a conduit configured to lead fluid from the damping chamber. The arrangement is configured to control the movement speed of the percussive element by adapting the pressure by which the fluid in the damping chamber affects the brake surface of the percussive element by leading fluid from the damping chamber only when a

predetermined reference point of the percussive element is located ahead of a

predetermined first position along an axial elongation of the damping chamber.

Since the arrangement is arranged to control the movement speed of the percussive element by containing a fluid in a damping chamber affecting a brake surface of the percussive element, the movement speed of the percussive element may be dampened in a very short distance without mechanically affecting the percussive element. Since the housing of the drilling machine further comprises a conduit configured to lead fluid from the damping chamber, the damping pressure, by which the fluid affects the brake surface and consequently the percussive element, may be reduced such that the damping effect on the percussive element is lowered or is made to level out. By reducing or completely removing the damping force on the brake surface, the percussive element may be made to continue striking even after the percussive piston has started to brake via the damping chamber. In contrast to prior art, where the percussive element is made to stop completely, or strikes with a forwardly displaced percussion position, the present invention allows the percussive element to strike with a reduced speed without displacing the percussion position. A consequence of a displaced percussion position is that the percussive element does not reach the adapter. This is mitigated in the present invention. By leading fluid from the damping chamber only when a predetermined reference point of the percussive element is located ahead of a predetermined first position along an axial elongation of the damping chamber, the damping force is not reduced until the percussive element has moved a predetermined distance in the damping chamber and has thereby had time to been braked to an adequate, predetermined speed. By a point of the percussive element being located“ahead” of a position in the damping chamber is hereby meant that the point of the percussive element is located deeper inside the damping chamber, i.e. along the axial elongation of the damping chamber. This also means that the point of the percussive element, and consequently the percussive element as a whole, is located closer to the adapter. The interaction between the reference point of the percussive element and the first position may be determined based on which drilling machine is used, on the properties of the damping fluid, on how the percussive element is designed, on the play between the percussive element and the wall of the damping chamber, and on the pressure level in the chamber. By correctly determining the first position and the reference point of the percussive element, a desired speed of the percussive element, and consequently of the adapter, is achieved when the adapter is in mechanical contact with the drilling machine forward in the housing of the drilling machine. The reference point of the percussive element may be a point somewhere along the axial elongation of the percussive element. The reference point may for instance be located in the plane defined by the brake surface in case the brake surface is orthogonally arranged relative to the elongation of the percussive element. This will be further described below. By allowing the fluid to be led through the conduit only when the reference point of the percussive element has passed the first position, the speed of the percussive element may advantageously be controlled more precisely and independently of manufacturing tolerances and wear of the adapter, since the control will not be affect by the length of the adapter or of the percussive element.

Thereby there is provided an arrangement which at drilling at a displaced adapter position allows the drilling machine to work at a precisely determined speed, which reduces the risk of damage to the machine and to the string.

According to some embodiments a lateral surface of the percussive element is configured to interact with the conduit such that when the reference point of the percussive element is behind the first position the lateral surface of the percussive element blocks fluid communication between the damping chamber and the conduit.

According to some embodiments wherein the lateral surface of the percussive element is configured to interact with the conduit such that when the reference point of the percussive element is ahead of the first position the conduit is uncovered, whereby the damping chamber is placed in fluid communication with the conduit. By“block” is herein meant to substantially prevent or block. There will always be a certain leakage since the percussive element has to have a small play in relation to the wall of the damping chamber to be able to move easily. Since the lateral surface of the percussive element is shaped to interact with the conduit so that the lateral surface of the percussive element blocks or prevents fluid communication between the damping chamber and the conduit when the reference point of the percussive element is located behind the first position, the percussive element will prevent fluid from flowing from the damping chamber through the conduit until the percussive element has moved a predetermined distance into the damping chamber. Thereby there is provided a robust and simple way of controlling the amount of fluid in the damping chamber, and thereby providing a robust and simple way of controlling the damping force with which the fluid affects the brake surface. By letting the shape of the percussive element control whether fluid may flow into the conduit, the desired result is attained with a minimum of comprised parts. There is no need of complex control equipment, valves or sensors. This is advantageous since it minimizes the risk of failure of any part of the system, which could lead to a halted operation, or in the worst case damaging a drilling machine and/or a drill string.

According to some embodiments the arrangement is configured to adapt the pressure by which the fluid in the damping chamber affects the brake surface of the percussive element by only leading fluid from the damping chamber when the reference point of the percussive element is located between the first position and a predetermined second position along the axial elongation of the damping chamber, wherein the second position is located ahead of the first position along the axial elongation of the damping chamber.

Since the arrangement is configured to only lead fluid from the damping chamber when the reference point of the percussive element is located between the first position and a predetermined second position along the axial elongation of the damping chamber, it is achieved that fluid is only led from the damping chamber when the percussive element moves so that its reference point is located between the first and the second position. In this way, the damping force is reduced only when the percussive element moves so that its reference point is located between the first and the second position, i.e. when the percussive element moves along the axial elongation of the damping chamber from the first point towards the second point. Since no fluid is led from the damping chamber when the reference point of the percussive element is located ahead of the second position, the damping force will increase quickly as soon as the percussive element is positioned so far into the damping chamber that the reference point is located ahead of the second position. Thereby, the part of the damping chamber located ahead of the second position may work as an extra brake and slow down the percussive element in case it moves too fast, too far into the damping chamber. Thereby, the risk that the percussive element strikes the adapter at a too high velocity, or when the adapter is located too far ahead in the drilling machine, is minimized, which could lead to damage of the drilling machine and of the drill string.

The lateral surface of the percussive element may be configured to interact with the conduit such that when the reference point of the percussive element is ahead of the second position, the lateral surface of the percussive element blocks fluid communication between the damping chamber and the conduit.

By letting the configuration of the percussive element control that fluid cannot flow into the conduit when the reference point of the percussive element is located ahead of the second position, the increased damping effect of the configuration of the percussive element is achieved. Thereby, no complex control system is required to achieve the desired result, which minimizes the risk of failure of any constituent component. This leads to increased running time and lower costs.

According to some embodiments the damping chamber constitutes an elongated, substantially cylindrical cavity in the housing of the drilling machine, along which the percussive element is configured to move.

The conduit may then comprise a first and a second end. Further, the lateral surface of the percussive element may be configured to interact with the conduit such that when the reference point of the percussive element is ahead of the first position the first end of the conduit is in fluid communication with a part of the damping chamber which is located ahead of the brake surface of the percussive element along the axial elongation of the damping chamber while the second end of the conduit is simultaneously in fluid communication with a part of the damping chamber which is located behind the brake surface of the percussive element along the axial elongation of the damping chamber.

By this configuration of the lateral surface of the percussive element, fluid may be led past the percussive element from a forward part of the damping chamber to a rear part of the damping chamber, wherein the rear part of the damping chamber is located behind the brake surface of the percussive element. The fluid located in the rear part of the damping chamber will thus not affect the brake surface of the percussive element with a damping force and the damping effect on the percussive element is reduced.

The damping chamber may comprise a first part having a first diameter and a second part having a second diameter, wherein the second diameter is greater than the first diameter.

Thereby, the second part will have a greater diameter than the first part.

Further, the damping chamber may in its forward axial end comprise a third part having a third diameter, wherein the third diameter substantially corresponds to the first diameter.

Thereby, there is provided a damping chamber where at least a part of the damping chamber has a greater diameter than two other parts of the damping chamber. The damping chamber may for instance have a main diameter and a part having a greater diameter than the main diameter.

The percussive element may be substantially cylindrically shaped and comprise a first part having a fourth diameter and a second part having a fifth diameter, wherein the fourth diameter is lesser than the first diameter and the fifth diameter substantially corresponds to the first diameter.

Thereby, the first part of the percussive element will have a smaller diameter than the first part of the damping chamber and the second part of the percussive element will have a diameter which corresponds to the first part of the damping chamber. The second part of the percussive element may then comprise the brake surface. The conduit may be constituted by the part of the damping chamber which has the second diameter.

The object is also achieved according to a second aspect by a drilling machine comprising the arrangement.

Since the drilling machine comprises the arrangement, the same advantages as above are provided. There is thereby provided a drilling machine which at drilling with reduced feed force/displaced adapter position may continue to operate while at the same time reducing the risk of damage to the machine and to the drill string.

The object is also achieved according to a third aspect by a method to control the movement speed of a percussive element arranged to be moved in a housing of a drilling machine, wherein the percussive element comprises a brake surface. The housing of the drilling machine comprises a damping chamber configured to contain a fluid provided to dampen the movement speed of the percussive element in the housing and a conduit configured to lead fluid from the damping chamber. The method comprises adapting the pressure by which the fluid in the damping chamber affects the brake surface of the percussive element by leading fluid from the damping chamber only when a

predetermined reference point of the percussive element is located ahead of a predetermined first position along an axial elongation of the damping chamber.

By utilizing this method, a drilling machine may continue to operate even at reduced feed force/displaced adapter position while at the same time reducing the risk of damage to the machine and to the drill string. According to some embodiments the method further comprises adapting the pressure by which the fluid affects the brake surface of the percussive element by preventing fluid communication between the damping chamber and the conduit when the reference point of the percussive element is located ahead of a predetermined second position along the axial elongation of the damping chamber, which second position is located ahead of the first position along the axial elongation of the damping chamber.

By utilizing this method the damping force is reduced only when the percussive element moves such that its reference point is located between the first and the second position. Since no fluid is led from the damping chamber when the reference point of the percussive element is located ahead of the second position, the damping force will quickly increase as soon as the percussive element is located so far into the damping chamber that the reference point is located ahead of the second position. Thereby, the part of the damping chamber that is located ahead of the second position may work as an extra brake and slow down the percussive element in case it moves at a too high velocity, too far into the damping chamber. Thereby, the risk of the percussive element striking the adapter at too high speed, or when the adapter is located too far ahead in the drilling machine, is reduced, which could lead to damage of the drilling machine and of the drill string.

DESCRIPTION OF DRAWINGS

Further objects and advantages of, as well as features of the invention will be apparent by the following description of one or more embodiments, which is provided with reference to the drawings, wherein:

Fig. 1 shows a side view of an arrangement in a housing of a drilling machine according to prior art, Fig. 2 shows a side view of an arrangement in a housing of a drilling machine according to prior art,

Fig. 3 shows a side view of an arrangement in a housing of a drilling machine according to the present invention,

Fig. 4a shows a side view of a percussive element in a first position in a damping chamber,

Fig. 4b shows a side view of a percussive element between a first and a second position in a damping chamber,

Fig. 4c shows a side view of a percussive element in a second position in a damping chamber,

Fig. 5a shows a damping chamber in a housing of a drilling machine,

Fig. 5b shows a percussive element for a drilling machine,

Fig. 6a shows a percussive element according to a disclosed embodiment,

Fig. 6b shows a side view of an embodiment of an arrangement in a housing of a drilling machine according to the present invention,

Fig. 7 shows a flow chart disclosing a method to control the speed of a percussive element in a housing of a drilling machine.

DETAILED DESCRIPTION

The present invention is described more in detail below with reference to the appended drawings showing exemplary embodiments. The invention should not be interpreted as being limited to the disclosed exemplary embodiments. Instead it is defined by the appended claims. Like numbers in the drawings refer to like elements throughout the description.

Figure 1 and 2 illustrate a part of a housing 1 of the drilling machine according to prior art. An adapter 3 is arranged in the housing 1. The adapter 3 is further arranged on a drill rod of a drill string (not shown) outside the housing 1. In figure 1 , the adapter is located in its position at the far right of the housing 1. The axial elongation D of the housing is marked with a dotted arrow. Unless otherwise noted, this direction defines directions or positions of this application.“Forward” in direction D thus corresponds to the left of figure 1 and“backwards” corresponds to the right of figure 1. The adapter 3 may thus be said to be located in its position in the far right of figure 1. In figure 1 , the adapter 3 rests against a rear stop 5. The stop 5 may be a mechanical stop which is mechanically supported by the housing 1. Alternatively, the stop may stop the adapter hydraulically. A percussive element 6 is arranged rearward in the housing 1 as compared to the adapter 3, i.e. to the right in figure 1. The percussive element may for instance be a percussive piston. The center axis of the percussive element may be parallel to the axial elongation D of the housing 1. The percussive element may be substantially elongated cylindri cally shaped. By“substantially elongated cylindrically” is herein meant that the percussive element 6 main shape is a cylinder, but parts of the percussive element 6 may deviate from the cylindrical shape. It may for instance comprise radially protruding elements or parts which will be further described below. During drilling, the percussive element 6 is made to strike the adapter 3 at a very high frequency. The impulse energy is transmitted via the adapter 3 down the drill string to a drill bit which crushes the rock. The percussive element 6 may for instance be made to strike through a hydraulic mechanism. This is a well-known technique and will not be further described here.

During drilling, the adapter is located in the position shown in figure 1 , herein also named adapter position. The percussive element 6 thereby attains a corresponding percussion position or percussion point. If the feed force is very low, or even pulling, i.e. it works in direction opposite to the direction D, the movement of the adapted 3 will be limited by stop 7 arranged in the forward part of the housing 1 , i.e. at a position in the housing 1 which is closer to the rock than the stop 5. The stop 7 may, just like stop 5, operate mechanically or hydraulically. As shown in figure 2, the adapter 3 is located at a distance A from its drilling position. If the drilling machine is allowed to strike when the adapter is located in this position, the percussive element 6 may cause serious damage to both the drilling machine and to the drill string since the force from the percussive element is transmitted in the housing 1 and in the drill string rather than via the drill string into the rock. Therefore, the speed of the percussive element 6 should be reduced when the adapter is in the forward position

A common way to reduce the speed of the percussive element is illustrated in figure 1 and 2. The housing 1 comprises a damping chamber 11 with an axial elongation E which may correspond to the axial elongation D. The percussive element 6 here comprises a radially protruding part 9, commonly called a piston brake. Thus protruding part 9 may for instance be arranged between a pair of piston rings. By letting this protruding part 9 enter the damping chamber 11 , a damping force is built up ahead of the protruding part 9 as the pressure of the fluid in the damping chamber 11 increases as the percussive element 6 moves into the damping chamber 11. To achieve this effect, the protruding part 9 should be configured to reach so far from the percussive element 6 towards the wall of the damping chamber 11 that the fluid in the damping chamber 1 1 cannot flow past the protruding part 9. As illustrated in figure 1 and 2, the protruding part 9 of the percussive element 6 has to be located at a distance d from the wall of the damping chamber 11 so that the percussive element 6 may move in the damping chamber 1 1 and not be damaged by mechanical interaction with the wall. The distance d is herein also called play d. Therefore, there will always exist a small leakage through the play d. To avoid damping of the percussive element 6 at normal drilling conditions, the forward part of the protruding part 9 of the percussive element 6 is located at a distance B from the damping chamber 11 , as shown in figure 1.

When the feed force is low and the adapter 3 is located far ahead in the housing 1 , such as shown in figure 2, the protruding part 9 of the percussive element 6 will move into the damping chamber 11. At a displacement of the percussive piston a distance C into the damping chamber 1 1 , i.e. a displacement of the percussive piston the distance B+C = A in relation to its position during normal drilling, the speed of the percussive element 6 will be determined by a combination of percussive power/percussive pressure, damping surface, size of the play d, viscosity of the fluid in the damping chamber 11 , weight of the percussive piston 6, etc.

The accuracy with which the distance A may be defined will be limited by, among other factors, manufacturing tolerances and will be reduced during operation due to wear of the adapter and of the stop 5 during operation. The percussion point is thereby displaced forwardly in the housing 1 , while the piston brake 9 is configured for the calculated percussion point. The percussive element will then be damped too much or even turn before the percussive element 6 hits the adapter 3. The percussive element 6 is then unable to disconnect parts of the drill string or loosen the drill string from the hole if the drill string is stuck. The solution disclosed in connection with figure 1 and 2 cannot solve this problem.

An exemplary solution to the problems described hereinbefore will now be described in connection with figure 3, figures 4a-4c and figures 5a-b. Like technical features will have like reference numbers as in figure 1 and 2. Some features are not shown in the figures, such as the stops 5 and 7, but it is understood that these features may be comprised in the solution described below. In figure 3 is shown an arrangement to control a movement speed of the percussive element 6 which is arranged to move in the housing 1 of the drilling machine. The percussive element 6 comprises a brake surface 13. The brake surface may extend in a plane whose normal is parallel to a center axis of the percussive element 6 and/or to the axial elongation D of the housing 1 and/or the axial elongation E of the damping chamber 1 1. Thereby it provides as large a surface as possible against which the damping fluid in the damping chamber 1 1 may work. The brake surface 13 is in figure 3 arranged on the protruding part 9. The arrangement further comprises a conduit 15 which is configured to lead a fluid from the damping chamber 1 1. The arrangement is configured to control the movement speed of the percussive element 6 by adapting the pressure by which the fluid affects the brake surface 13 of the percussive element by only leading fluid from the damping chamber 1 1 when a

predetermined reference point 17 (see figure 4a) of the percussive element 6 is located ahead of a predetermined first position 19 along the axial elongation of the damping chamber 1 1. The reference point 17 may, as shown in figure 4a, constitute a point on the brake surface such that when the brake surface has passed the position 19 along the axial elongation of the damping chamber 11 , the arrangement will lead fluid from the damping chamber through the conduit 15. The reference point 17 may obviously be chosen arbitrarily as long as the criterion that fluid may be led from the damping chamber 1 1 when the reference point 17 is located ahead of the first position 19 is fulfilled. The choice of the first position 19 must, however, be adapted according to the choice of reference point 17, or vice versa.

Figure 4a shows that a lateral surface of the percussive element 6 is configured to interact with the conduit 15 so that when the reference point 17 of the percussive element 6 is located behind the first position 19, the lateral surface of the percussive element 6 blocks fluid communication between the damping chamber 11 and the conduit 15. As previously pointed out, there will exist a certain leakage due to the play d, see figure 1. By blocking“fluid communication” is herein meant that fluid cannot flow from the damping chamber 1 1 through the conduit 15. However, the conduit 15 may be open towards the damping chamber 1 1 and fluid may exist in both the conduit 15 and in the damping chamber 1 1 , but no fluid flows in from the damping chamber 1 1. The lateral surface of the percussive element 6 may also be configured to interact with the conduit 15 so that when the reference point 17 of the percussive element 6 is located ahead of the first position 19, the conduit 15 is uncovered, whereby the damping chamber 1 1 is placed in fluid communication with the conduit 15. The damping chamber 11 may constitute an elongated substantially cylindrical cavity inside the housing 1 of the drilling machine. The conduit shown in figure 4a-4b comprises a first end 15a and a second end 15b. The lateral surface of the percussive element 6 is configured to interact with the conduit 15 so that when the reference point 17 of the percussive element 6 is located ahead of the first position 19 the first end 15a of the conduit is in fluid communication with a part of the damping chamber 11 which is located ahead of the brake surface 13 of the percussive element 6 along the axial elongation of the damping chamber 1 1 , while the second end 15b of the conduit is at the same time in fluid communication with a part of the damping chamber 11 which is located behind the brake surface 13 of the percussive element 6 along the axial elongation of the damping chamber 1 1. In figure 4a the percussive element 6 is located in a position where the reference point 17 is not yet behind the first position 19 in the damping chamber 1 1. The part of the lateral surface of the percussive element 6 which comprises the protruding part 9 in that position blocks fluid from flowing through the conduit 15 and out behind the protruding part 9. In figure 4b, the percussive element 6 has moved further forward in the damping chamber 1 1. Now the conduit 15 is no longer blocked by the lateral surface of the percussive element 6. Fluid may then flow from the damping chamber 1 1 , via the first end 15a of the conduit, through the conduit 15 and out via the second end 15b of the conduit, behind the brake surface 13. The damping pressure by which the fluid in the damping chamber affects the percussive element 6 is in this position much lower compared to the pressure by which the fluid in the damping chamber 11 affects the percussive element 6 in figure 4a, where the fluid is blocked from flowing through the conduit 15. The speed of the percussive element 6 will therefore, in a first stage, be significantly reduced, to thereafter level out or be reduced much more slowly. The percussive element 6 is thus not completely braked, but is allowed to have a speed which is adapted to allow controlled percussions even at a displaced position of the adapter 3.

Since the damping ceases or is significantly reduced, the percussive element 6 may have a relatively high velocity even inside the damping chamber. To avoid that the percussive element 6 strikes a bottom of the damping chamber 11 , the arrangement may be configured to further dampen the percussive element 6 in case it moves too far into the damping chamber. The arrangement is thus further configured to adapt the pressure by which the fluid affects the brake surface of the percussive element 6 by only leading fluid from the damping chamber 11 when the reference point 17 of the percussive element 6 is located between the first position 19 and a predetermined second position 20 along the axial elongation of the damping chamber 1 1. The second position 20 is located, as shown in figure 4c, ahead of the first position 19 along the axial elongation E of the damping chamber 11. The lateral surface of the percussive element 6 is configured to interact with the conduit 15 so that when the reference point 17 is located ahead of the second position 20, the lateral surface of the percussive element 6 blocks fluid communication between the damping chamber 1 1 and the conduit 15. Thereby, the fluid located in a forward part 22 of the damping chamber 1 1 may flow through the conduit 15. This fluid will therefore work at a high pressure against the brake surface 13 and significantly dampen the percussive element 6 if it moves far into the damping chamber 1 1.

In figure 5a is shown the damping chamber 11 without the percussive element 6. The damping chamber 11 may comprise a first part 50 having a first diameter d1 and second part 52 having a second diameter d2. The second diameter d2 is greater than the first diameter d1 ; d2 > d1. The conduit 15 may be constituted by the part of the damping chamber 11 having the second diameter d2, i.e. the second part 52. The damping chamber 11 may further, in its forward axial end, comprise a third part 54 having a third diameter d3. The third diameter d3 may substantially correspond to the first diameter d1. Thereby there is provided a damping chamber 1 1 having a depression. The depression corresponds to the second part 52 and consequently to the conduit 15. If the damping chamber is cylindrical, the second part 52 may extend around the whole cylinder surface. However, it is conceivable that only one or more depressions run along the longitudinal direction of parts of the wall of the damping chamber 11.

In figure 5b is shown the detached percussive element 6. The percussive element may be substantially cylindrically shaped. It may comprise a first part 56 having a fourth diameter d4 and a second part 58 having a fifth diameter d5. The fourth diameter d4 may be smaller than the first diameter d1 and the fifth diameter d5 may substantially correspond to the first diameter d1. The second part 58 of the percussive element 6 may comprise the brake surface 13. Thereby, the second part 58 of the percussive element 6 will be able to block fluid from flowing past the brake surface 13 of the percussive element 6 until the second part 58 of the percussive element is located in the part of the damping chamber 11 comprising the conduit 15. Since the part of the damping chamber 1 1 comprising the conduit has a diameter d2 surpassing the first diameter d1 and

consequently the diameter d5 of the second part 58 of the percussive element, fluid may flow past the brake surface 13 via the conduit 15. The percussive element 6 may also comprise a third part 60 having a sixth diameter d6.The sixth diameter d6 may be greater than the fourth diameter d4.

It has been described how the adaptation of the pressure in the damping chamber 1 1 is achieved by the configuration of the percussive element 6. The invention is, however, not limited to this embodiment. Other alternatives are conceivable. A sensor in the damping chamber might for instance interact with a valve in the conduit 15 whereby the valve is opened when the sensor detects that the reference point 17 is located ahead of the first position 19. Fluid may then be led from the damping chamber 11 through the conduit 15 to a space outside the damping chamber 1 1 in which space there is operating pressure.

In figure 6a is shown the percussive element with an extra protruding part 9’ having a large brake surface. This provides a large damping effect. In figure 6b is shown the percussive element 6 without the extra protruding part 9’. The percussive element 6 may then be configured with a track 61 or a groove 61 to provide the damping effect described above. The percussive element 6 may then comprise two protruding parts 62a, 62b having the same diameter. The track 61 is then constituted by a part of the percussive element 6 between the protruding parts 62a, 62b whose diameter is less than the diameter of the two protruding parts 62a, 62b. The protruding parts 62a 62b and the track 61 may then interact with the conduit 15 in the damping chamber 1 1 to control the movement speed of the percussive element 6. The fluid in the damping chamber 11 may then flow through the conduit 15 past the protruding part 62a when its forward surface has passed the first position 19 and before the forward part of the protruding part 62b has started blocking the conduit 15.

A method to control the movement speed of a percussive element 6 which is arranged to be moved in a housing 1 of the drilling machine will now be described with reference to figure 7. Optional steps of the method are marked with dotted lines in the figures.

The steps of the method described below may for instance be carried out by a control unit. Figure 7 shows an exemplary method to control the movement speed of a percussive element 6 which is arranged to be moved in a housing 1 of the drilling machine, where the percussive element 6 comprises a brake surface 13. The housing 1 comprises a damping chamber 1 1 configured to contain a fluid provided to dampen the movement speed of the percussive element in the housing 1. The housing further comprises a conduit 15 configured to lead fluid from the damping chamber 11. The method comprises adapting the pressure by which the fluid in the damping chamber 11 affects the brake surface 13 of the percussive element 6 by leading 701 fluid from the damping chamber 11 only when a predetermined reference point 17 of the percussive element 6 is located ahead of a predetermined first position 19 along an axial elongation E of the damping chamber 11.

The method may further comprise preventing 702 fluid communication between the damping chamber 1 1 and the conduit 15 when the reference point 17 of the percussive element 6 is located ahead of a predetermined second position 20 along the axial elongation E of the damping chamber 1 1 , which second position 20 is located ahead of the first position 19 along the axial elongation E of the damping chamber 1 1.