Abstract:
A rock drill includes a string of components interconnected by screw threads and connected to a shank. A percussion piston is reciprocated against the shank during a drilling operation, while a support device applies axial support to the rear of the shank, to enhance the drilling action. When the components are being unscrewed, the percussion piston is actuated to strike the rear end of the drill string, after the support device has at least partially released the axial support of the shank, to enable tensile stress to be generated in the components by the percussive impacts to facilitate unscrewing of the components.

Description:
TECHNICAL BACKGROUND 
   The invention relates to a method of opening joints between drilling components in a rock drill which is fed with respect to a feeding beam during drilling and which comprises a frame, a percussion device arranged in the frame. The conventional percussion device comprises a percussion piston which moves in the longitudinal direction, a shank which is arranged in the axial extension of the percussion piston for being struck by the percussion piston and to which the necessary drilling components, such as drill rods, a drill bit and the like are attached to form drilling equipment, and a rotating device which by means of a rotating bushing rotates the shank. The conventional rock drill further comprises at least one axial piston, which is positioned rearwardly of the shank and arranged to move in the axial direction by means of the pressure of a pressure medium fed onto its back surface during drilling, the axial piston acting on the shank by supporting it towards the front part of the drilling machine. The conventional method comprises supporting at least part of the drilling equipment against a supporting surface when threaded joints of the drilling components attached to the shank are opened and striking the drilling equipment with the percussion device, after which the shank is rotated by the rotating device in a direction opposite to normal drilling to open the desired joints. 
   The invention further relates to a rock drill which comprises a frame, a percussion device provided with a percussion piston which moves in the longitudinal direction, a shank which is arranged in the axial extension of the percussion piston, which is arranged to strike the rear end of the shank, and a rotating device which is arranged to rotate the shank by a rotating bushing arranged around the shank, and drilling equipment consisting of drilling components attached to the front end of the shank. The conventional drilling machine comprises at least one axial piston, which is arranged rearwardly of the shank, there being a pressure channel which extends rearwardly of the axial piston and allows feeding of a pressure medium rearwardly of the axial piston to move the axial piston in the axial direction, the axial piston being arranged to support the shank in the axial direction during drilling. 
   A drilling machine that comprises a percussion device and a rotating device is used in rock drilling. At the front end of the drilling machine there is a shank which is struck by a percussion piston of the percussion device and rotated by the rotating device during drilling. In the extension of the shank there are drilling components, such as one or more drill rods and a drill bit. The drill bit is the outermost part and comprises buttons for working the rock. The drilling components thus constitute the drilling equipment that is attached to the drilling machine and transmits the percussion force and the rotating force from the shank to the rock to be drilled. The drilling components are attached to the shank and further to other drilling components usually by means of a threaded joint. For example, when the drill bit is changed or drill rods are added/removed between the drill bit and the shank, joints have to be opened. When threaded joints between the drilling components are opened, the shank is rotated by a rotating motor in a direction opposite to normal drilling. To facilitate opening of a joint, the drilling equipment can be struck by the percussion device. In the case of damaged or stuck joints, in particular, it is very common to apply such striking in opening. 
   Finnish patent no. 98 401 describes a rock drill where the shank can be moved forward by means of axial pistons provided rearwardly of the shank when drilling is adjusted. In that case the shank is supported from behind either directly or by means of a bushing. The pressure acting rearwardly of the axial pistons is changed and thereby the travel of the pistons is adjusted and thus also the position of the shank. There are several axial pistons which are arranged in groups with different travel so that axial pistons with a longer travel can move over the optimal percussion point, to the front of it. It is a known fact that a stroke generated by the percussion piston produces a stress pulse that is reflected back from the rock. In the solution according to the publication, the pulse reflected from the rock is received by axial pistons that extend to the front of the optimal percussion point, and thus the backward movement of the shank and the drilling equipment is dampened. One purpose of this solution is to support the shank and the drilling equipment during the whole duration of drilling by means of axial pistons included in an axial bearing, in which case constant compression stress acts on the drilling equipment, which together with rotation continuously tightens the threaded joints during normal drilling, thus ensuring that the joints between the drilling components remain fastened. This way problems caused by loosened joints can be avoided. The threaded joint is typically damaged by vibration of a loosened joint. In the use of the drilling machine disclosed in the publication it has been noted that it is difficult to open the joints of the drilling components because as the drilling components are struck off, the axial bearing tries to ensure rock contact against the bottom of the drilling hole by supporting the shank during striking, in which case compression stress is ensured in the drilling equipment in the same way as in normal drilling. 
   OBJECT OF THE INVENTION 
   The object of the invention is to provide a new improved method for opening joints of drilling components and a rock drill according to the method. 
   SUMMARY OF THE INVENTION 
   The method of the invention is characterized by reducing the pressure of the pressure medium that pushes the axial piston towards the front end of the drilling machine when drilling components are struck off so that the shank and the drilling components in its extension are substantially not supported by the axial piston in the axial direction when drilling components are struck off, the compression stress caused in the drilling components by the stroke of the percussion piston being reflected at least partially from the front end of the drilling equipment, whereby said return pulse generates tensile stress in the drilling components and their joints. 
   The rock drill according to the invention is characterized in that the rock drill comprises means for reducing the pressure of the pressure medium leading to the axial piston when drilling components are struck off by a percussion device so that the shank and the drilling equipment are substantially not supported by the axial piston in the axial direction when drilling components are struck off. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail in the accompanying drawings, in which 
       FIG. 1   a  schematically illustrates a prior art rock drill where the solution according to the invention can be applied, and  FIG. 1   b  is a schematic side view of the rock drill according to the invention and drilling equipment attached to it, 
       FIG. 2  a schematic sectional side view of a detail of the rock drill according to the invention, and 
       FIGS. 3 and 4  are schematic sectional views of the structure of two other respective rock drills according to the invention at the axial bearing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1   a  is a simplified view of a rock drilling device, which comprises a movable base  1 , a boom  2  and a feeding beam  3  provided at the free end of the boom. The rock drill  4  can be moved with respect to the feeding beam and the necessary drilling equipment  5  is attached to the shank of the rock drill. The rock drill comprises a percussion device  6  for striking the shank and a rotating device  7  for rotating the shank. The shank transmits the percussion force and the rotational force to the drilling equipment, which transmits them further to the rock to be drilled. 
   In the solution shown in  FIG. 1   b  the drilling equipment  5  comprises a first drill rod  9   a  attached to the shank  8  and a second drill rod  9   b  attached to the extension of the shank. Furthermore, a drill bit  10  is attached to the outermost end of the drill rod string thus formed. Usually several extension rods are employed in drilling, but sometimes only one drill rod, in which case a drill bit is attached detachably or fixed to one end of the drill rod. In this application the drill rods, the drill bit and the like attached to the shank extension are commonly called drilling components. The rock drill  4  is mounted onto the feeding beam by means of a carriage  11 , or alternatively, if the structure of the drilling machine is rigid, it can be mounted in such a manner that it is movable with respect to the feeding beam  3  directly by slide blocks. The drilling machine is moved by feeding means known per se, such as pressure-medium-operated feed cylinders or the like, and by necessary power transmission members, such as wires. At the front end of the feeding beam or close to it, usually in connection with the front controller, there is ‘grip’  12  which enables clamping of the drilling component and holding of it when the joints between the drilling components are opened. A threaded joint is used between the drilling components, and the joint is opened by rotating the shank by the rotating motor in a direction opposite to normal drilling. In that case the grip generates counter torque for rotation, and thus the joint of the rotating drilling component in the grip opens. The grip may be generated by pressure-medium-operated clamping jaws or the like between which the drilling component is locked so that it cannot substantially move. The grip is known to a person skilled in the art and thus its detailed structure needs not be described here. Detached drilling components can be handled by suitable manipulators or the like between the component magazine and the drilling machine. 
   In  FIG. 1   b  arrows  60  illustrate how the stroke generated by the percussion piston produces compression stress in the drilling equipment  5 , which advances like a wave from the shank towards the drill bit  10 . Since the drill bit is not supported against the rock during striking in the same ways as in normal drilling, at least part of the compression stress is reflected back from the front surface of the drill bit and proceeds towards the shank in the desired manner as stress of the opposite sign, i.e. as tensile stress. On the other hand, during normal drilling as good rock contact of the drill bit as possible is to be maintained by means of the axial bearing so that the drill bit penetrates into the rock as well as possible and to avoid generation of disadvantageous tensile stress in the drilling equipment. 
     FIG. 2  illustrates a section of a rock drill  4  known per se. The rock drill comprises a frame, which in this case consists of frame parts  13   a  and  13   b  that are joined together. The percussion device includes a percussion piston  14 , which due to the influence of the percussion device moves to and fro in the axial direction and strikes the shank  15 . The shank is in the axial extension of the percussion piston at the front end of the rock drill. Furthermore, the shank is rotated by rotating a rotating bushing  16  around the shank by means of the rotating motor. The shank can move with respect to the rotating bushing in the axial direction. The structure and function of the percussion apparatus, rotating motor and the rotating bushing are known per se. The shank is supported axially rearwardly by a supporting bushing  17  to keep the shank in a suitable position in the axial direction to transmit the stroke and dampen the return movement, which follows the stroke. The expressions “behind” and “rearwardly of” refer to a position in relation to the normal drilling direction. The supporting bushing is provided with a slanted supporting surface  17   a  which touches the corresponding supporting surface  15   a  of the shank. Rearwardly of the supporting bushing there are several axial support pistons  18   a  and  18   b  arranged around the percussion piston. In the solution shown in the figure the axial pistons generate supporting force, which is transmitted to the supporting bushing  17  by means of bearing members  19   a  and  19   b  arranged coaxially with the pistons. For keeping the bearing members at the pistons there is a separate positioning bushing  20  positioned rearwardly of the supporting bushing. The positioning bushing also comprises limiting surfaces  21   a  and  21   b  that limit the movement of the axial pistons towards the front end of the drilling machine. If the support pistons  18   a ,  18   b  are arranged to act directly on the supporting bushing without any bearing members  19   a ,  19   b  or the like, the forward movement of the axial pistons can be limited by means of a separate limiting bushing or limiting surfaces provided in the frame. The axial pistons are located in cylinder spaces provided in the frame or in a separate member. Pressure channels  22   a  and  22   b  lead to the cylinder spaces. During drilling the drilling machine as a whole is fed with a certain force against the rock. If the shank is to be supported by the pistons  18   a ,  18   b , the total force via these pistons must exceed the feeding force. If this force does not exceed the feeding force the pistons will not support the shank but instead the pistons will be in their rearmost position. 
   In the construction shown in the figure there are several axial pistons which are divided into groups to form at least two separate piston groups with different travel or stroke to the front end of the drilling machine. As can be seen, the travel allowed for the upper piston  18   b  by the limiting surface  21   b  is longer than that allowed for the lower piston  18   a  by the limiting surface  21   a.    
     FIG. 2  further illustrates a simplified hydraulic coupling of the axial bearing. Pressure medium is supplied from a pump  23  along a pressure channel  22   b  to a pressure reducing valve  25 . A hydraulic accumulator  27  is preferably connected to the pressure channel  22   b . The arrangement further comprises a second pump  28 , which feeds pressure medium via a regulating valve  29  to the percussion apparatus  30 , which in turn generates the to and fro movement of the percussion piston  14 . The pressure medium that drives the percussion apparatus  30  is also connected to act directly on the axial pistons  18   a  via the pressure channel  22   a.    
   In the situation shown in the figure the regulating valve  29  is in a first position where no pressure medium is let from the pump into the percussion apparatus  30 , and thus pressure does not act on the axial pistons  18   a . The pressure of the pressure medium from the pump  23  is reduced in the pressure reducing valve  25 , after which it acts on the axial pistons  18   b . Consequently, the total pressure of the pressure medium acting rearwardly of the axial pistons  18   a  and  18   b  during striking is higher than the feed force of the rock drill, and therefore sufficient for keeping the shank in the optimal striking position, where the rock drill functions in the manner known per se. Instead of separate pumps  23  and  28 , it is naturally possible to use one common pump. In that case e.g. the pressure medium channel from the pump  28  is connected to the channel leading to the valve  25 . 
   The pressure channel  22   b  is further provided with a valve  32  for cutting off the pressure acting rearwardly of the axial pistons  18   b  when joints between the drilling components are struck off by means of the percussion device. When the valve  32  is in the lower position, no pressure is fed rearwardly of the axial pistons  18   b , but the pressure is released into a tank. In that case the axial pistons with a longer travel do not support the drilling equipment when joints are struck off, but the pulses caused by the strokes are reflected to the drilling equipment as tensile stress, which opens stuck joints between the drilling components. Furthermore, there may be a valve  32  arranged in the pressure channel  22   a  of the axial pistons  18   a  with a shorter travel for acting on the pressure which acts rearwardly of the pistons  18   a  during striking. 
   The pressure acting rearwardly of the axial pistons is supplied to the tank during striking, and thus zero or ambient pressure acts on the pistons, or a low pressure due to filters and similar throttling components in the return line. Alternatively, the pressure space rearwardly of the axial pistons is not connected to the pressure line towards the tank but the support provided for the drilling equipment by the axial pistons can be reduced by reducing the pressure acting rearwardly of the pistons with respect to the value used in normal drilling e.g. by means of a pressure reducing valve. Also when the axial pistons do not actively support the drilling equipment, the striking produces the desired tensile stress in the drilling equipment. 
   In practice, the user of the device turns the rock drill into the striking position by means of an operating switch  41  provided in a control panel  40  described below in  FIG. 3 . As a result, the valve  32  moves into the lower position and normal drilling control  42  of the drilling machine is bypassed; yet the full striking power is obtained, if necessary. 
   The rock drill and its axial bearing shown in  FIG. 3  correspond to those of  FIG. 2 , except that the axial pistons  18  are in direct contact with the supporting bushing  17  without separate bearing members. Furthermore, in the solution of  FIG. 3  all the axial pistons have the same travel. The movement of the axial pistons is restricted by means of a limiting bushing  33 . In this solution the pressure is also reduced or eliminated altogether rearwardly of the axial pistons when joints of the drilling equipment are struck open by the percussion piston. Since the shank is not supported when joints are struck open, tensile stress is generated in the drilling equipment, which facilitates opening of the threaded joints. 
     FIG. 4  illustrates a further construction where the invention can be applied. The operating principle of the axial bearing is similar to that illustrated in  FIG. 2 , except that, instead of several separate cylinder-like axial pistons, bushing-like pistons are used which are arranged around the percussion piston coaxially. In this case the pistons  18   a  and  18   b  are arranged so that the inner piston  18   b  can be moved forwards to the counter surface  21   b  by means of the pressure fed from the pressure channel  22   b . The piston  18   a  is positioned coaxially around the inner piston and pressure medium is supplied rearwardly thereof via the pressure channel  22   a . Like in the constructions described above, the pressure is reduced or eliminated only rearwardly of the piston  18   b  or rearwardly of both pistons  18   a  and  18   b  as shown in the figure when the joints of the drilling components are struck open. It should be noted that the construction may comprise only one bushing-like piston. Furthermore, between the bushing-like piston and the counter surface of the shank there may be a suitable bushing or a similar element that transmits the forces. 
   The basic idea of the invention is that when joints are opened while employing strokes generated by a percussion device, i.e. striking, the pressure of axial pistons, or of some pistons included in the axial bearing of the drilling machine, is cut off or at least reduced so that the axial bearing does not axially support the shank and the drilling equipment during striking. Consequently the compression stress generated in the drilling equipment by the stroke of the percussion piston is reflected back from the front surface of the drill bit and produces a return pulse, which generates tensile stress in the drilling equipment so that opening of the joints of the drilling components becomes easier after striking. Striking can be performed in such a manner that the drill bit is against the rock or a similar supporting surface, whereby all the joints of the drilling equipment are subjected to the tensile stress, or alternatively, only desired drilling components are struck, in which case striking is performed when the drilling equipment is supported by grip  12  provided in the feeding beam. 
   An advantage of the invention is that by generating tensile stress in the drilling equipment the joints between the drilling components can be opened easier. Also stuck threaded joints open without difficulty. The invention improves the efficiency of the rock drill because the time needed for changing drilling components or for extending/dismantling a drill rod string is shorter and thus more time will be available for actual drilling. The arrangement according to the invention is rather simple to install afterwards in existing rock drills or rock drilling devices. 
   The drawings and the related description are only intended to illustrate the inventive concept. The details of the invention may vary within the scope of the claims.