Abstract:
In a hydraulic rock drill the valve (27) is controlled by two control lines (37, 42), each control line having a plurality of branches with ports (38-41 and 43-46) which open into the hydraulic cylinder of the rock drill. 
     A valving pin (48) is slidable in a bore that intersects all of the branches of both control lines. By axially displacing the pin, the operator can pre-select the stroke length by deactivating some of the control lines and thereby the impact energy per blow.  The control lines are deactivated in a predetermined bound relationship to each other.

Description:
This invention relates to a hydraulically operated impact device, for example a rock drill, comprising a housing, a cylinder in the housing, an anvil means, a hammer piston which is reciprocably mounted in said cylinder and arranged to impact upon said anvil means, and port means in said cylinder cooperating with the hammer piston in order to control the reciprocation of the hammer piston and initiate the work stroke when the hammer piston reaches a predetermined variable rear position during its return stroke and initiate the return stroke when the hammer piston reaches a variable forward position during its work stroke. 
     In British Patent Specification No. 1,550,520, such a hydraulic impact device is described that has two sets of ports. The sets of ports are used independently of each other to vary the impact energy. The selection of ports of one of the sets is used to vary the stroke length and the selection of ports of the other set is used to vary the effective length of a work stroke, i.e. to retard the piston during a selected end portion of the work stroke. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide for a simple and efficient selection of the impact energy. This is achieved mainly by the provision of means for simultaneously varying the predetermined forward and rear positions defined above in a bound relationship. By this arrangement, the stroke length can be easily varied and the piston can be accelerated during its entire work stroke independently of the selected stroke length. As a result, the impact device maintains a high rate of efficiency when the stroke length is varied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic longitudinal section through a hydraulic jack hammer or rock drill according to the invention. 
     FIG. 2 is a schematic longitudinal section through another rock drill according to the invention. 
     FIG. 3 is a fragmentary longitudinal view showing an alternative design of a selector pin shown in FIG. 2 and an actuation device for the pin. 
    
    
     DETAILED DESCRIPTION 
     The impact device shown in FIG. 1 is a hydraulic rock drill, a hydraulic jack hammer or the like. It comprises a housing 11 forming a cylinder 12 in which a hammer piston 13 is reciprocable to impact upon an anvil element 14, for example a chisel, a rock drill stem or an adapter for a rock drill stem. A shoulder 15 on the anvil element takes support on a sleeve 16 that abuts against a recoil damping piston 17. The damping piston 17 is forced forwardly into its foremost position as shown by the hydraulic pressure in a cylinder chamber 18 that is constantly pressurized through a passage 19. The hammer piston 3 has two lands 20, 21 so that a front cylinder chamber 22, a rear cylinder chamber 23 and an intermediate cylinder chamber 24 are formed between the piston 13 and the cylinder 12. The piston 13 is driven forwardly by the pressure acting on its surface 25 and driven rearwardly by the pressure acting on its surface 26. A valve 27 is connected to an inlet 28 coupled to a source of high pressure hydraulic fluid and to an outlet 29 coupled to tank. Accumulators 30, 31 are coupled to the inlet 28 and the outlet 29. The intermediate cylinder chamber 24 is constantly connected to the outlet 29 by means of a passage 29a. The valve 27 is coupled to the rear cylinder chamber 23 by means of a supply passage 32 and to the front cylinder chamber 22 by means of a supply passage 33. The valve 27 has a valving spool 34 which in its illustrated position connects the rear cylinder chamber 23 to pressure and the front cylinder chamber 22 to tank. The spool 34 has cylindrical end portions 35, 36, the end faces of which have piston surfaces that are subject to the pressure in control passages 37, 42 that each are branched into four branches so that they each have four ports 38, 39, 40, 41 and 43, 44, 45, 46 respectively into the cylinder 12. A cylindrical bore 47 intersects all eight branches and a cylindrical pin 48 is slidable with a tight fit in the bore 47. This pin 48 has two recesses 49, 50 and it can be positively locked in four defined axial positions by means of a lock bolt 51. 
     The operation of the impact device of FIG. 1 will now be described. 
     The hammer piston 13 is shown in FIG. 1 moving forwardly in its work stroke (to the left in FIG. 1), and the valve spool 34 is then in its illustrated position. When the port 45 of the control passage 42 is opened to the rear cylinder chamber 23, the control passage 42 will convey pressure to the control piston 36 so that the valve spool 34 is moved to the right in FIG. 1. The valve spool 34 should preferably finish its movement at the very moment the hammer piston 13 impacts upon the anvil 14. Thus, the pressure existing from the moment of impact in the front cylinder chamber 22 moves the hammer piston 13 rearwardly until the branch 40 of the control passage 37 is opened to the front pressure chamber 22. Then, the control passage 37 conveys pressure to the control piston 35 which moves the valve spool 34 back to its illustrated position so that the rear cylinder chamber 23 is again pressurized. The pressure in the rear cylinder chamber 23 retards the hammer piston 13 and accelerates it forwardly again so that the hammer piston 13 performs another work stroke. 
     The valve spool 34 has annular surfaces 52, 53 and internal passages 54, 55 which hold the valve spool in position during the periods when the control pistons 35, 36 do not positively hold the piston. The annular surfaces 52, 53 are smaller than the end faces of the pistons 35, 36. 
     When the pin 48 is in its illustrated position, the port 40 of the control passage 37 and the port 45 of the control passage 42 are the ports that make the valve spool shift position. The other ports are inactivated. In the other three positions of the pin 48 one pin of the three pairs of ports 38, 43; 39, 44 and 41, 46 respectively is selected to cooperate to control the valve. 
     The first one of the ports 38-41 that is opened to the front cylinder chamber 22 during the return stroke of the hammer piston initiates the valve spool 34 to shift position. Thus, by adjusting the axial position of the pin, the operator pre-selects the stroke length of the hammer piston. The axial distances between the ports 43-46 are smaller than the corresponding distances between the ports 38-41. The axial positions of the ports 43-46 in the cylinder are such that for each stroke length the selected one of the ports 43-46 is uncovered a distance before the impact position of the hammer piston, and the distance is such that the valve spool has just moved to its position for pressurizing the front pressure chamber when the hammer piston 13 impacts the anvil 14. If the pump pressure is constant, the selected port is uncovered the same period of time before impact occurs independently of which one of the four ports is selected. 
     In FIG. 2, a rock drill is shown that has a hammer piston 13 with a single land 60. A shaft 61 is rotated by a non-illustrated hydraulic motor and coupled to rotate a chuck bushing 62. The drill steel adapter 14 has a non-circular widened portion 63 which engages with the chuck bushing 62 to rotate conjointly with the latter. The adapter 14 and other details that correspond to details in FIG. 1 have been given the same reference numerals in FIG. 2 as in FIG. 1, as for example the valve 27, the control passages 37, 42 and their branches with ports 38-41 and 43-46 respectively, the pin 48 and the supply passages 32, 33 to the front cylinder chamber 22 and to the rear cylinder chamber 23. The supply passage 32 is in this embodiment not controlled by the valve 27, but it is constantly pressurized from the inlet 28. The piston surface 26 is larger than the piston surface 25. The piston 13 is moved forwardly by the pressure acting on the surface 25 and it is moved rearwardly by the pressure acting on the differential area of the surfaces 26 and 25. Since, in contrast to FIG. 1, there is no intermediate cylinder chamber, the valve 27 is somewhat more complicated and the control passage 42 has another branch with a port 64 into the cylinder. The valve 27 has a plunger 65 that is separate from the valve spool 34. 
     The operation of the valve 27 will not be described, but reference is made to European Patent Application No. 79850095.5 (corresponding to European publication No. 0 010 532, and to U.S. Application Ser. No. 85,409, now U.S. Pat. No. 4,349,075 which is incorporated herein by way of reference and which describes the operation of the valve in detail. 
     In FIG. 2, the pin 48 is manually controlled, as in FIG. 1, but in FIG. 3 an alternative design is shown, in which the pin 48 is hydraulically remote controlled. On the end of the pin there is a piston 66 which is biassed to the right in FIG. 3 by means of a spring 67. 
     In FIG. 3, there is shown that there need not be a separate control line but that the outlet line 29 leading to tank can be used to convey the control pressure. This outlet line 29 can be pressurized through the pressure regulator 75. It is of course not possible to select the stroke length during drilling when the control system according to FIG. 3 is used, but it is usually not desirable to make the selection during drilling. 
     A valve 74 in the outlet line 29 holds normally the outlet line 29 open to tank, but it has an alternative position in which it is shown in FIG. 3. In this alternative position it connects a pressure regulator 75 to the outlet line 29. The pressure regulator 75 is coupled to the pump pressure. When the operation of the drill is interrupted and the valve 74 is shifted to its illustrated position, the lock pin 51 is released and the pressure from the pressure regulator 75 moves the piston 66 and thereby the selector pin 48 into an axial position in which the hydraulic pressure on the piston 66 balances the spring force. By manual adjustment of the pressure regulator 75, the axial position can be pre-selected. Then, when the valve 74 is switched back into its other position, the lock pin 51 moves into its position in which it positively locks the selector pin 48. In the inlet line 28, there is a manually operated supply valve 76. 
     As described with reference to FIG. 3, the outlet line 29 is used as a remote control line and the valve 74 and the pressure regulator 75 can be located at the operator&#39;s panel. Alternatively, a separate remote control line can of course be used and other remote control systems than the illustrated one can be used. It is, however, advantageous to reduce the number of lines leading to the rock drill. 
     There are prior art hydraulic rock drills that have a single control line instead of two control lines, as in the described embodiments. The invention can easily be applied to such designs and to most other designs of hydraulic percussive devices and it is not limited to the illustrated embodiments.