GB1588525A

GB1588525A - Hydraulic drive

Info

Publication number: GB1588525A
Application number: GB1636277A
Authority: GB
Original assignee: Joy Manufacturing Co
Current assignee: Joy Manufacturing Co
Priority date: 1976-04-28
Filing date: 1977-04-20
Publication date: 1981-04-23
Also published as: YU109877A; BR7702614A; MX147859A; SE7704440L; DE2719002A1; IT1076743B; JPS52133001A; FR2349747A1; AU520326B2; FI771344A; AU2438877A; SU784805A3; ES458572A1

Description

(54) IMPROVEMENTS IN OR RELATING TO A HYDRAULIC DRIVE (71) We, Joy MANUFACTURING COM- PANY, a corporation organised and existing under the Laws of the State of Pennsylvania, United States of America, located at Oliver Building, 535 Smithfield Street, Pittsburgh, State of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a hydraulic drive, and more particularly to a hydraulic drive for the hammer of a rock drill.

It is well known in the art of rock drills to provide a drill assembly with a fluid actuated hammer comprised of a linear percussion motor of the valveless distribution type wherein a hammer piston is self-excited for rapid linear reciprocation to repetitively impact a striking member for the purpose of drilling rock or other hard formations.

Although such drills have generally served the purposes intended, they have nonetheless been subject to various deficiencies. For example, some drills have been subject to unduly inefficient transfer of mechanical energy from the hammer to the striking member. Some prior hammers have been extremely sensitive to small changes in such parameters as fluid supply pressure or temperature, or location of the striking member impact end, and have thus been impractical for reliable day-to-day field use. Fluid cavitation of hydraulic fluid passing from a high pressure to a low pressure state has been a further problem in many prior art hydraulic hammers. Other hammers have been very difficult to start. Still other hammers have been of bulky and cumbersome design, and excessively difficult and expensive to manufacture.

According to one aspect of this invention there is provided a hydraulic drive of the valveless distribution type, for actuating a tool structure comprising: a body member having an elongated bore therein with one end of said bore being adapted to receive at least a portion of an actuatable tool structure internally thereof, an elongated piston axially reciprocal within said bore to deliver impact blows to such a tool structure, said bore having an axially intermediate formed portion, said piston having an axially extending intermediate head portion cooperable with said intermediate formed portion to define chamber portions within said bore on axially opposite sides of said intermediate head portion which chamber portions vary inversely in volume as said piston reciprocates, a first passageway means in said body member for communicating with both said chamber portions and having hydraulic fluid flow through portions thereof controlled by said piston upon reciprocation thereof to provide for selective admission of hydraulic fluid to said chamber portions, and second passageway means in said body member having hydraulic fluid flow through a portion thereof controlled by cooperation of the axially spaced ends of said head portion of said piston upon reciprocation thereof with an axially extending discharge port associated with the said second passageway means, to provide for discharge of hydraulic fluid from said chamber portions alternately.

Preferably said first passageway means includes axially spaced portions communicating with said bore at points outwardly adjacent each end of said intermediate formed portion and said second passageway means communicates with said formed portion of said bore axially intermediate said axially spaced portions.

Preferably, in use of the drive, said discharge is substantially continuous.

Conveniently wherein one end of said bore in the body member is open; said piston being cooperable with said first and second passageway means to place the one of said chamber portions intermediate said head portion and said one end of said piston in selective substantially continuous communication with one of said first and second passageway means throughout the reciprocation of said piston.

Advantageously said second passageway means includes at least one accumulator portion.

Preferably said head portion has an axial extent on the outer periphery thereof of the same length as the axial extent of said discharge port.

Advantageously said head portion has an axial extent on the outer periphery thereof, and said axial extent on said head portion is of a length with respect to the length of the axial extent of said discharge port to provide substantially continuous discharge of hydraulic fluid.

Conveniently said intermediate formed portion comprises an axially intermediate chamber section of greater cross-sectional extent than the cross-sectional extent of the adjacent sections of said bore axially outwardly thereof; said head portion of the piston having a central portion with an axially extending outer peripheral surface closely slidably received within said chamber section to define said chamber portions, the first passageway means in said body member having axially spaced fluid inlet port means in communication with said bore axially outwardly of the axially spaced ends of said chamber section, respectively; said piston having formed axially spaced means cooperable with said port means, respectively, for selective admission of hydraulic fluid to said chamber portions alternately; said second passageway means in said body member having said discharge port with an axial extent in communication with said chamber section; and said outer peripheral surface having the axially spaced ends thereof cooperable with said axial extent of said discharge port to effect said control of the discharge of hydraulic fluid from said chamber portions.

Advantageously said outer peripheral surface has an axial length equal to said axial extent of said discharge port means.

Preferably said outer peripheral surface has an axial length with respect to said axial extent of said discharge port means to provide, in use, an at least substantially continuous flow of fluid through said discharge port means.

Conveniently said body member has at least one accumulator chamber therein in continuous communication with one of said chamber portions.

Preferably said body member has a pair of axially spaced accumulator chambers therein in continuous communication with said chamber portions, respectively.

Conveniently said accumulator chamber is annular in form and encompasses said bore in spaced relationship thereto.

Advantageously said accumulator chamber is in continuous communication with said one of said chamber portions through a plurality of passageway portions spaced circumferentially with respect to said chamber section.

Preferably each of said accumulator chambers is annular in form and encompasses said bore in spaced relationship thereto.

Conveniently each of said accumulator chambers is in continuous communication with. said chamber portions through a plurality of passageway portions spaced circumferentially with respect to said chamber section.

Advantageously said discharge port extends axially between said chamber portions.

Preferably said outer surface has an axial length less than said axial extent of said discharge port so that the flow of fluid through said discharge port is continuous.

Conveniently said first passageway means includes a passageway within said body member for maintaining continuous communication between said inlet port means.

Advantageously said first passageway means has annular chamber portions within said body member spaced outwardly of said inlet port means, respectively, and said annular chamber portions encompass said bore.

Preferably said annular chamber portions are in communication with said inlet port means through axially spaced pluralities of passageways, respectively, and said pluralities of passageways being spaced circumferentially with respect to said bore with one end thereof forming said inlet port means, respectively.

Conveniently said formed axially spaced means of said piston include axially outermost portions cooperable with portions of said bore intermediate said chamber portions and said inlet port means, respectively, selectively to discontinue fluid flow communication between said chamber portions and said inlet port means during reciprocation of said piston.

Preferably said piston is symmetrical with respect to the central transverse plane thereof.

Conveniently hydraulic fluid in one of said chamber portions is effective upon said piston to move said piston through a work stroke and the hydraulic fluid in the other of said chamber portions is effective upon said piston to move said piston through a return stroke, with the hydraulic fluid in said one of said chamber portions being effective upon said piston for a longer period of travel during said work stroke without discharge of hydraulic fluid from said one of said chamber portions than the hydraulic fluid in said other of said chamber portions is effective upon said piston without discharge of hydraulic fluid from said other of said chamber portions during said return stroke.

Preferably said inlet port means are cooperable with said axially spaced means of said piston to provide unequal flow of hydraulic fluid to said chamber portions when the central transverse plane of said head section of said piston is coincident with the central transverse plane of said axial extent of said discharge port means.

Advantageously the rate of flow of hydraulic fluid to the chamber portion adjacent said one end of said bore is greater than the rate of flow of hydraulic fluid to the other of said chamber portions.

Preferably said body member has a plurality of hydraulic accumulators therein intermediate said fluid inlet port means which accumulators are in fluid flow communication with said bore.

Conveniently said first and second passageway means having respective annular chamber portions located laterally outwardly of said bore with an open extent at the outer surface of said body member, a shell member having an elongated bore therein, said shell member encompassing at least a portion of said body member to form hydraulic fluid chambers in conjunction with each of said annular chamber portions with the inner surface of said bore of said shell member forming tthe outermost extent of each of said fluid chambers.

Preferably the outer surface of said body member encompassed by said shell member and the inner surface of said bore of said shell member are cylindrical.

Conveniently, the outer surface of said body member engages the inner surface of said shell member to produce a stress within said shell member to retain said shell member on said body member.

Advantageously said chambers consist of all the fluid chambers utilised in reciprocating said piston.

Preferably said piston has guide portions extending in opposite axial directions with respect to said head section which guide portions are closely received within said bore and drain passageways cooperable with said guide portions respectively throughout the reciprocation thereof to discharge any flow of hydraulic fluid from said chamber portions along said guide portions.

Conveniently at least one of said drain passageways is connected to a reservoir.

Advantageously said drive has additional relatively movable parts having a leakage path therebetween to atmosphere and at least one of said drain passageway means communicating with said leakage path.

Preferably said formed axially spaced means of the piston are circumferentially continuous grooves.

Advantageously said piston has land portions extending axially from opposite ends of said head portion, respectively, to maintain a fluid pressure within said chamber portions during said discharge of hydraulic fluid, and said fluid pressure being of a magnitude to minimize cavitation in said chamber portions.

Conveniently said land portions are identical in configuration.

Preferably said land portions are tapered at an angle with respect to the central longitudinal axis of said piston.

Conveniently said taper is in the range of 5 to 15 degrees.

In one embodiment said piston has formed portions at the ends of said axially extending head portion of a configuration to provide for increased flow of hydraulic fluid through said second passageway means after initiation of flow there through during reciprocation of the piston.

Preferably said formed portions are identical in configuration.

Conveniently said formed portions extend axially outwardly from said head and are tapered at an angle with respect to the central longitudinal axis of said piston with the smaller end of each of said formed portions being axially spaced from said head.

Advantageously said taper is at an angle in the range of 5 to 15 degrees.

Preferably said configuration is of a form to maintain a pressure within the fluid in said chamber portions during such flow.

Advantageously said land or formed pom tions are coaxial with the central axis of said piston.

The invention also provides a method of moving an elongated impact piston forming part of a drive according to said one aspect of the invention through a work output stroke, thhe bore of the body member containing hydraulic fluid and said piston has an intermediate head portion within said bore whereby piston actuating and piston return chambers are formed within said bore on opposite sides of the head portion, said method comprising moving said piston through an initial portion of such a work output stroke by admitting pressurized hydraulic fluid to said actuating chamber throughout said initial portion while simultaneously pressurizing fluid in a predetermined volume in continuous hydraulic communication with said actuating chamber and discharging hydraulic fluid from said return chamber, discontinuing said admitting and moving said piston through an intermediate portion of such a work output stroke immediately subsequent to said initial portion by the hydraulic fluid in said predetermined volume and said actuating chamber while simultaneously discharging hydraulic fluid from said return chamber, and moving said piston through a final portion of such a work output stroke immediately subsequent to said intermediate portion by the hydraulic fluid in said predetermined volume and said actuating chamber while simultaneously discharging hydraulic fluid from said actuating chamber and discontinuing said discharging of said return chamber.

Preferably said method includes admitting pressurized hydraulic fluid to said return chamber throughout said final portion to provide at least in part pressurized hydraulic fluid for returning said piston to the position at which such a work output stroke is initiated.

Conveniently the method also includes simultaneously admitting pressurized hydraulic fluid throughout said final portion to a predetermined volume in continuous hydraulic communication with said return chamber.

Alternatively the method may include admitting pressurized hydraulic fluid to said return chamber throughout said final portion wh'ile simultaneously pressurizing hydraulic fluid in another predetermined volume in continuous hydraulic communication with said return chamber, moving said piston through an initial portion of a return stroke immediately subsequent to said final portion of such a work output stroke by continuing the admitting of pressurized hydraulic fluid to said return chamber and said another closed volume throughout said initial portion of such a return stroke while simultaneously discharging hydraulic fluid from said actuating chamber, discontinuing said last mentioned admitting and moving said piston through an immediate portion of such a return stroke immediately subsequent to said initial portion of such a return stroke by the hydraulic fluid in said another predetermined volume and said return chamber while simultaneously discharging hydraulic fluid from said actuating chamber, and moving said piston through a final portion of such a return stroke immediately subsequent to said intermediate portion of such a return stroke by the hydraulic fluid in said another closed volume and said return chamber while simultaneously discharging hydraulic fluid from said return chamber and admitting pressurized hydraulic fluid to said actuating chamber.

In one embodiment of the method said hydraulic fluid is discharged from said actuating chamber at a controlled rate whilst discontinuing said discharging of said return chamber.

In another embodiment of the method said hydraulic fluid is discharged from said return chamher at a controlled rate whilst admitting pressurized hydraulic fluid to said actuating chamber.

It will be appreciated that preferred embodiments of the invention may be embodied in a hydraulically operable self-porting hammer of compact and simplified design. The hammer having improved and simplified startup characteristics, and the hammer piston may have a longer useful life than ordinarily obtainable. The hammer may have improved efficiency of impact energy transfer and may have improved means to preclude pressure and fluid accumulations within portions thereof not intended to contain such pressure and fluid accumulations.

A control valve means may be operable to effect movement of the hammer piston to a starting position in its stroke from which self-excited - piston reciprocation is readily achieved upon introduction of motive fluid flow thereto, and the efficiency of operation may be insensitive to comparatively large variations in the location of the striking end of the impact receiving member.

The dwell time during which the hammer piston contacts the striking element during impact may be extended over that of known hammer.

As will be appreciated the hammer may have fluid energy absorbing or accumulating means communicating with the upstroke and downstroke sides of the hammer piston, and the hammer piston may be formed to control exhaust fluid flow.

In one embodiment the hammer piston may be selectively reversible in the piston bore such that either end thereof may be used for impacting; if desired such reversing of the hammer piston may also provide for selective varying of the hammer operating cycle.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a rock drill including hydraulically actuated hammer means constituting one embodiment of this invention; Figure 2 is an axial section of the drill shown in Figure 1 and taken on line 2-2 of Figure 3; Figure 3 is a transverse section taken on line 3-3 of Figure 2; Figure 4 is a fragmentary portion of the hammer means of Figure 1 showing the hammer piston in detail; and Figure 5 is a diagram of the relationship between hydraulic fluid pressure on the piston head and piston position in its stroke.

In Figure 1 a hydraulically actuated rock drill assembly 10 comprises a percussion head or motor portion 12 coaxially engaging a forward yoke portion 14. Suitably disc-like backhead and fronthead members 16 and 18 coaxially engage the rearward end of percussion head 12 and the forward end of yoke 14, respectively. Securing means such as a plurality of longitudinally extending side rods 20 rigidly clamp the hereinabove identified drill portions together to form the unitary drill assembly 10. Drill 10 is reversibly feedably mounted on an elongated feed frame 22, which frame 22 is in turn adjustably carried by any suitable mobile base such as a crawler frame and articulated boom assembly (not shown), and is supplied with motive fluid by suitable fluid hoses 24 communicating with drill 10 to actuate the drill 10 as hereinbelow described.

The yoke portion 14 (Fig. 2) comprises a generally annular yoke housing 28 having a generally annular, elongated chuck member 30 axially rotatably carried therewithin as by roller bearings 32. Chuck member 30 includes a plurality of circumferentially spaced gear teeth 34 coaxially encompassing an axially intermediate external peripheral portion thereof for engagement with a driving gear train (not shown) carried within housing 28 for rotation of chuck 30 as described hereinbelow.

The chuck carries coaxially therewithin an elongated annular rear bushing member 38 and an elongated annular drive member 40 forwardly adjacent bushing 38. Bushing 38 and drive member 40 are coaxially aligned with an annular forward bushing 46 secured within an inner peripheral portion 48 of front head 18 as by a nut 50 coaxially threadably engaging front head 18 whereby an elongated, generally cylindrical striking bar 42 extending coaxially within chuck member 30 and front head 18 has its axially opposed end portions longitudinally slidably supported within inner peripheral portions 44 and 45 of bushings 38 and 46, respectively.An externally splined intermediate portion 54 of striking bar 42 extending intermediate the respective supported end portions thereof is engageable within a cooperably splined internal peripheral portion 56 of drive member 40, and drive member 40 is non-rotatably splined to chuck 30 as at 58. Accordingly, striking bar 42 is axially rotatable as by a suitable rotation motor such as a pressure fluid actuated motor 36 which receives motive fluid through suitable supply lines (not shown) to drive the chuck and striking bar assembly in coaxial rotation through the above-mentioned gear train.

As indicated hereinabove striking bar 42 is axially slidable within chuck 30. In its extreme rearward position (Fig. 2) defined by abutment of cooperably formed, respective end portions 62, 66 of striking bar intermediate portion 54 and bushing 38, a rearwardmost end or impact surface 68 of striking bar 42 is positioned adjacent the forward-end of percussion head 12 to receive impact blows therefrom. Inasmuch as the hereinabove described yoke portion 14 forms no part of the present invention and is well known to those versed in the relevant arts, further detailed description thereof is omitted herefrom.

Percussion head 12 (Figs. 2 and 3) comprises an elongated formed member or cylinder 72 such as a machined steel casting, and an elongated cylindrical shell 74 coaxially rigidly encompassing cylinder 72 and axially coextensive therewith. Percussion head 12 has a plurality of chambers 76A to 76D, hereinafter collectively identified as chambers 76, and preferably formed as a plurality of axially spaced and aligned annular cavities 78 extending radially inwardly of the exterior periphery of cylinder 72 whereby an adjacent inner periphery 80 of shell 74 forms the radially outermost wall of the chambers 76. The chambers 76 are axially spaced apart by intervening radially outwardly extending partitions 82, each having an outer annular periphery 84 which sealing engages the inner periphery 80 of shell 74 to preclude fluid communication between adjacent chambers 76.Other radially outwardly extending partitions 82 are formed adjacent the forward and rearward axial end portions of cylinder 72 sealingly to engage respective axial end portions of periphery 80 thereby defining the outer or end walls of the end chambers 76A and 76D, respectively.

Shell 74 and cylinder 72 are preferably assembled by a shrink fitting process as by being initially formed for an interference fit therebetween at ambient temperature. For assembly the shell 74 is heated and/or the cylinder 72 cooled thermally to produce a diametrical clearance therebetween. After assembly the shell 74 and cylinder 72 equalize to ambient temperature to diminish the diametrical clearance therebetween and provide a continuous, fluid tight face seal as described without recourse to elastomeric sealing members and the like.

Cylinder 72 has an elongated annular liner assembly 90 retained within a stepped coaxial through bore 92 thereof and comprising an elongated member or sleeve 94 and an elongated buffer ring 102 coaxially disposed within a rearward and peripheral portion 100 of sleeve 94. The coaxially communicating inner peripheries of buffer ring 102 and sleeve 94 define a coaxial through bore 88 wherein an elongated, stepped cylindrical piston 70 is axially reciprocably disposed.

Bore 88 has respective axially spaced forward and rearward bearing portions 96, 104 which slidably support therewithin respective axially spaced forward and rearward stem portions 98, 981 of piston 70. An enlarged diameter intermediate portion 106 of the bore 88 extending intermediate the respective bearing portion 96, 104 has disposed therewithin a generally stepped cylindrical intermediate or head portion 108 of piston 70.

Respective variable volume upstroke and downstroke piston driving chambers 110, 112 are formed adjacent respective forward and rearward sides of piston head 108 by axially spaced annular peripheral clearance spaces between the head 108 and bore portion 106.

Piston 70 is cooperable with bore 88 to provide for porting of pressurized motive fluid alternately to and from driving chambers 1110, 112, for self-excitation of the piston 70 as described hereinbelow.

Backhead 16 is rigidly clamped by side rods 20 adjacent the rearward end of percussion head 12 in compressive axial engagement with suitably formed bearing surface portions of percussion head 12, for example coaxial bearing annuli 114 and 116 formed by rearwardly facing axial end portions of the cylinder 72 and shell 74, respectively.

Backhead 16 similarly engages the rearward end 118 of buffer ring 102 which, in turn has a forward end portion 120 thereof, axially engaging a cooperable, annular, rearwardly facing shoulder 122 formed upon the inner periphery of sleeve 94. Sleeve 74 seats within a rearward end portion of bore 92 by engagement of cooperably axially abutting shoulder portions formed on respective adjacent peripheral portions thereof as at 124 whereby the applied clamping forces of side rods 20 serve rigidly to seat liner assembly 90 within bore 92. The sleeve 94 and buffer ring 102 furthermore are nonrotatably affixed with respect to each other and cylinder 72 as by suitable keys or shear pins (not shown) fitted into cooperably formed keyways.

The drilling apparatus 10 further comprises a flushing fluid means generally indicated at 11 and comprising a tube 126 disposed within suitably formed delivery passageways extending coaxially within backhead 16, piston 70 and striking bar 42, and including a fluid inlet 128 in backhead 16 for directing flushing fluid such as air or water hereinto for cleaning detritus from the bore hole.

Drilling apparatus 10 has fluid supply means as follows for delivery of motive fluid to actuate piston 70. A motive fluid inlet connection 130 extends radially through shell 74 for communicating an external source of motive fluid flow such as a constant flow pump 132 via a fluid line 134 into chamber 76A which is of a volume to provide a reservoir of pressurized motive fluid for delivery to the upstroke and downstroke driving chambers 110, 112. When motive fluid is supplied to the respective driving chambers 110, 112 the fluid response is supplied primarily by the chamber 76A whereby the percussion head 12 need not depend directly on pump 132 for immediate fluid flow response and large pressure fluctuations in the supply line 134 are thus avoided.During inlet deadband cycle portions (to be described hereinbelow) when all fluid inlets to the chambers 110, 112 are closed, pump 132 recharges chamber 76A for the next fluid inlet opening. Chamber 76A communicates by means of a plurality of circumferentially spaced and generally radially extending bores 134 with a downstroke inlet annulus 136 extending radially outwardly of bore portion 104 axially rearwardly of the bore portion 106. The radial bores 134 are intersected by a respective plurality of axially extending passages 138 in cylinder 72 which in turn communicate via another plurality of radially extending bores 140 with an upstroke inlet annulus 142 extending radially outwardly of bore portion 96 axially forwardly of bore portion 106 whereby fluid communication between chamber 76 and the respective inlet annuli 136, 142 is constantly maintained.Similarly, the annular chambers 76B and 76D are in continuous fluid communication with the downstroke and upstroke driving chambers 112, 110 via axially spaced pluralities of circumferentially spaced and generally radially extending bores 144, 146, respectively, to provide respective downstroke and upstroke fluid energy accumulators for storing and releasing fluid pressure energy as described hereinbelow, and the remaining chamber 76C communicates via a similarly disposed plurality of radially extending bores 148 with an exhaust annulus 150 extending radially outwardly of bore portion 106 intermediate the axial ends thereof. The volume of chambers 110 and 112 and the associated chambers 76D, 76B is variable by movement of piston 70 alternately into and out of the chambers 110, 112.The percent volume variation is quite small, for example to the range of a fraction of 1% to approximately 5% in view of the limited compressibility of hydraulic fluids. The limits of percent volume variation may vary depending upon the particular fluid to be used. The respective pluralities of radial bores 144, 146 and 148 are circumferentially spaced intermediate the axial passages 138 (Fig. 3) to provide proper fluid flow as described.

Preferably, all of the respective pluralities of radially extending bores 144, 146 and 148 are spaced evenly about the circumference of cylinder 72 whereby the fluid flow therethrough to and from bore 88 produces no net side loading or torque upon the piston 70. Accordingly, piston 70 may readily be rotated by externally applied rotational impetus supplied for example by the rotating striking bar 42 during contact thereof with piston 70 at impact. Rotation of piston 70 within liner assembly 90 induces rotary viscous shear forces to provide a hydrodynamic lubricant fflm between the relatively rotating elements thereby improving the efficacy of piston lubrication to reduce wear and friction during piston reciprocation.

Additionally, the absence of torque and side loading on piston 70 as described permits continuing piston rotation during cycle portions between impact. To the extent that piston 70 is rotating concomitantly with striking bar 42 as each impact is initiated, the wear factor attributable to relative rotation between such impacting members during contact will be reduced. An exhaust outlet connection 152 communicates radially through shell 74 with exhaust chamber 76C and has a fluid line 154 connected thereto whereby exhaust fluid may be directed to a suitable fluid reservoir 156.

Because piston 70 (Fig. 4) is symmetrical about its medial transverse plane P-P, only one axial half portion of the illustrated piston, i.e., the upstroke half, will be described. The remaining piston half portion, i.e. the downstroke half, is the mirror image of the upstroke half. The reference characters applied to the downstroke half are primed characters to correspond to the hereinbelow described parts of the upstroke half of piston 70. The head portion 108 of piston 70 comprises a central axially extending annular land 158 axially slidable within bore portion 106 in cooperation with exhaust annulus 150 to provide exhaust porting or valving during piston reciprocation.A land 162 is formed with its largest diameter end portion 160, which is smaller than the diameter of land 158, located adjacent the axial end of land 158 and tapers radially and inwardly therefrom along its axial extent at a taper angle with respect to the central longitudinal axis of piston 70 in the range of about 5 to 150, preferable 10 , to provide controlled porting of exhaust fluid by uniformly increasing the outflow of pressure fluid to exhaust as the exhaust annulus 150 opens. Land 162 thereby reduces the possibility of undesirable fluid cavitation which might occur as a result of uncontrolled fluid pressure release to the exhaust.

Additionally, the taper on land 162 tends to promote non-turbulent flow of pressurized fluid from the respective driving chambers 110, 112 to the exhaust as piston head 108 alternately moves into each chamber 110, 112 during reciprocation, thereby reducing the tendency of the fluid within chambers 110, 112 to retard piston movement thereinto.

Axially spaced from the outer axial end of land 62 is an annular land 166 cooperable with an annular cavity 168 formed adjacent the interface of bore portions 106 and 96 to provide a fluid cushion in the event of excess piston over-travel during reciprocation. Extending axial intermediate the axially adjacent ends of lands 162 and 166 is an intervening portion 164 which may be formed with a uniform or a tapering diameter, depending upon the respective diameters of the portions of lands 162 and 166 joined thereby. Land 166 extends axially outwardly to terminate adjacent the stem portion 98. A radially inwardly extending annular inlet groove 170 to formed in stem portion 98 intermediate the axial ends thereof for providing fluid inlet porting or valving during piston reciprocation in cooperation with inlet annulus 142.The stem portion axially inward (or rearward) of groove 170 functions primarily as an inlet valve seat in cooperation with the respective portion of bore portion 96. The portion of stem 98 axially outward (or forward) of groove 170 is cooperable with the remainder of bore portion 96 for slidably supporting the piston 70 within bore 88. With the symmetrical piston 70 as described, the drill 10 may be assembled with either end of the piston 70 forward whereby an extended piston life is obtainable by reversing the piston 70 when the impact end thereof becomes worn after lengthy service. To this end, the bore portions 104 and 96 are arranged to engage equal axial lengths of the respective piston stem 981, 98 thereby ensuring the development of symmetrical wear patterns on the respective piston stem portions.That is, the axial lengths of bore portions 104 and 96 are proportioned with respect to the stroke of piston 70 to ensure that respectively axially opposed tip portions 13, 15 of respective stems 98, 981 are never slidably engaged within respective bore portions 96, 104 during piston reciprocation (Fig. 2).

Accordingly, after extended use the tip portions 13, 15 will have a larger diameter than the respective slidably supported stem portions 98, 981 which will have sustained measurable wear, and symmetrically located annular ledges (not shown) will thus have been developed therebetween whereby piston 70 may be reversed in its bore even after extended use without risk of mechanical interference between such annular ledges and the axially outward extremities of the bore portions 104, 96.

Drill 10 includes a percussion case draining means 172 (Figs. 2 and 3) employed in conjunction with annular wiper seals 174 encompassing piston stem portions 98, 981 intermediate the axial ends of each respective bore portion 96, 104. Limited fluid leakage past wipers 174 gradually accumulates adjacent the piston end portions as in cavity 176 and therefore, such cavities as 176 are suitably vented to preclude such fluid accumulations. In conjunction with each seal 174 an annular drain cavity 178 is formed in each bore portion 96, 104 axially inwardly of the respective seals 174 for containing any fluid which leaks axially outward along the periphery of the stem portions 98, 981 from the respective chambers 110, 112. Such fluid leakage is drained from each of the annuli 178 via one or more generally radially extending passages 180 to an axially extending passage 182 in cylinder 72.Passage 182 communicates with an annular drain cavity 184 which extends radially inward and outward of bearing annulus 114 intermediate cylinder 72 and backhead 16, and includes at least one radially extending slot 186 communicating radially across annulus 114 between the radially inward and outward portions thereof. Cavity 184 is isolated from the radially inwardly adjacent cavity 176 by the axial face seal 118 between backhead 16 and buffer ring 102. A drain connection 188 is provided in backhead 16 for correcting a fluid drain line 190 which leads to reservoir 156 with cavity 184 so that a fluid flow path is established for dissipating fluid pressure and directing fluid leakage away from the axially inner sides of seals 174.

In general, fluid leakage in drill 110 could cause dangerously high pressures therewithin, for example on backhead 16, thereby precipitating catastrophic failure of the front or back heads 18, 16 or side rods 20. Accordingly, the inclusion of annular cavity 184 in the drain path as described precludes any pressure buildup therewithin by venting cavity 184 to reservoir 56 as part of the percussion case drain means 172. A similar provision may be utilized to preclude pressure buildup in the yoke housing 28 or between portions of the cylinder 72 and yoke 14.

The radial drain passage 180 communicating with the forward drain annulus 178 includes an axially elongated annular cavity 192 formed radially intermediate the liner 94 and cylinder 72 wherein fluid may accumulate to be tapped off for lubricating various portions of the drill. For example, a network of passages 194 (Fig. 2) communicates from annulus 192 through cylinder 72, yoke housing 28 and front head 18 to deliver fluid leakage from annulus 192 for lubrication of relatively rotatable forward end portions of the chuck 30 and front head 18.

A four way, open center valve 196 (Fig.

2) is interposed in fluid lines 134, 154 between the drill 10 and the pump 132 and reservoir 156 for controlling motive fluid flow to the drill 10. Valve 196 is selectively operable to a position A to connect pump 132 to inlet 130 and exhaust outlet 152 to reservoir 156, or to a position C for connecting pump 132 to exhaust outlet 152 and inlet connection 130 to reservoir 156 for a purpose to be described hereinbelow. Valve 196 additionally provides for a third position B (not necessarily intermediate the positions A and C) wherein motive fluid flows freely from pump 132 to all ports of valve 196 and back to reservoir 156 to equalize the fluid pressures in the drill inlet and exhaust chambers 76A, 76C. The position B provides a neutral or idle operating mode for such purposes as purging of air or impurities from the fluid in drill 10.

Operation of drill 10 is illustrated in Fig.

5 by the relationship of fluid pressure in the driving chambers 110, 112 and the respective energy accumulators 76D, 76B to piston position in its reciprocal travel. As piston 70 reciprocates within bore 88 the upstroke and downstroke inlet grooves 170, 1701 alternately communicate respective inlet annuli 142, 136 with respective upstroke and downstroke chambers 110, 112 and the associated accumulators 76D, 76B (hereinafter referred to respectively as the "upstroke side" and the "downstroke side" of the piston) to act upon the differential areas formed by the diameter differential between land 158 and respective stems 98, 981. Likewise, during piston reciprocation land 158 alternately communicates axially opposed end portions of exhaust annulus 150 with the upstroke and downstroke sides of piston 70 to intermittently exhaust pressurized fluid therefrom.For illustrative clarity the piston displacement scale in Fig. 5 is greatly extended over the actual stroke of the piston in the described embodiment, which is a very short stroke on the order of Sg". Furthermore, the illustrated range of pressure values may be varied widely and is therefore not to be considered a limitation on the invention as described.

Drill 10 is of the valveless or self-exciting type wherein grooves 170, 1701 and land 158 of piston 70 cooperate with respective annuli 142, 136 and 150 to valve motive fluid to and from the upstroke side and downstroke side of the piston 70 in response to the position of piston 70 in its stroke. The respective inlet and exhaust ports thus formed provide fluid flow rate control over a continuous range from a fully open state to a fully closed state as indicated by continuously variable flow resistances R1 through R4 in Fig. 4. By virtue of peripheral clearances between piston 70 and bore 88 adjacent the respective inlet and exhaust ports a degree of fluid flow is maintained even when the ports are "closed" as at R for example.

A balanced or equilibrium piston position illustrated as being to the upstroke side of the midstroke position (Fig. 4) is defined for the piston 70 whereat the upstroke and downstroke sides of piston head 108 are subejected to equal and opposite motive fluid forces. In terms of the indicated flow resistance the balanced position of piston 70 is defined as that position for which R1/R2 equals R3/R4. That is, the ratio of inlet pressure drop to exhaust pressure drop on the downstroke side of piston head 108 is equal to the ratio of inlet pressure drop to exhaust pressure drop on the upstroke side.

In other words, the total pressure drop from inlet chamber 76A to exhaust chamber 76C is proportioned identically between the respective inlet and exhaust ports for both the upstroke side and the downstroke side of the piston. Since the total pressure drop from chamber 76A to chamber 76C is the same for any path therebetween, and since such pressure is identically proportioned between the respective inlet and exhaust ports on both the upstroke and downstroke sides of the piston head 108, the net effective pressure acting on either side of piston head 108 will be equal for the balanced piston position. There is no general requirement for equality among any of the flow resistances Rl through R4 at piston equilibrium so long as the indicated ratios hold.For example, assume hypothetically that R2 exceeds R1 and R4 exceeds R2 at the piston equilibrium position. It follows then that R3 + R4 exceeds R, + R2 (the total flow resistance from inlet 76A to exhaust 76C is greater for the downstroke side than for the upstroke side) and thus a larger proportion of the total fluid flow from inlet to exhaust will pass through the upstroke side. Nevertheless, so long as the piston 70 resides at the equilibrium position such that R1/R2 equals R3/R4, equal net effective pressures will act on each side of the piston head 108 in spite of the unequal flow, and the piston 70 thus will not be urged in either the upstroke or the downstroke direction by fluid pressure.

The above-described relationship of flow resistance is not dependent upon the particular dimensions or form of piston 70 and liner assembly 90. In general the various porting land and groove widths, circumferential clearances, port spacing, taper angles and the like may be varied to provide the described flow resistance relationships to define a suitable piston equilibrium position.

An additional requirement that R4 not be equal to R2 for the equilibrium position provides for simplified drill startup as hereinbelow described. For initial operation, pump 132 is providing fluid at the full flow rate for the neutral operating mode with valve 196 in the B position whereby fluid circulates freely from pump 132 through valve 196 and back to reservoir 156, and additionally to both inlet and exhaust chambers 76A, 76C to completely flood all fluid flow passages and equalize fluid pressure throughout drill 10. To begin piston reciprocation valve 196 is shifted from the B position to either the A or C position to port full motive fluid flow through drill 10.

In the A position chamber 76A is pressurized by pump 132 and chamber 76C is exhausted to reservoir 156 whereby the piston 70, which in general will not reside at the hereinabove defined equilibrium position, will be urged toward its equilibrium position by the inlet-to-exhaust pressure differential. As an example, assume that the piston 70 initially is positioned in the upstroke direction from its equilibrium position with valve 196 in the A position. It follows that Rl and R2 are greater (ports more fully closed) and R2 and R2 are less (ports more fully open) than when piston 70 is at equilibrium.Accordingly, R1/R2 will be greater than R/R4 (the proportion of total pressure drop through the upstroke side inlet exceeds the proportion of total pressure drop through the downstroke side inlet) whereby a net unopposed fluid pressure component acts on the downstroke side to urge piston 70 in the downstroke direction toward its equilibrium position. Similar considerations apply if piston 70 initially resides downstroke from its balanced position whereat Rl/R2 is less than R3/R, and an unopposed fluid pressure thus acts on the upstroke side to urge piston 70 toward the equilibrium position.In either case as the piston 70 approaches equilibrium, the ratios R1/R2 and R3/R4 approach equality and the net unopposed fluid pressure component acting on piston head 108 approaches zero.

From the above analysis it will be clear that if piston 70 overtravels its equilibrium position from either direction under the impetus of an unopposed fluid pressure component, an oppositely directed fluid pressure component will be established to urge piston 70 back toward the equilibrium position.

Such repetitive piston overtravel of the equilibrium position alternately in the upstroke and downstroke directions constitutes the normal self-exciting piston reciprocation mode. Therefore, when valve 196 is placed in the A position piston 70 may immediately begin self-excited reciprocation, in which case drill startup is completed, or may come to rest at the equilibrium position. Should this occur drill startup may be effected by shifting valve 196 to the C position to pressurize exhaust chamber 76C and connect inlet chamber 76A to reservoir 156. Inasmuch as R2 and R4 are not equal at the equilibrium position as hereinabove mentioned (in this case R4 exceeds R2), the initial fluid pressure surge from chamber 76C will more readily pressurize the upstroke side of piston head 108 thereby urging piston 70 further upstroke and away from equilibrium.

With chamber 76C pressurized and chambers 76A exhausted, the condition Rl/R2 equals R3 /R4 for piston equilibrium still applies. However, in this case it is a very precarious equilibrium wherein any deviation of piston 70 from equilibrium results in a net unopposed fluid pressure component in the direction of such deviation which increases with increasing deviation to urge the piston further from equilibrium. Accordingly, with valve 196 in the C position the initial pressure surge from chamber 76C urges piston 70 in the upstroke direction such that R1/R2 increases and Ra/R4 decreases, and an unopposed fluid pressure component thus develops on the upstroke side to urge the piston from equilibrium.As the piston 70 deviates from equilibrium the upstroke fluid pressure component increases to urge piston 70 to the full upstroke position at which point R1/R2 greatly exceeds R3/R4. Accordingly, upon shifting valve 196 back to the A position a large unopposed fluid pressure component will act upon the downstroke side to urge piston 70 toward and past equilibrium whereby self-excited piston reciprocation is established as hereinabove described.

Once having been started the piston 70 will continue in self-excited reciprocation according to the cycle depicted in Fig. 5.

Piston 70 begins its downward stroke from the full upstroke position 200 (left ordinate of Fig. 5) whereat the downstroke side is fully open to inlet chamber 76A and is pressurized to near peak inlet pressure, 2500 psi for example. The upstroke side is open to exhaust chamber 76C and is at the exhaust back pressure, for example 200 psi as shown at 202. Exhaust back pressure is maintained by the various flow restrictions between annulus 150 and reservoir 156, for example the changing cross sectional shape of the exhaust passages 148, the length of fluid line 154 and so forth. The exhaust back pressure in conjunction with the hereinabove described piston tapers 160 prevents cavitation of pressurized fluid flowing to exhaust by maintaining a positive exhaust path fluid pressure at all times.As piston 70 begins to accelerate toward impact (to the right in Fig. 5) pressure on the downstroke side begins to fall along line 204 as the moving piston head 108 vacates chamber 112 to increase the volume thereof. Simultaneously the accelerating piston decreases the volume of chamber 110; however, because the exhaust remains open to the upstroke side during this cycle portion the fluid pressure in chambers 110 and 76D does not significantly increase, but remains essentially constant as indicated by line 206.As piston 70 reaches point 208 on line 204 in its downstroke the rearward edge of groove 170l passes the forward edge of annulus 136 and the inlet to the downstroke side closes (R3 increases substantially). Thereafter the continued operation of pump 132 charges inlet chamber 76A up to peak pressure along line 210 during an inlet deadband portion of the cycle while the piston continues to accelerate under the impetus of fluid pressure energy stored on the downstroke side in chambers 112 and 7B. Because the volume of chamber 112 continues to increase as piston head 108 vacates it, the pressure of fluid therein and in communicating chamber 76B continues to drop along line 212.Simultaneously the fluid pressure on the upstroke side of the piston (chambers 110 and 76D) begins to rise along line 214 as the volume of chamber 110 continues to decrease before the encroaching piston head 108 and the exhaust area open to the upstroke side decreases (flow resistance R2 increases).

At point 216 on line 212 land 158 is centered on annulus 150 (R4 equals R2) and thus upon further piston movement the exhaust chamber 76C is opened to the downstroke side and simultaneously closed to the upstroke side of piston head 108. Accordingly, the fluid pressure on the downstroke side drops rapidly along line 218 to the exhaust back pressure as the remaining fluid energy in chambers 112 and 76B is exhausted to chamber 76C. The indicated exhaust back pressure, although only a fraction of the peak driving pressure, continues driving the piston toward impact.Also substantially simultaneously with or very shortly after 216 in the cycle as at 2161 the inlet deadband cycle portion ends as chamber 76A is opened to the upstroke side to charge inlet fluid pressure energy into chambers 110 and 76D whereupon the pressure in chamber 76A drops from its peak value 220 and subsequently equalizes with the upstroke side pressure at 222. As the volume of chamber 110 further decreases and charging of fluid into the upstroke side continues the fluid pressure in the upstroke side rises along line 224 toward its peak value as the piston impacts upon the striking bar 42, indicated at 226 on the right ordinate of the Fig. 5.

The axial position of piston 70 at impact is not a fixed parameter of the drill but may be varied over a comparatively broad range of locations because during the piston downstroke, chamber 76D absorbs much of the energy input generated by piston head 108 movement into chamber 110 and the pressure fluid inflow from chamber 76A thereby reducing the net fluid pressure resistance to further piston downstroke travel. Such fluid pressure resistance, if not reduced, would otherwise dissipate a significant part of the piston kinetic energy before impact and render the drill substantially more sensitive to impact point location.Storage of fluid pressure energy in chamber 76D during the piston downstroke also provides an extended dwell time or contact period between piston 70 and -striking bar 42 during impact for more efficient impact energy transmission, and additionally provides an initial store of energy for accelerating the piston in the upstroke direction after impact.

As the cycle continues, the piston rebounds from striking bar 42 and begins to accelerate toward its full upstroke position under the impetus of the fluid energy in chamber 76D and simultaneously supplied from chamber 76A through the open inlet to the upstroke side. As piston head 108 vacates chamber 110 to increase the volume thereof the pressure therein drops along line 228 to point 230 whereupon the inlet to the upstroke side closes. Substantially simultaneously or if desired very shortly thereafter as at 2301, exhaust chamber 76C is opened to the upstroke side and closed to the downstroke side, and accordingly the upstroke side pressure drops sharply along line 232 as the remaining fluid energy in chambers 110 and 76D is exhausted.Chambers 112 and 76B which have been gradually pressurized along line 234 to point 230l during the upstroke are further pressurized along line 236 as the piston head 108 encroaches upon chamber 112 through the inlet deadband cycle portion. Also during the inlet deadband portion inlet chamber 76A is again pressurized by flow from pump 132 along line 238 to peak inlet pressure at 240. As the piston reaches point 209 in its upstroke, which is the end of the inlet deadband portion, the pressure fluid inlet opens to the downstroke side to once again charge pressure fluid thereinto from chamber 76A, the pressure in which falls off along line 242 and equalizes with the downstroke side pressure which continues increasing along line 244.

Fluid pressure on the upstroke side, which remains open to exhaust chamber 76C, continues to decrease along line 232 to the exhaust back pressure as the piston 70 travels to the full upstroke position. Under the impetus of fluid pressure accumulated within chambers 76A, 110 and 76B, the piston decelerates to a stop at the full upstroke position thereof with peak inlet pressure refusing further upstroke movement as indicated at 200, and immediately accelerates toward another impact to begin another cycle.

The explanation hereinabove of piston startup and cycling represents the inventor's best understanding of some of the applicable theoretical considerations and is not to be construed as the complete, final or authoritative explanation of the physical laws governing operation of this embodiment of the invention.

The description hereinabove discloses an improved hammer for a rock drilling apparatus of simplified and compact design, and having means to easily start piston reciprocation from a neutral position thereof, and to prevent fluid cavitation, and an improved self-exciting hammer piston cycle which provides for improved impact energy transfer efficiency. The invention additionally provides among other novel features, means for draining leakage from closed cavities which could otherwise become dangerously pres surized.

Notwithstanding the disclosure of a particular preferred embodiment of the invention, it is to be understood that the invention is susceptible of various alternative embodiments and numerous modifications without departing from the scope thereof as defined by the appended claims. For example: the yoke portion and fluid circuit means may take any of various suitable forms; the relative sizes of the various accumulator chambers may be varied as by packing portions thereof with arcuate plates (not shown); the piston may comprise any of a wide variety of reversible or nonreversible symmetrical or asymmetrical designs, and particularly reversible asymmetrical designs wherein reversing the piston in its bore provides a modified operating cycle; various alternative porting arrangements offering modified operating cycles within the scope of the invention may be employed such as a cycle including a short positive exhaust deadband portion during which chamber 76C is isolated from both the upstroke and downstroke sides of piston head 108, or a short negative deadband portion during which chamber 76C is open to both the upstroke and downstroke sides; the upstroke side and downstroke side piston differential areas are not necessarily equal; and the like.

WHAT WE CLAIM IS : - 1. A hydraulic drive of the valveless distribution type for actuating a tool structure comprising: a body member having an elongated bore therein with one end of said bore being adapted to receive at least a portion of an actuatable tool structure internally thereof, an elongated piston axially reciprocal within said bore to deliver impact blows to such a tool structure, said bore having an axially intermediate formed portion, said piston having an axially extending intermediate head portion cooperable with siad intermediate formed portion to define chamber portions within said bore on axially opposite sides of said intermediate head portion which chamber portions vary inversely in volume as said piston reciprocates, a first passageway means in said body member for communication with both said chamber portions and having hydraulic fluid flow through portions thereof controlled by said piston upon reciprocation thereof to provide for selective admission of hydraulic fluid to said chamber portions, and second passageway means in said body member

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. and additionally provides an initial store of energy for accelerating the piston in the upstroke direction after impact. As the cycle continues, the piston rebounds from striking bar 42 and begins to accelerate toward its full upstroke position under the impetus of the fluid energy in chamber 76D and simultaneously supplied from chamber 76A through the open inlet to the upstroke side. As piston head 108 vacates chamber 110 to increase the volume thereof the pressure therein drops along line 228 to point 230 whereupon the inlet to the upstroke side closes. Substantially simultaneously or if desired very shortly thereafter as at 2301, exhaust chamber 76C is opened to the upstroke side and closed to the downstroke side, and accordingly the upstroke side pressure drops sharply along line 232 as the remaining fluid energy in chambers 110 and 76D is exhausted.Chambers 112 and 76B which have been gradually pressurized along line 234 to point 230l during the upstroke are further pressurized along line 236 as the piston head 108 encroaches upon chamber 112 through the inlet deadband cycle portion. Also during the inlet deadband portion inlet chamber 76A is again pressurized by flow from pump 132 along line 238 to peak inlet pressure at 240. As the piston reaches point 209 in its upstroke, which is the end of the inlet deadband portion, the pressure fluid inlet opens to the downstroke side to once again charge pressure fluid thereinto from chamber 76A, the pressure in which falls off along line 242 and equalizes with the downstroke side pressure which continues increasing along line 244. Fluid pressure on the upstroke side, which remains open to exhaust chamber 76C, continues to decrease along line 232 to the exhaust back pressure as the piston 70 travels to the full upstroke position. Under the impetus of fluid pressure accumulated within chambers 76A, 110 and 76B, the piston decelerates to a stop at the full upstroke position thereof with peak inlet pressure refusing further upstroke movement as indicated at 200, and immediately accelerates toward another impact to begin another cycle. The explanation hereinabove of piston startup and cycling represents the inventor's best understanding of some of the applicable theoretical considerations and is not to be construed as the complete, final or authoritative explanation of the physical laws governing operation of this embodiment of the invention. The description hereinabove discloses an improved hammer for a rock drilling apparatus of simplified and compact design, and having means to easily start piston reciprocation from a neutral position thereof, and to prevent fluid cavitation, and an improved self-exciting hammer piston cycle which provides for improved impact energy transfer efficiency. The invention additionally provides among other novel features, means for draining leakage from closed cavities which could otherwise become dangerously pres surized. Notwithstanding the disclosure of a particular preferred embodiment of the invention, it is to be understood that the invention is susceptible of various alternative embodiments and numerous modifications without departing from the scope thereof as defined by the appended claims. For example: the yoke portion and fluid circuit means may take any of various suitable forms; the relative sizes of the various accumulator chambers may be varied as by packing portions thereof with arcuate plates (not shown); the piston may comprise any of a wide variety of reversible or nonreversible symmetrical or asymmetrical designs, and particularly reversible asymmetrical designs wherein reversing the piston in its bore provides a modified operating cycle; various alternative porting arrangements offering modified operating cycles within the scope of the invention may be employed such as a cycle including a short positive exhaust deadband portion during which chamber 76C is isolated from both the upstroke and downstroke sides of piston head 108, or a short negative deadband portion during which chamber 76C is open to both the upstroke and downstroke sides; the upstroke side and downstroke side piston differential areas are not necessarily equal; and the like. WHAT WE CLAIM IS : -

1. A hydraulic drive of the valveless distribution type for actuating a tool structure comprising: a body member having an elongated bore therein with one end of said bore being adapted to receive at least a portion of an actuatable tool structure internally thereof, an elongated piston axially reciprocal within said bore to deliver impact blows to such a tool structure, said bore having an axially intermediate formed portion, said piston having an axially extending intermediate head portion cooperable with siad intermediate formed portion to define chamber portions within said bore on axially opposite sides of said intermediate head portion which chamber portions vary inversely in volume as said piston reciprocates, a first passageway means in said body member for communication with both said chamber portions and having hydraulic fluid flow through portions thereof controlled by said piston upon reciprocation thereof to provide for selective admission of hydraulic fluid to said chamber portions, and second passageway means in said body member

having hydraulic fluid flow through a portion thereof controlled, by cooperation of the axially spaced ends of said head portion of said piston upon reciprocation thereof with an axially extending discharge port associated with said second passageway means, to provide for discharge of hydraulic fluid from said chamber portions alternately.

2. A hydraulic drive as claimed in claim 1 wherein said first passageway means includes axially spaced portions communicating with said bore at points outwardly adjacent each end of said intermediate formed portion and said second passageway means communicates with said formed portion of said bore axially intermediate said axially spaced portions.

3. A hydraulic drive according to claim 1 or 2 wherein, in use, said discharge is substantially continuous.

4. A hydraulic drive as claimed in claim 1 or 2 or 3 wherein one end of said bore in the body member is open; said piston being cooperable with said first and second passageway means to place the one of said chamber portions intermediate said head portion and said one end of said piston in selective substantially continuous communication with one of said first and second passageway means throughout the reciprocation of said piston.

5. A hydraulic drive as claimed in any one of the preceding claims wherein said first passageway means includes at least on accumulator portion.

6. A hydraulic drive according to any one of claims 1 to 4 wherein said intermediate formed portion comprises an axially inter mediate chamber section of greater cross sectional extent than the cross-sectional extent of the adjacent sections of said bore axially outwardly thereof; said head portion of the piston having a central portion with an axially extending outer peripheral surface closely slideably received within said chamber section to define said chamber portions, the first passageway means in said body member having axially spaced fluid inlet port means in communication with said bore axially outwardly of the axially spaced ends of said chamber section, respectively; said piston having formed axially spaced means cooperable with said port means, respectively, for selective admission of hydraulic fluid to said chamber portions alternately; said second passageway means in said body member having said discharge port with an axial extent in communication with said chamber section; and said outer peripheral surface having the axially spaced ends thereof cooperable with said axial extent of said discharge port to effect said control of the discharge of hydraulic fluid from said chamber portions.

7. A hydraulic drive as claimed in claim 6 wherein said outer peripheral surface has an axial length equal to said axial extent of said discharge port means.

8. A hydraulic drive as claimed in claim 6 wherein said outer peripheral surface has an axial length with respect to said axial extent of said discharge port means to provide, in use, an at least substantially continuous flow of fluid through said discharge port means.

9. A hydraulic drive as claimed in any one of claims 1 to 8 wherein said body member has at least one accumulator chamber therein in continuous communication with one of said chamber portions.

10. A hydraulic drive as claimed in any one of claims 1 to 8 wherein said body member has a - pair of axially spaced accumulator chambers therein in continuous communication with said chamber portions, respectively.

11. A hydraulic drive as claimed in claim 9 wherein said accumulator chamber is annular in form and encompasses said bore in spaced relationship thereto.

12. A hydraulic drive as claimed in claim 11 wherein said accumulator chamber is in continuous communication with said one of said chamber portions through a plurality of passageway portions spaced circumferentially with respect to said chamber section.

13. A hydraulic drive as claimed in claim 10 wherein each of said accumulator chambers is annular in form and encompasses said bore in spaced relationship thereto.

14. A hydraulic drive as set forth in claim 13 as dependent upon claim 6 wherein each of said accumulator chambers is in continuous communication with said chamber portions through a plurality of passageway portions spaced circumferentially with respect to said chamber section.

15. A hydraulic drive as claimed in any one of claims 1 to 14 wherein said discharge port extends axially between said chamber portions.

16. A hydraulic drive as claimed in claim 6 wherein said outer surface has an axial length less than said axial extent of said discharge port so that the flow of fluid through said discharge port is continuous.

17. A hydraulic drive as claimed in any one of claims 6 to 16 wherein said first passageway means includes a passageway within said body member for maintaining continuous communication between said inlet port means.

18. A hydraulic drive as claimed in any one of claims 6 to 17 wherein said first passageway means has annular chamber portions within said body member spaced outwardly of said inlet port means, respectively, and said annular chamber portions encompass said bore.

19. A hydraulic drive as claimed in claim 18 wherein said annular chamber portions are in communication with said inlet.port means through axially spaced pluralities of passageways, respectively, and said pluralities of passageways being spaced circumferentially with respect to said bore with one end thereof forming said inlet port means, respectively.

20. A hydraulic drive as claimed in any one of claims 6 to 19 wherein said formed axially spaced means of said piston include axially outermost portions cooperable with portions of said bore intermediate said chamber portions and said inlet port means, respectively, selectively to discontinue fluid flow communication between said chamber portions and said inlet port means during reciprocation of said piston.

21. A hydraulic drive as claimed in any one of claims 1 to 5 wherein said head portion has an axial extent on the outer periphery thereof of the same length as the axial extent of said discharge port.

22. A hydraulic drive as claimed in any one of claims 1 to 5 wherein said head portion has an axial extent on the outer periphery thereof, and said axial on said head portion is of a length with respect to the length of the axial extent of said discharge port to provide substantially continuous discharge of hydraulic fluid.

23. A hydraulic drive as claimed in any one of the preceding claims wherein said piston is symmetrical with respect to the central transverse plane thereof.

24. A hydraulic drive as claimed in claim 1 or any claim dependent thereon wherein hydraulic fluid in one of said chamber portions is effective upon said piston to move said piston through a work stroke and the hydraulic fluid in the other of said chamber portions is effective upon said piston to move said piston through a return stroke, with the hydraulic fluid in said one of said chamber portions being effective upon said piston for a longer period of travel during said work stroke without discharge of hydraulic fluid from said one of said chamber portions than the hydraulic fluid in said other of said chamber portions is effective upon said piston without discharge of hydraulic fluid from said other of said chamber portions during said return stroke.

25. A hydraulic drive as claimed in claim 1 or any claim dependent thereon wherein said inlet port means are cooperable with said axially spaced means of said piston to provide unequal flow of hydraulic fluid to said chamber portions when the central transverse plane of said head section of said piston is coincident with the central transverse plane of said axial extent of said discharge port means.

26. A hydraulic drive as claimed in claim 25 wherein the rate of flow of hydraulic fluid to the chamber portion adjacent said one end of said bore is greater than the rate of flow of hydraulic fluid to the other of said chamber portions.

27. A hydraulic drive as claimed in claim 6, 7 or 8 wherein said body member has a plurality of hydraulic accumulators therein intermediate said fluid inlet port means which accumulators are in fluid flow communication with said bore.

28. A hydraulic drive as claimed in claim 1 or any claim dependent thereon, wherein said first and second passageway means having respective annular chamber portions located laterally outwardly of said bore with an open extent at the outer surface of said body member, a shell member having an elongated bore therein, said shell member encompassing at least a portion of said body member to form hydraulic fluid chambers in conjunction with each of said annular chamber portions with the inner surface of said bore of said shell member forming the outermost extent of each of said fluid chambers.

29. A hydraulic drive as claimed in claim 28 wherein the outer surface of said body member encompassed by said shell member and the inner surface of said bore of said shell member are cylindrical.

30. A hydraulic drive as claimed in claim 28 or 29 wherein the outer surface of said body member engages the inner surface of said shell member to produce a stress within said shell member to retain said shell member on said body member.

31. A hydraulic drive as claimed in claim 30 wherein said chambers consist of all the fluid chambers utilised in reciprocating said piston.

32. A hydraulic drive as claimed in claim 1 or any claim dependent thereon, wherein said piston has guide portions.extending in opposite axial directions with respect to said head section which guide portions are closely received within said bore and drain passageways cooperable with said guide portions respectively throughout the reciprocation thereof to discharge any flow of hydraulic fluid from said chamber portions along said guide portions.

33. A hydraulic drive as claimed in claim 32 wherein at least one of said drain passageways is connected to a reservoir.

34. A hydraulic drive as claimed in claim 32 or 33 wherein said drive has additional relatively movable parts having a leakage path therebetween to atmosphere and at least one of said drain passageway means communicating with said leakage path.

35. A hydraulic drive according to claim 6 or any claim dependent thereon wherein said formed axially spaced means of the piston are circumferentially continuous grooves.

36. A hydraulic drive according to any one of the preceding claims wherein said piston has land portions extending axially from opposite ends of said head portion, respectively, to maintain a fluid pressure within said chamber portions during said discharge of hydraulic fluid, and said fluid pressure being of a magnitude to minimise cavitation in said chamber portions.

37. A hydraulic drive as claimed in claim 36 wherein said land portions are identical in configuration.

38. A hydraulic drive as claimed in claim 36 or 37 wherein said land portions are tapered at an angle with respect to the central longitudinal axis of said piston.

39. A hydraulic drive as set forth in claim 38 wherein said taper is in the region of 5 to 15 degrees.

40. A hydraulic drive according to any one of claims 1 to 35 wherein said piston has formed portions at the ends of said axially extending head portion of a configuration to provide for increased flow of hydraulic fluid through said second passageway means after initiation of flow therethrough during reciprocation of the piston.

41. A hydraulic drive as claimed in claim 40 wherein said formed portions are identical in configuration.

42. A hydraulic drive as claimed in claim 41 wherein said formed portions extend axially outwardly from said head and are tapered at an angle with respect to the central longitudinal axis of said piston with the smaller end of each of said formed portions being axially spaced from said head.

43. A hydraulic drive as claimed in claim 42 wherein said taper is at an angle in the range of 5 to 15 degrees.

44. A hydraulic drive as claimed in claim 43 wherein said configuration is of a form to maintain a pressure within the fluid in said chamber portions, during such flow.

45. A hydraulic drive as claimed in any one of claims 36 to 46 wherein said land or formed portions are coaxial with the central axis of said piston.

46. A method of moving an elongated impact piston forming part of a drive according to any one of the preceding claims through a work output stroke, the bore of the body member containing hydraulic fluid and said piston has an intermediate head portion within said bore whereby- piston actuating and piston return chambers are formed within said bore on opposite sides of the head portion, said method comprising moving said piston through an initial portion of such a work output stroke by admitting pressurized hydraulic fluid to said actuating chamber throughout said initial portion while simultaneously pressurizing fluid in a predetermined volume in continuous hydraulic communication with said actuating chamber and discharging hydraulic fluid from said return chamber, discontinuing said admitting and moving said piston through an intermediate portion of such a work output stroke immediately subsequent to said initial portion by the hydraulic fluid in said predetermined volume and said actuating chamber while simultaneously discharging hydraulic fluid from said return chamber, and moving said piston through a final portion of such a work output stroke immediately subsequent to said intermediate portion by the hydraulic fluid in said predetermined volume and said actuating chamber while simultaneously discharging hydraulic fluid from said actuating chamber and discontinuing said discharging of said return chamber.

47. A method as claimed in claim 46 including admitting pressurized hydraulic fluid to said return chamber throughout said final portion to provide at least in part pressurized hydraulic fluid for returning said piston to the position at which such a work output stroke is initiated.

48. A method as claimed in claim 47 including simultaneously admitting pressurized hydraulic fluid throughout said final portion to a predetermined volume in continuous hydraulic communication with said return chamber.

49. A method as claimed in claim 46 including admitting pressurized hydraulic fluid to said return chamber throughout said final portion while simultaneously pressurizing hydraulic fluid in another predetermined volume in continuous hydraulic communication with said return chamber, moving said piston through an initial portion of a return stroke immediately subsequent to said final portion of such a work output stroke by continuing the admitting of pressurized hydraulic fluid to said return chamber and said another closed volume throughout said initial portion of such a return stroke while simultaneously discharging hydraulic fluid from said actuating chamber, discontinuing said last mentioned admitting and moving said piston through an immediate portion of such a return stroke immediately subsequent to said initial portion of such a return stroke by 'thk hydraulic fluid in said another predetermined volume and said return chamber while simultaneously discharging hydraulic fluid from said actuating chamber, and moving said piston through a final portion of such a return stroke immediately subsequent to said intermediate portion of such a return stroke by the hydraulic fluid in said another closed volume and said return chamb'er while simultaneously discharging hydraulic fluid from said return chamber and admitting pressurized hydraulic fluid to said actuating chamber.

50. A method as claimed in any one of claims 46 to 49 wherein said hydraulic fluid is discharged from said actuating chamber at a controlled rate whilst discontinuing said discharging of said return chamber.

51. A method according to claim 50 as dependent on claim 49 wherein said hydraulic fluid is discharged from said return chamber at a controlled rate whilst admitting pressurized hydraulic fluid to said actuating chamber

52. A hydraulic drive substantially as herein described with reference to and as shown in the accompanying drawings.

53. A method of moving an impact piston according to claim 46 and substantially as herein described with reference to the accompanying drawings.