CA1062989A - Hammer - Google Patents

Abstract

HAMMER
Abstract of the Disclosure A hydraulically operable hammer and more particularly a hydraulically operable hammer having improved cycling of a hammer piston reciprocable therewithin.

Description

06298~ : ~
This invention relates to a hydraulic drive for actuating tool.
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 repetltively impact a striking member for the purpose of drilling rock or other hard formations. `
Although such drills have generally served the purposes intel~ded, they have nonetheless been subject to various deficiencies.
I ~or example, some such drills have been subJect to unduly inefficient transfer of mechanical energy from the hammer to the '-. : .:
striking member. Some prior hammerR have been extremely sensitive to small changes in 6uch parameters as fluid supply pressure or temperature, or location of the striking member impact end, and I 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 . ~ . - .
~ 20 start. Still other hammers have been of bulky and cumbersome l . ~.
design, ~nd excessively difficult and expensive to manufacture.
i These and other shortcomings of prior hammers are ; ` alleviated by the instant invention according to which there i8 .
provided a~ impr~vded and simplified hydraulic hammer of the ~ ~ valveless or self-actuating type. ;

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Generally, the objects oE this invention are to provide: .
(a) a hydraulically operable self-porting hammer of compact and simplified design; : :
(b) an improved operating cycle for a hydraulically :
operable hammer;
(c) a hammer having improved and simplified startup .
characteristics;
(d) a hammer piston having a longer useful life than ordinarily obtainable;
(e) a hammer having improved efficiency of impact energy transfer;
(f) a hammer having improved means to preclude pressure and fluid accumulations within portions thereof not intended to contain such pressure and fluid ,.:
accumulations.

In its broadest form, thereEore, the prèsent invention ;~ ~ provides a hydrau1ic drive for actuating a tool structure ~: ; -;~

: comprising: a body member having an elongated bore therein with a ~central longitudinal axls and with one end of the bore being adapted to receive at least a portion of an actuatable tool :

structure internally thereof; an eIongated piston axially 1 : :
reciprocal within the bore to deliver impact blows to such a tool structure; the bore having an axially intermediate chamber section of grea~er cross-sectional extent than the cross-: sectional extent of the adjacent sections of the bore axially `: ~ :
outwardly thereof; the piston having a formed axially intermediate . head section; the head section having a central portion with an - .
: axially extending outer peripheral surface closely slideably ~ ~-ywl/~; - 2 -'` 1C1 6Z~38~
received l~ithin the chamber section to define chamber portions within the chamber section on axially opposite sides of the central portion of the head section which chamber portions vary inversely in volume as the piston reciprocates; first passageway means in the body member having axially spaced fluid inlet port ~ :
means in communication with the bore axially outwardly of the : `
axially spaced endsA~he chamber section, respectively; the piston .
having formed axially spaced means cooperable with the port means, respectively, for selective admission of hydraulic fluid to the chamber portions alternately; second passageway means in the body member having a discharge port means with an axial extent in communication with the chamber section; and the outer peripheral ~
surface having tha axially spaced ends thereof cooperable with ~ .
the axial extent of the discharge por~ means to control the dis- .
charge of hydraulic f.luid from the chamber portions.
The present invention may also be broadly defined as a method of moving an elongated impact piston through a work output .
stroke withi.n an elongated bore of a body member in which the . . `
bore contains hydraulic fluid and the piston has an intermediate head portion within the bore whereby pist-on actuating and piston return chambers are formed w~ithin the bore on opposite sides of ~. .
~: ~ the head portion comprising,-moving the piston through an initial : .: :
portion of such a work output stroke by admitting pressurized hydraulic fluid to the actuating chamber throughout the initial portion whiIe simultaneously pressurizing f-luid in a closed volume . . - :~
in continuous hydraulic communication with the actuating chamber and discharging hydraulic fluid from the return chamber, dis~
continuing the admitting and moving the piston through . .

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lO~Z9~39 an intermediate portion of such a work output stroke i~nediately subsequent to the initial portion by the hydraulic fluid in the closed volume and the actuating chamber while simultaneously discharging hydraulic fluid from the return chamber, and moving the piston through a final portion of such a worlc output stroke immediately subsequent to the intermediate portion by the hydraulic fluid in the closed volume and the actuating chamber while simultaneously discharging hydraulic fluid from the actuating chamber and discontinuing the discharging of the return chamber.
Other features and advantages of the instant invention are more fully specified in the following description with reference to the accompanying figures in which:
Fig. 1 is a perspective view of a rock drill including hydraulically actuated hammer means constructed according to the ` principles of this invention;
Fig. 2 is an axial section of the drill shown in Fig~ 1 and taken on line 2--2 of Fig.3;
Fig. 3 is a transverse section taken on line 3--3 of Fig. 2;
Fig. 4 is a fragmentary portion of the hammer means of Fig. 1 showing the hammer piston in detail; and Fig. 5 is a diagram of the relationship between hydraulic fluid pressure on the piston head and piston position in its stroke.
In Fig. 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 back head and front head members 16 and 18 coaxially engage the rearward end of percussion head 12 and the forward end of yoke .' ~ ' ~
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14, respectively. Securing means such as a plurality of longitudinally ex~ending side rods 20 rigidly clamp ~he 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 ~urn ad~ustably ~, carrled by any suitable mobile base such as a crawler frame and articulated boom a6sembly (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) comprlses a generally annular yoke housing 28 having a generally annular, elongated chuck member 30 axlally rotatably carried therewithin as by roller bearings 32. Chuck member 30 inrludes a plurality of circumferen- -tially 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 30 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 ~oaxially aligged 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 ~ `~
por-tions longitudiDally sl~dably supported within inner peripheral portions 44 and 45 of bushings 38 and 46, respectively. An externally splined intermèdiate portion 54 of striking bar 42 , ~ extendlng intermediate the respective supported end portions .~. .
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`` 106Z~89 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 ~triking bar as8embly 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 (~ig. 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 he~einabove 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 àn elongated formed member-or cylindqr 72 such ae a machined steel casting, and an elongated cylindrical shell 74 coaxially rigi~dly encompassing cylinder 72 and axially coextensive therewiJth. Percussion head 12 has a plurality of chambers 76A through 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 adJaoent inner periphery 80 of shell 74 forms the ; radially outermost wall of the chambers 76. The chambers 76 are axlally spaced apart by intervening radially outwardly extending . .. . .
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' 1062989 i partitions 82, each having an outer annular periphery 84 which sealing engages the inner periphery 80 of shell 74 to preclude fluld communication between adjacent chambers 76. Other radially outwardly extending partitions 82 are formed adjacent the forward and rearward axial end portions of cylinder 72 to seaiingly engage respective axial end portions of periphery 80 thereby deflning 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 initlally formed for an ;~
interference fit therebetween at ambient temperature. For assembly the shell 74 is h~aded and/or the cylinder 72 ie cooled -~
to thermally produce a diametrical clearance therebetween. Af~er assembly the shell 74 and c~ylinder 72 equalize to ambient tempera- ;
ture to diminish the diametrical clearance therebetween and ~ -provide a continuous,- fluid tight face seal as described without .
recourse to known elastomeric sealing members and the like.
~ Cylinde~ 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 end peripheral .
~ portion 100 of sleeve 94. The coaxially communicating inner ., peripheries of buffer ring 102 and sleeve 94 define a coaxial .
throug~ bore 88 wherein an elongated, stepped cylindrical piston ' I ~ tO ls axially reciprocably disposed.
f Bore 88 has respectively axially spaced forward and rearward bearing portions 96, 104 which slidably support there-within respectlve axially spaced forward and rearward stem por~ions 98, 98~ of pist'on 70. An enlarged diame$er intermediate portion 106 of the bore 88 extending intermediate the respective bearing /' :
~ - 6 -il dap / ~,~) " 'J""~', ' 10629~9 portions 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 rear-ward ends 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 110, 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 94 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 to rigidly seat liner assembly 90 within bore 92. The sleeve 94 and buffer~ring 102 furthermore are non-ro~atably 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 '-~ .

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: 106~989 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 thereinto for cleaning detritus from the bore hole. A full description of the flushing fluid means 11 may be found in copending Canadian application Serial No. 262,864, filed October 6, 1976 which is asslgned to the same assignee as the instant invention.
Drilliny apparatus 10 has fluid supply means as follows ~ `~
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for delivery of motive fluid to actuate piston 70. ~ 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 upstro]ce and downstroke driving ~ chambers 110, 112. When motive fluid is supplied to the respective drivlng 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 ir~ediate fluid flow response ., .
andlarge pressure fluctuations in the supply line 134 are thus avoided. During inlet deadband cycle portions (to be described .! .
l hereinbelow) when all fluid inlets to the chambers 110, 112 are 't~ : ~ 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 I 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 ,~j .

i axially extending passages 138 in cylinder 72 which in turn i' ~ ywl ~ ~ ~

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communicate via another plurality of radially extendiny bores 140 with an upstroke inlet annulus 142 extending radially outwardly of bore !

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, portion 96 axially forwardly of bore portion 106 whereby fluid conununication between chamber 76A 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 o~ 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 i6C 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 in the range of a fraction of 1% to approximately 5% in vlew of the limited compressibility of hydraulic fluids. The limits of percen~t Yolume 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 ' an externally applied rotational impetus supplied for example by '! the rotating striking bar 42 during contact thereof with piston 70 ; ~`
`~ ~ ~' i ''J ~ :' 6i~ 9~39 at impact. Rotation of piston 70 within liner assembly 90 induces - rotary viscous shear forces to provide a hydrodynamic lubricant film 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 uear 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 o~ piston 70 comprises a central, axially ~.
~ extending annular land lS8 axially slidable within bore portion j 106 ln cooperatlon 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 "
l ~ than the diameter of land 158~ located adJacent the axially end i of land 158 and tapers radially and inwardly therefrom along its ~ axial extent at a taper angle with respect to the central longi-. . . ;.
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tudinal axis of piston 70 in the range of about 5 to about 15 J ' ~' preferably 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 preesurized 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 move=ent thereinto.
Axially spaced from the outer axial end of land 162 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 overtravel during reciprocation. Extending axially 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 ~oined thereby. Land 166 extends axially outwardly to terminate , ad~acent the stem portion 98. A radially inwardly extending annular inlet groove 170 is 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 rParward) of groove 170 functions primarily as an inlet vaive 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 dap/( ~ :
, ~ 1062989 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 98', 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 tlp portions 13, 15 of respective stems 98, 98' 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, 98i 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 (Pigs. Z and 3) employed in con~unction with annular wiper seals 174 encompassing piston stem portions 98, 98' intermediate the axial ends of each respective bore portion 96, 104. Limited fluid . ,-leakage past wipers 174 gradually accumulates ad~acent the piston end portions as ln cavity 176 and therefore, such cavities as 176 ~:: :.
are suitably vented to preclude such fluid accumulations. In .
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- ~ iL062989 colljunction 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, 98l 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 outwàrd of bearing annulus 114 inter-mediate 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 communicating a fluid drain line 190 from reservoir 156 into cavity i .
1~4 whereby a fluid flow path is established for dissipating fluid ~ .
pressure and directing fluid leakage away from the axially inner " `
sides of seals 174. `
In genera'l, fluid leakage in drill 110 could cause :
. . .
dangerously high pressures therewithin, for example on backhead :

16, thereby precipitating catastrophic failure of the front or , - : backheads 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 reserv.oir :
5~ as part of the percussion case drain means 172. ~ similar provision may ~e 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 . - 13 - :
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`~ ~062989 forward drain annulus 178 includes an axially elongated annular cavity 192 formed radially inte~mediate the liner 94 and cylinder 72 wherein Eluid 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 fr7nt head 18.
A four way, open center valve 196 (Fig. 2~) is inter-posed in fluid lines 134, f54 between the drill 10 and the pump 132 and reservoir 156 for controlling motive fluid flow to the drill ;
10. ~alve 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, 170' alternately ;~
communicate respective inlet annuli 142, 136 with respective upstroke and downstroke chambers 110, 112 and the associated - 14 ~
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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, 98'. Likewise, during piston reciprocation land 15P, alternately communicates axially opposed end portions of exhaust annulus 150 with the upstroke and downstroke sides of piston 70 to intermittently exhaust pressurized 1uid therefrom. For illustrative clarity the piston displacement scale in Fig. 5 ls greatly extended over the actual stroke oE the piston in the described embodiment, which is a very short stroke on the order of 5/8". Furthermore, the illustrated range of pressure values ~,tay be varied widely and is therefore not to be considered a limitation on the invention described.
Drill 10 is of the valveless or self-exciting type where- ;~
. . .
in grooves 170, 170' 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 plston 70 in response to the position of piston 70 in its stroke. The respective inlet and exhaust ports thus formed provide flùid flow rate control overa continuous range from a fully open state to a fully closed state as indicated by continuously variable flow resistances Rl through R4 in Fig. 4. By virtue of peripheral clearances between piston 70 and bore 88 ad~,acent the respective inlet and exhaust ports a degree of fluid flow is maintained even when the ports are "closed" as a Rl for example.
A balanced or equilibrium piston position illustrated .. . .
as being to the upstroke aide of the midstroke position (Fig. 4) ~ is defined for the piston 70 whereat the upstroke and downstroke ;~ sides of piston head 108 are subjected to equal and opposite i ~ . .
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'` ~LO~;Z989 motive fluid forces. In terms of the indicated flow resistance the balanced position of piston 70 is defined as that position for which Rl/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 èxhaust chamber 76C
ls proportioned identically between the respective inlet and e~haust 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 R3 exceeds Rl and R4 exceeds R2 at the piston equilibrium position.
It follows then that R3 + R4 exceeds Rl ~ R2 (the total flow resistance from inlet 76A to exhaust 76C is greater for the down-stroke side than for the upstroke side) and thus a larger pro- `
....
portion of the total fluid flow from inlet to exhaust will pass : ; .
through the upstroke side. Nevertheless, so long as the piston 70 resldes at the equilibrium position such that Rl/R2 equals R3/R4, equal net effective pressures will act on each side of the piston head 108 in spite of the unequal fiows, and the piston 70 thus will not be urged in either the upstroke or the downstroke direction by fluid pressure.
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^~ ~062989 The above-described ~elationship 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 addltionally to both the 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 differen~ial. As an example, assume that the piston 70 initially is positioned in the upstroke direc~ion from its -i equilibrium position with valve 196 in the A position. It follows that Rl and R4 are greater (ports more fully closed) and R2 and R3 are less (ports more fully open) than when piston 70 is at equilibrium. Accordingly, Rl/R2 will be greater than R3/R4 (the proportion of total pressure drop through the upstroke side inlet _ 17 dap/l~
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~ ~0629~9 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 direc-tion toward its equilibrium position. Similar considerations apply if piston 70 initially resides downstroke from its balanced position whereat RllR2 is less than R3¦R4 and an unopposed fluid pressure component 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 Rl/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 reciprocatlon, 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 e~ceeds 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 ; - 18 -::
dap/ ~iv lQ6~:989 equilibrium.
~ith chamber 76C pressurized and chambers 76A exhausted, ~
the condition Rl/R2 equals R31R4 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 Rl/R2 increases and R3/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 Rl/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 main-tained by the various flow restrictions between annulus 150 and reservoir 156, for example the changing cross-sec$ional shape of .
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the exhaust passages 148, the length of fluid line 154 and so forth. The exhaust back pressure in conjunction with the herein-above described piston tapers 160 prevents cavitation of pressurized fluid flowlng to exhaust by maintaining a positive exhaust path fluld pressure a~ all times. As piston 70 begins to accelerate toward impact (to the right in Fig. 5) pressure on the downstroke side begins to all along line 204 as the moving piston head 108 vaci~tes chamber 112 to increase the volume thereof. Simultaneously the accelerating piston decreases the volume of chamber 110, how- -ever, 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 170' 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 ~`-'~ 20 pressure energy stored on the downstroke side in chambers 112 and '~ 76B. 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 1 ' , -decreases (flow resistance R2 increases).

At point 216 on line 212 land 158 is centered on annulus :1 .
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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 remain-ing fluid energy in chambers 112 and 76s is exhausted to chamber 16C. 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 216' the inlet deadband cycle portion ends as chamber 76A ls 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 sub-sequently equalizes with the-upstroke side pressure at 222. As -the volume of cha~nber 110 further decreases and charging of fluid ~
into the upstroke side continues the fluid pressure in the upstroke side rlses 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.
Tha axial position of piston 70 at lmpact 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 drlll substantially more sensitive to impact point location.

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Storage of fluid pressure energy in chamber 76D during the piston -~
downstroke also provides an extended dwell time or contact period between pis~on 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 cha~ber 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 230', 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 230' 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 lnlet 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 equalizes with the downstroke side pressure which continues increasing along line 244. Fluid pressure on the upstroke side, which remains open - ~ 22 _ dap/ ~ ~
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~ ~062989 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 theore~ical considerations and is not to be construed as the complete, final or authoritative explanation of the physical laws governing operation of this 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 r; pressurlzed~
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 broad spirit and scope thereof. 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 '.~, -: . . :

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~S` 106Z9~9 comprise any of a wide variety of reversible or non-reversible symmetrical or asymmetrical designs, and particularly reverslble 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. These and other embodiments and modifications having been envisioned and anticipated by the inventor, the invention should be interpreted broadly and limited -only by the cope o the claims appeDded hereto. ~

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Claims (41)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A hydraulic drive 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 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, first passageway means in said body member 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 said head portion of said piston upon reciprocation thereof to provide for discharge of hydraulic fluid from said chamber portions alternately with said discharge being at least substantially continuous.

2. A hydraulic drive as claimed in claim 1 wherein said first passageway means includes axially spaced portions communicating with said bore and said second passageway means communicates with said bore axially intermediate said axially spaced portions.

3. A hydraulic drive for actuating a percussive tool comprising: a body member having an elongated formed bore therein with one end of said bore being open; an elongated piston axially reciprocal within said bore with one end thereof being accessible through said one end of said bore; said piston including an intermediate head portion to define in conjunction with said bore variable volume chamber portions in said bore on axially opposite sides of said intermediate head portion; said body having first passageway means to provide for admission of hydraulic fluid to said chamber portions; said body having second passageway means to provide for exhausting of hydraulic fluid from said chamber portions; and 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.

4. A hydraulic drive as claimed in claim 3 wherein said head portion is cooperable with said second passageway means during piston reciprocation to place said second passageway means in communication with said chamber portions alternately and with said communication being continuous.

5. A hydraulic drive as claimed in claim 4 wherein said first passageway means includes at least on accumulator portion.

6. A hydraulic drive for actuating a tool structure comprising: a body member having an elongated bore therein with a central longitudinal axis and 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 chamber section of greater cross-sectional extent than the cross-sectional extent of the adjacent sections of said bore axially outwardly thereof; said piston having a formed axially intermediate head section; said head section having a central portion with an axially extending outer peripheral surface closely slideably received within said chamber section to define chamber portions within said chamber section on axially opposite sides of said central portion of said head section which chamber portions vary inversely in volume as said piston reciprocates; 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; second passageway means in said body member having a discharge port means 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 means to control the discharge of hydraulic fluid from said chamber portions.

7. A hydraulic drive as set forth 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 set forth 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 at least sub-stantially continuous flow of fluid through said discharge port means.

9. A hydraulic drive as set forth in claim 6 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 set forth in claim 6 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 set forth in claim 9 wherein said accumulator chamber is annular in form and encompasses said bore in spaced relationship thereto.

12. A hydraulic drive as set forth 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 set forth 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 wherein said accumulator chambers are in continuous communication with said chamber portions respectively, through a plurality of passage-way portions spaced circumferentially with respect to said chamber portions.

15. A hydraulic drive as set forth in claim 6 wherein said discharge port means extends axially between said chamber portions.

16. A hydraulic drive as set forth in claim 15 wherein said outer surface has an axial length equal to said axial extent of said discharge port means.

17. A hydraulic drive as set forth in claim 15 wherein said outer peripheral surface has an axial length with respect to said axial extent of said discharge port means to provide at least substantially continuous flow of fluid through said discharge port means.

18. A hydraulic drive as set forth in claim 6 wherein said first passageway means includes a passageway within said body member for maintaining continuous communication between said inlet port means.

19. A hydraulic drive as set forth in claim 6 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 encompassing said bore.

20. A hydraulic drive as set forth in claim 19 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.

21. A hydraulic drive as set forth in claim 9 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 encompasses said bore.

22. A hydraulic drive as set forth in claim 10 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.

23. A hydraulic drive as set forth in claim 6 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, to selectively discontinue fluid flow communication between said chamber portions and said inlet port means during reciprocation of said piston.

24. A hydraulic drive as set forth in claim 1 with said second passageway means having an axial extent communicating with said bore and said head portion having an axial extent on the outer periphery thereof of the same length as the axial extent of said second passageway means.

25. A hydraulic drive as set forth in claim 1 with said second passageway means having an axial extent communicating with said bore, said head portion having an axial extent on the outer periphery thereof, and said axial extent on said head portion being of a length with respect to the length of the axial extent of said second passageway means to provide such discharge of hydraulic fluid.

26. A hydraulic drive as set forth in claim 6 wherein said piston is symmetrical with respect to the central transverse plane thereof.

27. A hydraulic drive as set forth in claim 6 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.

28. A hydraulic drive as set forth in claim 6 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.

29. A hydraulic drive as set forth in claim 28 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.

30. A hydraulic drive as set forth in claim 6 where 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.

31. A hydraulic drive as set forth in claim 6 including 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.

32. A hydraulic drive as specified in claim 31 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.

33. A hydraulic drive as specified in claim 31 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.

34. A hydraulic drive as set forth in claim 31 wherein said chambers consist of all the fluid chambers utilized in reciprocating said piston.

35. A hydraulic drive as set forth in claim 6 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.

36. A hydraulic drive as specified in claim 35 where at least one of said drain passageways is connected to a reservoir.

37. A hydraulic drive as specified in claim 35 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.

38. A method of moving an elongated impact piston through a work output stroke within an elongated bore of a body member in which said bore contains 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 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 closed 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 closed 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 closed volume and said actuating chamber while simultaneously discharging hydraulic fluid from said actuating chamber and discontinuing said discharging of said return chamber.

39. The method as specified in claim 38 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.

40. The method as specified in claim 39 including simultaneously admitting pressurized hydraulic fluid throughout said final portion to a closed volume in continuous hydraulic communication with said return chamber.

41. The method as specified in claim 38 including admitting pressurized hydraulic fluid to said return chamber through-out said final portion while simultaneously pressurizing hydraulic fluid in another closed 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 closed 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.