CA1038725A

CA1038725A - Hydraulic percussion device

Info

Publication number: CA1038725A
Application number: CA246,078A
Authority: CA
Inventors: Gunter Klemm
Original assignee: FA ING GUNTER KLEMM SPEZIALUNTERNEHMEN fur BOHRTECHNIK
Current assignee: FA ING GUNTER KLEMM SPEZIALUNTERNEHMEN fur BOHRTECHNIK
Priority date: 1975-03-22
Filing date: 1976-02-19
Publication date: 1978-09-19
Also published as: ATA128176A; FR2305279B1; NL179351B; DE2512731C2; JPS5813311B2; SE410090B; ES446108A1; JPS51145977A; DE2512731A1; NL7602447A; CH595193A5; ZA76981B; IT1058625B; GB1536494A; FR2305279A1; FI760672A; AU500873B2; AT345218B; BE839858A; NL179351C

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention teaches a hydraulic per-cussion device which has a working piston displaceable in a working cylinder and a control part displaceable in a control cylinder. The control part has at least one control face on which the pressure of a control line connected with the working cylinder acts, and also has a pressure reservoir connected to the pressure line, and a pump device which delivers pressurized liquid into the pressure line. The invention is characterized in that, for producing impacts or blows the percussion force of which is constant, independently of the delivery capacity of the pump device, there is pro-vided a device which, subsequent to or during lift of the working piston, permits change-over or reversal of the control part into that position which corresponds to the impact movement of the working piston only when pressure and quantity in the pressure reservoir have exceeded a threshold value.

Description

7æs The invention relates to an hydraulic percussion device having a working piston displaceable in a working cylinder and a control part displaceable in a control cylinder, which said con-trol part has at least one control face on which acts thepressure of a control line connected with the working cylinder, and having also a pressure reservoir connected to the pressure line, and a pump device delivering liqu~d under pressurei~tothepressure line.
Hydraulic percuss~on devices are known (Swiss Specifi-cation No. 534,567, Atlas Copco, April 30, 1973), wherein control of the working piston is effected by a smaller control piston dis-placeable in a control cylinder. Displacement of the con~rol cylinder ~ 5 effected hydraulically via one or more control lines extending out of the working cylinder. The percussion devices have a hydro-pneumatic pressure reservo~r connected to the pressure line or to the outward-flow line, which said pressure reservoir is :
charged with pressure liquid during prédeterminsd phases of the working cycle o~ the working pis~on and gives up the said working ;:
quid again during other phase~ at which a higher degree of pressure energy is required. In the known devices, reversal of -20 ~ ~ the control piston takes place exclusively as a function of the position of the working piston. The working piston may have al- ;
ready reached the appropriate li~t position although the required quantity of liquid is no~ yet ~vailable in the pressure reservoir.
This is in~par~icular the case when the percussion device is em-ployed in combination with pump devices the de~ivery rateof which is relatively low,~i.e. such as deliver under the required pres-sure an inadequate quantity of liquid per unit time. In such cases, the known percussion devices operate with an ~lmost unre-duced number of blows, but the percussion strength decreasec con-siderably because there is not available in the pressure reser-voir su~ficient pressure li~uid to supply the piston from above with adequate percussion energy.

- 2 _ 7~25 Prequ~ntl~, the user o~ a percussion device has avail--a~le only specific pressure pumps~ It may ~appen that the delivexy capacity o~ the pressure pump is inadequate ~or impar-ting to t~e work~ng piStOII, during a blow, the degree o~ pex-cuss~on ~orce ~striking ~orce) which is required.
It is the object o~ the invent~on to provide a per~ -cussion device ~hic~ can ~e operated with constant percussion ~-~
~orce independently o~ t~e delivery capacity o~ the pressure pump connected thereto. For the solution o~ this problem, it is proposed according to t~e invention to provide a~ a percus-sion device`of the type mentioned at the outset a device which : , subsequent to or dur~ng li~ting o~ the working piston permits reversal o$ t~e control part into t~e position corresponding to the percussion movement o~ t~e ~orking piston only when pressure~
and quantity i~ the pressure rese:rvoir have exceeded a threshold value.
; The basic princ~ple of~the invention consists in that t i.5 guaranteed that the impact or the downward movement of the ~orking pi~ton does not take pIace until the necessary pressure~
~ ~has been generated by the pump in the pressure reservoir. The ; advantage of~this mode o~ operation conslsts in that the per~
cussîon ~orce required în each particular înstance îs obtained : : :ndependently o~ the deli~ery capacity o~ the pump employed and that it is necessary to operate only with a low percussion num~er when the delivery capacity of the pump îs inadequate.
There are various poss~ble modes o~ delaying the diis- ~-placement movement o~ the control piston. For example, there may be provîded in the contxol line e~tending to the control cylinder a valve operating as a function of pressure. In the control lîne, there obtained, towards the end of the return stroke of the ~orking piston, a pressure cor~esponding-to that of the pressure line and ~exewith also to the pressure o~ t~e pressure reser~oir.
I~ a valve operat~ng as a ~unct~on ~ pressure is provided in the control line, then "s~tch~ng-through" o~ the control line does not take place until t~e threshold ~alue of the val~e has been exceeded~ Onl~ then is t~e control part rerersed or changed ovex, produc~ng the down~a.rdl~ d~xected ~lo~ of the working piston. ~ further poss~ility ~or guaranteeing build-up o~
pressure in the pressure reservo~r consists in that a spring ~orce d~rected oppositely to t~e pressure o~ the control line is -~
caused to act on the control part~ Therewith, t~e control part is not able to tra~el until t~e pressure acting on its control face is greater than the spring ~ias, ~hich can be set. In the case~o~ such an embodiment, expediently, the control part is ~;
adapted to be subjected to pressure action~at ~oth its end faces, ;
the pressure forces su~stant~ally cancelling~each other out.
This p~essure compensat~on produces the~result~that the spring requires to be designed only to ~e su~ficientl~ strong to ensure that ~t will exert a ~orce corresponding to the pressure threshold - it~is not re~uired also to countera:ct an hydraulic 0 ~ bias. Additionally, the spring may also be hydraulically sup-ported by the arrangement whereby the pressure face operative out of the same direction as the spring is larger than the oppositely acting pressure ~ace.
A th~rd possible method of dela~ing reversal or change-over o~ the control piston ~s characterized in that there is prov~ded in one of the lines through which there is a flow during lift o~ the working piston a throttle location due to which the liquid stream is, on workingpiston li~t taking place, slowed down and the working piston is decelerated, and in that the throttle location is so dimensioned tha~ the working piston does not reach ~ lift or stroke position producing change-over - 4 ~

~o~
o~ t~e control part ~n~il t~e pressure reser~oir has ~een ~illed ~ith a predetermined minimum ~uantity o~ liquid.
Due to tRe throttle locat~on, the return stroke of the ~orking piston is slo~ed do~n~ so that the working piston xe-leases t~e control l~ne in delayed ~ashion ~or change-over of the control part. Dux~ng t~e phase o~ the slow return stroke, the pressure reservoir can ba replenished. The slowing down of the return stroke can Be e~ected by the arrangement whereby the throttle location is proviaad bither in the return ~low line or in the pressure line extend~ng into t~e working cylinder. In the second case, t~e throttle location is expediently bridged ~y a non-return val~e permeable ~or the liquid leaving the wor-king c~linder, or the throttle element itself is permeable in the return-~low direction.
.
~- The invention is d~scussed in detail hereinbelow~with reference to some examples o~ em~odiment and with reference to the figure~
FIGURE 1 shows O diagrammatically, a lonyitudinal section t~rough ~ first em~odiment o~ the percussion device 2Q ~ ~according to the inventi~n,~having a valve, adapted to be operated as a function of pressure, in the control line.
FIGURE ? shows a second example of embodiment, wherein the control sleeve is ~iased by a spring, the per-cussion device otherwise corresponding to that shown in Figure 1.
PIGURE 3 shows a ~urther example o~ embodiment having a control part biased by a spring, the percussion device corresponding other~ise to that of Figure 1.
FIGU~E 4 sh~ws an example o~ embodiment having a throttle location located in the return ~low line, the \

percuss~on dev~ce ot~exw~se correspondin~ to t~at o~ F~ur~ 1.
PIGURE 5 sho~ an ~xample o~ em~od.~ment having a throttle locat~on located ~n the pressu.re line, the percussion dev~ce other~ise corresponding to that ` o~ re.l, and FIGURE 6 shows a modP o~ carrying into e~ect the principle lllu~trated in ~igure 5.
Referring to ~igure 1, the working piston ll is .
arranged for longitudinal displacement in the working cylinder : 10~ Periodically it ~mpinges on the anvil 12 which may, for . ::
example, ~e connected wit~ a drilling linkage.
rough a supply line 13, ~ydraulic medium is intro~
duced into t~e.:~de~ice.. It ~lows via a pre-tensioned:non-return : valve 14:into t~e pressure line 15. ~The non-return llne~14~
: takes tRe~orm o~ a spring-~iased~sleeve 45. The sleeve has at Lts lower end a ollar 46 t~e rear side of which ls connect~ed ;with the pressureless return flo~ line 21. The spring 47,~
: ~hic~ urges the sleeve downwardly, is so dimensioned that the 20~ pressure~acting.on the lower end;~.ace 48 normally drives the sleeve 45 upwaxdly. The sleeve does not descend until the . pxessure in the pressure line falls below for example 70 bar and ;.
is no longer able to counteract the spring 47. The normal : : pres ure in the hydro-pneumatic pressure reservoir 43 connected :.
to the pressure li~e 15 is 5a bar.
Th.e pressuxe line 15 is continuously connected to the , lo~ex cham~er 16 o~ t~e cylinder 10, so that the hydraulic pressure always acts on t~e small annular ~ace 17 of the working pi~ton 11 and endeavours to drive t~e latter upwardly.

The upper cham~er 18 o~ t~a c~lindex is connected ~ia a line 19 ~ith t~e control Yal~e 20~ The latter connects the line 19 alternat~n~l~ w~t~ the pressure line 15 and with the return ~lo~ line 21. The upper cham~er 18 is delimited by the large annular ~ace 22 o~ t~e working piston. When, therefore, the ~ull p~essure acts ~ot~ on the lower annular face 17 and alsQ on thQ upper annular ~ace 22, the working piston is driven do~nwardly. ~hen, on the other hand, the pressure medium acts only on the smaller annular face 17 and the line l9 is connected with the pressureless return-flo~ line 21, the working piston e~fec~s i~s return stroke, during which it travels upwardly.
The hollo~ control sleeve 55 arranged in the control valve 20 is;formed externally,~in its rearward zone~ ~ith a wide annular groo~e 29 capable of connecting with each other the ;
bores, offset relati~e~to each other in the longitudinal direc~
; ~ t~on, of the lines l~ and 21. In the other end position shown in the draw~ngs, the connection to the return flow line 21 is shut off by the control sleeve, whereas the bore to the line l9 s free and is thus ccnnected with the interior of the continuous 20~ cllo~ ccntroI sleeve and with t~e pressure line 15. The inner ~; ~idth of the control sleeve is continuously constant. The rear end face~30~is, together vith the effective control~face 58~, largex~than the front end face 57 - alone, however, it is smaller than~the front end face 57. ;~
Control of the control sleeve 55 i5 ef~ected via the co~trol line 24 the pressure of which acts on the control face 58 o~ the annulax collar 56. The front delimiting face 59 of the annular collar îs continuously connected with the return flow l~ne 21. The control line 24 ~ranches, in the case of the present example of em~odiment, into t~ree first branch lines, 65, 66, 67, and ~our second branc~ lines 25, 26, 27, 28, each of ~LID3~7;æ5 :
which debouches into an annular ~roove ~ormed ~n the ~orking cylinder.
In the zone of the annular grooves connected with the ~;~
branch lines, the ~orking p~ston 11 has a luwer, thickened piston element 62 the d~ameter o~ whic~ is adapted to the ~-c~linder d~eter. The piston element 62 ~s able to close in sealing manner the ~ranc~ l~nes ~ith associated annular grooves.
Underneath, it is delimited ~y the annular face 17 and above by : :.
the annular ~ace 61~ Adjacent the annular face 61 is the wide ~;
; 10~ annular groove 69 and t~erea~ove is disposed a further thickened piston element 64 ha~ing t~e same diameter as the piston element :
62.~
In the zone of khe annu:lar~ ~roove 69 formed in the piston, a return-~flo~ line 52 extends out of the working cylinder.
The annular groove~69 o~ the work.ing piston is so dimensioned t~at Ln eac~ position o~ the ~orking~piston it engages the;return-lo~ line 52. ~When, there~ore, t~e workin~ piston has descended to s~uch aD extent that t~e upper annular face 61 o~ the plston part or element~62 passes into~position below the first opened 20~ ~ranah line 65 to 67, the associated branch line 65 to~67 is connected via~the annular groove 69 with the return-flow llne 52, ~hereby the control line 24 becomes pressureless.
The plugs 68 adapted to be inserted into the branch lines 65 to 67 so as to selectively seal of~ the latter, have the ta k o~ determining that piskon position in which the control line 24 is, during the piston movement, rendered pressureless for the ~irst time in the manner described, so that the pressure ~orce acting on the control ~ace 58 of the control sleeve ceases. ;
The second branch lines 25 to 28 are disposed below

3~ the ~irst branch lines and determine durin~ which piston stroke the pressure of the pressure line 15, which o~ta~ns continuously 3~72~i in the lo~er c~am~er 16 of t~e ~orkin~ c~linder~ is intxoduced into t~e control line 24.
Inserted into t~e control line 24 is a ~alve 34 which operates as a funct~on o~ pressure and wh~ch permits through-~low o~ the li~u~d stream in the direct~on towards the control slee~e 55 onl~ ~hen t~e pressure i5 a~ove a predekermined -CQpt~onally presetta~lel m~nimum ~alue. In the counter-airec-t~on, ~.e. ~rom the control sleeve to ~he working cylinder, the control line 34 is completel~ permeable, this ~ing illustrated ~; 10 b~ the non-return val~e 38 connected anti-parallel to the valve ~; ~ 34.
hen the pressure of the control line 24 exceeds the -minimum value set at t~e valve 34, it acts on the control face 5~8 of the control slee~e.
.
For~explanat'on of the~mode~o~ operatlon of~the per-; cussion tool, the discussion ~~ commence from~t~e position of the elements~s~ho~n in the dra~ings,~wherein the control sleeve 5~ connects the~;line l9 with~the~pressure line 15 and shuts-off ~ , , , ;th~ connectlon to~t~e return-~lo~ line 2l.~ ~Since the full ~ ~ ;
~ ~pressure~obtains both at~the upper,~large~annular~face 22 and also at~the lo~exl small;annular face 17, the working piston~ll -accelerated~downwardl~. As th~s takes place, the piston ~`
: . ~ :
part 62 first of~all continuousIy closes off the branch lines 28, 27, 26 and 25, and then fxees the~first branch lines 65, 66 and 67, provided that the~ are not sealed with a plug 68. On ~;
sealing the ~ixst branch line 2S, the control line 24 ls cut off from the high pressure and on ~reeing the uppermost branch line 65 it is connected with the pressureless return_flow line 52.
Thexeby~ liquid flows bac~ t~rou~h the valve 38, which in this position is permea~le, and the ~orce acting at the control ~ace 58 o~ the control slee~e i~ xeduced. Under the pressure of the _ g _ -~0~7~
~orce actin~ on t~e lar~er ~nd ~ace 57, the control p~ston 55 is displaced up~ardl~.
Dur~ng the do~n~ard ~ovement o~ the working piston, the pressure reservoix 43 has ~een discharged to a considerable degree, ~ecause t~e upper e~d ~ace 22 is relatively large and dux~ng impact a large quantit~ o~ liqu~d must ~e driven within a s~ort time into the wor~ing cylinder so as to achieve a high degree o~ impact energ~. Dur~ng the return stroke, following thereof, of the ~rking piston, the pressure liquid engages only the relatively small ~ace 17. Whilst the working piston -ascends, the pressure reservo~r 43 is simultaneously refilled.
Thereby1 the pressure ascends in the pressure line 15. This pxessure is routed via the control line 24 to the valve 34 oper-ating as a function o~ pressure~and, initially, the said valve ~ :
; ~ sti~ locks" for as lony~as t~e pressure has not yet reached the mlnimum value set. ~I~ t~e pressure pump ~not shown) con-~
:: nected to khe arrangement has::~ Led the pressure xeservoir 43 ~: to such an e~tent that the pressure threshold has been reached in the control line 24, then the valve 34 "switches through" and the ; pressure liquid ~lowing into the control cylinder 23 pushes: the control sleeve S5 ~ack:into the position shown in Figure l.
Due to the valve 34 operating in pressure-dependent : manner, it is guaranteed that the control sleeve 55 will not be ~` :
pressed into that position which corresponds to the downward --~;
movement of the working piston until the necessary pressure has built up in the pressure line 15, i.e, until the pressure reser-~oir 43 has been ~illed to the required degree. Therewith, constant percussion energy ~s made ready for each blow o~ the working piston, independentl~ o~ the delivery capacity of the pressure pump connected to the supply line 13.
~n the case o~ the example o~ em~odiment of Figure 2, -~L03~7~5 there i8 provided a spr.~ng~ased slee~e 551 the two end ~aces 30 and 57 o~ w~ch are o~ suB~tantiall~ equal size. The rear collar ~ace 58 o~ the collar 561 is, here again, subjected to the act~on o~ the pressure o~ the control line 24, in ~hich, how-ever, there is no Yal~e, and t~e ~ront collar ~ace 59 which is ~it~dra~n relakiYe to the end face 57 is disposed in an annular chamber connected to t~e pressureless return--flow line 21.
~ n order to generate pre-tension at the control sleeve 551, there is arran~ed in t~e interior thereo~ a helical spring 35 bearing at one end at the control sleeve 551 and at its other end at a set screw 36. The spring 35 imparts to the control sleeve 551 ~ias into the upper end position corresponding to :
o~ the working piston. The ~orce o~ the spring counter-acts the pressure engag~ng at the~ace 58 o~ ~he control line 24, ~Only when this pressure is aBle to overcome the spring orce is the~control sleeve displaced down~ardl~ and the path rom~the~pressure line lS through the control sleeve to the line~l9 freed, whereby the down~ardly directed impact mo~ement o~ the~working pisto~ is initiated.
;;2~ The~bias o~ t~e~spring is ad~usta~le by rotating~the ~ ;
set screw 36, s~o that t~e des.~red percus~ion energy can readily be set~hy varying the spring bias.
Refer~ring to the example of embodiment according to Figure 3, a control cylinder 552 biased by a spring 37 is provi-ded, The spring 37 is arranged substantially in a blind bore formed in the control cylinder. The lower end ~ace 572 consti-tu,tes, together with the base ~ace o~ the blind bore, the engage-ment face for the pressure of the pressure line 15, whereas the upper end ~ace 582 represents ~he control ~ace at which the pressure o~ the contxol line 24 is operative. The control face 582 is larger than the two lower pressure ~aces. ~dditionally, there is ~ormed ~n the control pi.ston an annular gxoove 2q2 which in one of the end position$ o~ the working piston connects . .
the line 19 wit~ the return-~lo~ line 21 and, in the other end position of the ~orking piston, connects the line 19 with a line 152 extend~ng to t~e pressure line 15.
After a ~lo~ o~ t~e ~or~ng piston, the spring 37 drives the control piston 582 into the upper end position. The pressuxe in the pressure l~ne 15 and inthe control line 24 has ~allen to a lo~ value. During t~e su~sequent lift o~ the :
~oxking piston, this pressure increases slo~ly. Only when the ::
pres9ure has become so large that t~e dif~erence in the forces actlng ~xom a~ove and ~elow on~the control piston 552 exceeds the force o~ the spring.37 is the control piston driven down~
~ardly, whereupon it connects tne line l9 with the line 152 extending to the pressure line.~ Thus, only then does the next ~impact or ~lo~ take place. Since, in the case o~ this example o$ em~odiment, the contxol ~ace 582 is only slightly larger than ~he two lo~er pressure faces of the control piston, the spring 37 may be designed to ~e relatively weak even ~f the pressure : 20 : ~ t~xes~old at whic~ changeover or reversal is to take place is ~;.
high. ~ ~.
The control sleeve 553 in the case of the example of embodiment according to Fiyure 4 corresponds exactly to that of the example of embodiment according ta Figure 1. Whereas in the case of the previously describ~d examples of embodiment, the working piston ls not impeded during its return stroke, and the ;-del~y in changeover o~ the control sleeve takes place as a func~
tion o~ the magnitude o~ tne control pressure, in the case of the device according to Figure 4 the return stroke o~ the working piston 11 is delayed in that a throttle element 40 is inserted into the return-~lo~ line 21. When the working piston per~orms its ret~rn stroke, liqu~d ~s pressed out o~ ~ie cham~ex 18 into the l~ne l9. The liquid ~lows throug~ the annulax yroove 29 into the return_~lo~ line 21. Due to the throttle location 40 provided in the return-~low line 21, ~low-of~ o~ the liquid is slo~ed_down, so that also t~e li~t o~ the working piston is !~.,'' slo~ed down. T~e work~ny piston t~us connects the line 24 in delayed manner ~it~ t~e pressure line 15. Due to this delay, the pressure xeservo~r 43 ~s gi~en the opportunity o~ ~ecom~ng c~arged, T~us, the up~ard movement o~ the w~rking piston is ~ntent~onally slowed~down in order t~at the pressure reservoir 43 may in the~intermediate period ~e ohargedO
lso, ~n the case o~ the device according to Figure 5, ~ ~-; the upward movement o~ the working~piston LS delayed by a ;~
throttle location. In th~s case-, the~throttle location ~41 is prov~ded in that~sect~on o~ t~he~pressure line 15 which is located betwean the contro1 s1ee~e 553 and ~the working cylinder 10.~ Connected in parallel with ~the throttle location 41 is a ncn-return valve 42 which is opened ~cr the flow-back which,~
dur~ng~the~downward~ vement of the working piston~, flows-back 20~-~ out o~t~e working cylinder, but is closed in the opposite direct~on. ~
During a working movement of the working piston, liquid flow~ cut of the working cy1inder into the pressure line 15. There~y, the non-return ~alve 42 permits throughflow~ so that the impact or blo~ is not hindexed. During the subsequent return stroke, the non-return valve 42 is closed. Thus, the liguid i~ required to pass through thethrottle location 41.~
Thus, working piston lift ta~es place so slowly that meanwhile the pres~u~e reservoir 43 can ~e recharged. In differentiation ~rom Figure 4~ in the case o~ Pi~ure S the throttle location 41 i8 arranged upstrea~ oX the woxking cylinder in the ~low direction~

In t~e case o~ the exa~le o~ emBod~ment accord~n~ to Fi~ura 6, the same control sleeve 553 is emplo~ed as ~n the embodiment accordin~ to ~igure 5, The pressure line 15 has, s~ortly ~efore its entr~ into the working cylinder, a right angle bend locat~on rearwardly o~ which a diaphragm 39 is inser-ted into $he line. T~e diaphra~m 39 has a narro~ central bore and const~tutes the t~rottle element. A ~ore 70 extending ~rom the interior to the d~aphragm 39 is sealed with a screw plug 71.
From the innex end ~ace o~ tne screw plug projeats a spring 72 -dri~ing the d~aphragm 39 with a small degree o~ pre-tension , .. .. . . . .. .
against its seat. ~he diaphragm 39 is displaceable against the action of the spring 72. In its other end position, into which it is pressed on pressure li~uid flowing out of the , working~-cylinder, the liquid~flo~s through longitudinal grooves 73 formed in the outer si;de of the diaphragm and extending over a part of the diaphragm length. ~ In the position of the dia~
phragm 39 shown in Figure 6, thé grooves 73 are clo~ed, so that for the return stroke of the worXing piston only the passage ~oross-section o the central bore of the diaphragm 39 is avaiI-~
20~ able. During~the impact, the diaphragm is pushed back, so that for escape o~ the pressure liquid out of the working cylinder the larger flow cross-section of the longitudinal grooves 73 i5 avaiIable.

Subsequent to screwing out o~ the screw plug 71, various t~pes o~ diaphragms can be inserted through the bore 70.
The selection o~ diaphragm si~e depends in each particular instance on the delivery capacity of the pump available and on the impact force required.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Hydraulic percussion device having a working piston displaceable in a working cylinder and a control part dis-placeable in a control cylinder, which said control part has at least one control face on which acts the pressure of a control line connected with the working cylinder, and having also a pressure reservoir connected to the pressure line, and a pump device delivering pressurized liquid into the pressure line, characterized in that, for producing impacts or blows the per-cussion force of which is constant, independently of the deli-very capacity of the pump device, there is provided a device which, subsequent to or during lift of the working piston, per-mits change-over or reversal of the control part into the posi-tion which corresponds to the impact movement of the working piston only when pressure and quantity in the pressure reservoir have exceeded a threshold value.

2. Percussion device according to claim 1, characterized in that a valve operating as a function or pressure is arranged in the control line extending to the control cylinder.

3. Percussion device according to claim 1, characterized in that a spring force directed oppositely to the pressure of the control line acts on the control part.

4. Percussion device according to claim 3, characterized in that the control part is adapted to be subjected to pressure action from both its end faces, the pressure forces substanti-ally cancelling each other out or the pressure force supporting the spring predominating.

5. Percussion device according to claim 1, characterized in that there is provided in one of the lines through which there is a flow during lift of the working piston a throttle location due to which the liquid stream is slowed down on working piston lift taking place and the working piston is delayed, and in that the throttle location is so dimensioned that the working piston does not reach a lift position producing change-over of the control part until the pressure reservoir has been filled with a predetermined minimum quantity of liquid.

6. Percussion device according to claim 5, characterized in that the throttle location is provided in the return-flow line.

7. Percussion device according to claim 5, characterized in that the throttle location is provided in the pressure line extending in the working cylinder and is bridged by a non-return valve permeable for the liquid leaving the working cylinder.

8. Percussion device according to claim 5, characterized in that there is disposed in the pressure line extending into the working cylinder a throttle element which is displaceable due to the pressure between two end positions and which has a by-pass line open only in one of the end positions.

9, Percussion device according to claim 8, characterized in that the throttle element is a diaphragm which is arranged at a bend location in the pressure line, is formed over a portion of its length with longitudinal grooves, and can be replaced by a bore extending to the bend location and sealable by a plug.

10. A percussion tool operable by pressure medium and comprising a working cylinder; a working piston reciprocable in said cylinder; a pressure medium supply line; a pressure medium reservoir connected to said supply line: a pressure medium dis-charge line; a control device for controlling the supply of pressure medium from the supply line to the cylinder and for controlling the discharge of pressure medium from the cylinder to the discharge line, whereby the working piston is operable to carry out reciprocation in the working cylinder; said control device comprising a control cylinder and a control member reciprocable in said control cylinder between a first control position in which the working piston is operable to carry out a working stroke, and a second control position in which the working piston is operable to carry out a return stroke; a control line connecting the working cylinder to the control cylinder; a control surface provided on said control member and arranged to apply a force to said control member urging the control member from said second position to said first position when there is a predetermined pressure of medium in the control line; and control means operable to effect, or permit, movement of the control member from said second position to said first position after the pressure in the pressure medium reservoir has built up to a predetermined threshold value, whereby substan-tially constant values of impact can be delivered by the tool independently of the delivery capacity of a pump for supplying pressure medium to the supply line when the pump is connected therewith.

11. A percussion tool according to claim 10, in which said control means is operable, subsequent to the return stroke of the working piston, so as to effect changeover or reversal of the control member to the end position which corresponds to the wor-king stroke of the working piston only when the pressure and quantity of pressure medium in the reservoir has exceeded a threshold value.

12. A percussion tool according to claim 10 in which said control means is operable, during the return stroke of the working piston, to effect changeover or reversal of the control member to the end position which corresponds to the working stroke of the working piston only when the pressure and quan-tity of pressure medium in the reservoir has exceeded a thresh-old value.

13. A percussion tool according to claim 10, in which a valve is arranged in the control line extending to the control cylinder, and is operable to open when a predetermined pressure obtains in the control line.

14. A percussion tool according to claim 10, in which a spring force biases the control member in a direction opposite to the action exerted on the control member, in use, by pressure medium in the control line.

15. A percussion tool according to claim 14, in which the control member is adapted to be subjected to pressure action from both its end faces, the pressure forces substantially cancelling each other out or the pressure force supporting the spring predominating.

16. A percussion tool according to claim 10, in which there is provided, in a pressure line through which pressure medium flows when the working piston carries out the return stroke, a throttle due to which the flow of pressure medium is retarded so as to slow down the working piston during the return stroke; and in which the throttle location is so dimensioned that the working piston does not reach a position producing changeover of the control member until the pressure reservoir has been filled with a predetermined minimum quantity of pressure medium.

17. A percussion tool according to claim 16, in which the throttle is provided in the discharge line.

18. A percussion tool according to claim 16, in which the throttle is provided in a pressure line extending to the work-ing cylinder and is bridged by a non-return valve openable in an opposite direction to permit pressure medium to be exhausted from the working cylinder.

19. A percussion tool according to claim 16, in which there is disposed in a pressure line extending to the working cylinder a throttle element which is displaceable due to the pressure between two end positions and which has a by-pass line open only in one of the end positions.

20. A percussion tool according to claim 19, in which the throttle is a diaphragm which is arranged at a bend location in the pressure line, and is formed over a portion of its length with longitudinal grooves.

21. A percussion tool according to claim 20, in which the diaphragm is replaceably mounted in a bore extending to the bend location, and sealable by a plug.

22. A percussion tool according to claim 10 and substan-tially as hereinbefore described with reference to any one of the embodiments illustrated in the accompanying drawings.