CA1246641A - Combined hydropneumatic brake cylinder/air reservoir device with automatic double-acting slack adjuster arrangement - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A combined hydropneumatic brake cylinder/air reservoir device having a hydraulic piston formed on the end of a push rod through which interconnected, different sized, pneumatic pistons operate to transmit the desired brake force. The hydraulic piston operates in a hydraulic cylinder having a slack adjuster piston arranged to supply or reduce the hydraulic fluid in the braking circuit in order to compensate for overtravel and undertravel of the pneumatic piston assembly due to brake shoe wear and brake shoe replacement respectively.
The double-acting slack adjuster action results in the stroke of the pneumatic piston assembly being maintained constant to assure continued optimum operating efficiency.

Description

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COMBINED ~YDROPNEUMATIC BRAKE CYLI~DER/AIR
RESE~OIR DEVICE WITH AUTOMATIC
RRANGEME~T

BACKGROUND OF THE INVENTION
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The present inven~ion is related to hydropneumatic type actuators~ and more particularly to brake actuators of the above type, which are suitable for use in .Ereight type railway bxake systems.
At the present time, railroads typically employ tradi-tional automatic air brake systems. Each car in a train iæ
normally equipped with auxiliary and emergency reservoirs which are charged with compressed air from a brake pipe extend-ing through the train, and a control val~e which responds to changes in the brake pipe pressure to control the fl~w o~
compressed air to and from the car brake cylinders. Since the railroad industry has standardized on relatively low braking pressures, and practical considerations limit the size of the car brake cylinders, it has become necessary to employ force-multi.plying linkages between the brake cylinder and brake shoes in or~er to obtain the high braking forces required. Such brake rigging arrangements inherentl~ increase the stroke of the brake cylinder piston required to move the brake shoes enough to take up the clearance space be~ween the , brake shoes and wheel treads. Accordingly, the plston ~troke required to bring the shoes into wheel contact, must be :

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relatively large and thus consumes a considerable amount of air. This in turn requires relativel~ large air reservoir~, which are space~consuming and thus impose al further restric-tion on the arPa needed ~or the force-multiplying brake rigging.
The gradual acceptance of hydraulic brake system~ in the rapid transit segment of the railway industry suggests the possibility o~ using hydraulics as a means of transmittirlg brake ~orces to the brake shoes in freigh~ type service. Such an approach would be advantageous in eliminating the need ~or the cumbersome, mechanical brake rigging presently employed on ~reight cars to transmit the brake cylinder ~orces to the brake shoes. Ideally, such an arrangement would require only a single hydropneumatic actuator de~ice on each car corres-15 ponding to the brake ylinder in a conventional mechanicalbrake rigging sy~tem. Mechanical ad~antage sufficient to obtain the desired high brake shoe forces would be obtained by a high ratio input to output hydropneumatic piston of the actuator device. Because of this high input to output ratio and the brake shoe clearance to be taken up, the stroke of the actuator piston in the pneumatic actuator portion would be necessarily large, and thus re~uire a considerable amount o~ air simply to bring the brake shoes into braking engagement with the wheel treads.

SUMMARY OF THE I~VENTIO~
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The object of the present invention is to pro~ide a compact, low-cost, yet highly reliable hydropneumatic actuator device having a -twc-stage pneumatic actuator portion to reduce the air required to take up the clearance between the brake shoes and wheel treads of a railway vehicle.
Another objec$ of the invention is to provide a hydro-pneumatic actuator of the above type, which automatically comp~nsates fox variations in the clearance between the brake shoes and wheel treads due to brake shoe wear or replacement, for example, in order to maintain the desired stroke of the pneumatic piston su~stantially constant to assure optimum operation of the two-stage pneumatic actuator portion.
Briefly, the hydropneumatic actuator according to the present invention converts an input force, such as the pneu-matic output of an air braka control valve device, intv a proportionally higher hydraulic pressure. During the first stage of operation, a relatively small pneumatic piston drives a hydraulic piston through a distance corresponding to the nominal brake shoe clearance in order to obtain a ~arge volu-metric displacement v hydraulic fluid with low air consump~
tion. The pneumatic piston and consequently the hydraulic piston, which is formed on the end of the pneumatic piston, is therefore displaced a distance corresponding to the nominal stroke of the pneumatic/hydraulic piston. FollGwing this, a second s~age of operation is initiated by venting air from the 6~4~

one side of the larger pneumatic piston, which is integral with the smaller pneumatic piston. This result~ in a hig~er pneumatic to hydraulic force multiplication being obtained after the brake shoe ~learance has heen taken up~ but with less air consumption since in the actuated po~ition of the larger pneumatic piston, the volume of air existing on the one side of the piston is reduced to a mini~um.
A slack ad]uster piston in a reference chambar of the hydraulic portion operates between opposite end stops as the pneumatic/hydraulic piston is actuated through its nominal stroke, thereb~ displacirlg a predetermined amount of hyclrau~ic fluid sufficient to take up the brake shoe clearance.
In the event brake shoe engagement occurs pxematurely, ~ such as during the initial application of the brakes following lnstallation of new brake shoes~ a high pressure is developed in the reference chamber due to brake shoe engagement occur-ring before complete displacement of hydraulic fluid has taken place. Accordingly, a one-way check val~e between the refex-ence chamber and a hydraulic reservoir is opened by the

2~ hydraulic pressure buildup, thereby displacing the excess hydraulic fluid from the reference chamber until the slack adjuster piston engages its one end stop. During a subsequent brake release, the braXe shoes will be retracted as the slack adjuster piston is returned to its original position against ~:

its other end stop, thereby esta~lishing the desired brake shoe clearance.
In the event overtravel occurs due to brake shoe wear, in which case hydraulic fluid in the r~ference chamber is completely displaced, consequent movement of the slack adjuster piston into engagement with the one end stop results in a one-way check valve opening a passageway through the slack adjuster piston via which passageway any additional hydraulic fluid necessary to maintain brake shoe engagement with shoe wear is supplied from the hydraulic cylinder. During a subsequent brake release, the slack adjuster piston i9 retracted to establish the desi.red brake shoe clearance, following which continued movement of the pneumatic-hydraulic piston to release position corresponding to overtravel during the preceding brake application draws makeup hydraulic fluid into the hydraulic cylinder from the hydraulic reservoir via another one-way check valve~

BRIEF DESCRIPTIOM OF THE DRAWING
The foregoing objects and additional advantages and features of the present invention will become apparent from the following more detailed explanation of the invention when taken in conjunction with the a~tendant single Figure drawing showing a sectional assembly view of a hydropneumatic actuator device, in which the pneumatic portion embodie~ a combined ,.

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brake cylinder/air reservolr design and the hydraulic portion provides a double-acting slack adjusterO
DESCRIPTION A3!~D OPERATIO:W
In a preferred embodiment o~ the invention~ there is shown a hydropneumatic brake actuator including a pneumatic portion 1 in the form of a combined brake cylinder/air raservoir device, such as disclosed in U0 S0 Patent ~oO
4055701810 assigned to the assignee of the present invention, and a hydraulic portion 20 Pneumatic portion 1 comprises a main cast mg 3 hav mg a partition forming a pressure head 4 with an opening 5 in pressure head 4O A back cover 6 is bolted to the one end of main casting 3 and an end casting 7 is bolted to the opposite end of maln casting 3. A cylindrical tube 8 extends longitudinally be~ween pressure head 4 and back cover 60 thereby o.rming an annular emergency chamber 9 about the outer periphery of ~ube 80 A fluted guide sleeve 10 is disposed within tube 8, such that its opposite ends also extend between pressure head 4 and back co~er 6. Longitudinal passageways 11 are formed by t~e fluted sleeve ~0 and tube 80 The guide slee~e 10 is aligned axially concentric with opening 5 and has an inside diameter substantially the same as the diameter of opening 5. Slidably disposed within guide sleeve 10 is a tubular-shaped positioning piston 12 that forms, in cooperation with sleeve 10 and back cover 6D a positioning chamber 13.

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Positioning piston 12 includes a piston head 14 integral with a tubular~shaped, elongated body lS that projects through opening 5 and to which i9 attached a p~wer pist~n 16 having a diameter larger than the diameter of positioning piston 12.
Being interconnected, positioning piston 12 and power piston 16 form a dual piston assembly that moves as a unit between a braXe release position and a brake application position.
stop bwmper 17 is carried by power piston 16 for engagement with pressure head 4 in the brake release position of the dual piston assembly, as shown. A cylindrical sleeve 18 i~ bolted to an inwardly projecting annular sleeve 19 of end casting 7 and carries on its outer periphery a sealing ring 20 with which the inner perlphery of tubular-shaped body 15 is engageable during movement of the dual piston assemblyO
An annular cylinder liner 21 is secured to main casting

3 between the main casting and end casting 7 to receive ~ower piston 16~ m e inside diameter of cylinder liner 21 is sub-stantially the same as the outside diameter of power piston 16.
By unbolting end cover 7 from main casting 3, the piston assembly and cylinder liner may be withdrawn from the main casting and, if different braking power is desired, replaced with a piston asse~bly having a different sized power pi~ton.
A cylinder liner having a diameter corresponding to the replacement power piston may then be installed in main casting ~, `

30 as exp].ained above, ~o accommodate the diferent sized power piston~
T~e end of cylinder liner 21 adjacent power piston 16 is spaced longikudinally ~rom the power pi~3ton when the dual S piston assembly is in release position, as shown. Until power piston 16 enters the cylinder liner 21 duri.ng an application stroka, free air ~low between opposite sides of power piston 16 oc~ur~. When the power piston entars the cylinder liner 21, an application chamber 22 is formed on one side of the power piston and a release chamber 23 is formed on the opposite side, it being understood that the combined volume of chambers 22 and 23 is substantially greater than the maximum volume o~
c~lamber 13. 'rhe volumetric dimension o~ chamber 13 increases by axial displacement of positioning piston 12 during movement of the dual piston assembly t~ward brake application posi-tion and is maximum in the application position. An annular pressure seal 24 is carried on the periphery of power piston 16 and makes sealing ~ngagement with the cylinder wall of liner 21 to establish a pressure seal between chambers 22 and 23. In the release position shown, fluid pres~ure com, munication is established between chambers 22 and 23, by way of the space between piston 16 and the adjacent end of liner 21. At this point, chambers 22 and 23 are,in effect~ a single cha~er~ until piston 16 enters into sealing engagemant with .

- 8 _ liner 21 during subseguent movement toward ~he brake applica~
tion position.
A ~luid pressure seal ring 25 is carried on pressure head

4 about the central opening 5, so as to engage tha outer peri-p~ery of the tubular-shaped body 15 of positioning piston 12 and thereby maintain a pressure_tight seal be~ween pressure chambers 22-23 and chamber 13. The fluid pressure seal ring 20 carried on the outer perip~ery of cylindrical sleeve 18 near its free end maintains a pressure_tight seal between chambers 22-23 and akmosphere.
A hollow piston rod 27 is connected, as by threads, at its one end to positioning piston 12 within tubular body 15.
The other end o~ piston rod 27 is ~ixed, as by welding~ to a ~ydraulic piston 28 that projects through a sealed opening 29 formed at the juncture of interconnected sleeves 18 and 19 and into a hydraulic chamber 30. Hydraulic chamber 30 is contained in a hydraulic cylinder 31 that is integral with end casting 7 and comprises hydraulic portion 2. A piston return spring 32 acts between the head 14 of positioning piston 12 and a spring seat 33 that abuts the seal end of sleeve 18 to urge the dual piston assembly toward release position.
A branch pipe 34 is c~nnected between a brake pipe 35 and a mounting boss 36 that receives a control valve device 38, which may be similar to that described in U. S. Patent No.
4,405,182. Mounting boss 36 includes a port 39 leading to ~- 9 --,~

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emergency chamber 9, a port 40 leading to chamber 13 via fluted passageway 11, and ports 41t 42 leading to the respec-tive application and release chambers 22, 23.
In additio~ to hydraulic chanber 30, ~hydraulic cylinder 31 also comprises a reference chamber 43 that is axially aligned with hydraulic cha~ber 30 and a hydraulic reservoir 44. A one-way check valve device 45, such as a conventional sprin~-biased ball check valve, is provided in a passage 46 be~ween reservoir 44 and chamber 30 via w~ich a source of hydraulic fluid is connected to the hydraulic system.
Operably disposed in reference chamber 43 i9 a slack adjuster piston 47 having a predetermined maximum stroke corras ponding to the desired clearance to be maintained, for example, between the bxake shoes (not shown) of a railway vehicle and the tread of the wheel to be braked (not shown). This pre-determined maximum stroke of slack adjuster piston 47 is estab_ lished by a stop at one end of chamber 43 formed by a shoulder 48 between chambers 30 and 43, and a stop at the other end of chamber 43 formed by a cover member 49 that closes this other end of the reference chamber 43. A return spring 50 be~ween piston 47 and end cover 49 normally maintains the slack adjuster piston 47 in engagement with shoulder 48. Contained in a throu~h passage 51 of slack adjuster piston 47 is a one-way check valve device 52, such as a conventional spring-biased ball check valve that is arranged ko permit fluid flow in a directlon from chamber 43 to chamber 30 when the foxce of . ;, , ~, fluid pressure in chal~ber 43 becomes substantially greater than the force required to open another one_way check valve 53.
This one-way check valve device 53 may also be a conventional spring-biased ball check valve that is disposed in a passage 54 between chamber 43 and reservoir 44. A pin 55 is set in cover member 49 so as to project into chaMber 43 ~o engage and unseat the spring-loaded ball valve o~ one-way check valve 52 when slack adjuster piston 47 is in engagement with its stop pro-vided by cover mQmber 49. An outlet 56 is provided in cover member 49 via which hydraulic fluid is transmitted ~rom chambers 30, 43 to an output device, such as the wheel brake cylinders (not shown), to which the aforementioned brake shoes may be attached. A high pressure, annular seal 57 is provided in the piston bore of chamber 43 between passage 54 and outlet 56 to isolate these passages when piston 47 is in its extreme right hand position.
During initial charging of the brake e~uipment, pressure chambers 9, 22 and 23 are stored with compressed air su~plied by wa~ of brake pipe 35, branch pip~ 34, control valve de~ice 38, and the respective ports 39, 41 and 42 of main casting 3~
The dual piston assembly comprising positioning piston 12 and power piston 16 is maintained in release position, as shown, under the in~luence o~ return spring 32.
In order ~o initiate a service brake application, air in brake pipe 35 is reduced at a~service rate. During the initial stage o~ a service brake application, control va}ve 38 is ., .~ , 6~

operative in rasponse to the brake pipe pressure reduction to connect com~ressed air from charged chambers 22 ana 23 to the previously evacuated positioning chamber 13. As this aix pres.sure in positioning chamber 13 builds up in attempting to equalize with the higher air prassur~ in chambers 22 and 23, a force is developed on positioning piston 12 in a right_ hand direction. When this force exceeds the force of return spring 32, the dual piston assembly will be forced in a brake application direction, carrying along piston rod 27. During this movement of the dual piston assembly, unrestricted flow of compressed air takes place from release chamber 23 to application chamber 22 via the space between the power piston 16 and the adjacent end o~ cylinder liner 21 to accommodate the displacement of power piston 16. A one~way chec~ valve (not shown) is also provided in control valve device 38 to accommodate displacement of air from release chamber 23 to application chamber 22 when piston 16 makes full sealing engagement with cylinder liner 21, as the dual piston assembly moves toward full application position.
As piston rod 27 is moved from release to application position, hydraulic piston 28 is forced into hydraulic chamber 30, thereby displacing hydraulic ~luid via outlet 56 to force the vehicle brake shoes into braking ~ngagement, as herein-after described more fully. Consequently, brake shoe clearance _ ~2 -~6i6~

is taken up with a low brake force r0quirement during this initial stage of a ~ervice brake application.
It will be further appreciated that in application posi-tion of power piston 16, the volume of application chamber 22 is increased and the volume of release chamber 23 is decreased.
In th2 second stage of this service brake application, the air in the reduced volume of chamber 23 is exhausted by control valve 38 an amount corresponding to the reduction of ir pres-sure in brake pipe 380 The resultant reduction of pressure in release chamber 23 establishes a relatively high di~erential force across power piston 16 in a brake application direction, since the pressure in chamber 22 is prevented from equalizing with the pressure in chamber 23 by seal ring 24. This high pneu~atic force is transmitted to the brake shoes ~ia the ~ydraulic fluid in the brake circuit, w~ich in effect becomes a solid link once brake shoe clearance has been taken up.
During a subsequent brake release operation, air in rel~ase chamber 23 is inikially connected by control valve device 38 to cha~ber 22 via a release control choke ~not shown) in control val~e device 38 to assure a controlled release of braking force. Concurrently, the pressure in positioning chamber 13 is vented to atmosphere. As the pressures acro~s power piston 13 become egualized, return spring 25 is efec-tive to ~orce the dual piston assembly toward the release position. Once power piston 13 move~ out of engagement with _ ~3 -Çi6~
cylinder liner 1~, unrestricted flow of air occurs between chambers 22 and 23 to hasten movement of the dual piston assembly and the car brake shoes to the xelea~e position. As the dual piston assembly is retracted, so is hydraulic piston

5 28, which relieves the hydraulic brake pressure to permit the brake shoes to be retracted from engagement with the wheel tread braking surface. In this release position, the brake shoes are spaced~apart from the wheel tread brake surface a distance correspsnding to tha desired brake shoe clearance, as maintained by operation of the hydraulic slack adjuster to be now explained.
It will be appreciated that the two-~tage operatîon o~
the pneumatic portion 1 minimizes air usage by initially taking up the brake shoe clearance through the use of a relatively small positioning piston 12 and subsequently obtaining the desired high braking forces by venting air from one side of a relatively large power piston 16 that is connectad to the positioning piston 13. Since only low force is required to move the brake shoes into engagement with the braking suxfaca o the wheel treads through the brake shoe clearance, whi~h ; constitutes the entire piston stroke, a reduced air require-ment is realized, as compared to arrangements employing a single, large piston. In addition, the high force output required for obtaining the desired brake force i5 obtained by ~enting only a minimum of air from the release chamber ~3, ~ 14 _ , ".

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since in the application position, the volume o~ this chamber 23 has been reduced to a minimum s.ize.
In the present invention, the pneumatic output force realized is converted into hydraulic pressure through hydraulic piston 28, which eliminates the need for mechanical brake rigging to obtain the desired output forces.
In order to maintain maximum efficien~y of the pneumatic portion 1 with respect to air consumptivn, hydraulic portion 2 is provid~d with a double-actiny slack adjuster arrangement to maintain the brake shoe clearance at a predetermined value.
During the initial stage of operation of pneumatic portion 1, hydraulic piston 28 advances into hydraulic chamher 30 as ~he dual piston assemhly comprising pistons 12 and 16 advances toward brake application position. During this stroke of t~e dual piston assembly, sufficient hydraulic fluid in hydraulic chamber 30 is displaced to ~orce slack adjuster piston 47 fxom i~s normal or release position against shoulder 48 right-wardly into engagement with cover member 49. During this displacement of slack adjuster piston 47, the hydraulic fluid in re~erence ~hamber 43 is forced to flow to the vehicle brake clrcult via outlet 56. The volume of hydraulic fluid dis-placed from referenca chamber 43 is selected such that the predetermined brake shoe clearance will be taken up by move-: ment of the brake shoes into brake engagement with the wheel treads. It will be understood that during takeup of this ~ 15 _ :
''~"' ` ' ' ' brake shoe clearance, the force buildup in reerence chamber43 is relatively low9 since brak~ shoe/wheel engagement has not ye-t occurred. This force buildup is less than the force required to overcome the spring bias o~ on~way chec}c ~alve 5 53, which accoxdingly remains closed to prevent hydraulic fluid from being forced into reservoir 44. Thus, the hydxau7ic fluid is forced to flow via outlet 56 to the vehicle brake circuit.
In the event brake shoe/wheel engagement occurs before the hydraulic fluid in reference chamber 43 is completely dis-placed; due to premature brake shoe engagement, as would be the case in applying the brakes ~ollowing instal~ation of new brake shoes, for example, khe pressure of the remaining hydraulic fluid in reference chamber 43 will increase ~u~fi-ciently to o~ercome the spring bias and open ona-way check valve 53 to allow the excess hydraulic fluid pre~iously accumulated in the hydxaulic brake circuit during wear-out of the repla~ed brake shoes to pass into reservoir 44. As this occurs~ slack adjuster piston 47 will xeach its rightward-most position abutting cover member 49. It should bs mentionedat this point that the spring bias of one~way check valve 52 in slack adjuster piston 47 is greater than the spring bias vf one~way check valve 53 to prevent back-~law of hydraulic fluid from chamber 43 to cha~ber 30 during this time.

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In this rig~tward~most position of slack adjuster piston 47, annular seal 57 engages the skirt of piston 47 tv isolate any further flow of high pressure hydraulic fluid into reser-voir 44 via one~way check valve 53. Concurrently, one~way check valve device 52 carried by piston 47 engages projecting pin 55 to cause the æprin~-biased ball valve to openO This allows additional hydraulic fluid to be delivered from hydraulic chamber 30 to the vahicle hrake circuit via the open one-way check valve 52 and outlet 56, in the even~ overtravel of the dual piston assemhly occurs due to braka shoe wear.
During a s~sequent brake release, hydraulic pi~ton 28 is retracted with the dual piston assembly under th~ influence of return spring 32, t~ereby relieving the high pressure developed in hydraulic chamber 30 during the preceding brake 1~ application. This permits return ~pring 50 to force slac~
adjuster pi~ton 47 in a leftward direction out o enyagement with co~er member 49, whereby the ball check valve of one-way check valve device 52 is closed by its bias spring. As slack adjuster piston 47 is shifted back to its normal release posi 2n tion into engagement with shoulder 48~ hydraulic fluid in the braking circuit is drawn back into reference chamber 43, thereby allowing the brake shoes to be retracted fr~m t~e wheeL tread braking surfaces. The volume of ~ydraulic fluid returDed to reference chamber 43 is such that the predeter-mined ~rake shoe clearance is established.

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The dual piston assembly and hydraulic piston 28 continue to be retracted toward release position a distance corres-ponding to any overtravel accumulated during brake application.
Since slack adjuster piston 47 is pre~ented from follow~
ing ~ydraulic piston 28 during this period o~ the release stro~e, a low pressure is created in chamber 30 sufficient to open check valve device 45 to admit mak~eup hydraulic fluid to flow from reservoir 44 to chamber 30 via passageway 46 and one-way check valve device 45 until complete retraction of the dual piston assembly occurs, as determined by engagement of bumpers 17 with pressure head 4. It wi~l be understood at this point that the spring bias of check valve device 52 is substantially greater than that of check valve device 45 so that back-flow o~ hydraulic fluid from chamber 43 to chamb~r 30 cannot occur as this makeup hydraulic fluid is drawn into chamber 30. Accordingly, brake shoe wear is automatically aompensated to maintain a predetexmined brake shoe clearance and thereby assure the optimum operating characteristic o pneumatic portion 1.

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Claims (13)

Having now descried the invention, what I claim as new and desire to secure by Letters Patent, is:

1. A hydropneumatic actuator device comprising:
(a) a pneumatic cylinder:
(b) a dual piston assembly operatively disposed in said pneumatic cylinder for axial movement between a release position and an application position defining a predetermined stroke including:
(i) a positioning piston cooperating with said pneumatic cylinder to form a first chamber on one side of said positioning piston, pres-surization of said one chamber producing a differential force on said positioning piston in the direction of said application position;
(ii) a power piston interconnected with said posi-tioning piston for joint operation therewith, said power piston cooperating with said pneu-matic cylinder to form second and third chambers on the opposite sides of said power piston in which compressed air is normally stored in said release position of said dual piston assembly, one of said second and third chambers being vented in said application position of said dual piston assembly to provide a force did-derential across said power piston in the direction of said application position;
(c) a reservoir in which hydraulic fluid is stored;
(d) a hydraulic cylinder having an inlet via which hydraulic fluid is supplied to said hydraulic cylinder from said reservoir, and an outlet;
(e) a hydraulic piston connected to said dual piston assembly and operably disposed in said hydraulic cylinder to displace hydraulic fluid in accordance with movement of said dual piston assembly between said release and application positions; and (f) double-acting slack adjuster means for adjusting the amount of hydraulic fluid in said hydraulic cylinder in accordance with overtravel and undertravel of aid dual piston assembly to maintain said predetermined stroke thereof constant.

2. A hydropneumatic actuator device, as recited in claim 1, wherein said hydraulic piston comprises a push rod con-nected at one end to one of said positioning and power pistons and having the opposite end projecting into said hydraulic cylinder through a sealed opening therein.

3. A hydropneumatic actuator device, as recited in claim 1, wherein said slack adjuster means comprises:

(a) a pair of axially spaced end stops located in said hydraulic cylinder between said inlet and outlet;
(b) a slack adjuster piston reciprocally disposed in said hydraulic cylinder between said pair of end stops, one side of said slack adjuster piston forming in cooperation with said hydraulic cylinder a refer-ence chamber;
(c) a return spring in said reference chamber between said one side of said slack adjuster piston and one of said pair of end stops to urge movement of said slack adjuster piston in the direction of the other of said pair of end stops;
(d) a first passageway in said slack adjuster piston via which hydraulic fluid in said hydraulic cylinder is connected to said reference chamber;
(e) a first one-way check valve in said first passageway to prevent flow of hydraulic fluid from said hydraulic cylinder to said reference chamber during movement of said slack adjuster piston from said other and stop into engagement with said one end stop, whereby a predetermined amount of hydraulic fluid is displaced from said reference chamber via said outlet;
(f) means for opening said first one-way check valve upon engagement of said slack adjuster piston with said one end stop, whereby an additional amount of hydraulic fluid in excess of said predetermined amount is displaced from said hydraulic cylinder via said first passageway and said outlet in the event said predetermined stroke of said dual piston assembly is exceeded; and (g) a second one-way check valve in said inlet via which makeup hydraulic fluid is drawn from said reservoir into said hydraulic cylinder in response to movement of said dual piston assembly toward said release position following engagement of said slack adjuster piston with said other end stop to thereby compensate for said overtravel of said dual piston assembly during the preceding movement thereof toward said application position.

4. A hydropneumatic actuator device, as recited in d aim 3, wherein the amount of said makeup fluid corresponds to aid additional amount of hydraulic fluid in excess of said pre-determined amount.

5. A hydropneumatic actuator device, as recited in claim 3, wherein said slack adjuster means further comprises:
(a) a second passageway between said reservoir and said reference chamber; and (b) a third one-way check valve in said second passage via which hydraulic fluid in said reference chamber flows into said reservoir when a predetermined back pressure develops at said outlet prior to said slack adjuster piston engaging said one end stop during operation of said dual piston assembly toward said application position to thereby compensate for said undertravel of said dual piston assembly.

6. A hydropneumatic actuator device, as recited in claim 5, further comprising means for interrupting fluid pressure communication between said outlet and said second passage when said slack adjuster piston is engaged with said one end stop.

7. A hydxopneumatic actuator device, as recited in claim 6, wherein said interrupting means comprises an annular seal in said reference chamber between said outlet and said second passageway with which said slack adjuster piston is engageable upon engagement thereof with said one end stop.

8. A hydropneumatic actuator device, as recited in claim 3, wherein said means for opening said first one-way check valve comprises a pin on said one end stop projecting into said reference chamber so as to engage and thereby open said first one-way check valve when said slack adjuster piston is engaged with said one end stop to thereby establish flow of hydraulic fluid from said hydraulic cylinder to said outlet via said first passageway.

9. A hydropneumatic actuator device, as recited in claim 3, wherein said first and second one-way check valves are biased in a closed direction to prevent hydraulic fluid flow from said hydraulic cylinder to said reservoir and to said reference chamber respectively, the bias of said first one-way check valve being greater than the bias of said second on-way check valve.

10. A hydropneumatic actuator device, as recited in claim 5, wherein said first and third one-way check valves are normally biased in a closed direction to prevent fluid flow from said hydraulic chamber to said reference chamber, and from said reference chamber to said reservoir respectively, the bias of said first one-way check valve being greater than the bias of said third one-way check valve.

11. A hydropneumatic actuator device, as recited in claim 3, further comprising means for urging said dual piston assembly toward said release position.

12. A hydropneumatic actuator device, as recited in claim 11, further comprising stop means for limiting axial movement of said dual piston assembly in a direction to establish said release position.

13. A hydropneumatic actuator device, as recited in claim 12, wherein said one and said other of said pair of end stops are spaced-apart from each other a distance corresponding to said predetermined stroke of said dual piston assembly whereby said application position of said dual piston assembly is established in accordance with engagement of said slack adjuster piston with said one end stop.