US3008454A

US3008454A - Locking actuator

Info

Publication number: US3008454A
Application number: US803988A
Authority: US
Inventors: Ralph M Wilkins
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-04-03
Filing date: 1959-04-03
Publication date: 1961-11-14
Anticipated expiration: 1978-11-14

Description

Nov. 14, 1961 R. M. wlLKlNs LOCKING 'AcTUAToR Fled'April 5, 1959 ,K2 ,QN no S N au Nu, .3. Q mw un om Q N A v. E m

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b my .5123275 3,008,454 Patented Nov. 1.4, 1961 3,008,454 LOCKING ACTUATOR Ralph M. Wilkins, 145 Golden Gate Drive,

` Fircrest, Wash. Filed Apr. 3, 1959, Ser. No. 803,988 8 Claims. (Cl. 121-40) This invention relates generally to tiuid actuators of the locking type, and more specifically to an improved locking mechanism incorporated therein.

Although the principles of the present invention may be included .in various uid actuators, a particularly usefulv application is made in a lluid actuator of l the p,

double-acting type. The principles of this invention may be incorporated in a double-acting cylinder having a single rod or having a double rod, and likewise may be incorporated in a single acting fluid actuator having either a single or a double rod.

When one of this type of devices is utilized in a given installation, it is frequently desirable that the rod be positively retained at one or both of its extremes of travel, and that such retention be maintained even though the force which caused such positioning be removed. Further, it is highly desirablel that the mechanism provided for locking the rod position should be automatically released wheneverlrelease is desired, but not otherwise.

The present invention, along with other features, in its preferred fonn, contemplates the utilization of an auxiliary piston which is provided with a cam surface which directly engages a number of locking keys so as to extend them into locking position, and so as to permit their retraction. A preferred form of the invention also includes a slide valve which is effective to admit fluid pressure to the main piston after the auxiliary piston having the cam surface has been moved in a key-unlocking direction.

Accordingly,'it is an object of the present invention to provide a uid actuator of the locking type.

Another object of the present invention is toprovide an improved simplified locking mechanism for a fluid actuator.

Yet another object of the present invention is to provide la fluid actuator whereininternal ow is timed, regulated, or metered to obtain a desired sequential application of forces, and movements. Y Many other advantages, and features and additional objects of the present invention will become manifest to those versed inthe art upon making reference to the detailed description and the accompanying sheet of drawings in which a preferred structural embodiment incorporating the principles'of the present invention is shown by way of illustrative example.

On the drawing:

FIGURE l is -a cross-sectional view, partly broken away and partly in elevation, of a locking actuator assembly provided with simplified locking means in accordance with the principles of the present invention;

FIGURE 2 is a cross-sectional view of the actuator shown in FIGURE l, and taken along line II-II; and

FIGURE 3 is a cross-sectional view of the structure shown in FIGURE l taken along line III-III thereof.

As shown on the drawings z` The principles of this invention are particularly useful 2 when embodied in a uid actuator assembly such as illustrated in FIGURE l, generally indicated by the numeral 10. The actuatorassembly lgenerally includes a cylinder 11 and end cap or gland 12, a second end cap or gland 13, a main piston assembly generally indicated by the numeral 15, and a pair of piston rods 16 and 17. The cylinder 11 is "provided with .a main bore 18 within which the main piston assembly is slidably disposed. It will be noted that the bore 18 extends into the end caps 12 and 13.

The gland or end cap 12 is provided with a liuid pressure port 19 which communicates in a generally radial direction with the bore 18. The port 19 is disposed at one end of the cylinderl 11, and =a similar port 20 is provided in the gland 13 at the opposite end of the cylinder. The end caps 12 and 13 are each provided with suitable threads 21 and an O-ring seal 22 which cooperate with the cylinder 11 to secure the end caps 12 and 13 thereon in sealed relation thereto in va known manner. The end caps 12 land 13 likewise have axial apertures through which the rods 16 and 17 respectfully extend and within which O-ring seals 23 are provided for sealing cooperation with the rods 16, 17.

The end cap 12 is provided with -a coaxial secondary bore 24 of reduced diameter which defines a radially extending abutment face 25. In like manner, the end cap 13 is provided with a coaxial secondary bore 26 den'ing a radially extending abutment face 27. The main bore 18 is provided with an internal bevelled shoulder 28 which is axially spaced from the end of lthe cylinder and from the vabutment face 25 whereby the abutment face Z5 is disposed intermediate the port A19 and the shoulder 28. Similarly, the end cap 13 is provided with an internal bevelled shoulder 29. Thus the bevelled shoulders 28, 29 are axially spaced from each other.

If desired, a support means such as a trunnion 30 may be carried by one of the end caps 13 for at least partial support of the `actuator 10. Thus, in this embodiment, it will be noted that the end cap 12 is substantially identical to the end cap 13 except for support or mounting provisions carried on the end cap 13.

The piston assembly 15 includes a main piston 31 which is provided with one or more sealing O-rings 32 each of which forms a slidingseal between the piston 31 and the main bore 18 of the cylinder 11. The piston 31 is adapted as by a set of threads 33 for driving connection to the rod 16, and as lby a set of threads 34 for driving connection to the rod 17. The main piston 31 is also provided with a pair of oppositely directed separate axial bores 35, 36, the bore 35 opening only toward that end of thel piston 31 which is directed toward the shoulder 28, and the bore 36 opening only in the direction of the bevelled shoulder 29. It is to be noted that the bores 35 and 36 are separate from each other and therefore -are not in fluid communication with each other. Each of the bores 35, 36 is provided with one or more radial guide apertures, the radial guide apertures being identified by the numerals 37-40 for the axial bore 35, and by the reference numeral 40-43 for the axial bore 36. Each of the axial guide apertures 37-43 is spaced from the adjacent end or face 44, of the piston 31 respectively.

Within the axial bore 35, there is disposed an auxiliary piston 46, while within the axial bore 36, there is diseach provided with a suitable O-n'ng seal which cooperates with the pistons 46, 47r respectively. Thus the auxiliary pistons 46, 47 are reciprocably and sealably disposedr within the axial bores 35, 36 of the main piston 31.

The auxiliaryk piston 46 is provided with a set of generally at cam surfaces 49 while the auxiliary piston 47 is similarly provided with a set of generally llat camsurfaces 50.

Within each of' the radial guide apertures 37-40 there vis disposed, and reciprocably supported therein, a locking key 51.v Each of the locking keys 51 is adapted at its radially inner end 52 to be engaged by one of the cam surfaces 49 of the auxiliary piston 46, while each of the keys 51 is adapted at its radially `outer end or surface 53 for sliding engagement with andv abutment against the shoulder 28 of the cylinder 11.

In likek manner, each of the radial guide apertures. 40-43 is provided with a locking key 54 which is reciprocably guided therein. Bach of the locking keys 54 is provided with an inner radial end 55 which is adapted to cooperate with and to engage one of the cam surfaces 50, yand is also provided with a radially outer end 56 which is adapted tor engage the shoulder 29 as shown in FIGURE 1.

Means are provided to bias the auxiliary piston 47 toward the end of the actuator wherein the port 20 is disposed, and means are also provided to bias the auxiliary piston 46 toward the end of the actuator 10 wherein the port 19 is disposed. In the disclosed embodiment, two such means are provided for each` of the auxiliary pistons 46, 47. A spring 57 acts between the inner end of the bore 36 and the auxiliary piston 47 to biasy the auxiliary piston 47 to the right as shown. Likewise, a spring 58 acts between the inner end of the axial bore 35 and the auxiliary piston 46 to provide a force urging the auxiliary 46 to the left as shown. In addition, a straight bleed passage 59 directly communicates the inner end of the axial bore 36 only with the face 44 of the piston 31, and a second straight bleed passage 60 similarly communicates the inner end of the axial bore 35 with the opposite face 45 of the piston 31; thus a uid pressure applied at the port 19 passes through the straight bleed passage 59 into the bore 36 to urge the auxiliary piston 47 to the right, thereby aiding the spring 57. It can be seen that so long as there is such a pressure present, and so long as it is of suicient magnitude, the spring 57 could` be omitted. However, a preferred embodiment.

of the valve bores 61 and 62 open outwardly and gen-v erally coaxially with the axial bores 35', 36 respectively.

Within the valve bore 61 there is disposeda slide valve member 67 while within: the valve bore 62 there is disposed a slide valve member- 68. The val-ve members 67, 68l lare reciprocably disposed within the: valveA bores, and have a generally sealing relationship therewith. The distal end of each ofthe slide valves 67, 68 isundercut as at 69 to admit uid pressure thereto when. the end engages the inner end ofthe valve bore as shown in' FIGURE l by the slide valve member 68. Each of the slide valve members 67, 68k has a longitudinal groove or cut or port 70 which permits fluid pressure to pass therethrough` after a predetermined amount of movement of the slide valve. members 67', 68. In this embodiment, the slide valve members 67, 68-'form anv integral part along with the, cam surfaces 49', 50of the auxiliary pistons 46, 47.

The remaining structural details will be more readily understood following a description of the operationl of this device. v

In FIGURE l, the parts are shown in the relationship that they take when a fluid pressure is applied to the port 19. Under this condition, fluid pressure passes through the bleed passage 59 to act .on the auxiliary piston 47 which is positioned so that its cam surfaces hold the locking key members` 54 in a radially outward position wherein their outer extremities are received to abutor engage the beveled shoulder 29 of the cylinder 11-'. Any uid pressure on the right hand. side of the piston. assembly 15 is drained off through the port 20 as explained more fully herein later. Fluid pressure at the port 19 also acts against the face 44 of the piston 31 to urge it toward the right. In the event that theA pressure should fail or be removed, any force tending to move therods 16, 17 to the left would be unable to move the rods with respect to the cylinder because the locking keys 54 would abut the shoulder 29. In tum, the keys 54 are held in this position bythe auxiliary piston 47.r The spring 57, in turn, insures that the auxiliary piston 47 remains in the position shown regardless of the attitude in which the device as a whole may be oriented. When pressure is no longer applied at the pont 19, the spring 58 returns the includes the spring 57 because in the event of failure of such pressure, the spring 57 insures that the auxiliaryY piston 47 is maintained in the position shown. It is to be observed that the bleed passages 59 and 60 communicate directly as described and do not pass through or intersect with the radial guide apertures 39 and 40. It is also to be noted that the bleed passages 59 and 60 act to vent the inner ends of the axial bores 36, 35 respectively when the auxiliary pistons 47, 46 move individually toward the inner end of their' respective bores. Thus both of the-bias means described each acts individually to bias the corresponding auxiliary piston toward one end of the actuator 10v and into engagement with the cooperating locking keys.

The main piston 31, at each of the threads 33 and 34, is providedywith a further bore which receives the threaded end portions 16', 17' of the rods 16 and 17. The end 16' of the rod 16 is provided with a valve bore 61 while the end 17 of the rod 17 is provided with a valve bore 62. In this embodiment, the valve bores 6l, 62 have been provided in a circular sectional configuration and are thus supported jointly by the rod and the main piston 31. The bore 61 terminates at an inner end 63 which has fluid communication through a vent opening 64 with the port 19. In like manner, the valve bore 62 terminates at an inner end which has fluid communication with the port 20 through a ven-t opening 66. 'Ihe opposite ends auxiliary piston 46 so that the cam surfaces 49 engage the inner end 52V of the locking keys 51. This tends to urge the keys in a radially outward direction and also closes the slide valve 67.

Thereafter, when a fluid pressure is applied to the port 20, the pressure passes through the opening 66 to the means 69 on the end of the slide valve 68 to urge it to the left as shown. The bleed passage 59 acts to prevent any hydraulic lock within the bore 36 and communicates with the port 19 as, shown. The abutment face 27 of the end cap 13 and the abutment face 45 of the main piston 31 form a reasonably close fit, so that while some uid pressure also tends to move the piston to the. left at this v instant, because of this lit, there is less than the entire effective area of the piston 31 subjected to the pressure at the port 20. However, the full pressure acts on the slide valve as described to compress the spring 57 and to thereby move the cam surfaces 50 to the left as shown. Before the auxiliary piston 47 has bottomed, the port 70 in the slide valve member 68 will have opened to directly admit uid pressure to a point 71 disposed intermediate the slide valve 68 and the auxiliary piston 47. This newly admitted pressure then passes through a bleed passage or passages 72 to bring the. fluid pressure directly to the abutment faces 45, 27 which are then in face-to-face engagement. Thus the'pre'ssure at the port 20 becomes elfective over the entire area of the piston 31 after the locking keys 54 have been unlocked. This pressure acts as a force to move the piston assemlbly 15 as a unit to the left, Ithe locking keys moving radially inwardly under the influence of the beveled shoulder 29 as it does so.

ly toward the end cap 13 while the cam surfaces 49 converge radially toward the end cap 12. Thus the cam surfaces 49 and 50 converge radially in a direction away from each other.

The movement of the piston assembly continues to the'left as described until a buffer ring 73 enters the secondary bore 24 of the end cap 12. As it does so, it forms a leaking seal therebetween, whereby fluid is trapped between the abutment surfaces 25 and 44. To permit continual gradual reduced-rate movement of the piston to the left, the buffer ring 73 is grooved as at 74 so as to permit the desired rate of fluid leakage, and hence so as to control the amount of bufiing or cushioning desired. Since the slide valve 67 is then closed, passages 75 which correspond tothe bleed passages 72 do no venting. On

the other hand, fluid pressure from the port 20 after reaching the point 71 and passing through the vent passages 72is immediately permitted to pass through the straight bleed passage 60 and to act on the auxiliary piston=46 to aid the spring 58 in urging the locking keys 51 yin a radially outward direction. As soon as the beveled outer end 53 of the locking keys 51 reaches the beveled shoulder 28, the cam surfaces 49 act to force the keys 51 into locking engagement with the shoulder 28.

When this condition has been reached, the device has been operatedthrough one-half cycle, the later half ofl which is identical to that described. During the opera- I tion of the later half of such' cycle, a lbuffer ring 76 similar to the ring 73 and' grooved at 77 enters the coaxial secondary ibore 26 so that the abutment faces 27, 45 define a uid trap, which is adapted as by the means 77 to gradually leak the fluid therefrom to the port 20. It is to be understood that the grooves 74, 77 could be disposed in the bores 24, 26. The bufmg rings 73, 77 are securely carried by the main piston 31 4and-therefore, in effect, comprise a portion of reduced diameter at each end thereof, which portion is spacedaxially from the adjacent guide apertures toward the port 19' and 20 respectively. Thus also the portions 73, 76 of the main piston 31 are snugly receivable Within the coaxial secondary bores 24, 26 to trap fluid between the pairs of abutment faces 25, 44 and 27, 45, such trap being adapted to bleed fluid along the reduced diameter vportion 73 or 76 respectively to the

adjacent port

19 or 20.

f. It is to'be noted that the locking

keys

51, 54 have a generally mushroom-shaped section` as shown `in FIGURES 2 and 3. It is also to be noted that each is provided with a beveled inner and outer end. vIt is to be understood that other configurations of locking key may be employed,

and that the device is operative even though no camshaped or beveled portions aretprovided on the locking key itself. However, the -beveled portions minimize galling and friction and are therefore preferable.

Although various minor modifications may be suggested by those versed to the art, it should be understood that I wish to embody within the scope ofthe patent warranted hereon all such embodiments as reasonably and properly come within the scope of my cont'bution 'tto the art.

I claim as my invention:

1. In combination: a cylinder adapted to transmit uid pressure through one end thereof, said cylinder having an internal bevelled shoulder adjacent to said end; a rod having a main piston reciprocablyY and sealably disposed municating that piston face in which said axial bore opens with one of saidvbores at a point intermediate said slide y pressure, said main piston having an axial bore opening within said cylinder for movement in response to said fluid pressure, said main pistonhaving an axial Abore opening only toward said one end, and a radial guide aperture intersecting said bore near the open end thereof;-one of said rod and saidmain piston having a valve bore axially spaced from andaligned with said axial bore, the inner end of said valve bore being in uid communication with said one end; an auxiliary piston reciprocably and sealably disposed within said axial bore of said main piston for relative axial movement, said auxiliary piston having a cam surface which converges radially inthe direction of said one end; and slide -valve means secured to said auxiliary piston for joint movement therewith and disposed to valve means and said auxiliary piston; a locking key reciprocably-disposed in said radial guide aperture, said' key being adapted at its radially outer end to engage said bevelled shoulder of said cylinder, and at its radially :inner end to directly engage said cam surface on said auxiliary piston; and means for biasing said auxiliary piston toward said one end against said key.

2. In combination: a cylinder having a port at one end thereof for transmitting pressure-fiuid therethrough,said cylinder having an internal bevelled shoulder adjacent to said end but axially spaced therefrom toward the opposite end of said cylinder, said cylinder having a radially extending abutment face disposed intermediate said port and said bevelled shoulder; a rod having a main piston reciprocably and sealably disposed within said cylinder for movement in response to said uid pressure, said main piston having an axial bore opening only in a face of said main piston directed toward said one end, and a radial guide aperture intersecting said bore near the open end thereof; said face of said main piston being generally sealably engageable with said abutment face of said cylinder, so that when said cylinder abutment face and said mainpiston face are in face-to-face engagement, the effective size of said main piston is reduced; an auxiliary piston reciprocably and sealably disposed within said axial bore of said main piston for relative axial movement, said auxiliary piston having a cam surface which converges radially in the direction of said one end; av locking key reciprocably disposed in said radial guide aperture, said key being adapted at its radially outer end to engage said bevelled shoulder of said cylinder, and at its radially inner end to directly engage said cam surface on said auxiliary piston; and means for biasing said auxiliary piston toward said one end against said key; whereby fluid pressure at said port will act to move said `auxiliary piston and unlock said key before the fiuid force is applied fully to said main piston.

3. In combination: a cylinder adapted to transmit fluid pressure through one end thereof, said cylinder having an internal bevelled shoulder adjacent to said end; a rod having a main piston reciprocably and sealably disposed within said cylinder for movement in response to said tiuid only toward said one end, and a radial guide aperture intersecting said bore near the open end thereof; an auxiliary piston reciprocably and sealably disposed within said axial bore of said main piston for relative axial movement, said auxiliary piston having a cam surface which converges radially in the direction of said one end; said auxiliary piston being movable in response to fluid pressure at said one end; valve means actuated by movement of said auxiliary piston for thereafter directing said uid pressure to act fully on said main` piston; a locking key reciprocably disposed in said radial guide aperture, said key being adapted at its radially outer end to engage said bevelled. shoulder of said cylinder, and at its radially inner end to directly engage said cam surface on said auxiliary piston; and means for biasing said auxiliary piston toward said one end against said key.

4. In combination: a cylinder having a port at one end thereof for transmitting pressure fluid therethrough, said -cylinder having a main bore with an internal bevelled shoulder therein adjacent to said end but axially spaced therefrom toward the opposite end of said cylinder, said cylinder having a coaxial secondary bore intermediate said port and said bevelled shoulder defining an abutment face on said cylinder; a rod having a main piston reciprocably said one end, and a radial guide aperture intersecting said bore near the open end thereof; said main piston having a portion of reduced diameter axially spaced from said radial guide aperture toward said port and defining an abutment face on said main piston, said portion of reduced diameter being snugly receivable in said secondary bore to define a -uid trap intermediate said abutment faces, said trap being adapted to bleed Huid therefrom along said reduced diameter portion to said port; an auxiliary piston reciprocably and sealably disposed within said' axial bore of said main piston for relative axial movement, said auxiliary piston having a cam surface which converges radially in the direction of said one end; a locking key reciprocably disposed in said radial guide aperture, said key being adapted at its radially outer end to engage said Abevelled shoulder of said cylinder, and at its radially inner end to directly engage said cam surface on said auxiliary piston; and means for biasing said auxiliary piston toward said one end against said key 5. In combination: a cylinderl having a pair of ports each respectively disposed at the ends thereof for transmitting pressure uid therethrough, said cylinder having a main bore with a pair of spaced internal bevelled shoulders therein each respectively disposed adjacent to one of said ends but axially spaced therefrom toward the opposite end of said cylinder, said cylinder having a pair of secondary bores coaxial'with said main bore and each respectively disposed intermediate one of said ports and the adjacent bevelled shoulder and each defining an abutment face on said cylinder directed toward the other; a rod having a main piston reciprocably and sealably disposed within said main bore of said cylinder for movement in response to said fluid pressure, said main piston having a pair of oppositely directed separate axial bores, each of said bores respectively opening toward one of said ends, and a pair of radial guide apertures each intersecting only one of said bores near the open end thereof; said main piston having a pair of portions of reduced diameter axially spaced from each other with said radial guide apertures being therebetween and each defining an oppositely directed abutment face on said main piston, each of said portions of reduced diameter being respectively snugly receivable in one ofV said secondary -bores to define fiuid traps intermediate each cylinder abutment face and each piston abutment face, said traps being respectively adapted to bleed uid therefrom along one of said reduced diameter portions to one f said ports; a pair of auxiliary pistons each being respectively reciprocably and sealably disposed within one of said axial bores of said main piston for relative movement, each of said auxiliary pistons having a cam surface which converges radially in a direction away from the other auxiliary piston; a vpair of locking keys each reciprocably respectively disposed in one of said radial guide apertures, each of said keys being adapted at its radially outer end to engage one of said bevelled shoulders of said cylinder, and at its radially inner end to directly engage said cam surface on one of said auxiliary pistons; and means for biasing each of said auxiliary pistons respectively against one of said keys.

6. In combination: a cylinder adapted at its ends to transmit uid pressure therethrough, said cylinder having a pair of spaced internal bevelled shoulders each respectively disposed adjacent to one of said ends; a rod having a main piston reciprocably and sealably disposed within said cylinder for movement in response to said uid pressure, said main piston having a pair of oppositely directed separate axial bores, each of said `bores respectively opening toward one of said ends, and a pair of radial guide apertures each intersecting only one of said bores near the open end thereof; a pair of auxiliary pistons each being respectively reciprocably and sealably disposed within one of said axial bores of said main piston for relative movement, each of said auxiliary pistons having a cam surface which converges radially in a direction away from the other auxiliary piston; said auxiliary pistons both being movable .with respect to said main piston in one direction in response to fluid pressure at one of said ends, and in the opposite direction in response to fluid pressure at the other of said ends; a pair of valve means each respectively opened by inward movement of one of said auxiliary pistons for thereafter respectively directing said fluid pressure to act fully on said main piston; and a pair of locking keys each reciprocably respectively disposed in one of said radial guide apertures, each of said keys being adapted at its radially outer end to engage one of said bevelled shoulders of said cylinder, and at its radially inner end to directly engage said cam surface on one of said auxiliary pistons; and means for biasing each of said auxiliary pistons respectively against one of said keys.

7. In combination: a cylinder adapted at its ends to transmit fluid pressure therethrough, said cylinder having a pair of spaced internal bevelled shoulders each respectively disposed adjacent to one of said ends; at least one rod having a main piston reciprocably and sealably disposed within said cylinder for movement in response to said fluid pressure, said main piston having a pair 0f oppositely directed separate axial bores, each of said bores respectively opening toward one of said ends, and a pair of radial guide apertures each intersecting only one of said bores near the open end thereof; a pair of valve bores each respectively disposed in one of said rod and said main piston, each of said valve bores being respectively axially spaced from and aligned with one of said axial bores, the inner end of 'each of said valve bores being respectively in fluid communication with one of said ends; a pair of auxiliary pistons each being respectively reciprocably and sealably disposed within one of said axial bores of said main piston for relative movementeach of said auxiliary pistons having a cam surface which converges radially in a direction away vfrom the other auxiliary piston; and a pair of slide valve means each respectively secured to one of said auxiliary pistons for joint movement therewith, and each respectively disposed to be reciprocably received in one of said valve bores; a pair of bleed passages each disposed in one of said rod and said main piston, and each respectively communicating one of the faces of said main piston with a point in that one of said axial bores which opens in said one face, said point being intermediate the corresponing slide valve means and the corresponding auxiliary piston; a pair of locking keys each reciprocably respectively disposed in one of said radial guide apertures, each of said keys being adapted at its radially outer end to engage one of said bevelled shoulders of said cylinder, and at its radially inner end to directly engage said cam surface on one of said auxiliary pistons; and means for biasing each of said auxiliary pistons respectively against one of said keys.

8. In combination: a cylinder having a pair of ports each respectively disposed at the ends thereof for transmitting pressure iiuidthercthrough, said cylinder having a main bore with a pair of spaced internal bevelled shoulders therein each respectively disposed adjacent to one of said ends but axially 4spacedtherefrom toward the opposite end of said cylinder, said cylinder having a pair of radially extending abutment faces each respectively diposed intermediate one of said ports and the closer one of said bevelled shoulders; a rod having a main piston reciprocably and sealably disposed within said cylinder for movement in response to said tiuid pressure, said main piston having a pair of oppositely directed separate axial bores, each of said bores respectively opening only in oppositely directed faces of said main piston, and a pair of radial guide apertures each intersecting only one of said bores near the open end thereof; said faces of said main piston being respectively generally sealably engageable with said abutment faces of said cylinder, so that when one of said cylinder abutment faces and one of said main piston faces are in face-to-face engagement, the effective size of that end of said main piston is reduced; a pair of auxiliary pistons each being respectively reciprocably and sea'lably disposed within one of said axial bores of said main piston for relative movement, each of said auxiliary pistons having a cam surface which converges radially in a direction away from the other auxiliary piston; a pair of lockingkeys each reciprocably respectively disposed in one of said radial guide apertures, each of said keys being adapted at its radially outer end to engage one of said bevelled shoulders of said cylinder, and at its radially inner end to directly engage said cam surface on one of said auxiliary pistons; and means for biasing'each of said auxiliary pistons respectively against 10 one of said keys; whereby fluid pressure at one off said ports will act to move one of said auxiliary pistons to unlock one of said keys before the u'id force is ap plied fully to said main piston.

References Cited in the tile of this patent UNITED STATES PATENTS Schakel Dec'. 30,