US2389661A - Compression riveter - Google Patents

Description

1945- H. R. FISCHER COMPRESSION RIVETER Filed Aug 2, 1940 5 Sheets-Sheet 1 a Y 2 4m W Nov. 27, 1945. FISCHER COMPRESSION RIVETER Filed Aug. 2, 1940 5 Sheets-Sheet 2 BY M, i A'II'TORNEY 27, 1945- H. R. FISCHER COMPRESSION RIVETER Filed Aug. 2, 1940 5 Sheets-Sheet 5 1945- H. R. FISCHER 2,389,661

COMPRES S ION RIVETER ATTORNEY 5- H. R. FISCHER 2,389,661

COMPRESS ION RIVETER Filed Aug. 2, 1940 5 Sheets-Sheet 5 I E:I.E.

INVENTOR era ,9. fiisc/ier Patented Nov. 27, 1945 I COMPRESSION RIVETER Howard B. Fischer, Detroit, Mich., assignor to Chicago Pneumatic Tool Company, New York, N. Y., a corporation of New Jersey Application August 2, 1940, Serial No. 349,820

2 Claims.

This invention relates generally to fluid operated presses for performing riveting and similar operations and more particularly to a pressure fluid distributing system,. and associated control elements, capable of use in a tool or device of this class.

In a device of the class to which the present invention relates a working plunger is moved toward and away from a stationary die, or through a compression and return stroke, to complete a single cycle of operation. The plunger is forced toward the stationary die by a power piston. acting either directly on the plunger or through intermediate pressure multiplying and transmitting elements, while any yielding means may be used to return the plunger. The power piston reciprocates within a cylinder, constructed as an integral part of the tool assembly or located remotely therefrom, and movement of the piston is eflected by pressure fluid directed to alternate ends of the cylinder at opposite sides of the piston head. The means for controlling the flow of pressure fluid to the power cylinder is the primary subject of this invention and it will be evident that this means is-applicable to any pressure operated tool of the class described.

Among the control elements comprised in the device of the present invention are a throttle valve, a distributing valve settable to a plurality of positions to control the flow of pressure fluid to the power cylinder, and means for effecting auto matic movement of the distributing valve from and to a normal position upon actuation of the throttle valve. Included in the latter means is a pressure responsive valve controlling communication between the high pressure end of the power cylinder and an area enclosing one end of the distributing valve whereby pressure fluid from the power cylinder may be directed against the distributing valve to shift the valve from one to another of its control positions. By manipulation of an adjusting screw a variable resistance against movement to open position may be given the pressure responsive valve, thereby to prevent automatic shifting of the distributing valve until a predetermined peak pressure is attained in the high pressure end of the power cylinder.

The present invention also provides a means for manually setting the distributing valve from one to another of its set positions in order that, when desired, only a part of a machine operation may be effected by actuation of the throttle valve.

One object of the invention is to increase the efiiciency and simplicity of operation of a compression tool through the provision of control mechanism adapted to initiate and complete a cycle of machine operation upon actuation of a single control lever.

Another object of the invention is to enable the operation of the tool to be stopped at any point during the compression stroke and to enable the tool to be then restored to normal condition without completing the compression stroke.

Another object of the invention is to effect automatic movement of the distributing valve, and thereby automatically initiate the return of the working plunger, in response to the attainment of a predetermined degree of pressure in the high pressure end of the power cylinder.

Another object of the invention is to vary the peak pressure obtainable in the high pressure end of the power cylinder.

v and partly in side elevation of a riveting tool embodying the structure and mechanism of the present invention;

Fig. 2 is a front end view of the tool shown in Fig. 1, with a cover plate removed and the lower end of the stationary yoke broken away;

Fig. 3 is a View in cross-section through the cylinder head of the tool, showing the operating controls of the pressure fluid distributing system, and is taken substantially along the line 3-3 of Fig. 1;

Fig. 4 is a diagrammatic representation of the pressure fluid distributing system and associated operating controls, showing all parts in normal position with the power piston in retracted position;

Fig. 5 is a view similar to Fig. 4, showing the throttle valve in open position and the power piston moving through its compression stroke;

Fig. 6 is a view similar to Figs. 4 and 5, showing the throttle valve in open position, the power piston at the end of its compression stroke and the distributing valve starting to move away from normal position;

Fig. 7 is a view similar to Fig. ,6, showing the distributing valve shifted out of normal position and the power piston returning toward retracted position; and

Fig. 8 is a detail view of an adjustable mechanism for limiting the movement of the working plunger away irom'the stationary die.

Since the invention is concerned primarily with means for controlling the application of pressure fluid in a compression tool, the major portion of the description will be directed to a disclosure of this control means. In Figs. 1, 2 and 3, the invention is shown embodied in a small riveting tool of the type adapted either to be mounted upon a pedestal, for use as a fixed machine, or held in the hands of the operator, for use as a portable tool. The tool is, further, of the class embodying rivet set mechanism, power mechanism and operating controls in a single unitary structure, connected by a pressure fluid conducting hose line to a source of power.

Referring to Fig. 1, the tool comprises several cooperating sections or sub-units including a head I!) and a cylinder intermediate the head In and an end portion, or casing, i2 to which is secured a yoke I3. Within the head are pressure fluid passages and control valves to be described in detail at a later point in the specification; while within the cylinder H is a piston |4 having a rod i5 secured thereto and extending through one end of the cylinder into the end portion l2. Fixed to the outer end of the rod 85 is a plate l6 bifurcated to form a pair of arms I1 (see also Fig. 2) between vhich are supported a pair of rollers H3. The rollers l8 are arranged one above the other and the upper one engages the lower surface of a guide block I9, fixed within the casing |2, while the lower one engages the upper edge of a lever 2| pivoted on a stud 22 supported between the side walls of easing |2. The lower edge of the lever 2| is formed at its outer end with a projection 23 which overlies the upper end of a downwardly extending plunger or rivet set 24 movable within a bushing 25 positioned in the upper end of the yoke I3. The lower end of the rivet set 24 lies above a die 26 set in the lower, work engaging, surface of the yoke I3 and the rivet set reciprocates vertically in a movement toward and away from the die 26 to complete a single riveting operation. The adjacent ends of the lower die 26 and an upper die 21, within the lower end of the rivet set 24, are concave in shape in order that the ends of the rivet may have an even rounded form when headed. A spring 28 mounted within the casing l2, in a manner not herein shown, is connected to the rivet set 24 and urges it upward into engagement with the projection 23 on lever 2|. Through the rivet set 24, the spring 28 acts also to maintain the upper edge of the lever 2| in contact with the lower one of the pair of rollers 8. The upper edge of the lever 2| is formed with a depression 29 near the pi ot point 22 and with an inclined surface 3| extending from a point near the depression 29 to the outer end of the lever. With the piston in retracted position, or in the lefthand end of the cylinder II as viewed in Fig. 1, the lower roller 8 takes a. position above the depression 29 allowing the lever 2| and rivet set 24 to rise from the fully operated position of Fig. 1 to a normal position wherein the dies 26 and 21 are spaced apart a sufficient distance to permit the application of the tool to the next rivet. As the piston |4 moves forwardly, or to the right as viewed in Fig. 1, the rollers I8 pass between the lever 2| and guide block I9 and act as a wedge, moving the lever 2| in a clockwise direction and thereby forcing the rivet set 24 downward toward the die 26. At the start of the forward or compression stroke the lower roller l8 rides out of the depression 29 and, in so doing, moves'the lever 2| and rivet set 24 quickly through the major portion of their travel toward fully operated position. This initial movement of the rivet set brings the die 21 into contact with the work and is usually termed the rivet setting portion of the stroke. As the piston |4v continues to move forwardly the lower roller I8 advances along the slightly inclined surface 3| and the rivet set 24 is pressed down slowly to the position shown in Fig. 1, completing the rivet heading operation.

The upper edge 3| of the lever 2| is substantially a straight line for the major portion of its length. Toward the front end, however, it has a slight curve, upwardly and forwardly. The shape of the edge 3|, which may be determined either empirically or mathematically, is preferably such as to prevent substantial variation in the mechanical advantage, or instantaneous speed ratio, between the primary power piston I4 and the working plunger 24 during the final travel, say, the last one-eighth inch, of plunger 24. -As a result, the maximum, or final, riveting pressure will be proportional to the maximum or ultimate cylinder air pressure, notwithstandingvariations of, say, one-eighth inch, in the thickness of the plates being riveted together.

The front of the end portion, or casing, H is closed by a plate 32, which, as shown in Fig. 8. supports an adjustment means whereby the distance which the dies 26 and 2'! are spaced apart in the normal position of the rivet set 24 may be varied to compensate for differences in the length of the rivets or the thickness of the sheets being joined. This adjustment means comprises a settable element 33 having a stem extending into a bracket 34 secured to the outside surface of the plate 32. A thumb nut 35, supported by the bracket 34 and plate 32, has a threaded engagement with the stem of the element 33 and by manipulation of the thumb nut the settable element may be raised or lowered relatively to the bracket 34. The lower end of the element 33 extends inwardly through a slot in the plate 32 and has mounted therein a plunger 36 pressed, by means of a spring 31, toward the outer end of the lever 2|. The end of the plunger 36 is positioned above a cut-out portion 38 (Fig. 1) in the upper end of the rivet set 24 and is adapted to engage this cutout portion to limit the movement of the rivet set toward normal position. By adjustment of the thumb nut 35 the plunger 36 may be variably set with respect to the cut-out portion 38 to vary the extent of upward movement permitted the rivet set 24.

Reciprocation of the piston I4 is effected by introducing pressure fluid into the cylinder H at alternate points to the rear and the front of the piston. The pressure fluid, in the present instance compressed air, is directed, by means of a hose line 4| (Fig. 1), from a source (not shown) to an inlet passage 42 formed in an extension 43 of the'head H), which extension serves as a handle for the tool. The passage 42 opens into a bore 44 in the head "I and the air flowing into the bore 44 is admitted, under the control of a throttle valve 45, to a compressed air distributing system whereby it is caused, upon actuation of the valve 45, to drive the piston l4 forwardly and, when a predetermined peak pressure is reached within the cylinder II in back of the piston H, to return the piston to normal position. In Fig. 1, the throttle valve 45 is shown in open positionand the piston l4 at the end of its compression stroke.

Referring also to Fig. 3, which is a view incross section through the head l0, it will be seen that the several control elements of the compressed air distributing system include, in addition to the throttle valve 45, a distributing valve 48, a pressure relief valve 41 and a check valve 48 (Fig. 1). The valves 45, 46 and 41 are movable vertically within respective bushings 49, and 52 which are, in turn, positioned within bores cut vertically through the head Ill. The throttle valve 45 is settable to an open and closed position to permit and prevent the flow of air through the tool; the distributing valve 46.is settable to two operative positions, in one of which it directs the air to one end of the cylinder II, and in the other of which it directs air to the opposite end of the cylinder; the pressure relief valve 41 controls the automatic movement of the distributing valve 46 from one set position to the other to initiate return movement of the piston l4; and the check valve 48 serves a purpose incidental to the automatic operation of the distributing valve, and which will be later described.

The flow of air through the tool and the operation of the valvular control mechanism will be best understood by consideration of the diagrammatic drawings, Figs. 4-7, and these views should be read in conjunction with Fig. 3 in following the operationsdescribed below. In the diagrammatic views the cylinder H is represented as a bore or compression chamber in the head i 0, and, for convenience of illustration, the piston I4 is shown as constructed of a single piece, with the stem thereof cooperating directly with the work engaging surface of the yoke l3. Terms of position and direction, such as upper, lower, and the like are used hereinafter to simplify description of the diagrammatic views and refer only to the position and direction of motion of the parts as illustrated in these views.

The upper end of the throttle valve 45 extends above the bushing 49 and underlies a set screw 53 movable with an operating lever 54 pivoted to the head it). The lower end of the valve 45 extends below the lower end of the bushing 49 and has secured thereto a valve disc 55 having an annular groove formed in the upper surface thereof in which is positioned a sealing ring 55 engageable with a flange 57 formed on the lower end of the bushing 49 to cut off communication between the interior of the bushing and the bore 44. A compression spring 58 positioned within the bore 44, between a cap 59 closing the lower end of the bore and a plate 58 bearing on the lower surface of disc 55. urges the throttle valve assembly upward to closed position wherein the sealing ring 56 contacts the flange 51. In the operation of the tool the lever 54 is pressed downward to the position shown in Figs. 1 and 5, whereupon the disc 55 is moved out of engagement with the flange 51 and live air flows from the bore 44 around a reduced portion of the valve 45 to the interior of bushing 49. A set of ports 62 establish communication between the interior of bushing 49 and an annular groove 53, formed in the outer surface of the bushing, and the air passes through these ports and around groove 63 to a passage 64 (Figs. 4-7), leading through a chamber 65 to an annular roove 56 in the distributing valve bushing 5i. From groove 66 the air flows through a set of ports 61 in the bushing 5|, then passes around an upper reduced portion 68 of the valve 45 and out a second set of ports 69 to another annular groove H. A passage 12 communicates with groove H and conducts the air therefrom to the upper end of compression chamber H where it acts on the upper surface of piston l4 to drive it downwardly toward the work. In Fig. 5, the piston is approaching the rivet but has not yet contacted it. In Fig. 6 the piston has engaged and headed the rivet and is shown just prior to the start of the return stroke.

Throughout the compression and return strokes the throttle valve 45 is held depressed and air continues to flow from the bore 44, through the bushing 49, to the passage 64 and chamber 65. Thus at the end of the downward movement of the piston l4, air is still directed to the upper end of the compression chamber ll.

However, when a predetermined peak pressure is reached within the chamber I I, an automatic return operation is initiated by means hereinafter described, the first step of which is a movement of the distributing valve/46 downward from the position shown in Figs. 4 and 5 to the Fig. 7 position. This movement is accomplished against the pressure of a spring 13, urging the valve 46 upward, and, as shown in Fig. 7, results in placing the head of the valve opposite the admission ports 61, to close these ports, and in placing a lower reduced portion 14, of the valve, opposite a set of ports 15, to open these ports. The ports 15 communicate, through an annular groove 16, with a passage 11 leading from the previously mentioned chamber 65 which receives live air directly from the passage 64. The check valve 48 is positioned within the chamber 65 between the passages 64 and 11 and, during the compression stroke, cuts off communication between these passages. With the set of ports 6! closed, however, the air directed to the upper end of chamber 65 is held momentarily by the resistance of the check valve 48 and then rises in pressure and is forced past the valve to passage 11 and groove 16. Passing through the ports '15 the air then flows around the reduced portion 14 of valve 45' and out another set of ports 18 to another groove 19. From groove 19 the air is conducted, through a passage 8|, to the lower end of compression chamber II and acts on the lower surface of the head of piston l4 to return the piston upward to the normal position of Fig. 4. In Fig. 7 the piston is moving upwards, but has not yet completed its return stroke. The operating le-' ver,54 may be released at any time following the return movement of the piston l4. When the lever is released the throttle valve 45 and associated elements 55 and 6! are returned to closed position by the spring 58 thereby cutting off the supply of air to the feed passage 64. Duringthe return stroke the distributing valve 46 is held in the lower position of Fig. '7 by live air flowing from passage 64 through a port 82, of reduced diameter, which, with the distributing valve in its lower position, establishes communication between the groove 66 and a chamber 83, above the valve, formed through the cooperation of the upper end of the valve, the bushing 5i and a cap 84 (see Fig. 3) closing the upper end of the bushing. Upon the closing of the throttle valve 45 the flow of air to the chamber 83 is interrupted and the spring 13, pressing upon the lower end of valve 46, is permitted to return the valve to its upper normal position. In the upper position of the distributing valve, the port 82 is closed.

In order that movement of the piston l 4 and of the distributing valve 46 may not be impeded by the resistance of trapped air within the chambers ii and 83, a means is provided for connecting these chambers to exhaust. The opposite ends of the chamber H are connected alternately to exhaust under the control of the distributing valve 46. During the compression stroke, or while the piston I4 is moving downward and the valve 46 is in its upper position, the lower end ofchamber H is open to exhaust. During the return stroke, or while the piston l4 is moving" upward and the distributing valve'is in its lower position, the upper end of chamber II is open to exhaust. A main exhaust port 65 is connected, by way of a passage 86 to an annular groove 81 formed in the periphery of the distributing valve bushing 5i at a point between the grooves H and 19. The groove 81 opens into the interior of the bushing 5| through a set of ports 98 and communicates alternatively with the grooves II and 19, according to the position of valve 45. With the valve 46 in the normal, or Fig, 4, position communication is established between the grooves 81 and 19 around the lower reduced portion 14 of the valve, thereby connecting passage 8| and the lower end of chamber II to exhaust. With the valve 46 in the lower position of Fig. '1 communication is established between the grooves 01 and 1i, around the upper reduced portion 66 of the valve, thereby connecting passage 12 and the upper end of chamber I I to exhaust. Chamber 83, above distributing valve 46, is connected constantly to exhaust port 85 by a passage 89 and port 9| opening directly into the chamber. A metering valve 92 positioned in the passage 89 reduces the rate of air is supplied to the chamber through port 62 the pressure therein is maintained at a relatively high level but when the throttle valve 45 is closed to cut off this supply, the pressure in the chamber drops rapidly, allowing the spring 13 to return the valve 46 upward. A third passage 93, opening into exhaust port 85, leads from a groove 94 in throttle valve bushing 49 and connects the interior of this bushing to exhaust when the throttle valve 45 is closed. The groove 94 opens into the bushing 49 through a set of ports 95 positioned just below the upper full portion of throttle valve 45. When the throttle valve is depressed to open position, the upper end thereof, which has a sliding fit with the bushing 49, closes the ports 95 and prevents the live air flowing from bore 44 from passing to exhaust through the passage 93. Upon release of the valve 45 the ports 95 are opened and the air 45 trapped in chamber 65, feed passage 64, and within bushing 49 can then pass to exhaust. Port 82 thereby supplements the restricted orifice 92 in venting the space 83 above valve 46 during its initial upward movement.

The means for controlling the shifting of the distributing valve 46 from its upper to lower position includes the pressure relief or automatic return valve 41. This valve is formed with a stem 41A movable within an innerreduced 55 portion of the bushing 52 and with a head 41B movable within the larger inner portion of the bushing and engageable with an annular shoulder 90 defined between the reduced and larger ends of the bushing to prevent the passage of air from one portion of the bushing to the other. The upper, and larger, portion of the bushing 52 communicates, by means of a port 96, with a passage 91 leading to the upper end of distributing valve bushing 5| where it opens, through a port 98, into the chamber 83. The lower reduced portion of bushing 52 is connected, through a port 99 and passage IOI, directly to the upper end of the compression chamber II. The stem 41A, of the valve 41, is formed with a longitudinal bore I02 open at its lower end and having a port I03 at its upper end positioned just below the head 413. Thus, in the normal closed position of the valve 41 (Fig. 4) the passages 91 and I! are out 01f i'rom one another since the port I03 is then positioned below the annular shoulder 90, and the head 41B is below port 96. Upward movement of the valve 41 to the position shown in Fig. 6 serves to place the port I 03 above the shoulder 90 and the head 413 in a position un- 41, this air pressure extends to a larger area.

The valve moves therefore with a pop-oil action.

A means is provided for imparting a variable resistance to the valve in order that it may be actuated only when a predetermined peak pressure is reached in the compression chamber during the compression stroke. This means resides in a plunger I04 movable Within the upper end of bushing 52 and pressed into engagement with the upper end of the valve 41 by a compression spring I05 extending upward into the stem of a manually operable adjustment screw I06. By

5 turning the screw I06 the pressure with which flow from the chamber 83. Thus, as long as live the spring I05 urges the plunger I04 downward may be increased or lessened, thereby varying the pressure opposing upward movement of the valve 41. When the pressure within the cham- 80 her I I becomes sufliciently high to force the valve 41 oil its seat live air is transmitted through the ports and passages leading to the chamber 83 where it acts on theupper end of the valve 46 to force it downward to the position shown in Fig. 1

and thereby initiate an automatic return stroke in the manner previously described. In Fig. 6 the valve 41 is open and air pressing upon the upper end of valve 46 has started it downward, although, at this time, air is still flowing through .the ports 61 and into the upper end of chamber As soon as the valve 46 moves to its lower position the upper end of chamber II is connected to exhaust, thereby relieving the pressure below the valve 41, and air is admitted from feed passage 64, through port 82, to the chamber 83. The air so directed to the chamber 83 acts to hold the valve 46 in shifted position and serves an additional purpose in that it flows back along the passage 91 and presses upon the upper end of the valve 41 to return this valve and hold it positively on its seat throughout the return stroke. At the same time, the air acting on the upper end of the valve 41 presses against the lower end of plunger I04 and, as shown in Fig. '1, holds it in an upper position. The purpose of the plunger I04 is to remove the possibility of a flutter of the valve 41, which might occur if the spring I05 made direct contact with the valve.

The check valve 48, by delaying the passage of air through the ports and passages leading to the lower end of chamber II permits a strong burst of pressure through port 82 to the chamber 83,

r resulting in an immediate return of the pressure ing 52. Passage 91 is vented, through restricted orifice 92, when the parts return to the normal or Fig. 4 position.

Following an operation of the tool, all parts stand normally in the position shown in Fig. 4.

Upon actuation of thelever 54, the throttle valve air from chamber ll, being now free to flow past I the valve 41 to the chamber 83, has started the valve 46 toward its lower position. In Fig. 7 the valve 46 has reached lower position and the return stroke has been initiated. During the return stroke there takes place, concomitantly, three distinct operations, namely: the connecting of the upper end of chamber II to exhaust; the positive return of relief valve 41 to closed position, and the passage of return air by the check valve 48 and around the lower reduced portion 14 of the valve 46 to the lower end of chamber Ii. Fig. 7 and the piston I4 is nearing the end of its return stroke. When the piston reaches the position shown in Fig. 4, the operating lever 54 may be released and all the parts held out of normal position, by the pressure of live air, will be restored to the control of their respective springs.

Through the provision of the automatic return valve 41 and the adjustment means whereby the pressure opposing actuation of this valve may be varied, the flexibility and efliciency of the tool is greatly increased and a standard, uniform quality of work may be attained irrespective of possible variations in factory line pressure or the limited capabilities of an inexpert operator. Each particular size (diameter) and type of rivet has an individual requirement as to the amount of pressure needed properly to head it. Pressure in excess of this requirement may dam- I'he'valve 4! has been returned in age the work, and in any case, is a waste of power. Pressure less than that required results in a poorly headed rivet which, as the least of its faults, breaks the uniform quality and appearance of a finished series of rivets. In using the present tool the definite relation between the pressure requirement of the rivet to be headed and the opposing pressure acting on the valve 41 is determined and the adjusting screw I06 set accordingly to hold the valve on its seat until the proper pressure is reached within the compression chamber H. Variations in thickness of the plates being riveted do not interfere with the uniformity of rivet heading pressure, since the mechanical movement between the piston 14 and die 21 is so arranged that the rivet pressure is proportional to the cylinder air pressure within ordinary operating limits. In the operation of the tool the operator has merely to depress the lever 54 and hold it depressed until he sees that the piston M has returned to normal position.

The flexibility of the tool is further extended, by reason of its construction, in that it is possible to stop the piston M at any point during the compression stroke, and then cause it either to return to normal or continue on its downward stroke. By this means the position of the rivet or the eifect thereon of the rivet set may be observed at any point in the driving operation. By

opening the throttle valve 45 gradually the rivet set can be brought down to the rivet slowly and upon release of the lever 54 the piston I4, and, therefore, the rivet set, will stop and remain in the position to which they had advanced when the throttle valve closed. If it is desired then to go through and drive the rivet the throttle lever 54 is depresed a second time'and the riveting operation, including the automatic return stroke, will be completed. Should it be desired to have the rivet set return without driving the rivet, the disand its upper end extending outside the head it) adjacent the lever 54. In initiating a return stroke by this means the plunger I08 is depressed first, and then the throttle lever 54.

The return stroke adjuster illustrated in Fig. 8 is claimed in a divisional application, Serial No.

' 572,565, flled January 12, 1945.

What is claimed is:

1. In a device of the class described, the combination of a pressure fluid distributing system including a pressure chamber, a distributing valve settable to'a plurality of positions to control the flow of pressure fluid through said system, a passageway for conducting pressure fluid from said pressure chamber to said distributing valve in a direction to shift said valve from one to another of its set positions, means normally preventing the flow of pressure fluid through said passageway and movable in response to the force of accumulated pressure within said chamber to an open position with respect to said passageway, and means controlled by said distributing valvefor applying simultaneously a holding pressure on said valve to retain it in shifted position and a return pressure on said pressure responsive means to return said means to closed position.

2. A fluid pressure system comprising a cylinder, a piston reciprocable therein, a distributing valve, a source of pressure fluid, a supply passageway leading from said source to the distributing valve, a manually controlled throttle valve arranged to open and close said passageway, a front and a rear passage leading from the dis- .tributing valve to the front and rear ends respectively of the cylinder, said distributing valve being movable to a first position in which it connects the rear cylinder passage to the supply passageway to drive the piston forward, and movable to a second position in which it connects the front cylinder passage with the supply passageway to drive the piston back, valve shifting means including a tripping passage and a pressure relief valve opening the same for automatically moving the distributing valve to the second position upon the attainment of an adjustable predetermined pressure in the rear end of the cylinder, valve holding means for holding the distributing valve in the second position during and subsequent to'its operation by the shifting means while supplying pressure fluid through said tripping passage to hold the relief valve closed, and means for automatically restoring the distributing valve to its first position upon the closing of the throttle valve subsequent to the operation of the shifting and holding means.

- HOWARD R. FISCHER.

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