US3442338A

US3442338A - Hydraulic circuit for mobile hydraulic hammer

Info

Publication number: US3442338A
Application number: US610884A
Authority: US
Inventors: Dean E Broderson
Original assignee: DEAN E BRODERSON
Current assignee: DEAN E BRODERSON
Priority date: 1967-01-23
Filing date: 1967-01-23
Publication date: 1969-05-06
Anticipated expiration: 1986-05-06

Description

y 6, 1969 D. E. BRODERSON 3,442,338

HYDRAULIC CIRCUIT FOR MOBILE HYDRAULIC HAMMER Sheet Filed Jan. 25, 1967 INVENTOR DEA/V E. BRODBQSON ATTORNEYS y 1969' D. E. BRODERSON 7 3,442,338

HYDRAULIC cmcur: FOR MOBILE HYDRAULIC HAMMER Filed Jan. 25, 1967 Sheet 1? of 5 IL I v INVENTOR 054 5. 58005250 ATTORNEYS Sheet 3 of 3 May 6, 1969 o. E. BRODERSON HYDRAULIC CIRCUIT FOR MOBILE HYDRAULIC HAMMER Filed Jan. 23, 1967 United States Patent 3,442,338 HYDRAULIC CIRCUIT FOR MOBILE HYDRAULIC HAMMER Dean E. Broderson, 10116 Forest,

Kansas City, Mo. 64131 1 Filed Jan. 23, 1967, Ser. No. 610,884 Int. Cl. B25d 9/06, 11/00; E21c 11/02 US. Cl. 173-86 9 Claims ABSTRACT OF THE DISCLOSURE Brief description of invention a This invention relates to an improved hydraulic circuit and system for use with power hammers, and similar devices, which incorporate hydraulic lift cylinders. The improved hydraulic circuit and system provides for a dual range of speed and force variations for a hydraulic lift cylinder, and the changes in speed and forces are accomplished through a novel system which changes the hydraulic lift cylinder from a rod displacement type of unit to a piston displacement type of unit.

In the copending application Ser. No. 522,715 filed Jan. 24, 1966, now Patent No. 3,384,186 there is described a mobile power hammer device which includes a hammer means which can be lifted and dropped onto objects which are to be broken up or compacted. The hammer means is carried on a vehicle which can be driven or wheeled from place to place, and in the conventional construction of such devices, a hydraulic power means is used to raise the hammer in a tower for dropping the hammer in a free fall hammering action. Such prior deviceshave been used for breaking up pavement, for earth compaction where highways or driveways are being recovered or rebuilt, and in connection with ditching or trenching operations. In the device described in the above identified copending application, the hammer means is mounted for reciprocation in a movable tower which is carried at one end of a self-propelled vehicle. The hammer means is lifted and dropped by means of a hydraulic system, and control devices associated therewith, and a novel means is described for actuating the hammer while at the same time protecting the hydraulic devices associated with it. The tower which supports the hammer is mounted at one end of the vehicle so that it can be preferably traversed back and forth in various positions relative to the vehicle chassis. In this manner, the tower can be moved to a desired location for dropping the weighted hammer, or other tool, at a specified point transversely of the position of the vehicle. Also, the copending application describes an improved arrangement whereby hydraulic hoses are protected and not interrupted in their operation by transverse movements of the tower relative to the vehicle. Further, there is described a system for hydraulically and automatically controlling the height to which the hammer is lifted and the dwell time between thetime the hammer is released for a hammering action and the time its elevation commences. Although the application Ser. No. 522,715 describes an improved hydraulic control system for use with a somewhat conventional hydraulic lift cylinder, it has been found that a further improvement can be made in the system by including an improved hydraulic lift cylinder in the previously described system. The present invention is directed to an improvement of a hydraulic lift cylinder, and the improvement contemplates the provision of a dual range cylinder which can be selectively operated at a low speed range and at a high speed range, with different forces being available at the different speed ranges.

In a typical use of a mobile hydraulic hammer, it is necessary to provide a sufiicient hydraulic force to raise the hammer from a work surface to its uppermost position within the tower mounted on a mobile vehicle. The usual force which is required to raise a hammer from the work surface is equal to whatever force is required to accelerate the weight of the hammer in an upward direction at the desired speed of movement. However, it has been found that occasionally there is a requirement for an increased force to be applied to the lifting cylinder associated with the hammer for the purpose of returning the hammer to its uppermost position. Most often, the requirement for an increased upward force for lifting the hammer is a result of a tool becoming wedged into the surface which is being worked, and then, it is neces sary to apply considerable force beyond what is normally required to lift the hammer away from the work surface. Also, tamping tools may become stuck in soft earth, and again, it is desirable to be able to apply an increased lifting force to the hammer means associated with the device.

The present invention. solves the problem of providing an increased lifting force, when necessary, through a novel construction for a hydraulic lifting cylinder associated with the device. The lifting cylinder may be of any type having a piston and rod means which is movable within a cylinder in response to hydraulic forces applied to opposite sides of the piston and rod means. The present invention improves such hydraulic cylinder constructions by providing a novel circuiting arrangement for the flow of hydraulic fluid in a system which controls the movement of a piston withinthe cylinder. The novel arrangement includes means for diverting hydraulic fluid in one of two directions from one side of the piston and rod combination. The fluid is diverted by a selector valve which may be manually operated to cause hydraulic fluid to flow either back into a reservoir or alternatively into a conduit which conveys fluid to an opposite surface of the piston and rod construction. When the hydraulic fluid is caused to flow back into the reservoir from one surface of the piston, and while hydraulic pressure is being applied to an opposite side of the piston, there is a relatively large total force applied to the piston and the rod connected thereto. On the other hand, when hydraulic fluid is caused to flow from one side of a piston to an opposite side of the same piston, there is a lesser total force applied to the piston. In this way, the hydraulic system can be utilized to provide two separate force ranges for operating the hydraulic lift device, and the lift device operates at a higher speed when a lower total force is being applied to the piston as compared'to when a greater total force is applied to the piston.

The improved system and means for operating the same are easily constructed and adapted to existing equipment. Also, the system is easily operated, and a hydraulic lifting cylinder can be quickly shifted from its normal high speed range of operation to a lower speed range having a greater force available for lifting or moving whatever devices are associated with the cylinder and piston arrangement. These and other advantages of the invention will become apparent in the more detailed discussion which follows, and in that discussion reference will be made to the accompanying drawings in which:

FIGURE 1 is a side elevational view of a mobile hammering vehicle, utilizing a hammer tower and a hydraulic control system in accordance with this invention;

FIGURE 2 is a partial elevational view of the FIG- URE 1 vehicle showing an end view of the tower structure associated with the vehicle together with a hydraulic lift cylinder arrangement for raising and dropping a hammer contained within the tower;

FIGURE 3 is a schematic layout of a typical hydraulic'control system which utilizes a hydraulic lift cylinder with features described in the copending application Ser. No. 522,715, filed Jan. 24, 1966;

FIGURE 4 is a detailed view in partial cross section of an improved hydraulic system for use with the hydraulic lift device shown in FIGURE 3, and showing the system operating in a high speed, lower total force range; and

FIGURE 5 is a view similar to FIGURE 4, but showing the use of the improved system to operate a lift cylinder device at a lower speed, high total force range.

Detailed description of invention Referring to FIGURE 1, a mobile power hammer unit is illustrated, and such a mobile unit may utilize the improved hydraulic control system of the present invention. The unit illustrated in FIGURE 1 is described in detail in the copending application Ser. No. 522,715, and the subject matter of that application is incorporated herein by reference to that application. The mobile unit includes a vehicle mounted on wheels, and an engine is provided for propelling the vehicle over the ground. Mounted at one end of the vehicle is a tower 12 which serves as a support frame and guide for reciprocations of a hammer means 14, or other tools which are mounted therein. The tower is pivotally mounted on a horizontal axis 16 so that it can be moved from an inoperative horizontal position (dotted lines) to an operative vertical position, as shown. A hydraulic piston and cylinder means 18 may be interconnected between a portion of the vehicle 10 and structure associated with the tower 12 so that extension and retraction of a piston within the cylinder will serve to lower and raise the tower 12 about its pivotal axis 16. Any suitable control devices and hydraulic circuits may be provided for controlling a flow of fluid into the piston and cylinder means 18, such devices being of any well known construction.

The hammer means 14 includes a relatively large weighted portion 20 which is guided for reciprocation within the upright guides 22 of the tower 12. The hammer means also includes a tool or working portion 24 which is attached at a lower end of the weighted portion 20. Thus, the hammer can be lifted to a desired height in the vertically positioned tower, and then dropped so that the weighted portion 20 will drive the working element 24 into the work surface, such as pavement or earth which is being worked. The hammer means 14 is raised to its upper positions within the tower 12 by a sheave and cable system which is described in the above mentioned copending application, and a hydraulic lifting means is provided for extending and retracting a cable which is connected to the hammer means 14. The hydraulic means for controlling the cable is cgntained entirely within a part of the tower structure which is never moved while the tower and its hammer are in operation.

Referring to both FIGURES 1 and 2, it can be seen that the tower 12 is mounted for movement transversely across an end of the vehicle 10 by a chain and sprocket means which is controlled independently of the cable and sheave means mentioned above. The illustrated embodiment provides for a full traversing movement of the tower structure relative to the vehicle without interfering with or binding any hydraulic conduits used for lifting and releasing the hammer means within the tower. This improved arrangement is a result of providing a short hydraulic conduit 26 which does not move with the traversing movement of the tower, but which does provide for all of the hydraulic flow of fluid which is necessary to raise and release the hammer. Thus, the illustrated construction permits a free movement of the tower transversely across the end of the vehicle without a requirement for long lengths of costly hydraulic conduits running between the vehicle and the tower. A chain and sprocket means for moving the tower transversely is not illustrated, but any suitable construction may be utilized. For example, the improved arrangement described in the copending application Ser. No. 522,715 may be utilized to actuate traversing movement of the tower relative to an end of the vehicle. As shown in FIGURE 1, the tower is mounted on guideways 28 (dotted lines), and the guideways may be in the form of hollow metal rails having square cross sections which contact mating surfaces contained within a portion of the tower structure.

A lifting and releasing means for the hammer is contained within the housing structure 29 associated with the end of the vehicle upon which the tower is mounted. As illustrated, the housing structure 29 is constructed to pivot about the axis 16 when the entire tower struc ture is lowered or raised, however, alternative arrangements may be used for housing the hydraulic system which controls the lifting and releasing of the hammer. The lifting and releasing means includes a cable and sheave system which can be hydraulically extended to move a cable around a plurality of sheaves in such a way that the hammer means is lifted to one of its upper positions in the tower 12.

Sheave members

30 and 32 are arranged at opposite ends of the housing 29, and a hydraulic lift cylinder 34 is interposed between the sheaves for moving one of the sheave members relative to the other. For example, the sheave member 30 may be mounted in a fixed position at one end of the housing structure 29, while the second sheave member 32 may be mounted to be moved toward and away from the sheave member 30. The hydraulic lift cylinder device 34 is installed between the sheaves so that a rod member 36 extending from a cylinder 38 of the device can be actuated to move the sheave member 32 away from the fixed sheave 30. This movement causes a movement of the cable 40 about the plurality of sheaves associated with each sheave member, and the hammer means 14 is lifted in accordance with an extension movement of the cable 40. Of course, the number and arrangement of individual sheaves may be modified to accommodate desired mechanical advantages for the system.

Looking to the details of FIGURE 2, it can be seen that one end of the cable 40 is attached at 42 to the weighted portion 20 of the hammer means. An opposite end of the cable 40 is connected to a link 43 mounted on the tower structure, and thus, for all transverse movements of the tower there is a relative movement of the cable 40 about the large pulley 44. This relative movement of the cable with all transverse movements of the tower provides for a compensation in the length of cable which is supporting the hammer means 14, and in this manner, the hammer is maintained at a substantially constant level for all transverse movements of the tower. Thus, there is provided a system for lifting and releasing a hammer means with n a ower struc ur and fur he the system accommodates all transverse movements of the tower relative to a vehicle upon which it is mounted. As will be discussed with reference to FIGURES 3 through 5, a hydraulic system is provided for conducting hydraulic fluid to opposed surfaces of a piston within the cylinder 38 of the hydraulic lift device 34. When hydraulic fluid is applied to one surface'of a piston contained within the cylinder, the rod 36 is caused to move outwardly away from the cylinder 38, and this extension movement results in a lifting movement of the hammer means through the action of the cable 40. After the hammer means has been lifted to an upper position within the tower, hydraulic fluid may be dumped from the pressure side of the piston within the cylinder 38, and the hammer means is thereby released for being dropped onto a work surface.

FIGURE 3 illustrates a hydraulic control system for automatically controlling the lifting and dropping of the hammer means 14, and this control system is described in detail in the copending application Ser. No. 522,715.

The control system includes an automatic control together with a manual control for lifting and releasing the hammer means at desired time intervals. The hydraulic lift cylinder means 34, which actuates the

sheaves

30 and 32, is shown in the lower left hand corner of the schematic layout of the hydraulic system. A pumping means 50 is connected to a source of power for being driven and for pumping hydraulic fluid through the system. From the pumping means 50, fluid is caused to flow through the line 52 and through a manually operated valve 54 which is normally set to permit a passage of fluid from the line 52 and into a line 56 when it is desired to raise the hammer means to an upper position in the tower. A solenoid valve means 58 is interposed between the line 56 and the line 26 which leads into the hydraulic cylinder 38. It will be recalled that the line 26 may be in the form of a short flexible hose which is the only requirement for such a hose in the system shown in FIGURE 1. When the hammer is being raised, by an extension of the rod or arm member 36 outwardly (in the direction of the arrow) away from the cylinder 38, it is necessary for the valve means 58 to be closed to the line 60 so that hydraulic fluid can be pumped directly into the cylinder 34. The solenoid valve 58 is of a conventional construction and is automatically controlled by electric switching means which are shown schematically at 62. Suitable connections are provided between the switching means and the electrically operated solenoid valve 62 so that the valve can be actuated to a first position which blocks the flow of fluid to line 60, thus forcing pressurized fluid into the cylinder 38, or to a second position which permits the dumping of fluid out of the cylinder 38 through the line 60. The automatic switching means 62 includes controls for determining the length of time the valve is held in a position which permits a flow of fluid to the cylinder 38 and for the length of time that the valve is moved to a position which dumps fluid from the cylinder38. These two separate controls are shown respectively at 64-and 66. In a preferred control arrangement, the control 64 determines the length of time that current is off relative to the solenoid operated valve means 58. When the current is off, the valve 58 is set to permit a pumping of fluid into the hydraulic cylinder 38 through the lines 52, 56 and 2-6. By controlling the length of time that fluid can be pumped into the cylinder 38, there is also a control of the extent of movement of the sheave 32 away from the sheave 30. This control determines the maximum height to which the hammer may be lifted within the tower 12, and it is desirable to be able to control the height from which the hammer can be dropped for any particular job requirement. When the hammer reaches the predetermined height, electric current is caused to actuate the solenoid valve means 58 into a position which permits the dumping of hydraulic fluid out of the cylinder by way of line 26 together with the line 60. At the same time, the pumping of fluid is continued through the valve means 58 and through line 60 to the reservoir, and it will be appreciated that once the line 60 is open for dumping fluid, the weight of the raised hammer and its force upon the cable means 40 will cause a return movement of the arm 36 into the cylinder 38. Of course, it is to be understood that a piston means is contained within the cylinder 38 and is aflixed to the end of the rod or arm 36. The control switch 66 determines the amount of dwell time that remains between the time the hammer is released for a hammering action and the time that its lifting is commenced. This control feature permits a control of the rate of a hammering operation, and thus, the system permits an automatic repetition of preset hammering cycles in accordance with the positions of the switches 64 and 66. A manually operated control lever 68 provides for a manual turning of the valve device 54 to a position which stops further pumping of hydraulic fluid through the line 56 and into the hydraulic cylinder 38. The manual control arm 68 opens a dump line 70 which returns to the reservoir of the system, and when the control lever is moved in the direct-ion of the arrow, the line 70 is open for a dumping of hydraulic fluid from the cylinder 38 through the lines 26 and 56. This dumping action can be made to take place even though the automatic control system 62 is in operation, and this permits the operator to override the automatic operation of the hammer device. FIGURE 3 also illustrates, in dashed lines, the improvement of the present invention, and it can be seen that the improved circuiting arrangement for a hydraulic control system can be easily added to existing control systems. Of course, it is to be understood that all of the features of FIGURE 3 may not be required in a particular use of a hydraulic system, and the improve ment of the present invention may be applied to any system which utilizes a piston and rod arrangement within a cylinder.

FIGURES 4 and 5 illustrate in detail the improvement of the present invention, and the improved system may be utilized with the arrangement shown for FIGURE 3 or in any other hydraulic system for operating a piston and cylinder combination. As shown in the figures, a piston 72 is mounted for reciprocation within the cylinder 38, and the rod 36 is secured to the piston 72 so as to be moved by movements of the piston within the cylinder. A conduit 26 provides for a flow of hydraulic fluid against a first surface 74 of the piston 72 by an introduction of fluid into the chamber 76. When the chamber 76 is filled with hydraulic fluid to the maximum point, the piston and rod combination are extended outwardly away from the cylinder 38 for a maximum extension. FIGURE 4 illustrates a circuiting arrangement for use when the lift cylinder device 34 is to be used in normal operations requiring a high speed movement of the rod 36 outwardly away from the cylinder 38. The normal operation requires a total force for moving the piston and rod which is suflicient to raise the hammer associated with the hydraulic lift device. Accordingly, the system of FIGURE 4 is arranged to provide an adequate total force for rapidly lifting the hammer to an uppermost position for dropping the same. In the FIGURE 4 system, a hydraulic flow path is provided from a second chamber 78, on an opposite side of the piston from the chamber 76, back to the main line 26 which introduces fluid into the first chamber. The flow line is made up of conduits 80 and 82 with a selector valve 84 interposed between the two conduits so as to selectively control the flow of fluid out of the chamber 78 into a desired flow path. The FIGURE 4 arrangement shows the selector valve 84 in a position which permits a flow of fluid from the chamber 78, through the conduit 80, through a portion of the valve 84, through the conduit 82, and into the main line 26. Therefore, the total hydraulic force available for pushing the piston outwardly away from the cylinder 38 is a factor of the per square inch pressure of the hydraulic fluid times the difference between exposed

surfaces

74 and 84 on Opposite sides of the piston 72. For example, in a typical operation hydraulic fluid is introduced into the chamber 76 at a pressure of 2500 psi. With the flow line 80 and 82 open, as shown in FIGURE 4, the hydraulic pressure within the second chamber 78 will likewise be 2500 p.s.i. However, the surface area of the piston surface 74 may be 9.62 square inches as compared to a much smaller surface area of approximately 2.55 square inches for the second face 84. The difference between the two surface areas as multiplied by the pounds per square inch of pressure results in a total force of approximately 18,000 pounds (9.62-2.55 x2500) being applied outwardly on the piston and rod combination. Of course, it should be recognized that the flow of hydraulic fluid from the chamber 78 combines with a given flow of fluid through the line 26 to increase the total quantity per unit time of fluid being introduced into the chamber 76. Therefore, the chamber 76 is filled at a more rapid rate than would result from only the given flow through line 26, and the piston and rod are moved at a relatively high speed for the force that is required to lift a hammer.

FIGURE illustrates a low speed, high force range for the system of FIGURE 4, and it can be seen that the selector valve 84 has been moved to a position which causes hydraulic fluid to flow out of the chamber 78 and back into the reservoir of the system instead of into the feed line to the chamber 76. This arrangement results in a zero pressure per square inch in the chamber 78 since no pumping pressure is available to that chamber, and therefore, the total force available against the piston and rod combination would be approximately 24,000 pounds (9.62 2500) for the dimensions given in the FIGURE 4 example. Also, it is to be noted that the flow of fluid out of the chamber 78, when the piston is being moved outwardly from the cylinder 38, does not combine with the flow of fluid into the chamber 76. This means that the rate of flow for hydraulic fluid into the chamber 76 is slower in the FIGURE 5 circuiting as compared to the FIGURE 4 circuiting. Accordingly, the FIGURE 5 arrangement provides a slower movement of a piston outwardly from its associated cylinder while at the same time providing an increased force against the piston and the connecting rod carried thereby. The low speed, higher force range is useful for lifting a hammer and tool combination when the tool has become wedged or stuck into a work surface, and in such instances, the usual lifting force for lifting the hammer is insuflicient to dislodge the tool. The selector valve 84 is preferably manually operated, and positioned in a place where an operator of the mobile unit can simply push or pull the actuator for the selector valve so as to select a range of movement for the lift system. In normal operations, the valve 84 is left in the position shown for FIGURE 4, and an operator is required to change the valve position only when additional forces are required to lift the hammer. Of course, it is to be understood that the dimensions given above are by way of example only, and the force and speed ranges for a system can be varied by adjusting the dimensions of the system. Further, it is possible to arrange the illustrated flow line in any suitable way to accomplish the desired results. For example the return of fluid into the chamber 76 from the chamber 78 may be accomplished by a conduit communicating directly into the chamber 76, and of course separate conduits with separate valves may be used to determine the alternative flow paths which have been illustrated in FIGURES 4 and 5.

Although the invention has been described with reference to a single embodiment of an improved hydraulic control system, it will be appreciated that variations in the described system will become obvious to those skilled in the art, and such variations are intended to be included within the scope of this invention.

What is claimed:

1. In a hydraulic system for actuating a piston and cylinder combination and for applying a dual range of forces to the piston and to a rod connected thereto, said rod being further connected to a sheave and cable system which lifts a hammer for a power hammer system, the improvement comprising:

a main conduit in communication with a first chamber within said cylinder and with a first surface of said piston which is exposed in the first chamber, said conduit being connected to a source of hydraulic fluid which is under pressure and which can be pumped into said first chamber for expanding said chamber so as to move said piston and a rod connected to a second surface thereof,

a secondary conduit means in communication with a second chamber which is formed within said cylinder adjacent a second surface of the piston which is opposed to said first surface, said second chamber being on a side of the piston to which said rod is connected,

a selector valve means for diverting flow of hydraulic fluid to and from said secondary conduit means,

separate conduits between said selector valve means and to said main conduit and to a reservoir included in the hydraulic system, said selector valve means functioning to direct a flow of hydraulic fluid through said secondary conduit means and alternatively to the main conduit or to said reservoir, whereby a flow of fluid from said second chamber into said main conduit provides a relatively high speed and low force movement of said piston as compared to a flow of fluid from said second chamber into said reservoir.

2. The improvement of claim 1 wherein said system includes means for recirculating hydraulic fluid from said reservoir and through a pumping means for conveying hydraulic fluid into said main conduit.

3. The improvement of claim 1 and including means for dumping hydraulic fluid from said first chamber when it is desired to return said piston and rod combination to a beginning position.

4. The improvement of claim 1 and including means for introducing hydraulic fluid into said second chamber while releasing fluid from said first chamber when it is desired to return said piston and rod combination to a beginning position.

5. In a hydraulic system for actuating a piston and cylinder combination and for applying a dual range of forces to the piston and to a rod connected thereto, said rod being further connected to a hoisting system which lifts a hammer means in a power hammer device when said piston is moved in a first direction, the improvement comprising:

a main conduit for conducting hydraulic fluid into said cylinder for applying a hydraulic pressure to a first surface of said piston so as to push said piston and rod in a first direction relative to said cylinder,

secondary conduit means connected to said cylinder for conducting hydraulic fluid to and from a second surface of said piston which is smaller in surface area than said first surface and which is opposed to said first surface,

means for selectively conducting hydraulic fluid through said secondary conduit means to (a) said main conduit when hydraulic pressure is being applied to said first surface of the piston or to (b) a reservoir associated with said hydraulic system, whereby pressure is reduced in said cylinder and against the said second surface of the piston when fluid is conducted through said secondary conduit means and into said reservoir.

6. The improvement of claim 5 wherein said last named means comprises a valve means which can be actuated to alternatively conduct hydraulic fluid to said main conduit or to said reservoir.

7. The improvement of claim 6 wherein said secondary conduit means comprises a single conduit communicating with said valve means.

8. The improvement of claim 6 wherein said secondary conduit means includes separate conduits communicating separately with said main conduit and said reservoir, and wherein there is provided a separate valve means in each separate conduit.

9. In a hydraulic system for actuating a piStOn and cylinder, said piston being connected to a rod which is further connected to a sheave and cable system which lifts a hammer means for a power hammer system, said hydraulic system including a solenoid valve for directing fluid flow from a pump and reservoir to either said cylinder or said reservoir and an electrical control system for operating said solenoid valve in such a manner that the time periods during which fluid is pumped into or dumped from said cylinder, respectively, may be manually controlled, the improvement comprising:

a main conduit, connected to said solenoid valve, in

communication with a first chamber within said cylinder and with a first surface of said piston which is exposed in said first chamber,

a secondary conduit in communication with a second chamber formed within said cylinder adjacent a second surface of said piston which is opposed to and smaller than said first surface, said second chamber being on a side of said piston to which said rod is connected,

a selector valve means connected to said secondary conduit means, and

separate conduits between said selector valve means and to said main conduit and to said reservoir, said selector valve means functioning to direct a flow of hydraulic fluid through said secondary conduit means and alternatively to said main conduit or to said reservoir, whereby a flow of fluid from said second chamber into said main conduit provides a relatively high speed and low force movement of said piston as compared to a flow of fluid from said second chamber into said reservoir.

References Cited UNITED STATES PATENTS 523,419 7/1894 Thorpe 91-436 X 2,254,477 9/ 1941 Ferguson 173-86 X 2,985,142 5/1961 Aiken 91-436 ERNEST R. PURSER, Primary Examiner.

US. Cl. X.R.