US3796050A

US3796050A - High energy rate actuator

Info

Publication number: US3796050A
Application number: US00254482A
Authority: US
Inventors: A Fisk; A Harvey
Original assignee: Foster Miller Inc
Current assignee: Vencore Services and Solutions Inc
Priority date: 1972-05-18
Filing date: 1972-05-18
Publication date: 1974-03-12
Anticipated expiration: 1991-03-12

Abstract

An hydraulic actuator including a cylinder, a sleeve slidable in the cylinder, and a piston slidable in the sleeve with an impacting end extending out of the cylinder. A first chamber formed in the cylinder surrounds the impact end of the piston, and energy storage means is provided in the housing at the other end, which energy storage means is in communication with the sleeve and piston. A collar is carried on the piston having one face exposed to the pressure on the first chamber and having a second opposed face which in part defines a second chamber. Means are connected to the first chamber for increasing the pressure therein to retract the sleeve and piston in unison so as to develop stored energy in the energy storage means, and means are provided for equalizing the pressures on opposite faces of the collar as the piston and sleeve retract to release the stored energy to drive the piston in a power stroke in the direction of the impacting end and also causing the stored energy to drive the sleeve in the same direction. The second chamber is constantly open to the atmosphere.

Description

United States Patent 1 1 1111 3,796,050

Fisk et al. Mar. 12, 1974 HIGH ENERGY RATE ACTUATOR a [57] ABSTRACT [75] Inventors: Allan T. Fisk, Newton Center;

Andrew Harvey, waltham, both An hydraulic actuator including a cylinder, a sleeve f Mass slidable in the cylinder, and a piston slidable in the sleeve with an impacting end extending out of the cyl- [73] Asslgnee: Foster'Mluer Assoclates, Waltham1 inder. A first chamber formed in the cylinder sur- Massrounds the impact end of the piston, and energy storage means is provided in the housing at the other end,

[22] Filed: May 18, 1972 I which energy storage means 1s in communication with [21 1 Appl' 254,482 the sleeve and piston. A collar is carried on the piston having one face exposed to the pressure on the first 52 u.s.- Cl 60/371, 60/413, 92/134 chamber and having a Second pp face which in [51] Int. Cl B30b l/30 P defines a Second Chamber- Means are Connected 5g Fie|d f Search 0 3 9 371 4 3; 91/ 7 to the first chamber for increasing the pressure therein 91 49 422 234; 92 34 to retract the sleeve and piston in unison so as to develop stored energy in the energy storage means, and l 56 References Ci means are provided for equalizing the pressures on op- UNITED STATES PATENTS posite faces of the collar as the piston and sleeve retract to release the stored energy to drive the piston in a power stroke in the direction of the impacting end 3:431:725 3/l969 Chen 60/413 x also causm? the stored energy to Sleeve m the same direction. The second chamber 1s constantly open to the atmosphere.

Primary ExaminerEdgar W. Geoghegan Attorney, Agent, or FirmWolf, Greenfield & Sacks 15 Claims, 14 Drawing Figures HIGH ENERGY RATE ACTUATOR BACKGROUND OF THE INVENTION This application relates to high velocity hydraulic actuators and more particularly comprises an improvement over the actuator disclosed in application Ser. No. 91,651 filed Nov. 23, 1970, now abandoned assigned to Foster-Miller Associates, Inc.

The actuator of the present invention is suitable for use in many'applications including rock drills, pavement breakers, punch presses, forging hammers, nail drivers, etc. The actuator may be operated either as a single or repetitive cycle device.

High velocity actuators normally deliver very high instantaneous power during the power stroke. However, high velocities are not compatible with conventional steady flow hydraulic systems for reasons both of excessive fluid velocity losses and very high instantaneous power. Consequently, actuators are designed to store energy at modest power levels to be released at a high rate during the power stroke.

Prior to the invention disclosed in application Ser. No. 91,651, supra, actuator design was very complex. The complex designs are particularly detrimental in those applications which require an impact as the useful power output, because the complex designs are not readily capable of withstanding the very substantial shocks which are imposed on the systems in those applications. The invention disclosed in application Ser. No. 91,651 represents a substantial improvement over the prior complex designs for it reduces the number of moving parts essentially to three; namely, a piston, sleeve, and valve, and it accepts a constant flow rate as the motivating force so as to enable the actuator to be powered by commonly available fixed displacement power supplies.

Nevertheless, the design of the actuator shown in the earlier application Ser. No. 91,651, now abandoned had certain disadvantages. For example, in that system the exhaust flow of motivating fluid fluctuates from zero to some high value during each cycle. The high value may be as much as several times the normal inflow rate, depending upon the cycle rate of the actuator. Furthermore, the central valve forming part of the preferred embodiment of the actuator of that application represents a potential source of maintenance problems, particularly because it must cycle once for every cycle of the piston-sleeve assembly.

One important object of the present invention is to provide an actuator with a very simple mechanical operation, which permits energy storage in a compressed gas and quick release of that energy without excessive fluid velocities or pressure surges. Consequently, the simple mechanical configuration is conducive to the adoption of simple, rugged piston and sleeve shapes suitable for impacting service.

Another important object of this invention is to provide a high velocity actuator which is free of external control valves that are potential sources of maintenance problems.

BRIEF FIGURE DESCRIPTION FIG. 1 is a cross section view of an actuator constructed in accordance with this invention and schematically shown connected to a hydraulic control circuit;

FIGS. 2-7 are schematic drawings of the actuator of FIG. 1 sequentially illustrating the positions of the various parts of the actuator during a single cycle; and

FIGS. 8-14 are schematic drawings, some frag mented, of other embodiments of this invention.

DETAILED DESCRIPTION The actuator assembly shown in FIG. 1 essentially includes three parts, namely, housing 10, sleeve 12, and piston 14. For simplicity, piston rings and other forms of seals which may be utilized have been omitted in the drawing. Housing 10 and sleeve 12 are shown to have upper diameters smaller than their lower diameters, unlike the embodiments illustrated in earlier application Ser. No. 91,651. However, this particular arrangement is not essential to the operation of this invention, and the diameters may be uniform throughout.

Housing 10 includes an inlet 16 adjacent its bottom wall 18, and an exhaust port 20 approximately centrally located between the top 22 and bottom 18 of the cylinder. Cylinder top wall 22 defines with the top of sleeve 12 and piston 14 an enclosed gas chamber 24. Top wall 22 of the housing is fitted with a check valve 26 used to charge the chamber 24 with gas.

The upper cylindrical portion 30 and lower cylindrical portion 32 which together define the housing 10 are joined at their inner surfaces by frustoconical section 34, which with sleeve 12 defines an annular hydraulic volume 36 in constant communication with the port 20. The upper smaller diameter portion 38 of sleeve 12 is joined to lower cylindrical portion 40 of the sleeve by frustoconical section 42, which in turn defines with the piston a second annular hydraulic volume 44.

Hydraulic volumes

36 and 44 are in constant communication with one another through the ports 46 formed in frustoconical section 42 of sleeve 12.

Piston 14 as shown in the drawing is essentially rod shaped but includes a collar 48 of greater diameter than either the upper portion 50 or lower portion 52 of the piston.

A chamfered seat 54 is formed in the lower end of sleeve 12, that serves as a valve seat for the beveled edge 56 of collar 48 which mates with it. When the sleeve and piston are in the position shown in FIG. 1, volume 44 is sealed from the lower chamber 58 formed in housing 10 about the lower end 52 of the piston.

Sleeve 12 slides freely in housing 10, and piston 14 slides freely in the sleeve, and a sliding seal exists between upper cylindrical portion 38 of the sleeve and upper portion 30 of the housing. Similarly, a sliding seal is effected between upper portion 50 of the piston and the inner surface of the upper portion 38 of the sleeve.

To assist in understanding the description of the operation of the actuator which follows, certain areas of the sleeve and piston have been labeled with letters which are used in the equations below. Thus, in FIG. 1, area A represents the upper end of piston 14, B represents the area of the upper end of sleeve 12, C represents the area of the collar 48 exposed to the pressure chamber 58, D represents the areaof the lower end of sleeve 12 exposed to the pressure chamber 58, and E represents the area of the exposed end of piston 14.

In FIGS. 27, the operating cycle of the actuator of FIG. 1 is illustrated. In describing this operation it is assumed that the inlet 16 is connected to a constant source of flow as may typically be established by a gear pump. And at all times during the cycle, the discharge port 20 is open to the atmosphere.

In FIG. 2 the pressure P (pressure in chamber 58) drives piston 14 and sleeve 12 upwardly against the gas pressure P in chamber 24 by acting on area C of collar 48 and area D of sleeve 12. Thus the piston and sleeve combination move upwardly in unison.

To assure that the piston and sleeve are held together the following inequalities must be established.

P58 C P24A and 24 ss Because in the embodiment shown area C is larger than area A and area B is greater than area D, the proper relationship is provided so as to insure the joint travel of the piston and sleeve as suggested in FIG. 2.

The piston-sleeve combination rises as a single body as flow into chamber 58 continues as suggested in FIG. 2 while the relationships set forth in equations 1 and 2 are maintained. As joint upward travel of piston and sleeve continues, relief passages 60 formed on the inner surface of cylindrical portion 32 of the housing are exposed by the lower end of sleeve 12 to the chamber 58. When this occurs,

volumes

44 and 36 are placed in communication with the chamber 58 and the valve seat 54 and collar 48 are bypassed. This results in the equalization of the pressures on the opposite side of the collar 48 acting'on areas C and C. When the pressures on opposite sides of the collar 48 are balanced, then the pressure P on area A creates an unbalance, and the piston is driven downwardly, breaking the seal at 54, 56 and free flow is established from chamber 58 through volume 44, passages 46, volume 36, to discharge port 20. Downward travel of the piston is impeded only slightly by flow of hydraulic fluid in the chamber 58 about collar 48 through annular area D. Consequently downward movement of the piston is very rapid, with the accelerating force approximately equal to P A.

With the seal at the valve seat 54 broken, the pressure drop across 'collar 48 from area C to C instantaneously becomes zero. If the cross sectional areas A and E of the upper and lower portions of the piston are equal, downward motion of the piston does not displace fluid, and the piston moves downwardly until stopped by some useful impact as suggested in FIG. 5. With the piston and sleeve separated, the hydraulic fluid in chamber 58 is free to exhaust through volume 44, passages 46, volume 36, to discharge port 20. Consequently the pressure 58 is sharply reduced, assuming a fixed inlet flow. If volume 44, passages 46, volume 36 and port are large, the gas pressure in chamber 24 acting on area B will rapidly drive the sleeve 12 down as shown in FIG. 6. The descent rate of the sleeve depends on the restriction of those passages and volumes. The sleeve descends, passing the relief passages 60 until a seal is again formed at the valve seat 54 as suggested in FIG. 7.

The sleeve is able to form a seal at valve seat 54 to close off flow at that area because of the force relationships expressed in equations [1] and [2] above. That is, the downward force on the sleeve closes the valve seat until the pressure in chamber 58 (P rises sufi'iciently to create an upward force on the piston and sleeve areas C and D causing the piston and sleeve to rise again as a unit in the manner suggested in FIG. 2.

In FIG. 1 the system which includes the actuator is suggested schematically. A pump 70 is shown connected to inlet duct 16 for providing a constantinlet flow to the chamber 58. The pump is shown supplied by a reservoir 72 which in turn is connected to discharge duct 20. The actuator may be shut off by shunting the housing 10 as suggested by line 74 in the system, which contains an on-off valve 76.

The present invention is susceptible to several modifications. Some of these are suggested in FIGS. 8 to 14 described below.

In FIG. 8, a flexible membrane is shown extending across the chamber 24 so as to positively seal that chamber. Consequently, hydraulic fluid may act on the top of the sleeve and piston through the diaphragm, and a positive seal is provided to prevent any loss of the fluid in chamber 24.

FIG. 9 shows another embodiment of the invention wherein the reciprocating motion of the piston 84 is employed to power a self-contained compressor which feeds the gas chamber 86 so as to maintain the pressure in the chamber above some minimum level. In accordance with this embodiment, housing 88 is provided with an upper extension 90 having a bore 92 which slidably receives the upper end of piston rod 94 carried by the piston 84. The upper end of the bore 92 is closed by adjustable plug 96, and a port 98 is provided in the extension in communication with the bore.

As the piston 84 moves downwardly during the power stroke, air flows into bore 92 through inlet port 98 when the upper end of rod 94 drops below the port, and when piston 84 moves upwardly and rod 94 again closes port 98 the entrapped air in the bore is compressed and ultimately discharges through the check valve 100 located in exhaust port 102.

A second bore 104 closed at its outer end communi cates directly with chamber 86, and a check valve 106 in the bore controls flow from the bore 104 to the chamber 86. When the pressure in chamber 86 falls below a selected value, the check valve 106 will open to permit air to pass into chamber 86'. This will occur only when the piston and sleeve are at the bottom of their stroke. Consequently, pressure in the chamber 86 is maintained at a minimum predetermined value corresponding to the maximum pressure within the bore 92. Plug 96 may be adjustable to vary the volume of the bore 92 so as to adjust the predetermined minimum pressure value for chamber 86.

When starting the actuator, it may be necessary to cycle the device a few times to achieve the desired pressure level. The variable bore volume 92 provided by plug 96 may be adjusted to limit the peak pressure within the compressor which, in turn, and in cooperation with

check valves

100 and 106 become the minimum gas pressure in chamber 86.

In FIG. 10 yet another modification is shown. In this embodiment, the diameter A of upper piston 110 is greater than the diameter E of lower piston portion 111. Consequently, when the piston descends during the power stroke, it must displace fluid in chamber 58, and this tends to be accommodated by an upward acceleration of the sleeve. As a result, sleeve descent is delayed, which counteracts piston recoil effects.

In FIG. 11 another embodiment of the invention is shown wherein the cross sectional area A" is smaller than the cross sectional area E" of the lower end of the piston (an arrangement opposite to that shown in FIG. 10). In this configuration, as the piston 114 descends during the power stroke, the negative change in volume of the piston in chamber 58 tends to draw fluid into the chamber, which is accommodated by a downward acceleration of sleeve 12 until piston 114 stops. This configuration results in a shorter cycle time than would otherwise exist;

In FIG. 12 still another embodiment is shown. In accordance with this embodiment, the relief passages 60 are omitted, and a stop 116 is provided in the chamber 24 of housing to arrest upward movement of the piston during the retraction portion of the cycle. It will be apparent that when the piston retracts to a position wherein it engages stop 116 as shown in FIG. 12, con tinued inflow of hydraulic fluid into chamber 58 will cause the sleeve to continue to rise and unseat collar 48 from valve seat 54. Consequently, when the seal is broken, the piston will immediately fire in the power stroke of the cycle in accordance with the teachings of the other embodiments.

The embodiment of FIG. 13 includes means for controlling the descent rate of sleeve 12 which accordingly controls the cycle time of the actuator. In accordance with this embodiment, the upper end 122 of piston 124 is somewhat elongated, and a partition 126 is provided in chamber 128 in sealing engagement with the piston extension. Sleeve 12 defines a volume 130 about the piston below the partition 126. a

A diaphragm 132 is also shown in FIG. 13 extending across chamber 128 in the manner of the embodiment shown in FIG. 8. However, that diaphragm is not essential to the operation of the device, but merely assures a positive seal in the chamber.

As piston 124 and sleeve 12 move upwardly together in housing 120, fluid in volume 130 is free to flow into chamber 128 below diaphragm 132 through

ducts

134 and 136 connected by check valve 138 which is free flowing in only the direction indicated by its arrow. After the piston is fired in the manner described in connection with FIG. 1, sleeve 12 is restricted in its downward motion because fluid can enter the volume 130 only through the orifice 140 which also joins

ducts

134 and 136. By adjusting the orifice size, the velocity of the sleeve in its downward travel may be varied.

Further control of sleeve descent may be achieved by the addition of a duct 142 which joins the volume 130 somewhat below the top of sleeve 12 when in its retracted position. That is, when the sleeve 12 is in its uppermost position, the duct 142 is-closed so as to bar flow from chamber 128 below diaphragm 132 to the volume 130 through it. However, when the sleeve descends in its downward stroke and uncovers the duct 142, the orifice 140 is bypassed and flow is allowed freely from chamber 128 below diaphragm 132 to volume 130. Thus, the sleeve descent rate will increase after duct 142 is uncovered by the upper end of the sleeve.

Referring again to FIG. 13, yet another means of achieving a delay of sleeve descent is suggested. In accordance with this embodiment, the duct 142 may be eliminated as well as orifice 140. In the absence of orifice 140 and duct 142, the descent of sleeve 12 would be delayed until the top of piston extension 122 passed below the point 144. When the piston drops below that point, free communication is established between chamber 128 below diaphragm 132 and volume 130, and sleeve descent would thereupon be unrestricted as in the embodiment of FIG. 1.

In FIG. 14 yet another modification is shown. In accordance with this embodiment, the constant flow actuation is replaced by an alternating power supply in the form of a reciprocating pump 150. The pump is connected to chamber 58 by duct 152, and the discharge port 20 is connected to reservoir 72' which in turn is connected to duct 152 through check valve 154. The check valve limits flow only in the direction from the reservoir to the duct as suggested by its arrow.

. In accordance with this embodiment, displacement of the pump in a direction to drive fluid into chamber 58 is sufficient to cock the piston-sleeve assembly, and the excess fluid present in chamber 58 during the compression stroke of the piston is discharged through port 20 after the piston 14 has fired and separated from sleeve 12. The flow in duct 152 may oscillate back and forth between the pump and chamber 58 as the pump reciprocates. If desired, a check valve (not shown) may be inserted in duct 152 to permit flow only in one direction, namely, from pump to chamber 58 lt will be appreciated that in all embodiments where I the two ends of the piston are of the same diameter, the piston may be made precisely symmetrical so as to permit reversing the piston to double its life. It will also be appreciated that the frequency of the cycle may be controlled by providing throttling valves in the discharge port or by introducing restrictions in the passages 46. Alternatively, frequency may be controlled by providing means for varying the inlet flow rate to chamber 58 from the pump.

It will be appreciated that the actuator disclosed in this present application is of simple design capable of producing high impact velocities. In each embodiment the velocity is achieved by initially compressing a gas or some other energy storage means and then applying that force of stored energy to the piston without requiring rapid liquid displacement. The actuator lends itself to a variety of control techniques as suggested above, and it may be operated to provide fully automatic, continuous operation,semi-automatic operation, or single blow operation.

It will also be appreciated that the foregoing description is intended merely to be illustrative of the invention and that other embodiments may occur to those skilled in the art. Therefore, it is not intended to limit the breadth of this invention to the embodiments illustrated and described. Rather, it is intended that the scope of this invention be determined by the appended claims and their equivalents.

What is claimed is:

. 1. An hydraulic actuator comprising a cylinder,

a sleeve slidable within the cylinder and forming a seal with the inner surface of the cylinder at one end thereof,

apiston slidable in and forming a seal with the sleeve at said one end and having an impacting end extending out the other end of the cylinder,

a first chamber in the cylinder at said other end defined in part by said other end of the cylinder and the other end of the sleeve and surrounding the impacting end of said piston,

a sealed gas filled second chamber at said one end of the cylinder and defined in part by the adjacent ends of the sleeve and piston,

a collar carried on the piston and having a face which is exposed to the first chamber,

means connected to the first chamber for increasing the pressure therein to retract the sleeve and piston in unison in the direction of the sealed chamber to develop stored energy in the sealed chamber by the compression of the gas therein,

a second face on the collar of the piston opposed to the first recited face and a third chamber defined in part by the second face, said third chamber being constantly open to the atmosphere, and

means for equalizing the pressures on the opposite faces of the collar as the piston and sleeve move toward the sealed second chamber to cause the pressure in the sealed chamber to drive the piston rapidly in a power stroke in the direction of the impacting end of said piston, said last-mentioned means causing the pressure in the first chamber to drop sharply and thereby permitting the pressure in the sealed chamber to drive the sleeve in the direction of said first chamber.

2. An hydraulic actuator as described in claim 1 further characterized by said means for equalizing the pressure in the second chamber being a fluid duct bypassing the collar when the piston and sleeve are retracted beyond a predetermined position within the houing.

3. An hydraulic actuator as described in claim 1 further characterized by said means for equalizing the pressure in the second chamber being a stop provided in the housing for physically limiting the extent the piston may be retracted in the housing.

4. An hydraulic actuator as described in claim 2 further characterized by said fluid duct being formed in the inner surface of the housing. r

5. An hydraulic actuator as described in claim 1 further characterized by the inner diameter of the cylinder at said one end which defines the sealed chamber being smaller than the diameter of the other end of thecylinder which defines the first chamber, the end of the sleeve in part defining the first chamber forming a sliding seal with the inner surface of said other end of said cylinder. 6. An hydraulic actuator as described in claim 1 further characterized by said sleeve in part defining said third chamber and forming a valve with the collar separating the third chamber from the first chamber as the sleeve and piston retract in unison in the housing, said valve opening to connect the first and third chambers when the sleeve and piston move relative to one another. 7. An hydraulic actuator as described in claim 6 further characterized by a relatively large constantly open port in the sleeve communicating with the third chamber and connecting the third chamber to the atmosphere. 8. An hydraulic actuator as described in claim further characterized by said sleeve in part defining said third chamber and forming a valve with the collar separating the third chamber from the first chamber as the sleeve and piston retract in unison in the housing, said valve opening to connect the first and third chambers when the sleeve and piston move relative to one another.

9. An hydraulic actuator as described in claim 8 further characterized by v a relatively constantly open port in the sleeve communicating with the third chamber and connecting the third chamber to the atmosphere.

10. An hydraulic actuator as described in claim 6 further characterized by the area of the end of the piston exposed in the sealed chamber being substantially equal to the area of the impact end of the piston extending out of the chamber.

1 1. An hydraulic actuator as described in claim 6 further characterized by the area of the end of the piston exposed to the sealed chamber being greater than the area of the impact end of the piston extending out of the chamber.

12. An hydraulic actuator as described in claim 6 further characterized by the area of the end of the piston exposed to the sealed chamber being smaller than the area of the impact end of the piston extending out of the chamber.

13. An hydraulic actuator comprising a cylinder,

a sleeve slidable within the cylinder,

a piston slidable in the sleeve and having an impacting end extending out of the cylinder,

a first chamber in the cylinder defined in part by said cylinder and sleeve and surrounding the impacting end of said piston,

energy storage means in the housing associated with the remote end of the piston,

means connected to the first chamber for increasing the pressure therein to retract the sleeve and piston in unison in the direction of the energy storage means to develope stored energy in said means,

a second face on the collar of the piston opposed to the first recited face and another chamber defined in part by the second face, said other chamber being constantly open to the atmosphere, and

means for equalizing the pressures on the opposite faces of the collar as the piston and sleeve retract to permit the stored energy to drive the piston rapidly in a power stroke in the direction of the impacting end of said piston, said last mentioned means causing the pressure in the first chamber to drop sharply and thereby permitting the stored energy to drive the sleeve in the direction of said first chamber.

14. An hydraulic actuator as described in claim 13 further characterized by said sleeve in part defining said other chamber and forming a valve with the collar separating the other .chamber from the first chamber as the sleeve and piston retract in unison in the housing,

said valve opening to connect the first and other chambers when the sleeve and piston move relative to one another.

15. An hydraulic actuator as described in claim 14 further characterized by a relatively large constantly open port in the sleeve communicating with the other chamber and connecting the other chamber to the atmosphere. t

Claims

1. An hydraulic actuator comprising a cylinder, a sleeve slidable within the cylinder and forming a seal with the inner surface of the cylinder at one end thereof, a piston slidable in and forming a seal with the sleeve at said one end and having an impacting end extending out the other end of the cylinder, a first chamber in the cylinder at said other end defined in part by said other end of the cylinder and the other end of the sleeve and surrounding the impacting end of said piston, a sealed gas filled second chamber at said one end of the cylinder and defined in part by the adjacent ends of the sleeve and piston, a collar carried on the piston and having a face which is exposed to the first chamber, means connected to the first chamber for increasing the pressure therein to retract the sleeve and piston in unison in the direction of the sealed chamber to develop stored energy in the sealed chamber by the compression of the gas therein, a second face on the collar of the piston opposed to the first recited face and a third chamber defined in part by the second face, said third chamber being constantly open to the atmosphere, and means for equalizing the pressures on the opposite faces of the collar as the piston and sleeve move toward the sealed second chamber to cause the pressure in the sealed chamber to drive the piston rapidly in a power stroke in the direction of the impacting end of said piston, said last-mentioned means causing the pressure in the first chamber to drop sharply and thereby permitting the pressure in the sealed chamber to drive the sleeve in the direction of said first chamber.

4. An hydraulic actuator as described in claim 2 further characterized by said fluid duct being formed in the inner surface of the housing.

5. An hydraulic actuator as described in claim 1 further characterized by the inner diameter of the cylinder at said one end which defines the sealed chamber being smaller than the diameter of the other end of the cylinder which defines the first chamber, the end of the sleeve in part defining the first chamber forming a sliding seal with the inner surface of said other end of said cylinder.

6. An hydraulic actuator as described in claim 1 further characterized by said sleeve in part defining said third chamber and forming a valve with the collar separating the third chamber from the first chamber as the sleeve and piston retract in unison in the housing, said valve opening to connect the first and third chambers when the sleeve and piston move relative to one another.

7. An hydraulic actuator as described in claim 6 further characterized by a relatively large constantly open port in the sleeve communicating with the third chamber and connecting the third chamber to the atmosphere.

8. An hydraulic actuator as described in claim 5 further characterized by said sleeve in part defining said third chamber and forming a valve with the collar separating the third chamber from the first chamber as the sleeve and piston retract in unison in the housing, said valve opening to connect the first and third chambers when the sleeve and piston move relative to one another.

9. An hydraulic actuator as described in claim 8 further characterized by a relatively constantly open port in the sleeve communicating with the third chamber and connecting the third chamber to the atmosphere.

11. An hydraulic actuator as described in claim 6 further characterized by the area of the end of the piston exposed to the sealed chamber being greater than the area of the impact end of the piston extending out of the chamber.

13. An hydraulic actuator comprising a cylinder, a sleeve slidable within the cylinder, a piston slidable in the sleeve and having an impacting end extending out of the cylinder, a first chamber in the cylinder defined in part by said cylinder and sleeve and surrounding the impacting end of said piston, energy storage means in the housing associated with the remote end of the piston, a collar carried on the piston and having a face which is exposed to the first chamber, means connected to the first chamber for increasing the pressure therein to retract the sleeve and piston in unison in the direction of the energy storage means to develope stored energy in said means, a second face on the collar of the piston opposed to the first recited face and another chamber defined in part by the second face, said other chamber being constantly open to the atmosphere, and means for equalizing the pressures on the opposite faces of the collar as the piston and sleeve retract to permit the stored energy to drive the piston rapidly in a power stroke in the direction of the impacting end of said piston, said last mentioned means causing the pressure in the first chamber to drop sharply and thereby permitting the stored energy to drive the sleeve in the direction of said first chamber.

14. An hydraulic actuator as described in claim 13 further characterized by said sleeve in part defining said other chamber and forming a valve with the collar separating the other chamber from the first chamber as the sleeve and piston retract in unison in the housing, said valve opening to connect the first and other chambers when the sleeve and piston move relative to one another.

15. An hydraulic actuator as described in claim 14 further characterized by a relatively large constantly open port in the sleeve communicating with the other chamber and connecting the other chamber to the atmosphere.