EP1242746A1

EP1242746A1 - Method and apparatus for generating hydraulic pressure

Info

Publication number: EP1242746A1
Application number: EP00983611A
Authority: EP
Inventors: Jan Ericsson
Original assignee: J TEC; J-Tec
Current assignee: Hyfotec Sweden AB
Priority date: 1999-12-07
Filing date: 2000-11-29
Publication date: 2002-09-25
Anticipated expiration: 2020-11-29
Also published as: DE60013324D1; DE60013324T2; SE9904464D0; WO2001042662A1; ATE274645T1; AU2034301A; EP1242746B1

Abstract

The present invention relates to a method of operating an apparatus (1) for generating high hydraulic pressure and having a low pressure piston (12) being displaceable in a low pressure cylinder (2). Said low pressure piston carries a high pressure piston (13) that is displaceable in a high pressure cylinder (3) forming a high pressure chamber (16). The inner wall of the high pressure cylinder carries stationary fluid seals (21, 24, 43) for sealingly engaging the high pressure piston. A continuous restricted outflow of hydraulic fluid is provided from the high pressure chamber (16) at least during the initial phase of the working stroke of the apparatus to thereby relieve the high pressure seal (43). The invention also relates to an apparatus (1) for generating high hydraulic fluid pressure.

Description

METHOD AND APPARATUS FOR GENERATING HYDRAULIC PRESSURE

TECHNICAL FIELD

The present invention relates generally to the generation of fluid pressure for use in diverse applications, and specifically concerns the boosting of hydraulic fluid pressure.

BACKGROUND

Traditionally high hydraulic pressure is generated in an apparatus generally known as a pressure intensifier, a pressure amplifier or a pressure booster, and such high pressure is gene- rated in different pressure ranges and for a great number of purposes. A conventional apparatus for intensifying hydraulic fluid pressure comprises a low pressure piston moveable in a low pressure cylinder and carrying a high pressure piston moveable in a high pressure cylinder and having a significantly smaller effective area than the low pressure piston. As is normal in such an apparatus the low pressure piston divides the low pressure cylinder into a working stroke chamber having a low pressure fluid inlet, and a return chamber likewise having a low pressure fluid inlet. The high pressure cylinder is provided with a low pressure inlet and a high working pressure outlet. During the working stroke the movement of the low pressure piston is directly transmitted to the high pressure piston, to multiply the pressure of the fluid present in the high pressure chamber in direct relation to the differential area of the low and high pressure pistons. Such an apparatus is disclosed i.e. in DE-A-42 06 759.

In such pressure intensifiers, seals are provided between the high pressure and low pressure sides, to eliminate fluid leakage from the high pressure cylinder, thereby effectively securing the intended raised fluid pressure level in the high pressure chamber. In applications where different fluids are used on the high pressure and low pressure sides it is also important to secure that high pressure fluid does not escape past the seals, since it might otherwise mix with the low pressure fluid and adversely affect the efficiency of the intensifier apparatus. For the continued effective operation of the pressure intensifier, it is therefore vital to provide appropriate seals.

In pressure intensifiers working in lower and medium pressure ranges reliable sealing around the periphery of the high pressure piston may be provided using traditional ring seals, such as O-rings and lip seals. The problem of fluid leaking past the seals may be taken care of by providing fluid leakage outlets in the cylinder wall, between seals or behind all of the seals, such as disclosed in WO96/41692. Such fluid leakage outlets also serve the purpose of allowing monitoring of the seals and their function.

On the other hand, when designing pressure intensifiers generating very high hydraulic pressures, say in the pressure range of up to approximately 2000 bar, special measures must inevitably be taken to increase the effectiveness as well as the useful life of the seals and to thereby prevent the unwanted leakage. A specific problem related to the high pressure seals in conventional pressure intensifiers is the fact that at least some of the seals are directly exposed to the pressure in the high pressure cylinder during build up of the high pressure, that is before the seals engage the high pressure piston. Such exposure to the pressure build-up in the high pressure chamber may cause damage to the seal and may even tend to dislocate seals from their seats. For that reason, the seals are presently regarded to set the practical upper pressure limit for pressure intensifiers. As was mentioned above said upper limit is in the range of approximately 2000bar, although attempts are frequently made to operate pressure intensifiers with higher pressures. However, such high pressure operation inevitably results in very short service intervals since the high pressure seals will have a useful life of only a few hours.

Within the above mentioned very high hydraulic pressure range and in applications requiring relatively large volumes of hydraulic fluid, such as in hydroforming processes, an additional problem occurs when the very high pressure shall be relieved again. In the hydroforming process used to exemplify the problem as well as in other high pressure applications, it is highly desirable to perform a gradual, controlled lowering of the pressure in the consumer apparatus and in the high pressure chamber. Yet another problem related to the operation of an apparatus working in the discussed, high pressure ranges is the air that will be present in the fluid medium and that may be trapped in the high pressure cylinder.

SUMMARY The invention overcomes the above discussed problems in an efficient and satisfactory manner. It is a general object of the invention to provide a solution to the problem of effectively and reliably generating high hydraulic pressure.

In particular, it is an object of the invention to provide an improved method of operating an apparatus for generating high hydraulic fluid pressure, securing effective long-term operation thereof with a minimum of shutdowns for service and repair. Briefly, this is achieved in a favorable manner by eliminate the exposure of the high pressure seals to the very high pressure during the initial phase of the working stroke of the apparatus. Specifically, this is accomplished by providing a restricted outflow of hydraulic fluid from the high pressure chamber during the initial phase of the working stroke of the apparatus.

In an embodiment of the invention the continuous restricted outflow of hydraulic fluid is provided from the start of the working stroke of the apparatus, said outflow being blocked when the forward end of the high pressure piston has passed the high pressure seal. In this manner, the seal will be effectively protected during the entire working stroke of the apparatus.

In a further embodiment of the invention, the continuous restricted outflow of hydraulic fluid from the high pressure chamber as well as from the consumer is provided during the final phase of the return stroke of the apparatus. Hereby, the very high pressure generated in the consumer as well as in the high pressure cylinder, by the working stroke of the apparatus, will be relieved in a very favorable manner, causing no damage to either the apparatus or to associated equipment.

In another embodiment of the invention the continuous restricted outflow of hydraulic fluid from the high pressure chamber is provided during a phase of flushing the high pressure chamber with hydraulic fluid immediately prior to performing the working stroke. This controlled outflow of high pressure fluid from the high pressure cylinder before the working stroke allows air from the pressure medium to be flushed out from the high pressure cylinder prior to the working stroke.

Another object of the invention is to provide an improved and very effective apparatus for generating high hydraulic fluid pressure, securing effective long-term operation thereof with a minimum of shutdowns for service and repair. Briefly, this is achieved in a favorable manner by providing a central bore extended through the high pressure piston of the apparatus, said bore having a restrictor for permitting fluid flow from the high pressure chamber, a check valve for blocking fluid flow from the high pressure chamber and a valve operating rod for selectively opening the check valve. Hereby a controlled outflow of fluid from the high pressure chamber is permitted during at least the initial phase of the working stroke of the apparatus to eliminate the exposure of the high pressure seals to very high pressure during said initial phase.

These and further objects of the invention are met by the invention as defined in the appended patent claims.

In summary, the present invention provides for the following advantages over the state of the art:

Permits controlled build-up of the pressure in the high pressure cylinder; thereby

Significantly increases the useful life of the high pressure seals, to permit longer uninterrupted operation of the apparatus and of processes supplied thereby;

Permits smooth, controlled relief of the very high fluid pressure produced in the apparatus as well as from the consumer;

Makes it possible to work with extremely high output pressures without undue wear or damage to the seals; and

- Permits reliable venting of air from the high pressure cylinder.

Other advantages offered by the present invention will be readily appreciated upon reading the below detailed description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which: Fig. 1 is a partial plan view from one side, illustrating an embodiment of an apparatus according to the present invention,

Fig. 2 is a longitudinal section through the apparatus of fig. 1 ,

Fig. 3 A is an end view of the apparatus as seen from the high pressure side thereof being to the left in figs. 1 and 2,

Fig. 3B is an end view of the apparatus as seen from the low pressure side thereof being to the right in figs. 1 and 2,

Fig. 4A-B are enlarged views of the high pressure piston illustrated in two positions,

Fig. 5A is a plan view from one side, illustrating an embodiment of a simplified pressure intensifying apparatus according to the present invention, and

Fig. 5B is an end view of the end of the apparatus being to the left in fig. 6 A.

DETAILED DESCRIPTION

Fig. 1 illustrates the general design of an embodiment of the inventive apparatus 1 for genera- ting high fluid pressure. Basically, the apparatus consists of a low pressure cylinder 2 provided to the right in figs. 1 and 2 and a high pressure cylinder 3 provided to the left in figs. 1 and 2. In this specification the terms "front" and "rear" are used to indicate the positions of parts or portions of the apparatus relative to the direction of the working stroke of the apparatus, meaning that a "front" portion is positioned towards the high pressure side of the apparatus compared to a "rear" portion. It should also be emphasized that the term "low pressure" as used herein - i.a "low pressure fluid", "low pressure chamber" etc., refers to a pressure that is low compared to the high pressure output from the apparatus, but that is not necessarily low in other respects or applications.

A rear end wall 4 closes the low pressure cylinder 2 outwardly and a front end wall 5 closes the high pressure cylinder 3 outwardly. The low pressure and high pressure cylinders 2 and 3 are separated by an intermediate wall 7 provided with a central, through bore 7A for sealingly receiving an axially displaceable high pressure piston 13 firmly attached to a low pressure piston 12, as will be described more closely below with reference specifically to fig. 2.

The low pressure cylinder 2, which is illustrated with a portion thereof cut away in figs. 1 and 2, is clamped between the rear end wall 4 and the intermediate wall 7 by means of first connecting rods 10, in the illustrated embodiment twelve rods evenly distributed around the outer circumference of the cylinder 2. The rods 10 are passed through bores (not shown) in the end wall 4 and their threaded front ends 10A are screwed into corresponding threaded blind bores (not shown) in the intermediate wall 7. The likewise threaded rear ends 10B of the rods 10 extend out through corresponding bores in the rear end wall 4 and are engaged by nuts 11 tightened against the end wall 4. The front end of the low pressure cylinder 2 is guided by a projection 7C formed by a step in the rear end face of the intermediate wall 7, and the rear end thereof is likewise guided by a projection 4A formed by a step in the front end face of the rear end wall 4. O-rings or equivalent seals are provided between the respective surfaces of the cylinder wall 2 and the projections 7C, 4A for providing a fluid seal therebetween.

The low pressure cylinder 2 displaceably receives the low pressure piston 12 which is provided with the appropriate seals 12 A, such as O-rings, at its outer circumference for sealingly engaging the inner wall of the cylinder 2. By means of the piston 12, the low pressure cylinder 2 is divided into a rear working chamber 18 and a front return chamber 17.

Inlet-outlet openings 19, 20 for low pressure working fluid are provided in the rear wall 4 and in the intermediate wall 7, respectively and serve to supply working fluid to the working chamber 18 and return fluid to the return chamber 17, respectively.

As was mentioned above, the low pressure piston 12 carries a high pressure piston 13 protruding from the front face thereof and extending into the central bore 7A of the intermediate wall 7. Said central bore 7A is provided with a low pressure seal 21 surrounding and sealingly engaging the high pressure piston 13 and positioned in the rear portion of the bore7A, closest to the low pressure cylinder 2. In the illustrated embodiment, the low pressure seal 21 is a multi-lip seal of a synthetic composite material, but other standard seals suitable for the pressures on the low pressure side may likewise be employed. In front of the low pressure seal 21, adjacent the latter, is positioned a distribution ring or bushing 22 containing radial inlet openings 22A (only one is illustrated), communicating with a low pressure fluid inlet 49, through which fluid is introduced into the high pressure cylinder 3, as will explained further below. In front of the distribution ring 22 is provided a further bushing 23 carrying a similar multi-lip seal 24 of a synthetic composite material, and serving as a final seal against a high pressure chamber 16. The high pressure chamber 16 is formed in the high pressure cylinder 3 and partly in the bore 7A in the intermediate wall 7, in front of the high pressure piston 13, and by a central bore 5 A in the front end wall 5.

The high pressure piston 13 is firmly attached to the low pressure piston 12, and may preferably be secured with a press fit in a central opening provided in the low pressure piston.

The piston 13 is provided with a central through bore 25 illustrated in greater detail in figs. 4A-B. The through bore 25 opens into the working fluid chamber 18 at one end and into the high pressure chamber 16 at the other end. The front end of the high pressure piston 13 is provided with a one way restrictor 26 having a displaceable valve cone 27 cooperating with a valve seat 30 provided in a valve body 29. The valve cone 27 is pressed against the seat 30 by a spring 29C acting between a spring retainer 29B and the valve cone, and the valve cone, the spring and spring retainer are maintained in position in the valve body 29 by a retainer ring in a conventional manner. The valve body 29 itself is locked in position in a recess in the end face of the piston 13 in a similar manner. Further, the valve cone 27 is provided with a narrow central restriction channel 28 through which a restricted fluid flow is permitted in the direction from the high pressure chamber 16 to the piston bore 25, even when the valve cone is seated against the seat 30. In addition to a central bore 29D in the valve body 29, intended for receiving the valve cone 27 and the spring 29C, the valve body 29 is also provided with a number of air discharge channels 29A (only one illustrated in figs. 4A-B) serving to discharge air from the radially outer parts of the high pressure chamber 16, as will be apparent.

In the rear end of the high pressure piston 13 is provided a check valve 31 similar to the one way restrictor 26, but lacking the restriction channel. Thus, the check valve 31 completely blocks fluid flow in the direction from the high pressure chamber 16 to the working fluid chamber 18 of the low pressure cylinder 2 when its displaceable valve cone 32 engages the valve seat 33 provided in the valve body 34. During the first phase of the working stroke of the apparatus 1 as well as during the last phase of its return stroke the valve cone 32 is lifted from the valve seat 33 by a valve operating rod 35. The valve operating rod 35 is secured to the front face of the rear wall 4 by means of a bolt 37 connected to a bushing 36 screwed into a recess provided in said front face of the rear wall 4. The rod 35 extends into the working chamber 18 and is aligned with the through bore 25 of the high pressure piston 13 and with the outlet opening in the valve body 34 so that it will open the check valve 31 to allow fluid flow from the high pressure chamber 16 to the working fluid chamber 18 during the mentioned operational phases. For this purpose it is evident that the rod 35 has an outer diameter that is slightly smaller than the inner diameter of the outlet opening in the valve body 34, to allow fluid flow from the through bore 25 to the working fluid chamber 18, past the outside of the rod 35.

The front wall 5 of the high pressure cylinder 3 is provided with a central high pressure outlet 14 through which a high pressure fluid consumer (not shown) is connected to the portion of the high pressure chamber 16 formed by the bore 5 A within the front end wall 5. In the illustrated embodiment, the front end wall 5 is provided with an adapter 6 for direct attachment to the consumer by means of bolts 6A. The outer end of the adapter 6 is provided with a bushing 6B carrying seals 6C; the indicated, ordinary O-rings will be sufficient in most cases, for providing an appropriate seal against the consumer as well as against the bushing 6B. Such an embodiment, where the apparatus 1 is "docked" directly in the consumer, is preferred in many applications with very high pressures, such as in the hydroforming techniques, since it will eliminate the need for hose or pipe connections.

In order to withstand the very high pressures generated in the high pressure chamber 16 the high pressure cylinder 3 is clamped between the front end wall 5 and the intermediate wall 7 by means of second, heavier connecting rods 8 evenly distributed around the outer periphery of the high pressure cylinder 3. In the illustrated embodiment eight connecting rods 8 are provided, the threaded rear ends 8B of which are screwed into corresponding threaded blind bores 7B provided in the front face of the intermediate wall 7 and the likewise threaded front ends 8A of which are extended through corresponding through bores 5B in the front end wall 5 and are engaged by nuts 15 tightened against the end wall 5. The connecting rods 8 carry at least one support ring 9 surrounding and provided in close engagement with the outer periphery of the high pressure cylinder 3. In figs. 1 and 2 the high pressure cylinder 3, like the low pressure cylinder 2, is illustrated with a mid-portion thereof cut away, and it should be emphasized that one or more additional support rings 9 may be provided along the cut-away portion, the number of support rings 9 required depending i.a. upon the stroke length of the apparatus 1 and upon the strength of the wall of the high pressure cylinder 3.

As is illustrated in fig. 2 the high pressure cylinder 3 consists of separate, coaxial inner and outer casings 38 and 39 respectively. The inner casing 38 is provided closely fitting into the outer casing 39, and a pressure fluid space 40 is provided between the two casings, extending over a major portion of the axial length of the inner casing 38 and around the entire outer periphery thereof.

The inner casing 38 of the high pressure cylinder 3 is provided with a high pressure seal assembly 43. The seal assembly 43 basically consists of a number of primary and secondary metal seal rings that are clamped between the front wall 5 and the intermediate wall 7, in the latter case through the bushings 22, 23, by means of the connecting rods 8. The metal seal rings are preferably manufactured from hardened steel, have a rectangular cross section and are formed with parallel circumferential grooves on their radially outer and inner surfaces. The purpose of said relief grooves is to perform an initial sealing between the high pressure cylinder and the high pressure piston, and to produce a pressure drop across each groove, thereby gradually reducing the fluid pressure. The secondary metal seal rings are provided with a recess in which a "soft seal" is received.

The illustrated seal assembly 43 is disclosed in greater detail in our Swedish Patent Application No. 9904463-8 and the disclosure of said Patent Application is included herein by reference. Such a seal is specifically suitable for sealing against very high fluid pressures, such as those employed in the hydroforming techniques and in some press equipment and ranging from about 2000 bar upwards. The purpose of the double casing of the high pressure cylinder 3 as well as of the support ring 9 is to permit controlling the gap between the seal 43 and the high pressure piston, as is disclosed in detail in our Swedish Patent Application No. 9904464-

6, and the disclosure of said Patent Application is likewise included herein by reference. The operation of the above described apparatus shall now be explained. Prior to the actual working stroke of the apparatus, hydraulic fluid of system pressure is normally introduced into the high pressure cylinder through the fluid inlet 49 and the radial inlet openings 22A in the distribution ring 22. For the sake of this explanation, it may be assumed that the system pressure is approximately 350 bar. This fluid is used to flush the high pressure side as is normal in this kind of apparatus, and to fill up the same. At this stage the low and high pressure pistons 12, 13 are in the position illustrated in fig. 4 A, that is farthest to the right in fig. 2, with the valve cone 32 lifted from its seat 33 by the rod 35. This means that air from the high pressure side, together with the hydraulic fluid, will be flushed out through the central restriction channel 28 of the restrictor 26, through the central bore 25 and the opened check valve 31 to the working chamber 18 of the low pressure cylinder 2. The above mentioned air discharge channels 29A assist in discharging air from the radially outer areas of the high pressure cylinder. This discharge of air will be most effective in case the apparatus 1 is positioned vertically, with the high pressure side down.

Next, the working stroke is initiated by introducing working fluid, normally the system fluid mentioned above, to the working chamber 18. The low pressure and high pressure pistons 12, 13 begin moving forward, i.e. to the left as seen in fig. 1. In the illustrated embodiment this initial forward movement does not cause any substantial increase in the high pressure cylinder 3 fluid pressure, since fluid is still discharged through the through bore 25 in the described manner, thereby equalizing the pressures in the high and low pressure cylinders 2, 3.

With the positioning of the distribution ring 22 and its radial inlet openings 22A at the rear end of the high pressure cylinder 3, behind the high pressure seal assembly 43, the high pressure piston 13 blocks the fluid entering through the radial inlet openings 22A after a short forward movement. This represents an important improvement over the traditional, sensitive check valves normally used in high pressure side fluid inlets outside the apparatus.

When the pistons 12, 13 have reached the position where the rod 35 is disengaged from the valve cone 32 of the check valve 31 , the check valve 31 blocks further discharge of fluid from the high pressure cylinder 3 through the through bore 25 and into the working chamber 18.

The length of the rod 35 is adapted to the axial position of the high pressure seal 43 in the high pressure cylinder. In other words the axial length of the part of the rod 35 protruding from the rear end wall 4 shall at least be equal to the axial distance between the front end of the high pressure piston 13 and the front end of the first (i.e. farthest to the left in fig. 2) metal ring of the high pressure seal 43, when the pistons 12, 13 are in their rearward position as illustrated in fig. 2. This means that the forward end of the high pressure piston 13 passes the first metal ring of the seal 43 before the rod 35 is disengaged from the valve 31. With this arrangement the very favorable effect is obtained, that the seal 43 is not subjected to any fluid pressure significantly higher than the system pressure until it is engaged fully by the piston. This will have a very favorable effect upon the useful life of the high pressure seal, irrespective of the type of seal employed.

At the same time, with the fluid discharge through the bore 25 closed, the pressure of the fluid in the high pressure chamber 16 will be multiplied, in the present embodiment approximately 20 times, corresponding to the ratio of the areas of the high and low pressure pistons. With the above given system pressure of 350 bar this will mean an output pressure in the order of 7000 bar. With pressures of this magnitude, and even higher, there is an immediate danger that "soft" seals would become damaged or even dislocated. Any such tendency is eliminated with the above discussed inventive proposals.

When the apparatus has performed its full working stroke, supplying high pressure fluid to a consumer that may preferably be a hydroforming machine, system pressure is supplied to the return chamber 17 through the fluid inlet 20. In order to obtain a smooth return stroke, a regulated counter-pressure is normally maintained in the working chamber 18. As the pistons 12, 13 move rearwardly, the rod 35 once more engages the check valve 31 to allow fluid flow from the high pressure chamber 16 to the working chamber 18, and this effects an even further enhanced pressure relief.

In fig. 5A-B is illustrated an embodiment of a simplified pressure intensifier 100 suitable for applications requiring pressure fluid in lower pressure ranges of up to approximately 2000 bar, such as for pressurizing the space 40 between the casings 38, 39 of the high pressure cylinder

3 in the apparatus 1 of the first embodiment. Like the apparatus 1, the pressure intensifier 100 consists of a low pressure cylinder 102 with fluid inlets 119, 120 and a high pressure cylinder 103 with a fluid inlet 149 and a fluid outlet 114. The cylinders are provided with rear and front end walls 104 and 105 respectively. Like in the previous embodiment connecting rods 110 are in this case employed to clamp the low pressure cylinder 102 between the rear wall

104 and the high pressure cylinder 103, which in this embodiment is formed integral with the intermediate wall. The front wall 105 is attached to the high pressure cylinder by means of bolts 108.

A low pressure piston 112 is slidable in the low pressure cylinder 102 dividing it into a working chamber 118 and a return chamber 117. A high pressure piston 113 is fixed to the low pressure piston 112, extending into a high pressure chamber 116 in the high pressure cylinder 103. Like in the previous embodiment the high pressure piston 113 is provided with a central bore 125 opening into the working chamber 118 and the high pressure chamber 116. The bore 125 is provided with the front one way restrictor 126 and the rear check valve 31 with its valve cone 32 being engaged and moved by the rod 135.

Being intended for a lower pressure range the simplified pressure intensifier 100 is provided with a high pressure seal 143 being a multiple lip seal like the low pressure seal 121. Like in the previously described embodiment the high pressure side is initially flushed by fluid of system pressure supplied through the fluid inlet 149, in a position immediately in front of the high pressure piston 113. By means of the valves 126 and 131 in the bore 125 of the high pressure piston 113, the high pressure seal 143 is once more protected from the high output pressure, until supported by the piston 113.

It will be understood by those skilled in the art that various other modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.

Claims

PATENT CLAIMS

1. A method of operating an apparatus (1; 100) for generating high hydraulic pressure and having a low pressure piston (12; 112) being displaceable in a low pressure cylinder (2; 102), said low pressure piston carrying a high pressure piston (13; 113) being displaceable in a high pressure cylinder (3; 103) forming a high pressure chamber (16; 116) being connected to a high pressure fluid consumer, the inner wall of the high pressure cylinder carrying stationary fluid seals (21, 24, 43; 121, 143) for sealingly engaging the high pressure piston characterized by providing a continuous restricted outflow of hydraulic fluid from the high pressure chamber (16; 116) at least during the initial phase of the working stroke of the apparatus to thereby relieve the high pressure seal (43; 143).

2. A method according to claim 1, characterized by providing the continuous restricted outflow of hydraulic fluid from the high pressure chamber (16; 116) at least from the start of the working stroke of the apparatus (1; 100) and by blocking said outflow when the forward end of the high pressure piston (13; 1 13) has passed the high pressure seal (43; 143).

3. A method according to claims 1 or 2, characterized by providing the continuous restricted outflow of hydraulic fluid from the high pressure chamber (16; 1 16) as well as from the consumer during the final phase of the return stroke of the apparatus, thereby relieving the very high pressure generated by the working stroke of the apparatus (1; 100) for generating high hydraulic pressure.

4. A method according to any of claims 1-3, characterized by providing the continuous restricted outflow of hydraulic fluid from the high pressure chamber (16; 116) during a phase of flushing the high pressure chamber (16; 116) with hydraulic fluid, said flushing phase immediately preceding the working stroke.

5. A method according to any of claims 1-4, characterized by providing the continuous restricted outflow of hydraulic fluid from the high pressure chamber (16; 116) through a restricted bore (25; 125) in the high pressure piston (13; 113).

6. A method according to any of claims 1-5, characterized by supplying fluid to the to the high pressure chamber (16; 116) adjacent a rear end of the chamber, immediately in front of the forward end of the high pressure piston (13; 113) in its fully retracted position, the piston (13; 113) thereby blocking the fluid inlet as it is extended.

7. Apparatus (1; 100) for generating high hydraulic fluid pressure, having a low pressure piston (12; 112) being displaceable in a low pressure cylinder (2; 102) forming a working chamber (18; 118) in turn being connected to a low pressure fluid supply, said low pressure piston carrying a high pressure piston (13; 113) being displaceable in a high pressure cylinder (3; 103) forming a high pressure chamber (16; 116) being connected to a low pressure fluid supply and to a high pressure fluid consumer, the inner wall of the high pressure cylinder carrying stationary fluid seals (21, 24, 43; 121, 143) for sealingly engaging the high pressure piston characterized in that the high pressure piston (13; 113) is provided with a central through bore (25; 125) in which is provided a one way restrictor (26; 126) permitting a constant, restricted fluid flow from the high pressure chamber (16; 116), and a check valve (31; 131) normally blocking the fluid flow from the high pressure chamber (16; 116) and in that a valve operating rod (35; 135) is provided, extending into the bore (25; 125) to engage and open the check valve at least in the initial phase of the working stroke of the apparatus (1; 100) and in the final phase of its return stroke, thereby permitting a controlled outflow of fluid from the high pressure chamber (3 ; 103) during said phases.

8. An apparatus (1; 100) for generating high hydraulic pressure according to claim 7, characterized in that the check valve (31; 131) comprises a valve cone (32; 132) blocking the fluid flow from the high pressure chamber (16; 116) when seated against a valve seat (33) thereof and in that the operating rod (35; 135) is adapted to engage the check valve by lifting the valve cone (32; 132) from its seat (33) during said phases.

9. An apparatus (1; 100) for generating high hydraulic pressure according to claim 7 or 8, characterized in that the central bore (25; 125) extends through the low and high pressure pistons (12; 112 and 13; 113, respectively) and opens into the high pressure chamber (16; 116) of the high pressure cylinder (3; 103) as well as into the working chamber (18; 118) of the low pressure cylinder (2; 102).

10. An apparatus (1; 100) for generating high hydraulic pressure according to any of claims 7-9, characterized in that the rod (35; 135) is firmly attached to and protrudes forwardly from the front face of a rear end wall (4; 104) of the low pressure cylinder (2; 102) and in that the axial length of the part of the rod (35; 135) protruding from the rear end wall (4; 104) is at least equal to the axial distance between the front end of the high pressure piston (13; 113) and the front end of a high pressure seal (43; 143), when the pistons (12, 13; 112, 113) are in their rearward position.

11. An apparatus (1; 100) for generating high hydraulic pressure according to any of claims 7-10, characterized in that the one way restrictor (26; 126) comprises a displaceable valve cone (27) cooperating with a valve seat (30) provided in a valve body (29), in that the valve cone (27) is pressed against the seat (30) by a spring (29C) and in that the valve cone (27) is provided with a narrow central restriction channel (28) through which a restricted fluid flow is permitted in the direction from the high pressure cylinder (3; 103) to the piston bore (25; 125), even when the valve cone is seated against the seat (30).

12. An apparatus (1; 100) for generating high hydraulic pressure according to any of claims 7-11, characterized in that a fluid inlet (49, 22A; 149) to the high pressure chamber (16; 116) is positioned in the rear end of the chamber, immediately in front of the forward end of the high pressure piston (13; 113) in its fully retracted position, the piston (13; 113) thereby blocking the fluid inlet as it is extended.

13. An apparatus (1) for generating high hydraulic pressure according to claim 1 1 or 12, characterized in that the valve body (29) is provided with a number of air discharge channels (29 A) serving to discharge air from the radially outer parts of the high pressure chamber (16,

116).