Patent Publication Number: US-10788061-B2

Title: Hydraulic actuator with cartridge pressure amplifier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage application of International Patent Application No. PCT/EP2017/076112, filed on Oct. 12, 2017, which claims priority to European Patent Application No. 16197319.3, filed on Nov. 4, 2016, each of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The invention relates to a hydraulic actuator comprising a cylinder housing, a piston with a piston rod being displaceably arranged inside the cylinder housing and a pressure amplifier comprising an inlet section with a pressure inlet port, an active section with a high pressure outlet port, a low pressure chamber and a high pressure chamber. 
     BACKGROUND 
     Such hydraulic actuators are known and used in different industrial sectors. They are, for example, used to drive mechanical members for pressing, cutting or the like. In such applications said mechanical members encounter a resistance induced by the work piece to be pressed or cut. This resistance may well vary during the working process. Therefore, it is important that the hydraulic actuator can provide sufficient working pressure during all stages of the working process. As the pressure needed does depend on the resistance induced by the working piece, also the pressure demand to be provided by the hydraulic actuator varies. 
     In order to avoid a shortfall of pressure during the working process, it is known to make use of pressure amplifiers in connection with the hydraulic actuator. Said pressure amplifiers comprise an inlet section with an inlet port. Hydraulic fluid used to operate the hydraulic actuator enters the inlet section through the inlet port. The hydraulic fluid passes through the low pressure chamber. The pressure of the hydraulic fluid is subsequently enhanced. It then passes through the high pressure chamber and exits the pressure amplifier via the high pressure outlet port of the active section. Thereby, an amplification of the pressure of the hydraulic fluid inside the hydraulic actuator can be achieved. An increased pressure demand of the hydraulic actuator can be met. 
     However, it is also apparent that additional elements, such as the pressure amplifier with its pressure inlet port, inlet section, active section and high pressure outlet port need to be added to the hydraulic actuator. A fluid communication between the hydraulic actuator and the pressure amplifier has to be established. Typically, in order to achieve this, the technical design of the hydraulic actuator needs structural modifications or additional parts. Such a modified technical design makes construction and assembly cumbersome and expensive. The hydraulic actuator and the pressure amplifier need to be assembled concomitantly. The different parts of the hydraulic actuator and the pressure amplifier need to be machined for each other. 
     SUMMARY 
     It is therefore an objective of the present invention to provide a hydraulic actuator with a modular pressure amplifier. 
     This objective is achieved in that the hydraulic actuator comprises a cartridge pressure amplifier comprising a sleeve being arranged at least partially inside the piston rod, and wherein the pressure amplifier is stationarily arranged inside the sleeve. 
     A modular design of the pressure amplifier thus becomes possible by means of the cartridge pressure amplifier. The cartridge pressure amplifier can be fully assembled independently of the hydraulic actuator. The inlet section and the active section of the pressure amplifier are arranged inside the sleeve: the cartridge pressure amplifier can thus be easily assembled and then be inserted into the piston rod as a hole. It only remains to establish a fluid communication between the pressure amplifier and the cylinder housing. To this end, the sleeve is arranged at least partially inside the piston rod. Thus, hydraulic fluid exiting the high pressure outlet port of the pressure amplifier can enhance the pressure supplied by the piston of the hydraulic actuator. Moreover, arranging the sleeve at least partially inside the piston rod also eliminates the necessity for additional constructional features associated with the hydraulic actuator. The common features of the hydraulic actuator such as the piston rod can be maintained. No additional parts are needed. 
     In an embodiment, the sleeve is arranged concentrically with the piston rod and fixes a position of the inlet section relative to a position of the active section. The cartridge pressure amplifier consists of two sections: the inlet section and the active section. This is due to the assembly of its internal parts such as the low pressure chamber and the high pressure chamber. In order to achieve a proper function of the pressure amplifier, it is necessary to hold these two sections together with an external force. To this end, the sleeve is used to fix a position of the inlet section relative to a position of the active section. The sleeve might therefore force-fittingly fix the inlet section and the outlet section relative to each other. However, also a form-fit is possible. Both sections may then be inserted simultaneously into the piston rod. Modular assembly becomes possible. The sleeve is concentrically arranged inside the piston rod. Thus, imbalances in the moving piston rod are avoided. Assembly of the cartridge pressure amplifier inside the piston rod is facilitated. 
     In another embodiment, the pressure inlet port and the high pressure outlet port are coaxially arranged at opposite axial ends of the sleeve. This arrangement facilitates the supply of the cartridge pressure amplifier with hydraulic fluid. It is, for example, possible to arrange the pressure inlet port in the vicinity of a piston eye. The channels supplying the cartridge pressure amplifier with hydraulic fluid via the pressure inlet port may then be arranged inside the piston rod and the piston eye. Alternatively, the pressure inlet port may as well be arranged inside the cylinder housing itself. In this way the cylinder housing may contain the channels supplying the hydraulic fluid via the pressure inlet port. The pressure inlet port and the high pressure outlet port are coaxially arranged in order to avoid imbalances. This also achieves an effective transmission of hydraulic fluid from the cartridge pressure amplifier to the hydraulic actuator. 
     In another embodiment, the inlet section comprises a pilot sequence valve being in fluid communication with the pressure inlet port and being arranged in an axial direction of the inlet section. The pilot sequence valve may be thread-mounted in the axial direction into the inlet section. The bottom of the pilot sequence valve is therein connected to the pressure inlet port through a main inlet channel. The pilot sequence valve is normally closed. In this way, it allows for full flow of hydraulic fluid inside the main inlet channel. The axial arrangement of the pilot sequence valve allows for an easy and compact assembly. 
     In yet another embodiment, the pilot sequence valve is pressure-activated when the pressure at the pressure inlet port exceeds a preset value, thereby opening a pilot channel from the pressure inlet port to the low pressure chamber. The bottom of the pilot sequence valve is connected to the pressure inlet port through the main inlet channel. It is connected through the first pilot channel to a first control valve pin. The first control valve pin forms part of the fluid connection from the pilot sequence valve via the pilot channel to the low pressure chamber. The pilot sequence valve is normally closed. In this state, it blocks the fluid communication associated with the first control valve pin to the low pressure chamber. Once the pressure of the hydraulic fluid in the inlet section reaches a preset value, the pilot sequence valve opens. Thereby, the pilot channel from the pressure inlet port to the low pressure chamber opens. The pressure of the hydraulic fluid is subsequently amplified in view of the increased pressure demand. The setting of the pilot sequence valve to a preset value can be adjustable. The setting of the pilot sequence valve may also be fixed to a certain preset value. 
     In another embodiment, the active section comprises an over-center valve establishing a fluid communication between the pressure inlet port and the high pressure outlet port and being arranged in an axial direction of the active section. The over-center valve comprises multiple parts which are integrated inside the active section in an axial direction thereof. Once the inlet section and the active section are mounted with respect to each other, it is no longer possible to set a pressure level of the over-center valve. Therefore, proper setting is achieved by several types of springs. These springs form part of the multiple parts of the over-center valve. The over-center valve can provide a full flow from the pressure inlet port to the high pressure outlet port. Moreover, it may provide a load holding function at the high pressure outlet port, thus meeting an increased pressure demand in the hydraulic actuator. Eventually, the over-center valve may also provide a controlled lowering function from the high pressure outlet port to the pressure inlet port, thus avoiding too steep pressure drops. The over-center valve comprises three connection ports: an over-center valve inlet port associated with the main inlet channel, an over-center valve outlet port associated with a second high pressure channel as well as an over-center valve pilot port associated with a pilot line. The pilot line connects the over-center valve with the main backflow channel. In a direction from the pressure inlet port to the high pressure outlet port, the over-center valve provides a full flow of hydraulic fluid through the main inlet channel. This can be achieved by means of a check valve integrated in the over-center valve. In the opposite flow direction, from high pressure outlet port to pressure inlet port, the over-center valve blocks flow of hydraulic fluid. However, once the pressure applied to the pilot line exceeds a certain preset value, the over-center valve opens a fluid path from the high pressure outlet port to the main backflow channel. 
     In yet another embodiment, the over-center valve is mounted on a first axial end face of the inlet section, wherein the first axial end face of the inlet section abuts a first axial end face of the active section. The over-center valve comprises multiple parts such as several types of springs. These parts are mounted in the axial direction of the active section in a space-saving manner. Therein, a dividing plane is constituted by the abutment of the first axial end face of the inlet section and the first axial end face of the active section. All parts of the over-center valve are mounted on the first axial end face of the inlet section, i.e. from the dividing plane. Correct positions of all parts of the over-center valve can therefore be achieved by covering the first axial end face of the active section with the first axial end face of the inlet section. There is no need for thread-mounting of the over-center valve. No thread in the active section is needed. Assembly and manufacturing of the cartridge pressure amplifier becomes easy and inexpensive. 
     In another embodiment, the low pressure chamber comprises a low pressure piston and a low pressure piston bushing, wherein the low pressure piston is displaceably arranged relative to the low pressure piston bushing. The low pressure piston bushing is an easy and cost-efficient way of increasing the lifetime of the low pressure piston. This is achieved by decreasing the friction between the low pressure piston and circumferential walls of the low pressure chamber of the inlet section. The low pressure piston bushing may, for example, be molded into the inlet section or may be mounted with a press fitting (depending on the material used for the bushing). It may consist of one piece. It may also consist of different pieces. The different pieces are then molded into the inlet section one after the other. Gaps between the different pieces are to be avoided. The correct position of the different pieces may be controlled by a jig during the molding process. After the molding process, the low pressure piston bushing needs to be machined to a certain inside diameter. 
     In another embodiment, the high pressure chamber comprises a high pressure piston and a high pressure piston bushing, wherein the high pressure piston is displaceably arranged relative to the high pressure piston bushing. The high pressure piston bushing is an easy and cost-efficient way of increasing the lifetime of the high pressure piston. This is achieved by decreasing the friction between the high pressure piston and the circumferential walls of the high pressure chamber of the active section. The high pressure piston bushing comprises two parts with different length: a first high pressure piston bushing element and a second high pressure piston bushing element. The correct position of the different bushings may be controlled by a jig during the molding process. After the molding process, the high pressure piston bushing needs to be machined to a certain inside diameter. The bushing could also be mounted with a press fitting (depending on the material used for the bushing). 
     In yet another embodiment, the high pressure piston bushing comprises an aperture opening a second pilot channel establishing a fluid communication between the high pressure chamber and a control valve. The high pressure piston bushing may comprise the first high pressure piston bushing element and the second high pressure piston bushing element. Between these bushings, the aperture is located. The aperture opens the second pilot channel, once the high pressure piston has reached an axial end position at the far end of the inlet section inside the high pressure chamber. The lifetime of the cartridge pressure amplifier can be increased by means of the bushing, while at the same time ensuring its proper function. The high pressure piston bushing can be implemented without the need for modifying the constructional features of the cartridge pressure amplifier. 
     In another embodiment, the cartridge pressure amplifier is fixed to the piston rod such that the piston rod and the cartridge pressure amplifier are mutually displaceable. To this end, the cartridge pressure amplifier may be mounted fully inside the piston rod. It may be mounted concentrically with the piston rod. This makes assembly of the hydraulic actuator easy. The cartridge pressure amplifier may be assembled separately from the hydraulic actuator. It may then be integrated into the piston rod, before assembly of the hydraulic actuator is completed. A modular assembly of hydraulic actuator and cartridge pressure amplifier becomes feasible. 
     In another embodiment, the cartridge pressure amplifier comprises an internal adapter establishing a fluid communication between the pressure inlet port and a piston inlet port. The pressure inlet port may be arranged inside the piston eye. The piston inlet port may be a drilled hole inside the piston eye. The piston inlet port may be concentrically arranged with the piston rod. The internal adapter connects the piston inlet port with the pressure inlet port and hence the cartridge pressure amplifier. The internal adapter may be a tube. The internal adapter constitutes an easy way to establish a fluid communication between the hydraulic actuator and the cartridge pressure amplifier. The length of the internal adapter may vary depending on the stroke of the piston rod. All parts necessary for establishing such a fluid communication may therefore be assembled inside the piston rod. 
     In yet another embodiment, the internal adapter comprises a radial sealing concentrically fixing the internal adapter relative to the piston rod. This makes assembly easy and effective. The radial sealing may be a sealing ring. As the piston inlet port as well as the cartridge pressure amplifier may be arranged concentrically with the piston rod, a concentric fixing of the internal adapter relative to the piston rod is advantageous. A space-saving assembly can be achieved. Fluid communication between the cartridge pressure amplifier and the hydraulic actuator is established. 
     In another embodiment, the cartridge pressure amplifier is fixed to the cylinder housing such that the piston is displaceable relative to the cartridge pressure amplifier. The cartridge pressure amplifier is mounted in the cylinder housing concentrically with the piston rod. The cartridge pressure amplifier is at least partially arranged inside the piston rod. However, in this embodiment the cartridge pressure amplifier does not follow the movement of the piston, but stays stationary relative to the cylinder housing. As the cartridge pressure amplifier is still arranged at least partially inside the piston rod, the overlap between the cartridge pressure amplifier and the piston rod varies during the stroke of the piston. 
     In a final embodiment, the pressure inlet port is arranged inside the cylinder housing establishing a fluid communication between the pressure inlet port and a housing inlet port. The housing inlet port may be arranged in the cylinder housing as a drilled hole. The pressure inlet port may be arranged coaxially with the piston rod. It connects the cartridge pressure amplifier with the hydraulic fluid supply of the hydraulic actuator via the housing inlet port. The high pressure outlet port of the cartridge pressure amplifier is arranged at the axially opposite end of the cartridge pressure amplifier relative to the pressure inlet port. Therefore, during most of the stroke of the piston, the high pressure outlet port will be arranged inside the piston rod. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall be described with reference to different embodiments in connection with the figures in the forth-coming paragraphs. Therein, 
         FIG. 1  depicts a hydraulic actuator with a cartridge pressure amplifier according to a first embodiment of the invention; 
         FIG. 2  depicts a hydraulic actuator with a cartridge pressure amplifier according to a second embodiment of the invention; 
         FIG. 3  depicts a first embodiment of the cartridge pressure amplifier; 
         FIG. 4  depicts a second embodiment of the cartridge pressure amplifier; 
         FIG. 5  depicts a third embodiment of the cartridge pressure amplifier; 
         FIG. 6  depicts a fourth embodiment of the cartridge pressure amplifier. 
     
    
    
     DETAILED DESCRIPTION 
     A hydraulic actuator  1  comprises a cylinder housing  2 . The cylinder housing  2  comprises at its first axial end a cylinder eye  3 . It further comprises a cylinder head  4  sealing an inner volume of the cylinder housing  2  in a fluid-tight manner. The hydraulic actuator  1  comprises a piston  5  with a piston rod  6  being displaceably arranged inside the cylinder housing  2 . The piston rod  6  engages with the cylinder head  4 . The piston rod  6  comprises a piston head  7  at its first axial end and a piston eye  7   a  at its second axial end. A working chamber  8  of the hydraulic actuator  1  is arranged at the side of the piston head  7  opposite the piston eye  7   a . The piston head  7  comprises a piston side port  9 . The piston side port  9  is arranged coaxially with the piston rod  6 . It establishes a first fluid communication between the working chamber  8  of the hydraulic actuator  1  and a cartridge pressure amplifier  10 . The cartridge pressure amplifier  10  is arranged inside the piston rod  6 . It comprises a sleeve  10   a . The sleeve  10   a  as well as the cartridge amplifier  10  are arranged coaxially with the piston rod  6 . The piston rod  6  further comprises a piston rod side port  11  establishing a second fluid communication between the cartridge pressure amplifier  10  and the inner volume of the cylinder housing  2 . 
     At an axial end of the cartridge pressure amplifier  10  in the vicinity of the piston eye  7   a , an internal adapter  12  is arranged. The internal adapter  12  is fixed to its position inside the piston rod  6  by means of a radial sealing  13 . The radial sealing  13  fixes the internal adapter  12  coaxially with the piston rod  6 . The internal adapter  12  establishes a fluid communication between the cartridge pressure amplifier  10  and a piston inlet port  14 . The piston inlet port  14  is arranged inside the piston eye  7   a . A piston outlet port  15  corresponding to the piston inlet port  14  is also arranged inside the piston eye  7   a.    
     In the embodiment of  FIG. 1  the cartridge pressure amplifier  10  is concentrically mounted inside the drilled piston rod  6 . The cartridge pressure amplifier  10  is arranged closer to the piston head  7  than to the piston eye  7   a . The piston inlet port  14  and the piston outlet port  15  are arranged inside the piston eye  7   a  as drilled holes. They provide hydraulic fluid with a certain, preset pressure. The pressurized hydraulic fluid is provided by an external pump (not shown), for example. The piston inlet port  14  is arranged coaxially with the piston rod  6 . It is connected to the internal adapter  12 . The internal adapter  12  is connected to the cartridge pressure amplifier  10 . 
     The internal adapter  12  may be a tube. It is located coaxially with the piston rod  6  inside the drilled piston rod  6 . The internal adapter  12  may change according to the stroke of the piston  6 . The internal adapter  12  may be fixed in its position by means of the radial sealing  13 . The radial sealing  13  may be a sealing ring. The radial sealing  13  keeps the internal adapter  12  in its position coaxially with the piston rod  6 . Assembly becomes easy and effective. The piston rod  6  has a diameter larger than the diameter of the internal adapter  12 . Thus, an annular piston channel opens a fluid communication between the cartridge pressure amplifier  10  and the piston outlet port  15 . This annular piston channel is used for backflow of hydraulic fluid from the cartridge pressure amplifier  10  to the piston outlet port  15 . 
     Now, the pressurized hydraulic fluid is provided in the piston inlet port  14  and the internal adapter  12  to the cartridge pressure amplifier  10 . The pressure of the hydraulic fluid thus provided to the cartridge pressure amplifier  10  is enhanced by means of the cartridge pressure amplifier  10 . The high pressure hydraulic fluid exits the cartridge pressure amplifier  10  via the piston side port  9  into the working chamber  8  of the hydraulic actuator  1 . Thus, enhanced pressure can be supplied for the hydraulic fluid inside the hydraulic actuator  1 . 
     In the embodiment of  FIG. 2  the cartridge pressure amplifier is arranged in a different manner. The cartridge pressure amplifier  10  here is concentrically mounted in the bottom of the cylinder housing  2 . The bottom of the cylinder housing  2  is the axial end face of the inner volume of the cylinder housing  2  opposite the cylinder head  4 . A housing inlet port  14   a  and a housing outlet port  15   a  are now arranged inside the cylinder housing  2 . The housing inlet port  14   a  provides pressurized hydraulic fluid, e.g. by means of an external pump (not shown), to the cartridge pressure amplifier  10 . It therefore serves the same purpose as piston inlet port  14 . The housing inlet port  14   a  is arranged coaxially with the piston rod  6 . It is connected to the cartridge pressure amplifier  10 . In this embodiment, no need for an internal adapter  12  arises. The backflow of hydraulic fluid from the cartridge pressure amplifier  10  is achieved by means of the housing outlet port  15   a . It thus serves the same purpose as the piston outlet port  15 . 
     As the cartridge pressure amplifier  10  is stationarily mounted in the cylinder housing  2  according to the embodiment of  FIG. 2 , more differences to the embodiment of  FIG. 1  arise. The cartridge pressure amplifier  10  is no longer arranged stationarily relative to the piston rod  6 . It is, however, arranged stationarily relative to the cylinder housing  2 . This means, the piston rod  6  overlaps with the cartridge pressure amplifier  10  to a varying degree depending on the stroke of the piston rod  6 . As the pressurized hydraulic fluid enters the cartridge pressure amplifier  10  via the cylinder housing  2 , the amplified hydraulic fluid exits the cartridge pressure amplifier  10  through the piston side port  9  into the inside of the piston rod  6 . 
     Moreover, the embodiment of  FIG. 2  does not rely on the piston rod side port  11  being arranged in a radial direction of the piston rod  6 . Instead, the piston rod side port  11  is arranged inside the cylinder housing  2 . It establishes a fluid communication to a cylinder external pipe  16 . Said cylinder external pipe  16  is in fluid communication with housing outlet port  15   a.    
     Otherwise, the working principle of the hydraulic actuator  1  according to the embodiments of  FIG. 1, 2  are identical and known in the state of the art. 
     The embodiment of  FIG. 3  shows a pressure amplifier  17 . The pressure amplifier  17  comprises an inlet section  18  as well as an active section  19 . The division of the pressure amplifier  17  into an inlet section  18  and an active section  19  is due to the assembly of its internal parts. The inlet section  18  and the active section  19  are held together by external force in order to assure proper function of the pressure amplifier  17 . The external force is provided by the sleeves  10   a  of the cartridge pressure amplifier  10 . 
     The inlet section  18  comprises a pressure inlet port  20 . The pressure inlet port  20  is connected to the internal adapter  12  of the embodiment of  FIG. 1  or the housing inlet port  14   a  of the embodiment of  FIG. 2 . Thereby, pressurized hydraulic fluid is provided to the pressure amplifier  17 . The pressurized hydraulic fluid flows inside a main inlet channel  21 . The main inlet channel  21  connects the pressure inlet port  20  to a high pressure outlet port  22 . The high pressure outlet port  22  is connected to the piston side port  9  of the hydraulic actuator  1 . Thereby, hydraulic fluid with an amplified pressure can be provided to the hydraulic actuator  1 . The high pressure outlet port  22  is arranged inside the active section  19  of the pressure amplifier  17 . 
     The active section  18  also comprises a backflow inlet port  23 . The backflow inlet port  23  is connected to a main backflow channel  24  leading to a backflow outlet port  25 . The backflow inlet port  23  is connected to the piston rod side port  11  of the hydraulic actuator  1 . The backflow outlet port  24  is connected to the piston outlet port  14  or the housing outlet port  14   a , respectively. 
     The working principle of the pressure amplifier  17  is as follows. 
     When there is no demand for hydraulic fluid with an amplified pressure, the hydraulic fluid enters through the pressure inlet port  20  and passes through the main inlet channel  21 . An over-center valve  26  is arranged in the main inlet channel  21  inside the active section  19 . When there is no demand for hydraulic fluid with amplified pressure, a check valve inside the over-center valve  26  allows full flow of hydraulic fluid through the main inlet channel  21  to the high pressure outlet port  22 . An amplification of pressure does not occur. At the same time, the backflow of hydraulic fluid is going directly from the backflow inlet port  23  to the backflow outlet port  25  via the main backflow channel  24 . 
     Once an increased external load is applied to the hydraulic actuator  1 , the pressure of the hydraulic fluid is also increasing at the pressure inlet port  20 . When the pressure of the hydraulic fluid exceeds a certain preset value, a pilot sequence valve  27  opens a first pilot channel  28 . Thus, the pilot sequence valve  27  is closed, as long as the pressure of the hydraulic fluid does not exceed the preset value. Once the pilot sequence valve  27  opens, however, hydraulic fluid passes through the first pilot channel  28  and exerts pressure on a first control valve pin  29  of a control valve  30 . The pressure applied to the first control valve pin  29  moves the control valve  30  to a position in which hydraulic fluid may pass through it and into a low pressure piston channel  31 . 
     The low pressure piston channel  31  leads to a low pressure chamber  32 . In said low pressure chamber  32  a low pressure piston  33  is slidably arranged. The low pressure piston  33  comprises a low pressure piston surface  34 . The hydraulic fluid acts on said low pressure piston surface  34  and the low pressure piston  33  starts moving in a direction opposite the low pressure piston channel  31  and toward a low pressure working chamber  35 . The low pressure piston  33  is connected via a low pressure-high pressure piston rod  36  to a high pressure piston  37  inside a high pressure chamber  38   a.    
     The high pressure piston  37  comprises a high pressure piston surface  38 . Said high pressure piston surface  38  has a smaller area than the low pressure piston surface  34 . Hence, the pressure acting on the low pressure piston surface  34  is amplified by the ratio of the two surfaces, when the high pressure piston  37  acts on hydraulic fluid inside a high pressure working chamber  39 . The pressure-amplified hydraulic fluid exiting the high pressure working chamber  39  passes through a first check valve  40  opening in a direction toward the high pressure outlet port  22  by means of a first high pressure channel  41 . The first high pressure channel  41  leads to a second high pressure channel  42  of the main inlet channel  21 . 
     Once the low pressure piston  33  (and therefore the high pressure piston  37 ) has thus reached its end position, an aperture  43  opens a fluid communication with a second pilot channel  4 . The second pilot channel  44  is connected to a second control valve pin  45  of the control valve  30 . As the surface area of the second control valve pin  45  is larger than the one of the first control valve pin  29 , the control valve  30  moves to its previous position. After this, the first check valve  40  closes down. As now both the pilot sequence valve  27  as well as the first check valve  40  are closed, pressure is applied to a second check valve  46 . The second check valve  46  opens a fluid communication from the main inlet channel  21  to the high pressure working chamber  39 . The pressure applied to the high pressure working chamber  39  begins to force the high pressure piston  37  toward the low pressure chamber  32 . An annular channel  47  connects the low pressure working chamber  35  to the control valve  30 . Thereby, the pilot sequence valve  27  eventually returns to its original position and the cycle is repeated. 
     The embodiment of  FIG. 4  shows how the pilot sequence valve  27  can be thread-mounted in an axial direction of the inlet section  18 . The bottom of the pilot sequence valve  27  is then connected to the pressure inlet port  20  through the main inlet channel  21 . A side port of the pilot sequence valve  27  is connected via the first pilot channel  28  to the first control valve pin  29 . Setting of the pilot sequence valve  27  can be adjustable or fixed to a certain preset value. 
     As can also be inferred from  FIG. 4 , the pressure amplifier consists of two separate sections: the inlet section  18  and the active section  19 . The inlet section  18  comprises a first axial end face  48  and a second axial end face  49 . The active section  19  comprises a first axial end face  50  and a second axial end face  51 . Therein, the first axial end face  48  of the inlet section  18  and the first axial end face  50  of the active section  19  abut. Hence, in order to achieve a proper function of the pressure amplifier  17 , the inlet section  18  and the active section  19  are held together by external force exerted by the sleeve  10   a.    
     In the embodiment of  FIG. 5  the position of the over-center valve  26  inside the active section  19  is exemplified. The over-center valve  26  consists of multiple parts which are arranged in an axial direction of the active section  19 . All such parts are mounted from the first axial end face  48  of the inlet section  18 . The correct position of all the parts is achieved by covering of the inlet section  18 . Hence, there is no need for a thread inside the active section  19 . Once the inlet section  18  and the active section  19  are mounted together, it is not possible to set the pressure level on the over-center valve  26 . Therefore, such setting is done by several types of springs. 
     The over-center valve  26  can provide a full flow from the pressure inlet port  20  to the high pressure outlet port  22 . It can provide a load holding function at the high pressure outlet port  22 . It can furthermore provide a controlled lowering function from high pressure outlet port  22  to pressure inlet port  20 . 
     The over-center valve  26  has three connection ports: an over-center valve inlet port associated with the main inlet channel  21 ; an over-center valve outlet port associated with the second high pressure channel  42 ; and an over-center valve pilot port associated with a pilot line  52 . The pilot line  52  connects the over-center valve  26  with the main backflow channel  24 . In a direction from the pressure inlet port  20  to the high pressure outlet port  22 , the over-center valve  26  provides a full flow function by means of an integrated check valve. In the opposite direction, the over-center valve  26  is kept blocked until sufficient pressure is applied to the pilot line  52 . The over-center valve  26  is also connected to a bypass-channel  53 . 
     In the embodiment of  FIG. 6 , the pressure amplifier  17  is shown with a low pressure piston bushing  54  and a high pressure piston bushing  55 . Such integrated bushings are a proper way to increase the lifetime of both the low pressure piston  33  as well as the high pressure piston  37 . The low pressure piston bushing  54  decreases the friction between the low pressure piston  33  and the walls of the low pressure chamber  32 . The high pressure piston bushing  55  decreases the friction between the high pressure piston  37  and the walls of the high pressure chamber  38   a.    
     The low pressure piston bushing  54  is molded into the inlet section  18 . The proper position is controlled by jig during molding process. There is a use for machining of the low pressure piston bushing  54  to a certain diameter after molding. 
     The high pressure piston bushing  55  comprises a first high pressure piston bushing element  56  and a second high pressure bushing element  57 . The assembly process is the same as for the low pressure piston bushing  54 . However, the first high pressure piston bushing element  56  and the second high pressure piston bushing element  57  are arranged such that the aperture  43  is arranged between them. The first high pressure piston bushing element  56  may be shorter than the second high pressure piston bushing element  57 . 
     While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.