Patent Publication Number: US-11396894-B2

Title: Hydraulic shield support system and pressure intensifier

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This Application is a 35 USC § 371 US National Stage filing of International Application No. PCT/EP2019/025065 filed on Mar. 8, 2019 which claims priority under the Paris Convention to European Patent Application No. 18162613.6 filed on Mar. 19, 2018. 
     TECHNICAL FIELD 
     The present disclosure generally relates to a hydraulic shield support system and a pressure intensifier for use therein, in particular, to a hydraulic shield support system for use in underground mining. 
     BACKGROUND 
     In underground mining systems, various hydraulic assemblies are used, for example, for controlling hydraulic functions of roof supports used in underground longwall mining. For example, a self-advancing roof support system may include at least two adjustable-length hydraulic props provided on base shoes and supporting a shield. In particular, such hydraulic supports are used to keep the face or working area free and to support the roof. Generally, the canopy or shield of the roof support is supported by double acting hydraulic props supported on the base shoes. 
     In view of a constant demand for longer faces and higher capacity systems, the roof surface area to be supported by the roof supports increases constantly. To support the rock, it is therefore necessary to increase the load that can be supported by the shields. 
     The disclosed systems and methods are directed at least in part to improving known systems. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect, the present disclosure relates to a hydraulic shield support system adapted for underground mining. The system comprises a plurality of length-adjustable hydraulic props configured to support a shield, and a hydraulic fluid supply configured to supply hydraulic fluid at a first pressure. A plurality of pressure intensifiers are fluidly connected between the hydraulic fluid supply and each of the hydraulic props. Each of the plurality of pressure intensifiers is configured to supply hydraulic fluid at an increased second pressure to the associated hydraulic prop. A plurality of control valves are configured to selectively supply the hydraulic fluid from the hydraulic fluid supply to the respective pressure intensifiers to operate the same. Further, a plurality of pressure sensors are configured to measure the pressure of the hydraulic fluid supplied to each of the hydraulic props by the associated pressure intensifier. A control unit is configured to set a plurality of desired pressures of the hydraulic fluid to be supplied to the plurality of hydraulic props, at least two of the set desired pressures being different from each other. The control unit is further configured to receive the pressures measured by the plurality of pressure sensors, and to switch the plurality of control valves to stop supplying fluid to each of the pressure intensifiers when the measured pressure reaches the set desired pressure for the associated hydraulic prop. 
     In another aspect, the present disclosure relates to a method of operating a hydraulic shield support system adapted for underground mining, the system comprising a plurality of length-adjustable hydraulic props configured to support a shield, a hydraulic fluid supply configured to supply hydraulic fluid at a first pressure, a plurality of pressure intensifiers fluidly connected between the hydraulic fluid supply and each of the hydraulic props, each of the plurality of pressure intensifiers being configured to supply hydraulic fluid at an increased second pressure to the associated hydraulic prop, and a plurality of control valves configured to selectively supply the hydraulic fluid from the hydraulic fluid supply to the respective pressure intensifiers to operate the same. The method comprises setting a plurality of desired pressures of the hydraulic fluid to be supplied to the plurality of hydraulic props, at least two of the set desired pressures being different from each other, measuring the pressure of the hydraulic fluid supplied to each of the hydraulic props, and switching the plurality of control valves to stop supplying fluid at the first pressure to each of the pressure intensifiers when the measured pressure reaches the set desired pressure for the associated hydraulic prop. 
     In yet another aspect, the present disclosure relates to a pressure intensifier for use in a hydraulic shield support system. The pressure intensifier comprises a housing including a low-pressure input configured to receive hydraulic fluid at a first pressure, and a high-pressure output configured to output the hydraulic fluid at an increased second pressure. The pressure intensifier further comprises an intensifier piston movably disposed in the housing and defining a low-pressure chamber and a high-pressure chamber on opposite sides of the piston, the intensifier piston being configured to increase the pressure of hydraulic fluid in the high-pressure chamber by moving into the high-pressure chamber when hydraulic fluid at the first pressure is supplied to the low-pressure chamber. A directional control valve is movably disposed in the pressure intensifier, the directional control valve being movable between a first control valve position in which the low-pressure chamber is fluidly connected to the low-pressure input and a second control valve position in which the low-pressure chamber is fluidly connected to a drain. A switching valve is configured to switch the directional control valve between the first control valve position and the second control valve position, wherein the switching valve is configured to switch the directional control valve from the first control valve position to the second control valve position when the intensifier piston reaches a predetermined position in the high-pressure chamber. 
     Other features and aspects of the present disclosure will be apparent from the following description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic side view of a shield support in accordance with the present disclosure; 
         FIG. 2  shows a schematic diagram of a hydraulic circuit of a shield support in accordance with the present disclosure; 
         FIG. 3  shows a schematic representation of a pressure intensifier in accordance with the present disclosure. 
         FIG. 4  shows a schematic diagram of another hydraulic circuit of a shield support in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following is a detailed description of exemplary embodiments of the present disclosure. The exemplary embodiments described herein are intended to teach the principles of the present disclosure, enabling those of ordinary skill in the art to implement and use the present disclosure in many different environments and for many different applications. Therefore, the exemplary embodiments are not intended to be, and should not be considered as a limiting description of the scope of protection. Rather, the scope of protection shall be defined by the appended claims. 
     The present disclosure may be based in part on the realization that the increased pressures required for the hydraulic props of a hydraulic shield support system can be achieved by utilizing a plurality of pressure intensifiers, each pressure intensifier being associated with one of the hydraulic props to increase the pressure of the hydraulic fluid that is supplied to the same. In this respect, it has been realized that it is advantageous to be able to individually configure the pressure that is supplied to each hydraulic prop, by a corresponding control of the pressure intensifiers associated with the same. In particular, it has been realized that it is advantageous to provide a pressure sensor for each of the hydraulic props, and control the operation of the individual pressure intensifiers based on the measurement results from the plurality of pressure sensors. In this case, a control unit can monitor the pressures measured by the plurality of pressure sensors, and individually switch off the pressure intensifiers when the desired pressure for the corresponding hydraulic prop has been reached. In this manner, an appropriate pressure profile for the plurality of hydraulic props can be obtained. 
     The present disclosure is further based on the realization that, by producing the increased pressure directly at the hydraulic prop, the hydraulic pressure for the remaining functions of the hydraulic system can be reduced, i.e., a lower input or system pressure is sufficient to operate said remaining functions. In this respect, it has been realized that by mounting the pressure intensifiers directly at the hydraulic prop, without the need for hoses or the like, the pressure intensifiers can be arranged in a particularly advantageous manner. In this way, the pressure intensifier functions similar to a valve, which can be controlled to directly supply the hydraulic fluid at the desired pressure to the associated hydraulic prop. 
     The present disclosure is also based in part on the realization that, in some cases, it may be advantageous to provide the pressure intensifier in series with a hydraulically releasable non-return valve that is associated with each hydraulic prop. In this case, the pressure intensifier does not need to be configured such that it can sustain the high pressure from the hydraulic prop, as this function is already performed by the non-return valve. In this configuration, it has been realized that it is advantageous to provide a further non-return valve parallel to the pressure intensifier. 
     The present disclosure is further based in part on the realization that, in order to obtain a reliable operation of the shield support system, in particular, the pressure intensifiers of the same, it is necessary to provide a pressure intensifier with a configuration that can be realiably operated to increase the low system pressure to the increased pressure required by the hydraulic props. Here, it has been realized that it is advantageous that the pressure intensifier includes a directional control valve that is movably disposed in the pressure intensifier and can selectively connect the working chamber of the pressure intensifier either with the pressure supply that supplies the system pressure or with a drain that discharges fluid to a pressure sink such as a tank or reservoir. 
     It has been realized that it is advantageous that a further switching valve is provided in the pressure intensifier to reliably switch the directional control valve between its two configurations. In this manner, once the system pressure is supplied to the pressure intensifier, the same can continue to operate in an autonomous manner, until the maximum obtainable pressure or the desired pressure for the associated hydraulic prop has been reached. In this respect, it has also been realized that a reliable switching of the switching valve can be achieved when the same is configured as a mechanically actuated valve that is actuated when the intensifier piston reaches a predetermined position. This advantageous configuration results in a mechanically actuated 3/3 way valve that controls the operation of the pressure intensifier. 
     The present disclosure is further based on the realization that, in the pressure intensifier having the above-described configuration, it is advantageous when the switching valve is configured as a non-return valve. With this configuration, in case the pressure intensifier stops its operation, i.e., the intensifier piston stops its reciprocating movement, the pressure intensifier can be reactivated by applying the system pressure to the drain of the same, while the input that usually receives the system pressure is connected to the tank or reservoir. This allows re-establishing a predetermined initial configuration of the pressure intensifier, from which it can again start operating normally. 
       FIG. 1  shows a schematic representation of a hydraulic shield support system  100  for use in deep mining operations. A shield support  1  includes two base runners or shoes  3  located alongside each other on a floor  2 , and a shield  5  underpinning the so-called roof  4  and protruding to the working or coal seam (not shown). Shield support  1  further includes a backshield  6  screening the face area. Backshield  6  is articulated to floor shoes  3  by two arms  7 . Arms  7 , together with two hydraulic props  8  supported on foot joints on base shoes  3 , apply sufficient forces to shield  5  to keep the face area free. Hydraulic props  8  arranged, for example, as a pair alongside each other and supported on respective base shoes  3  are telescopic, for example, in several stages, and may be subjected to pressure at either end. 
     A hydraulic fluid may be supplied either to a pressure chamber in hydraulic props  8  through pipes  13  to press shield  5  against roof  4 , thus setting shield support  1  (hereinafter referred to as “set condition”), or to an annulus to retract hydraulic props  8  for removal of hydraulic shield support  1 . 
     Shield support  1  is actuated by an electronic control unit  80 , by means of which directional control valves in a valve control bank  40  can be actuated to control operation of shield support  1 . Control bank  40  includes a plurality of selectively positionable control valves  41 ,  47  (see  FIG. 2 ) for each hydraulic prop  8 , each of which can be positioned in one or more control positions. A valve chest  14  is mounted on each hydraulic prop  8  and contains a non-return valve  51  (see  FIG. 2 ). Hydraulic pressure is supplied to the hydraulic prop  8  by a pressure supply  12  configured as a pressure pipe, for example, pipe  13 . Hydraulic fluid may also be supplied to the annulus of hydraulic prop  8  via another pressure pipe  54  (see  FIG. 2 ). A pressure intensifier  21  (see  FIG. 2 ) is provided for each hydraulic prop  8 . In some embodiments, pressure intensifier  21  is mounted to hydraulic prop  8  and/or non-return valve  51  through a mounting portion  15  configured as, for example, a mounting flange connected to or provided integrally with a housing of pressure intensifier  21 . In other embodiments, pressure intensifier  21  may be mounted to pipe  13 , for example, by a screw connection or the like. 
     In the shield support system of the present disclosure, at least two hydraulic props  8  are provided. Further, in a deep mining application, the face area is supported by a plurality of hydraulic shield supports  1  located alongside each other. In between each shield support  1  and the working face is a winning system such as, for example, a coal plough or drum cutter loader with a chain conveyor. The winning system can be advanced towards the working face by an advancing ram  16 . An angle cylinder  9  is interposed between back shield  6  and shield  5 . The supply of pressure to all hydraulic shield supports  1  takes place through a hydraulic supply system not shown in detail, wherein a pump may be provided for one or more of shield supports  1  to provide hydraulic fluid to the hydraulic props  8  of shield supports  1 . 
     As will be described in more detail below with respect to  FIGS. 2 and 3 , a plurality of pressure intensifiers  21  are provided for the plurality of hydraulic props  8 .  FIG. 2  shows a schematic representation of a hydraulic circuit of hydraulic shield support system  100  configured to supply hydraulic fluid to one of the plurality of hydraulic props  8 . 
     As shown in  FIG. 2 , system  100  includes a hydraulic fluid supply  12  configured to supply hydraulic fluid at a first hydraulic pressure P, which may correspond to the system pressure, to pressure intensifier  21  via control valve  41  and pressure pipe  13 . As also shown in  FIG. 2 , system  100  also includes a pressure sink T, such as a tank or reservoir, to which hydraulic fluid from hydraulic prop  8  can be discharged via pressure pipe  54  and control valve  47 . 
     As shown in  FIG. 2 , each pressure intensifier  21  is fluidly connected between hydraulic fluid supply  12  and hydraulic prop  8 , and configured to supply hydraulic fluid at an increased pressure HP to associated hydraulic prop  8 . As shown in  FIG. 2 , pressure intensifier  21  has a low-pressure input E via which hydraulic fluid supplied from hydraulic fluid supply  12  is supplied to pressure intensifier  21 , a high-pressure output A via which hydraulic fluid at an increased pressure is supplied to hydraulic prop  8 , and a drain R, via which hydraulic fluid is discharged to pressure sink T. The operation of pressure intensifier  21  will be described in more detail in the following. 
     As shown in  FIG. 2 , hydraulic fluid from hydraulic fluid supply  12  at system pressure P is supplied to low-pressure input of pressure intensifier  21  via control valve  41  and pipe  13 . Control valve  41  may be movable between two valve positions, under a control of control unit  80 . In a first valve position, not shown in  FIG. 2 , control valve  41  fluidly connects low-pressure input E of pressure intensifier  21  with hydraulic fluid supply  12  to supply hydraulic fluid at pressure P. In a second position, which is shown in  FIG. 2 , control valve  41  fluidly connects low-pressure input E of pressure intensifier  21  with the pressure sink T via a return line  22 . 
     As further shown in  FIG. 2 , high-pressure output A of pressure intensifier  21  is fluidly connected to a pressure chamber  18  of hydraulic prop  8 , which pressure chamber is defined between a housing  19  and a bottom surface of a piston  17  provided in housing  19 , by a pressure pipe  52 . Piston  17  and housing  19  further define a second chamber, for example, an annulus, of hydraulic prop  8 , in a manner that is known to the skilled person. Said annulus is fluidly connectable to pressure sink T via pressure pipe  54  and control valve  47 . In this manner, hydraulic fluid in the annulus of hydraulic prop  8  can be selectively discharged to pressure sink T via a corresponding operation of control valve  47  by control unit  80 . Control valve  47  is also configured as a valve with two positions. In a first position, which is not shown in  FIG. 2 , the annulus of hydraulic prop  8  is fluidly connected to hydraulic fluid supply  12  to receive the system pressure P, and in the second position shown in  FIG. 2 , the annulus is fluidly connected to pressure sink T. As shown in  FIG. 2 , an assembly including non-return valve  51  and pressure intensifier  21  is mounted to housing  19  of hydraulic prop  8 , for example, via an appropriate mounting flange of mounting portion  15 . 
     As also shown in  FIG. 2 , non-return valve  51  is arranged between pressure pipe  13  and pressure pipe  52 , i.e., between control valve  41  and hydraulic prop  8 . In addition, non-return valve  51  is configured to be hydraulically releasable by the hydraulic pressure of the hydraulic fluid in pressure pipe  54 , in particular, when the hydraulic pressure in pressure pipe  54  is the system pressure P. 
     System  100  also comprises a pressure sensor  61  configured to measure the pressure of hydraulic fluid that is supplied to pressure chamber  18  of hydraulic prop  8 . Pressure sensor  61  may be arranged along pressure pipe  52  at a position downstream of pressure intensifier  21 , and is configured to measure the pressure of the fluid supplied to pressure chamber  18  and output a corresponding measurement result to control unit  80 . Control unit  80  is configured to set a desired pressure of the hydraulic fluid to be supplied to hydraulic prop  8 , receive the pressure measured by pressure sensor  61 , and switch control valve  41  to stop supplying fluid at system pressure P to pressure intensifier  21  when the measured pressure reaches the desired pressure for associated hydraulic prop  8 . 
     It will be appreciated that control unit  80  is configured to set a plurality of desired pressures for the plurality of hydraulic props  8 , in particular, such that at least two of the set desired pressures are different from each other. With this configuration, a pressure profile with different pressures for different hydraulic props  8  can be obtained, by switching off the respective pressure intensifiers  21  when the desired pressures have been reached. Therefore, it is understood that a pressure sensor  61  is provided for each hydraulic prop  8  and configured to detect the pressure of hydraulic fluid supplied to the same. Likewise, control unit  80  is configured to receive all pressures measured by the plurality of pressure sensors  61 , and individually actuate the respective control valves  41  and, optionally,  47 . 
     An exemplary operation of the system shown in  FIG. 2  will be explained in the following. At the start of supplying pressure to hydraulic prop  8  to set the same, control unit  80  actuates control valves  41 ,  47  such that the system pressure P is supplied to low-pressure input E of pressure intensifier  21 . Further, control valve  47  is actuated to fluidly connect the annulus of hydraulic prop  8  to pressure sink T. In this configuration, the drain R of pressure intensifier  21  is also fluidly connected to pressure sink T via its connection to pressure pipe  54 . Pressure intensifier  21  therefore begins operating to increase system pressure P to the desired high pressure HP. In particular, hydraulic fluid at an increased pressure is supplied to pressure chamber  18  of hydraulic prop  8  from high-pressure output A of pressure intensifier  21 . Accordingly, piston  17  of hydraulic prop  8  begins to extend from housing  19  of hydraulic prop  8 . A back flow of hydraulic fluid at the increased pressure from the pressure chamber of hydraulic prop  8  is prevented by non-return valve  51 . 
     Pressure sensor  61  detects the value of the increased pressure that is supplied to pressure chamber  18  of hydraulic prop  8 , and outputs the measurement result to control unit  80 . Control unit  80 , which has previously set a desired pressure for the hydraulic fluid to be supplied to hydraulic prop  8 , receives the measured pressure and compares the same to the previously set desired pressure. When the measured pressure reaches the desired pressure, control unit  80  actuates control valve  41  to fluidly connect low-pressure input E of pressure intensifier  21  with tank or reservoir T. Accordingly, the system pressure P is no longer supplied to pressure intensifier  21 , and the same stops its operation. Therefore, the high pressure HP will no longer increase. 
     When piston  17  is to be retracted, control unit  80  actuates control valve  47  to fluidly connect the annulus of hydraulic prop  8  to hydraulic fluid supply  12 . The pressure P in line  54  actuates non-return valve  51 , and piston  17  retracts. 
       FIG. 3  shows an exemplary embodiment of pressure intensifier  21 . As shown in  FIG. 3 , pressure intensifier  21  includes a housing  71  and an intensifier piston  72  moveably disposed in housing  71 . Intensifier piston  72  defines a low-pressure or working chamber  73  and a high-pressure chamber  74  on opposite sides of the same. Intensifier piston  72  is configured to increase the pressure of hydraulic fluid in high-pressure chamber  74  by moving into the same when hydraulic fluid at system pressure P is supplied to low-pressure chamber  73 . In the exemplary embodiment shown in  FIG. 3 , intensifier piston  72  is generally cup-shaped, with the annular side wall  89  of the same moving into the correspondingly annular-shaped high-pressure chamber  74 . 
     Further, pressure intensifier  21  includes a valve assembly accommodated in a valve housing  83  that is mounted to the end of housing  71  that is opposite to low-pressure chamber  73 . In particular, valve housing  83  defines the inner surface of annular high-pressure chamber  74 . A pair of seals  85 ,  97 , which will be described in more detail below, are provided between the outer surface of valve housing  83  and an inner peripheral surface of wall  89  of intensifier piston  72 . An inner space  99  is defined between an inner bottom surface  81  of intensifier piston  72  and the opposing outer bottom surface of valve housing  83 . 
     As shown in  FIG. 3 , a reduced diameter distal end portion is formed in side wall  89  of intensifier piston  72  and provided in high-pressure chamber  74 . At least one radial bore  87  is formed in the reduced diameter distal end portion of side wall  89  to be in fluid communication with high-pressure chamber  74 . 
     Valve housing  83  comprises a fluid inlet  90  formed in an outer peripheral surface of valve housing  83  that defines an inner surface of high-pressure chamber  74 . Fluid inlet  90  is provided between seals  85 ,  97 . Seals  85 ,  97  and radial bore  87  are provided at positions such that, when intensifier piston  72  has reached its end position in low-pressure chamber  73  (the rightmost position in  FIG. 3 ), fluid inlet  90  is fluidly communicated with high-pressure chamber  74  via radial bore  87 , with inner space  99  defined between intensifier piston  72  and valve housing  83  being fluidly separated from high-pressure chamber  74  by seal  97 . As intensifier piston  72  moves into high-pressure chamber  74 , it reaches a position where radial bore  87  moves past seal  85  to fluidly separate fluid inlet  90  from high-pressure chamber  74 . 
     As shown in  FIG. 3 , valve assembly  88  includes a directional control valve  75  and a switching valve  77 . Directional control valve  75  is movably disposed in pressure intensifier  21 , i.e., valve housing  83 , and is movable between a first control valve position in which low-pressure chamber  73  is fluidly connected to low-pressure input E of pressure intensifier  21 , and a second control valve position in which low-pressure chamber  73  is fluidly connected to drain R. In some embodiments, directional control valve  75  is concentrically arranged inside intensifier piston  72 . Further, switching valve  77  is configured to switch directional control valve  75  between the first control valve position and the second control valve position. In particular, switching valve  77  is configured to switch directional control valve  75  from the first control valve position to the second control valve position when intensifier piston  72  reaches a predetermined position in high-pressure chamber  74 . 
     In the exemplary embodiment, switching valve  77  is a mechanically actuated valve that is mechanically actuated by intensifier piston  72  upon reaching the predetermined position. As will be described in more detail below, in the exemplary embodiment shown in  FIG. 3 , the predetermined position of intensifier piston  72  is its end position within high-pressure chamber  74 . In this end position, bottom surface  81  of intensifier piston  72  contacts a contact element  82  of switching valve  77  and actuates the same to move from a first valve position to a second valve position to switch directional control valve  75  from the first control valve position to the second control valve position. This will be described in more detail below. 
     As shown in  FIG. 3 , inner space  99  is fluidly connected to drain R. Further, switching valve  77  is fluidly connected between a return line  76  that connects inner space  99  with drain R, and a control chamber  93  of directional control valve  75 , which will be described in more detail below. In the first valve position, when contact element  82  is not contacted by intensifier piston  72 , switching valve  77  fluidly separates control chamber  93  from return line  76 . On the other hand, in the second valve position, when intensifier piston  72  contacts contact element  82 , switching valve  77  fluidly connects return line  76  to control chamber  93 . 
     Directional control valve  75  is, in the exemplary embodiment, a 3/2 directional control valve. Directional control valve  75  includes a movable element  33  having a first pressure receiving surface  91  and a second pressure receiving surface  92  with an area that is greater than an area of the first pressure receiving surface  91 . First pressure receiving surface  91  is exposed to hydraulic fluid at system pressure P, and second pressure receiving surface is exposed to hydraulic fluid in control chamber  93 . As already explained, control chamber  93  is selectively in fluid communication with fluid inlet  90  or drain D, depending on the switching state of switching valve  77 . A non-return valve  94  is arranged between fluid inlet  90  and return line  76 . 
     As shown in  FIG. 3 , in the first control valve position, directional control valve  75  fluidly connects low-pressure input E to low-pressure chamber  73 . On the other hand, in the second control valve position, directional control valve  75  fluidly connects drain R to low-pressure chamber  73 . Therefore, in the first control valve position, low-pressure chamber  73  is supplied with hydraulic fluid at system pressure P, whereas in the second control valve position hydraulic fluid in low-pressure chamber  73  is discharged towards drain R. 
     A working cycle of exemplary pressure intensifier  21  will be explained in the following. 
     In an initial position of pressure intensifier  21 , intensifier piston  72  is fully retracted into low-pressure chamber  73 . In this state, intensifier piston  72  is not in contact with contact element  82  of switching valve  77 . Accordingly, switching valve  77  is in the position shown in  FIG. 3 , i.e., does not connect return line  76  to control chamber  93  of directional control valve  75 . Radial bore  87  is positioned between seals  85 ,  97  and fluidly connects high-pressure chamber  74  to control chamber  93  of directional control valve  75  via fluid inlet  90 . As second pressure receiving surface  92  of directional control valve  75  is greater than first pressure receiving surface  91 , which is exposed to fluid at system pressure P, and second pressure receiving surface  92  is also exposed to fluid at system pressure P via fluid inlet  90 , directional control valve  75  is in the position shown in  FIG. 3 . Accordingly, low-pressure chamber  73  is connected to low-pressure inlet E via directional control valve  75 . In this state, high-pressure chamber  74  is completely filled with hydraulic fluid at system pressure P. As the area of the bottom surface of intensifier piston  72  is greater than the annular front surface of wall  89  of the same, intensifier piston  72  begins moving towards high-pressure chamber  74 . 
     Accordingly, the pressure of the fluid in high-pressure chamber  74  increases, and the fluid at the increased pressure is supplied to hydraulic prop  8  via high-pressure output A. Once intensifier piston  72  has moved into high-pressure chamber  74  by a predetermined amount, radial bore  87  moves past seal  85 . Accordingly, control chamber  93  of directional control valve  75  is fluidly separated, and fluid at system pressure P remains inside control chamber  93 . Therefore, directional control valve  75  remains in the position that is shown in  FIG. 3 . In addition, low-pressure chamber  73  continues to be fluidly connected to low-pressure inlet E. Therefore, intensifier piston  72  continues to move into high-pressure chamber  74 . This configuration is shown in  FIG. 3 . 
     When intensifier piston  72  reaches a predetermined position, in particular, its end position in high-pressure chamber  74 , bottom surface  81  of intensifier piston  72  contacts contact element  82  of switching valve  77 . Due to this, control chamber  93  of directional control valve  75  is fluidly connected to drain R. Therefore, the pressure acting on pressure receiving surface  91  can move directional control valve  75  to its second valve position, to thereby fluidly connect low-pressure chamber  73  to drain R. 
     In some embodiments, the fluid connection between low-pressure chamber  73  and drain R can be via a hollow piston rod along which intensifier piston  72  moves. For example, the hollow piston rod may be connected to or integrally formed with directional control valve  75 . 
     In this configuration, fluid at system pressure P enters high-pressure chamber  74  and acts on the annular front surface of wall  89  of intensifier piston  72 . Accordingly, intensifier piston  72  moves towards low-pressure chamber  73 , and high-pressure chamber  74  is filled with fluid at system pressure P. In this state, control chamber of directional control valve  75  remains at the pressure of pressure sink T. Likewise, directional control valve  75  remains in its second valve position. 
     As soon as radial bore  87  passes seal  85 , control chamber  93  of directional control valve  75  is again fluidly connected to high-pressure chamber  74 . Accordingly, fluid at system pressure P acts on second pressure receiving surface  92 , resulting in that directional control valve  75  is again moved to its first valve position (the position that is shown in  FIG. 3 ). As a consequence, low-pressure chamber  73  is again fluidly connected to low pressure inlet E, and intensifier piston  72  again begins its movement into high-pressure chamber  74  to increase the pressure of fluid therein. 
     As will be readily appreciated by the skilled person, the reciprocating movement of intensifier piston  72  in housing  71  results in fluid at high pressure HP being delivered to pressure chamber  18  of hydraulic prop  8 , either until a maximum obtainable or allowable pressure is reached, or control unit  80  actuates control valve  41  when the set desired pressure for hydraulic prop  8  has been reached, in response to the measurement by pressure sensor  61 . 
     With the above-described configuration, a desired pressure profile can be obtained for the plurality of hydraulic props  8  of hydraulic support system  100  by controlling the individual pressure intensifiers  21  associated with the plurality of hydraulic props  8  in an appropriate manner. 
       FIG. 4  shows an alternative embodiment of hydraulic shield support system  100  including a plurality of pressure intensifiers  21  respectively associated with a plurality of hydraulic props  8 . The configuration of the system shown in  FIG. 4  is essentially the same as for the system shown in  FIG. 2 , such that only the differences will be described. 
     As shown in  FIG. 4 , the system in the alternative embodiment differs from the system shown in  FIG. 2  in that pressure intensifier  21  is fluidly connected in series between non-return valve  51  and control valve  41 . Accordingly, it is advantageous to provide an additional non-return valve  55  that is connected between control valve  41  and non-return valve  51  in parallel to pressure intensifier  21 . The reason for this is that sufficient flow-rate is required in order to avoid impacting the cycle time of pressure intensifier  21  in a negative manner. In some embodiments, the additional pressure intensifier non-return valve  55  has a flow-rate that is preferably greater than or equal to the flow-rate of non-return valve  51  and/or control valve  41 . Otherwise, the same effects that are obtained for the embodiment shown in  FIG. 2  can be obtained by the embodiment shown in  FIG. 4 . 
     INDUSTRIAL APPLICABILITY 
     The industrial applicability of the systems and methods disclosed herein will be readily appreciated from the foregoing discussion. One exemplary application is an application in an underground mining system, for example, in a self-advancing roof support system of an underground mining system. 
     It will be appreciated that the foregoing description provides examples of the disclosed systems and methods. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of disclosure more generally. All methods described herein may perform in any suitable order unless otherwise indicated herein or clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalences of the subject-matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or clearly contradicted by context. 
     Although the preferred embodiments of this disclosure have been described herein, improvements and modifications may be incorporated without departing from the scope of the following claims.