Abstract:
A pneumatic control assembly includes a valve forming a bore that is openably blocked by a spring-biased, first plug. An inner rod is movable in the bore with a lower end engageable with the first blockage plug and an upper end forming a second plug. The inner rod is spring biased to separate from the first plug. An intermediate rod forms a channel having an opening confronting and separated from the second plug by a basing spring. A vent hole is defined in the intermediate rod and in fluid communication with the channel. A piston is formed around the intermediate rod between the opening and vent hole. An outer rod is supported on the piston by a resilient member. When the outer rod is forced toward the intermediate rod, the resilient member is deformed, driving the intermediate rod toward the inner rod and the second plug blocks the opening of the intermediate rod. The engagement between the intermediate and inner rods drives the inner rod to urge the first plug to open the bore. Working fluid is thus allowed to flow through the bore of the valve. When the outer rod is released, the first plug is returned and blocks the bore again. The opening of the intermediate rod is separated from the second plug to allow the working fluid residual in the bore to vent through the vent hole.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the control of hydraulic power devices and in particular to a pneumatic control assembly for automatically shutting down the supply of high pressure hydraulic fluid when the hydraulic power device is in a returning stroke so as to enhance the operation safety of the hydraulic device. 
     BACKGROUND OF THE INVENTION 
     Heavy power devices which output great work or are capable of moving heavy objects are commonly used in for example construction sites or steel workshops. Examples of the heavy power devices are rear-dump trucks and hydraulic crane tracks. To obtain a great power output, most of the heavy power devices are operated hydraulically. A hydraulic power system requires a pump to pressurize the hydraulic fluid and thus supply the high pressure hydraulic fluid that is needed in operating the hydraulic power device. The pump may be driven by means of an electrical motor or an engine. The pump has to be turned on before the hydraulic power device is operated or the pump has to maintain continuous operation in order to supply the high pressure hydraulic fluid. The pump has to be stopped once the supply of high pressure hydraulic fluid is uo longer needed and this may be done by means of for example a clutch or the like coupled between the pump and the motor/engine. 
     In a regular hydraulically operated device, a controller is provided for the operator to control the supply of the high pressure hydraulic fluid and the moving direction of the hydraulic device. Such a controller may be electrically or pneumatically operated. For certain hydraulic power devices, pneumatic power is more readily available for control purpose, such as a rear-dump truck which itself is equipped with an air compressor or similar device. In such a case, a pneumatic control assembly has advantages over the electrically operated controller. 
     The control assembly of a hydraulic power device usually comprises two parts, one of which controls the supply of the hydraulic fluid and the other controls the moving direction of the hydraulic power device. It often happens that when the operator switches the direction control to the retracting direction to move the hydraulic power device in the returning stroke which in certain cases requires no supply of the hydraulic fluid, the operator inadvertently leaves the power control in the engaged position which makes the pump continuing supplying the hydraulic fluid to the hydraulic power device. In such a case, damage to the hydraulic power device may occur or even worse, the hydraulic power device may be accidentally actuated and thus causing property and live casualty/damage. 
     To overcome such a problem, devices that couple the power control of a pneumatic control assembly to the direction control, especially in moving the hydraulic power device in the returning stroke, are known, such as U.S. Pat. No. 6,065,497 to the current applicant. The known device, however, comprises a direction control valve that has a complicated structure, increasing costs of manufacturing and maintenance. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a pneumatic control assembly comprising a direction control valve having a simple structure. 
     Another object of the present invention is to provide a pneumatic control assembly comprising a direction control valve having low costs of manufacturing and maintenance. 
     In accordance with the present invention, there is provided a pneumatic control assembly comprising a simplified direction control valve. The direction control valve comprises a body forming a bore and inlet and outlet ports in fluid communication with the bore. The bore has a throat openably closed by a spring-biased, first blockage plug. An inner rod is movable in the bore with a lower end engageable with the first blockage plug. The inner rod is spring biased to separate from the first blockage plug. An intermediate rod forms a channel having an opening defined in a lower end of the intermediate rod confronting a second blockage plug mounted to an upper end of the inner rod. The intermediate rod is spring biased to separate from the second blockage plug. A vent hole is defined in an upper end of the intermediate rod and in fluid communication with the channel. An expanded piston is formed around the intermediate rod between the opening and vent hole. An outer rod is supported by a biasing spring retained on the piston. When the outer rod is forced toward the intermediate and inner rods, the biasing spring thereof is compressed, driving the intermediate rod toward the inner rod. The opening of the intermediate rod is closed by the second blockage plug. The engagement drives the inner rod to move with the intermediate rod and urges the first blockage plug to open the throat. Working fluid is thus allowed to flow from the inlet port, through the bore, toward the outlet port. When the outer rod is released, the first blockage plug is returned by its biasing spring to block the throat. The opening of the intermediate rod is separated from the second blockage plug to allow the working fluid residual in the bore to vent through the vent hole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from the following description of a preferred embodiment thereof with reference to the attached drawings, wherein: 
     FIG. 1 is a perspective view showing a pneumatic control assembly constructed in accordance with the present invention; 
     FIG. 2 is a partially exploded perspective view of the pneumatic control assembly in accordance with the present invention, showing the detailed structure of a power control rod of the pneumatic control assembly; 
     FIG. 3 is a partially exploded perspective view of the pneumatic control assembly in accordance with the present invention, showing the detailed structure of a direction control rod of the pneumatic control assembly; 
     FIG. 3A is a cross-sectional view showing a second rod of an acting rod assembly of a direction control valve of the pneumatic control assembly of the present invention; 
     FIG. 3B is a cross-sectional view showing a third rod, as well as the second rod, of the acting rod assembly of the direction control valve of the pneumatic control assembly of the present invention; and 
     FIGS. 4-10 are cross-sectional views showing different operation conditions of the pneumatic control assembly of which the drawings which carry the suffix “A” are associated with a power control valve of the pneumatic control valve, suffix “B” a first direction control valve, and suffix “C” a second direction control valve. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings and in particular to FIG. 1, wherein a pneumatic control assembly in accordance with the present invention, is shown, the pneumatic control assembly comprises a power control rod  12  and at least one direction control rod  14 . The pneumatic control assembly is particularly suitable for the control of a hydraulic power device (not shown), which receives high pressure hydraulic fluid from a supply source (not shown) under the control of the power control rod  12  so as to move in a desired direction in response to the operation of the direction control rod  14 . 
     The power control rod  12  is movable between an engaged position and a disengaged position to establish and cut off the supply of the high pressure hydraulic fluid to the hydraulic power device. The direction control rod  14  is movable among a forward position, a neutral position and a backward position with the neutral position in between the forward position and the backward position. When the direction control rod  14  is moved in a first direction toward the forward position, the hydraulic power device is actuated to move in a “forward direction” in a working stroke. When the direction control rod  14  is moved in a second, opposite direction toward the backward position, the hydraulic device is driven in a “backward direction” in a returning stroke. The neutral position is a position where the hydraulic device is kept stationary temporarily. 
     The terms “forward direction” and “backward direction” as used herein are referred to two generally opposite directions that a hydraulic power device may be moved and may be regarded as the working stroke and returning stroke of the hydraulic power device. For example, a rear-dump truck is equipped with a hydraulic lifter to raise one side of a tipper container for dumping objects received therein. The “forward direction” may be the direction where the tipper container is raised by the hydraulic lifter (the working stroke), while the “backward direction” is the direction where the tipper container is lowered down back to its resting condition (returning stroke). The neutral position of the direction control rod  14  is thus associated with a condition where the hydraulic power device (the tipper container) moves neither in the forward direction, nor in the backward direction, but is kept stationary. 
     Although in the following description, only a single direction control rod is illustrated, yet it is apparent to those having ordinary skill to extend the following description to a pneumatic control assembly having more than one direction control rod. For example, there are cases where it requires actuating the hydraulic device to move in two orthogonal directions and in such cases, two direction control rods may be used to respectively move the hydraulic power device in the two orthogonal directions. The two direction control rods may share a common power control rod or alternatively, they may have respective power control rod associated therewith and these are all potentially envisioned by those skilled in the art and should be regarded as part of the invention. 
     Preferably, the power control rod  12  and the direction control rod  14  are arranged on and supported by a support member  16  which is fixed to a control panel of a control room or cabin (not shown). Fixing the support member  16  to the control panel may be achieved by any suitable known means, such as bolts (not shown) extending through holes  18  in the support member  16  and tightened to the control panel. 
     The power control rod  12  is mechanically coupled to a power control valve  20 . When the power control rod  12  is moved to the engaged position, the power control valve  20  is open and allows a working fluid (for example compressed air) to flow therethrough for activating the operation of the hydraulic fluid supply source that supplies the high pressure hydraulic fluid to the hydraulic power device. When the power control rod  12  is moved to the disengaged position, the power control valve  20  is closed, the flow of the working fluid (compressed air) is cut off and the supply of the high pressure hydraulic fluid to the hydraulic power device is stopped. 
     The direction control rod  14  is mechanically coupled to two direction control valves  22  and  24  that respectively control two hydraulic fluid ports (not shown) of the hydraulic power device by means of flows of the working fluid, which may be a gaseous fluid, such as compressed air, through the direction control valves  22  and  24 . For example, when the first direction control valve  22  is open, one of the hydraulic fluid ports of the hydraulic power device which may be for example a hydraulic cylinder is open to allow the high pressure hydraulic fluid to flow into the hydraulic power device and forcing the hydraulic power device to move in the working stroke for doing work, such as moving a heavy object in the forward direction. On the other hand, when the second direction control valve  24  is open which causes the other hydraulic fluid port of the hydraulic device to open, the high pressure hydraulic fluid inside the hydraulic power device may then be properly expelled out thereof, allowing the hydraulic power device to move in the backward direction in the returning stroke. To avoid the ports of the hydraulic power device to be opened at the same time, causing problem and confusion in controlling the hydraulic power device, when the direction control rod  14  is at the neutral position, both the first direction control valve  22  and the second direction control valve  24  are closed. When the direction control rod  14  is at the forward position, the first direction control valve  22  is opened and the second direction control valve  24  is closed. When the direction control rod  14  is at the backward position, the first direction control valve  22  is closed and the second direction control valve  24  is opened. In other words, the first and second direction control valves  22  and  24  may not be opened at the same time. 
     Since the supply of the high pressure hydraulic fluid is controlled by the power control valve  20 , when the first direction control valve  22  is opened (namely, the direction control rod  14  is at the forward position), the power control valve  20  must be opened too to supply the high pressure hydraulic fluid to the hydraulic power device. When the first direction control valve  22  is closed and the second direction control valve  24  is opened (namely, the direction control rod  14  is at the backward position), the power control valve  20  may be closed if the design of the hydraulic power device does not require a back pressure (namely, the hydraulic pressure acting on the hydraulic device to move it in the backward direction) to move the hydraulic device in the backward direction or the power control valve  20  may be opened if the hydraulic power device needs a back pressure to move in the backward direction. For example, in certain cases, the hydraulic power device may be moveable in the backward direction by being acted upon by the gravity and in such cases, the power control valve  20  may be closed. However, in other cases, a backpressure is needed for the hydraulic power device to move and work in the backward direction. 
     The power control rod  12  and the direction control rod  14  are coupled to each other whereby in the cases that no back pressure is needed to move the hydraulic power device in the backward direction, when the direction control rod  14  is moved from the neutral position toward the backward position, the power control rod  12  is driven thereby to shift from the engaged position toward the disengaged position in order to automatically cut off the supply of the high pressure hydraulic fluid. The shift of the power control rod  12  from the engaged position to the disengaged position is done in a sudden and quick “leap”. In other words, the power control rod  12 , once actuated to move beyond a pre-set transition point between the engaged position and the disengaged position, moves abruptly toward the disengaged position. Thus, when the direction control rod  14  is manually moved to the backward position, the power control rod  12  reaches the disengaged position earlier than the direction control rod  14  reaching the backward position. 
     The coupling between the power control rod  12  and the direction control rod  14  comprises a peg  26  (see FIG. 2) extending from the power control rod  12  in a direction toward the direction control rod  14  and a projection  28  (see FIG. 3) formed on the direction control rod  14  and positioned corresponding to the peg  26 . The peg  26  and the projection  28  are dimensioned and positioned so as to contact each other when the direction control rod  14  is moved from the neutral position toward the backward position. Thus, the projection  28  of the direction control rod  14  engages and drives the peg  26  of the power control rod  12  to move therewith, thereby moving the power control rod  12  toward the disengaged position. However, when the direction control rod  14  is moved from the neutral position toward the forward position, the peg  26  and the projection  28  do not interfere with each other whereby the movement of the direction control rod  14  toward the forward position does not affect or cause any movement of the power control rod  12  that is located at the engaged position. 
     The power control rod  12  is constructed so tat when it is moved from the engaged position toward the disengaged position by being driven by the movement of the direction control rod  14 , it moves substantially synchronously with the direction control rod  14  and once it reaches or gets slightly beyond a pre-set transition point between the engaged position and the disengaged position, the power control rod  12  leaps suddenly and quickly from the transition point toward the disengaged position, as mentioned above. Thus, the power control rod  12  moves much faster than the direction control rod  14  after the transition point and reaches the disengaged position much earlier than when the direction control rod  14  reaches the backward position. In this way, before the direction control rod  14  reaches the backward position, the supply of the high pressure hydraulic fluid to the hydraulic power device has already been cut off by the power control valve  20  controlled by the power control rod  12  reaching the disengaged position. 
     The quick movement of the power control rod  12  from the transition point toward the disengaged position imposes a greater momentum on the power control rod  12  so as to allow the power control rod  12  to overcome any likelihood of being stopped or slowed down by friction or other resistance applied to the power control rod  12  during its movement toward the disengaged position and the power control rod  12  may move in a non-stopped manner directly toward the disengaged position to effectively and positively cut off the supply of the high pressure hydraulic fluid. In other words, an operator of the pneumatic control assembly needs only to manually move the direction control rod  14  from the neutral position toward the backward position a distance sufficient to position the power control rod  12  slightly beyond the transition point, the power control rod  12  may then automatically “leap” to the disengaged position very quickly and the likelihood of being stopped midway between the transition point and the disengaged position is significantly reduced or eliminated. The time elapse between when the power control rod  12  reaches the disengaged position and when the direction control rod  14  reaches the backward position provides a safer way for operating the pneumatic control assembly in controlling the hydraulic power device. This will be further described. 
     The movements of the power control rod  12  and the direction control rod  14  are respectively guided and controlled by first and second elongated slots  160 ,  162  formed in the support member  16  with the power control rod  12  and the direction control rod  14  movably received therein and extending therethrough. 
     The first elongated slot  160  has a predetermined length and a predetermined width (which is the dimension in a direction normal to the length) and has two lengthwise ends respectively corresponding to the engaged position and the disengaged position of the power control rod  12 . The power control rod  12  comprises an elongated bar  164  having a cross-sectional size receivable within the width of the first elongated slot  160  so as to allow the power control rod  12  to be movable relative to and along the first elongated slot  160  between the two lengthwise ends which provides means for guiding the power control rod  12  to move between the engaged position and the disengaged position. 
     The lengthwise end of the first elongated slot  160  corresponding to the disengaged position comprises an expanded hole  166  having a diameter greater than the width of the first elongated slot  160 . The power control rod  12  comprises a hollow cylindrical locking member  168  which is movably fit over the elongated bar  164  with a biasing member, such as a helical spring  172  encompassing the elongated bar  164  received within the hollow interior space  170  of the locking member  168 . The elongated bar  164  has a threaded top end  174  to which an inner-threaded retainer  176  is threadingly mounted to retain the spring  172  and the locking member  168  on the elongated bar  164 . The spring  172  is pre-compressed between the retainer  176  and the locking member  168  to bias the locking member  168  toward the support member  16  and force an end of the locking member  168  that faces toward the support member  16  against the support member  16 . The locking member  168  comprises a locking ring  178  fixed to the end of the locking member  168  that faces toward the support member  16 . The locking ring  178  surrounds and is movable along the elongated bar  164 . The locking ring  178  has a diameter greater than the width of the first elongated slot  160 , but smaller than the diameter of the expanded hole  166  so that the locking ring  178  is supported and movable on the support member  16  when the power control rod  12  is moved along the first elongated slot  160 . Once the power control rod  12  reaches the disengaged position, the biasing spring  172  forces the locking ring  178  into the expanded hole  166  and thus locking the power control rod  12  at the disengaged position and prohibiting the power control rod  12  from moving relative to the first elongated slot  160 . This forms locking means for fixing the power control rod  12  at the disengaged position. To release the power control rod  12  from the disengaged position, manually forcing the locking member  168  against the biasing spring  172  toward the retainer  176  so as to disengage the locking ring  178  from the expanded hole  166  allows the power control rod  12  to resume relative movability with respect to the first elongated slot  160 . This is the un-locking operation of the power control rod  12 . 
     The locking means that fixes the power control rod  12  at the disengaged position provides a safety in operation, which prevents the power control rod  12  from being unexpectedly shifted to the engaged position by being accidentally contacted. Furthermore, the locking means also serves as a measure to stop and precisely position the power control rod  12  at the disengaged position when the power control rod  12  is quickly moved from the transition point to the disengaged position. 
     It should be noted that the power control rod  12  is not provided with a locking device or a large diameter hole at the end of the elongated slot  160  that corresponds to the engaged position. Thus, the power control rod  12  may be readily moved away from the engaged position. Such an arrangement allows the power control rod  12  to be moved from the engaged position toward the transition point with the movement of the direction control rod  14  by means of the engagement between the peg  26  of the power control rod  12  and the projection  28  of the direction control rod  14  when the direction control rod  14  is moved from the neutral position toward the backward position and no manual un-locking operation is needed. 
     Similarly, the direction control rod  14  comprises an elongated bar (not shown) extending through and movable along the second slot  162  to allow the direction control rod  14  to be selectively positioned in any one of the forward position, the neutral position and the backward position. The second slot  162  has two ends, respectively corresponding to the forward position and the backward position and each having an expanded hole formed thereon to serve as locking means. A third expanded hole is formed on the second slot  162  between the two ends of the second slot  162  to define the neutral position. Similar to the power control rod  12 , the direction control rod  14  is provided with a locking member  180  which is manually movable along the elongated bar of the direction control rod  14  for selectively engaging the expanded hole of any one of the forward position, the neutral position and the backward position to lock the direction control rod  14  threat. Such a locking member allows the operator to secure/release the direction control rod  14  at/from the forward position, the neutral position or the backward position by operating the locking member  180 . 
     Due to the fact that when the direction control rod  14  is moved to the backward position of the second slot  162 , it is secured threat by the locking member  180  and due to that the peg  26  of the power control rod  12  is engaged by the projection  28  of the direction control rod  14  when the direction control rod  14  is not released from the backward position, even though the power control rod  12  is released from the locked condition at the disengaged position, the power control rod  12  is still incapable of moving away from the disengaged position toward the engaged position by being held in position by the projection  28  of the direction control rod  14  which engages the peg  26  of the power control rod  12 . 
     Further, the direction control rod  14  is preferably provided with a biasing spring  29  (see FIG. 3) which helps urging the direction control rod  14  to the neutral position and securing the direction control rod  14  threat. Such an arrangement provides a further operation safety in case that the direction control rod  14  is moved to be very close to but not precisely locked at the backward position (due to the inadvertency of the operator). Under such a situation, if the power control rod  12  is accidentally moved toward the engaged position, owing to the engagement between the peg  26  of the power control rod  12  and the projection  28  of the direction control rod  14 , the direction control rod  14  is driven by the power control rod  12  toward the neutral position and by means of the biasing spring  29 , the direction control rod  14  is urged into and securely held in the neutral position when the power control rod  12  is approaching the engaged position. This provides a further operation safety of the pneumatic control assembly. 
     With reference to FIG. 2, which shows an exploded view of the power control valve  20  for the explanation of the power control valve  20  and also referring to FIG. 4A, the power control valve  20  comprises a cam holder  30  which is a hollow member having an interior space accommodating therein a cam  32 . The cam  32  is pivotally supported inside the cam holder  30  by means of a pivot  34  to be rotatable therein. The cam  32  has a camming surface  36  having a contour drivingly engaging an expanded end  50  of an acting rod  38  of the power control valve  20 . The expanded end  50  of the acting rod  38  serves as the cam follower of the cam  32 . The power control valve  20  comprises a body having a first section  40  and a second section  42 . The first section  40  has a central bore  43  with a circumferential shoulder  44  formed therein for supporting an end of a helical spring  48 . The spring  48  encompasses the acting rod  38  and has an opposite end supported on an under side of the expanded end  50  thereof for supporting the acting rod  38  inside the bore  43  of the first section  40 . The first section  40  is fixed to a lower opening (not shown) of the cam holder  30  to allow the expanded end  50  of the acting rod  38  to extend into the cam holder  30  for engaging the camming surface  36  of the cam  32 . The movement of the power control rod  12  between the engaged position and the disengaged position rotates the cam  32  about the pivot  34  which in turn drives the acting rod  38  against the spring  48  by means of the camming action between the camming surface  36  of the cam  32  and the expanded end  50  of the acting rod  38 . This moves the acting rod  38  relative to the valve body. 
     The second section  42  is fixed to the first section  40  to have a bore  52  of the second section  42  in fluid communication with the bore  43  of the first section  40 . A plug  56  is movably received within the bore  52  of the second section  42  and is supported by a spring  58  inside the bore  52 . The spring  58  has an end supported on a circumferential shoulder  95  inside the bore  52  and an opposite end engaging and supporting the plug  56 . The second section  42  is provided with an inlet port  60  that extends from the bore  52  to outside the valve body to allow the working fluid (gas) to flow into the bore  52  of the second section  42 . The plug  56  is biased by the spring  58  to block the connection between the bore  52  of the second section  42  and the bore  43  of the first section  40  (as shown in FIG. 4A) for preventing the working fluid from getting into the first section  40 . 
     The acting rod  38  has a length such that when the acting rod  38  is acted upon by the camming surface  36 , a remote end, which is the end of the acting rod  38  opposite to the expanded end  50  that engages the camming surface  36 , is driven to contact and urge the plug  56  away from and thus opening the connection between the bores  43  and  52 . As consequence, the working fluid is allowed to flow into the first section  40 . The first section  40  is provided with an outlet port  62  which is connected to external piping to conduct the working fluid that flows into the power control valve  20  toward a desired point to activate the supply of the high pressure hydraulic fluid. 
     To prevent leakages of the working fluid occurring in the first section  40 , the bore  43  of the first section  40  is provided with a sealing ring  64  surrounding the acting rod  38  to eliminate leakage between the acting rod  38  and the bore  43  of the first section  40 . A seal holder  46  is provided to fix the sealing ring  64  in position inside the bore  43 . 
     Thus, when the power control rod is manually moved from the disengaged position to the engaged position, the cam  32  that is coupled thereto is driven thereby to have the acting rod  38  moved against the spring  48  by means of the camming engagement between the camming surface  36  of the cam  32  and the expanded end  50  of the acting rod  38  and the remote end of the acting rod  38  engages the plug  56 . The engagement between the acting rod  38  and the plug  56  effectively moves the plug  56  against the spring  58  to open the fluid communication channel between the inlet port  60  and the outlet port  62  and as a consequence, the working fluid of the power control valve  20  is then conducted to activate the supply of the high pressure hydraulic fluid. 
     FIGS. 5A,  6 A,  7 A and  8 A show the power control valve  20  in the open condition and FIGS. 9A and 10A show the power control valve  20  in the closed condition. 
     The direction control valves  22  and  24  may have the same construction as the power control valve  20  with the only difference in the contours of the cams thereof because the timing of opening/closing the valves  20 ,  22  and  24  may not be the same. 
     Alternatively, the direction control valves  22  and  24  may be of a more sophisticated construction as shown in FIG.  3  and will be described hereinafter. However, since the construction of the two direction control valves  22  and  24  are taken as the same in the preferred embodiment illustrated herein, the description may be, in certain paragraphs, directed to one of the two direction control valves  22  and  24 , but is equally applicable to the other one of the two valves  22  and  24 . 
     The direction control valves  22  and  24  have a common cam holder  66 , which is a hollow member for receiving therein cams  68  and  70  of the direction control valves  22  and  24 . Since the first direction control valve  22  and the second direction valve  24  are designed not to open at the same time, the cams  68  and  70  have contours that are opposite to and preferably substantially symmetrical with each other about a contour center (see FIGS. 4B and 4C) which may be regarded as the neutral position of the direction control rod  14  and in this case, the backward position and the forward position of the direction control rod  14  is substantially symmetrical about the neutral position. 
     Although it is shown in the preferred embodiment illustrated in the drawings that the cams  68  and  70  of the first and second direction control valves  22  and  24  share the same cam holder  66 , it is possible to provide each of the cams  68  and  70  with an individual cam holder for independently accommodating the cams  68  and  70 . 
     The cams  68  and  70  are pivotally supported inside the cam holder  66  by means of a pivot  72  to be rotatable therein. In the preferred embodiment illustrated, the pivot  72  is common to both direction control valves  22  and  24 , but in the case that the cams  68  and  70  have their own cam holder, then there could be separate (but preferably co-axial) pivots for supporting the cams  68  and  70  within respective cam holders. 
     Each of the cams  68  and  70  is provided with a cam contour which defines a camming surface  74  or  76  engaging an acting rod assembly  78  of the respective one of the two direction control valves  22  and  24  for switching of the valves  22  and  24  between open condition and closed condition. The two cams  68  and  70  are commonly coupled to the direction control rod  14  so as to be controlled by the movement of the direction control rod  14  to rotate about the pivot  72  for opening/closing the direction control valves  22  and  24 . 
     The direction control valve  22  (direction control valve  24  being the same) comprises a valve body having a first section  80  and a second section  82 . The first section  80  defines a bore  84  having circumferential shoulder  86  formed therein serving as a stop to be described. The acting rod assembly  78  is movably received within the bore  84  of the first section  80 , comprising a first rod  88 , a second rod  90  and a third rod  92 . 
     The first rod  88  has an internal channel  94  for movably receiving a portion of the second rod  90  therein. The first rod  88  has a circumferential shoulder  96  formed on an outer surface thereof for retaining an end of a first spring  98  which has an opposite end supported on an expanded piston  100  (also see FIG. 3A) of the second rod  90 . A second spring  110  is arranged on the opposite side of the piston  100  and is retained between an underside of the piston  100  and a circumferential shoulder  862  formed in the bore  84  of the first section  80  for supporting the piston  100  and the second rod  90 . This will be further described. 
     The first rod  88  has an end on which a follower surface  102  engaging the camming surface  74  (or  76 ) of the cam  68  (or  70 ). When the direction control rod  14  is moved (for example from the neutral position to the forward position for the first direction control valve  22  or from the neutral position to the backward position for the second direction control valve  24 ), the first rod  88  of the acting rod assembly  78  is moved toward the second rod  90  and compresses the first spring  98  and thus the second spring  110 . The piston  100 , as well as the second rod  90 , is moved toward and eventually gets into contact with the circumferential shoulder  86  inside the bore  84  of the first section  80 , as shown in FIGS. 6B and 10C. The movement of the piston  100  is stopped by the shoulder  86 . 
     A seal ring  104  (FIG. 3A) is provided between the piston  100  and the bore  84  of the first section  80  for preventing leakages therebetween. A blind bore  112  is defined in the second rod  90  and extending in an axial direction of the rod  90 . The bore  112  forms a throat  115  in the lower end of the second rod  90 . A radially-extending hole  91  is defined in the second rod  90  and in communication with the blind bore  112 . 
     Also referring to FIG. 3B, the third rod  92  is movably received in the bore  84  of the first section  80  and has an expanded upper end  108  and a reduced lower end (not labeled) extendible through a lower opening or throat  87  of the bore  84  of the first section  80  and into a port  114  of an internal channel  106  of the second section  82 . A third spring  118  is retained between the expanded upper end  108  of the third rod  92  and a circumferential shoulder  861  for biasing the third rod  92  in a direction away from the port  114  of the second section  82 . Preferably, a C-ring or a clip  128  is mounted to the lower end of the third rod  92  for counteracting the biasing force of the third spring  118  thereby retaining the third rod  92  in position inside the bore  84  of the first section  80 . 
     A first blockage plug  116  is movably received in the port  114  of the second section  82  and biased by a spring  126  against and thus blocking the throat  87  of the bore  84  of the first section  80 . This makes the valve a normally closed valve. 
     The expanded upper end  108  of the third rod  92  defines a receptacle (not labeled) receiving and retaining a second blockage plug  120 . The second spring  110  that supports the second rod  90  separates the throat  115  of the bore  112  of the second rod  90  thereby opening the throat  115 . 
     The first spring  98  has a spring constant greater than that of the second and third springs  110  and  118 . When the direction control rod  14  is shifted from the neutral position to the forward position (for the first direction control valve  22 ) or the backward position (for the second direction control valve  24 ), the acting rod assembly  78  of the direction control rod  22  or  24  is depressed, causing the first rod  88  to move toward the second rod  90  and third rod  92  which compresses the first spring  98  and urges the piston  100  and the second rod  90  toward the third rod  92 . With the movement of the second rod  90  toward the third rod  92 , the throat  115  of the second rod  90  engages the second blockage plug  120  of the third rod  92 , driving the third rod  92  to move with the second rod  90  and eventually forcing the first blockage plug  116  away from and thus opening the throat  87  of the bore  84  of the first section  80 . The working fluid is thus allowed to flow from the second section  82  into the first section  80 . The engagement between the second rod  90  and the third rod  92  also blocks the throat  115  of the bore  112  of the second rod  90  to prevent the working fluid from flowing into the bore  112  and thus leaking through the radially-extending hole  91  of the second rod  90 . 
     The second section  82  is fixed to the first section  80 . An inlet port  122  is formed on the second section  82  and in fluid communication with the internal channel  106  and thus the port  114  of the second section  82 . An output port  124  is formed on the first second  80  and in fluid communication with the bore  84  of the first section  80 . The output port  124  is located between the piston  100  of the second rod  90  and the port  114  of the second section  82  whereby when the acting rod assembly  78  is actuated and drives the first blockage plug  116  to open the port  114 , the working fluid of the direction control valve  22  or  24  flows into the valve  22  or  24  via the inlet port  122  and flows out of the valve  22  or  24  via the outlet port  124  to control the direction of supply of high pressure hydraulic fluid to the hydraulic power device. 
     The configuration of the control valve provides means for preventing the related parts from being damaged due to over-pressure of the working fluid and this is commonly known as “constant pressure valve”. In case that when the first blockage plug  116  is opened and the working fluid (gas) flowing into the control valve has a pressure greater than a predetermined threshold which is dependent upon the first spring  98 , the total force that is applied by the gas pressure on the piston  100  will be greater than the biasing force of the first spring  98  and thus compress the first spring  98  to move the piston  100  upward. This reduces the compression of the second and third springs  110  and  118  caused by the first spring  98  and thus the first blockage plug  116  is allowed to move back to block the throat  87  of the first section  80 . Thus, any over-pressure flowing into the direction control valve  22  or  24  will cause the direction control valves  22  and  24  to be automatically shut off for protection purpose. 
     When the direction control rod  14  is moved to the neutral position, the first spring  98  is not compressed and the first blockage plug  116  blocks the through  87  of the first section  80 . The throat  115  of the second rod  90  is separated from the second blockage plug  120 , allowing the working fluid that is present inside the bore  84  of the first section  80  to flow out of the valve  22  or  24  via the radially-extending hole  91  and a vent hole  109  of the first section  80 . 
     To obtain the desired sealing effect, the blockage plugs  116 ,  120  are preferably made of elastically deformable materials, such as rubber. 
     Quite apparently, the valve configuration that was described with reference to the direction control valves  22  and  24  may also be adapted as the power control valve  20 . 
     In the attached drawings, FIGS. 4B,  5 B,  6 B,  7 B,  8 B,  9 B and  10 B are illustrations of the first direction control valve  22  of which FIG. 6B shows the open condition and the remaining drawings show the closed condition. FIGS. 4C,  5 C,  6 C,  7 C,  8 C,  9 C and  10 C are illustrations of the second direction control valve  24  of which FIG. 10C shows the open condition and the remaining Figures are the closed condition. Also, FIGS. 4A,  5 A,  6 A,  7 A,  8 A,  9 A and  10 A are corresponding drawings associated with the power control valve  20 . 
     The operation of the pneumatic control assembly of the present invention will be described with reference to FIGS. 4-10. As mentioned, the labels of these drawings with the suffix “A” are associated with the power control valve  20 , suffix “B” the first direction control valve  22 , and suffix “C” the second direction control valve  24 , each drawing being related with a predetermined angle of the valve with the angle shown on the drawing. The numbering of these figures is given in order for a predetermined operation of the valves and will be described in that order hereinafter. 
     Referring to FIG. 4, the power control rod  12  is located at the disengaged position which is referred to as −35 degrees with reference to a given reference base line (not shown) and the power control valve  20  is closed. The direction control rod  14  is located at the neutral position that is 10 degrees with respect to the reference base line with both the first and second control valves  22  and  24  closed. Next, in FIG. 5, the power control rod  12  is moved to the engaged position which, in the embodiment illustrated, is 5 degrees with respect to the reference base line to open the power control valve  20 , as shown in FIG.  5 A. The first and second direction control valves  22  and  24  are maintained closed. Thereafter, in FIG. 6, under the condition that the power control valve  20  is open, the direction control rod  14  is moved toward the forward position which is 30 degrees with respect to the reference base line and the first direction control valve  22  is open, as shown in FIG.  6 B. The power control valve  20  and the second direction control valve  24  remain unchanged. Next, the direction control rod  14  is moved back to the neutral position (10 degree position) and the first direction control valve  22  is closed, as shown in FIG.  7 B. (It should be noted this situation is exactly the same as that shown in FIG. 5.) 
     Then, the direction control rod  14  is moved from the neutral position (10 degree position) to the backward position that is −30 degrees with respect to the reference base line and when the direction control rod  14  is moved from the neutral position toward the backward position, the projection  28  that is provided on the direction control rod  14  is brought into contact with the peg  26  on the power control rod  12  and thus urges the power control rod  12  to move with the direction control rod  14 . The power control rod  12  is thus moved toward the transition point. As shown in FIG. 8, when the direction control rod  14  is moved backward a distance of 10 degrees which is measured as −10 degrees with respect to the reference base line, and reaches the position corresponding to 0 degrees with respect to the reference base line, the power control rod  12  is also driven to move −10 degrees and reaches the position corresponding to −5 degrees. In the embodiment illustrated, the −5 degree position defines the transition point of the power control rod  12 . 
     In accordance with the present invention, the camming surface  36  of the cam  32  of the power control valve  20  is designed in such a way that when the power control rod  12  is moved in the backward direction to reach the transition point (−5 degree position in this case), it will fast move to the disengaged position (−35 degree position) by means of the cam  32  and the biasing spring  48 . For example, the camming surface  36  may have a concave contour  36 ′ (see FIG. 4A) which provides no physical contact or forcible engagement with the expanded end  50  of the acting rod  38  or is ineffective in controlling the movement of the acting rod  38  so that it allows the acting rod  38  to fast move upward to close the power control valve  20 . As shown in FIG. 9, due to the quick movement of the power control rod  12  from the transition point toward the disengaged position, when the direction control rod  14  is moved toward the backward position, but does not exactly reach the backward position yet, the power control rod  12  will already reach the disengaged position (−35 degree position) earlier than the arrival of the direction control rod  14  at the backward position and thus a time elapse exists therebetween. 
     Further moving the direction control rod  14  in the backward direction will eventually have the direction control rod  14  arrive at the backward position (−30 degree position) and at this time, the second direction control valve  24  is open, as shown in FIG. 10C, and the power control valve  20  and the first direction control valve  22  are closed. 
     Although the invention has been described by means of the preferred embodiments thereof, it is apparent to those skilled in the art that many changes, variation and modifications are possible without departing from the scope of the invention as defined in the appended claims.