Abstract:
The present invention provides a pressure control apparatus for controlling the pressure at an output port without reducing the effective sectional area by controlling a main valve through the switching operation of a small-sized directional control valve, wherein the main valve has any of the following functions, i.e. a function of-providing a valve full-open position, a function of reducing pressure, and a function of providing a valve full-closed position. When a diaphragm chamber is placed at the secondary pressure by switching the directional control valve, the primary pressure is reduced by the main valve, and a predetermined reduced pressure is outputted. When the diaphragm chamber is at the primary pressure, the main valve is fully closed. When the diaphragm chamber is at the atmospheric pressure, the main valve fully opens, and hence the primary-side fluid flows to the output port as it is.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a pressure control apparatus for controlling the pressure at an output port by controlling a main valve through the switching operation of a directional control valve.  
           [0003]    2. Discussion of Related Art  
           [0004]    [0004]FIG. 5 shows a conventional pressure control apparatus for controlling the pressure at an output port to either of two pressures, i.e. high and low pressures, by controlling a main valve and a discharge valve through the switching operation of a directional control valve [see Japanese Utility Model Application Public Disclosure (KOKAI) No. Hei 1-72612]. In FIG. 5, a directional control valve  140  has a small input port  141 , a small output port  142  and a small discharge port  143 . In a state where the small output port  142  is communicated with the small discharge port  143 , a pressure fluid is supplied to a supply port P. A pressure regulating member  125  is in the position shown in the figure. The supplied pressure fluid opens a main valve element  114  to flow to an output port A and also passes through a feedback passage  120  to flow into a feedback chamber (diaphragm chamber)  119 . Consequently, the fluid pressure in the feedback chamber  119  rises. When the pressure in the feedback chamber  119  reaches a first set pressure (a set low pressure), a diaphragm  118  moves to a neutral position, causing the main valve ( 114 ,  113 ) to be closed. When the fluid pressure at the output port A becomes lower than the first set pressure, the main valve ( 114 ,  113 ) opens to allow the pressure fluid to flow to the output port A. When the fluid pressure at the output port A becomes higher than the first set pressure, the main valve ( 114 ,  113 ) is closed, and a discharge valve ( 122 ,  121 ) opens. Consequently, the pressure fluid at the output port A is discharged. In this way, the fluid pressure at the output port A is controlled to the first set pressure.  
           [0005]    Next, when the directional control valve  140  is switched to allow the small output port  142  to communicate with the small input port  141 , the pressure fluid from the supply port P flows into a pressure setting chamber  127  through the small input port  141  and the small output port  142 . Consequently, the fluid pressure in the pressure setting chamber  127  rises, causing the pressure regulating member  125  to move downward. The pressure regulating member  125  stops when it abuts against an engagement portion  136  of a high-pressure setting member  135 . The downward movement of the pressure regulating member  125  compresses a pressure setting spring  128  and hence increases the urging force of the pressure setting spring  128  acting on the diaphragm  118 . The increase in the urging force of the pressure setting spring  128  allows the fluid pressure at the output port A to be controlled to a second set pressure (a set high pressure).  
           [0006]    With the prior art shown in FIG. 5, a pressure fluid at either of the first and second set pressures is obtained, and each set pressure is controlled to a constant pressure by feedback control. However, the feedback control is not always needed for some use applications of the fluid pressure cylinder. A first example of such use applications is as follows. During a stroke L 1  of the full stroke L of the piston of the fluid pressure cylinder, the main valve is fully opened to move the piston at high speed, thereby shortening the stroke time. During the remaining stroke L 2 , the piston is moved at a set pressure to control pressing force (e.g. a caulking machine). Alternatively, during the stroke L 1 , the piston is activated at a set low pressure to ensure safety. During the remaining stroke L 2 , the main valve is fully opened to activate the piston at high pressure (e.g. an automatic door). In a second example, a fluid at a set pressure is supplied for a desired period of time. Thereafter, the flow of the fluid is stopped (e.g. an air blower or a fluid pressure motor). The prior art shown in FIG. 5 has functions more than is needed for the first and second examples. Therefore, application of the prior art to the first or second example is a waste of its capabilities.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention was made to eliminate the above-described waste.  
           [0008]    A first object of the present invention is to provide a pressure control apparatus for controlling the pressure at an output port by controlling a main valve through the switching operation of a directional control valve, wherein the main valve has any two or three of the following functions, i.e. a function of providing a valve full-open position, a function of reducing pressure, and a function of providing a valve full-closed position.  
           [0009]    A second object of the present invention is to simplify the arrangement of the above-described pressure control apparatus and to reduce costs.  
           [0010]    To attain the above-described objects, the present invention is applied to a pressure control apparatus having a main valve and a diaphragm provided in a pressure control apparatus body. The pressure control apparatus body is provided with a directional control valve (outside or inside the pressure control apparatus body or at an intermediate position between the outside and inside of it). The main valve is disposed in a passage for communication between a supply port and an output port. The diaphragm forms a diaphragm chamber. The main valve is urged in a direction in which it is closed by a valve spring. The main valve is also urged in a direction in which it opens by a pressure regulating spring acting on the diaphragm. Force acting on the diaphragm changes in accordance with the switching operation of the directional control valve, and hence the pressure at the output port changes.  
           [0011]    According to a first arrangement of the present invention, the diaphragm chamber and a small output port of the directional control valve are communicated with each other. In accordance with the switching operation of the directional control valve, the diaphragm chamber is placed at any one of the secondary pressure (pressure at the output port), the atmospheric pressure, and the primary pressure (pressure at the supply port). When the diaphragm chamber is at the secondary pressure, the primary pressure flows to the output port after being reduced to a set pressure by the main valve. When the diaphragm chamber is at the atmospheric pressure, the main valve fully opens, and hence the pressure at the output port becomes equal to the primary pressure. When the diaphragm chamber is at the primary pressure, the main valve is fully closed.  
           [0012]    According to a second arrangement of the present invention, a shell is disposed in the center of the diaphragm in the first arrangement. A stem is connected at the proximal end thereof to a main valve element of the main valve and disposed so that the distal end of the stem abuts against the shell.  
           [0013]    According to a third arrangement of the present invention, the directional control valve in the first or second arrangement has two small input ports. One of the small input ports is communicated with the atmosphere, and the other of the small input ports is communicated with the output port of the pressure control apparatus body.  
           [0014]    According to a fourth arrangement of the present invention, an adapter is provided on a side of the pressure control apparatus body in the third arrangement. The directional control valve is connected to the outside of the adapter. The small output port of the directional control valve is communicated with the diaphragm chamber through a first A output communicating passage formed in the adapter and a first B output communicating passage formed in the pressure control apparatus body. One of the small input ports of the directional control valve is communicated with the atmosphere through a first input communicating passage formed in the adapter. A second A input communicating passage is formed in the adapter, and a second B input communicating passage is formed in the pressure control apparatus body. The second A input communicating passage and the second B input communicating passage are communicated with each other through a communicating pipe. The other of the small input ports of the directional control valve is communicated with the output port of the pressure control apparatus body through the second A input communicating passage, the input communicating pipe and the second B input communicating passage.  
           [0015]    According to a fifth arrangement of the present invention, the directional control valve in the first or second arrangement has two small input ports. One of the small input ports is communicated with the supply port of the pressure control apparatus body. The other of the small input ports is communicated with the output port of the pressure control apparatus body.  
           [0016]    According to a sixth arrangement of the present invention, an adapter is provided on a side of the pressure control apparatus body in the fifth arrangement. The directional control valve is connected to the outside of the adapter. The small output port of the directional control valve is communicated with the diaphragm chamber through a first A output communicating passage formed in the adapter and a first B output communicating passage formed in the pressure control apparatus body. One of the small input ports of the directional control valve is communicated with the supply port of the pressure control apparatus body through a third A input communicating passage formed in the adapter and a third B input communicating passage formed in the pressure control apparatus body. A second A input communicating passage is formed in the adapter, and a second B input communicating passage is formed in the pressure control apparatus body. The second A input communicating passage and the second B input communicating passage are communicated with each other through a communicating pipe. The other of the small input ports of the directional control valve is communicated with the output port of the pressure control apparatus body through the second A input communicating passage, the input communicating pipe and the second B input communicating passage.  
           [0017]    In addition, the present invention is applied to a pressure control apparatus having a main valve and a diaphragm provided in a pressure control apparatus body. The pressure control apparatus body is provided with directional control valve (outside or inside the pressure control apparatus body or at an intermediate position between the outside and inside of it). The main valve is disposed in a passage for communication between a supply port and an output port. The diaphragm forms a diaphragm chamber. The main valve is urged in a direction in which it is closed by a valve spring. The main valve is also urged in a direction in which it opens by a pressure regulating spring acting on the diaphragm. Force acting on the diaphragm changes in accordance with the switching operation of the directional control valve, and hence the pressure at the output port changes.  
           [0018]    According to a seventh arrangement of the present invention, a central communicating passage is formed in the passage for communication between the main valve and the output port. Adapter connecting openings are formed in two side surfaces, respectively, of the pressure control apparatus body. The adapter connecting openings are communicated with the central communicating passage through respective pressure communicating passages. An adapter is connected to one of the adapter connecting openings. A pressure gauge is provided in the adapter. The pressure at the output port is transmitted to the pressure gauge through the central communicating passage and the pressure communicating passage. The other of the adapter connecting openings is closed with a blanking plate assembly.  
           [0019]    According to the first and second arrangements, the pressure control apparatus controls the pressure at the output port by controlling the main valve through the switching operation of the directional control valve. When the diaphragm chamber is at the secondary pressure, the primary pressure flows to the output port after being reduced to a set pressure by the main valve. When the diaphragm chamber is at the atmospheric pressure, the main valve is fully open. Consequently, the pressure at the output port becomes equal to the primary pressure. When the diaphragm chamber is at the primary pressure, the main valve is fully closed. Because the pressure control apparatus does not need a high-pressure setting device, a pressure-setting chamber, etc. as required in the prior art, the structure is simplified, and the costs are reduced.  
           [0020]    In the pressure control apparatus according to the third and fourth arrangements, when the diaphragm chamber is at the secondary pressure, the primary pressure flows to the output port after being reduced to a set pressure by the main valve. When the diaphragm chamber is at the atmospheric pressure, the pressure at the output port becomes equal to the primary pressure. The pressure control apparatus is applicable to a caulking machine, an automatic door, etc.  
           [0021]    In the pressure control apparatus according to the fifth and sixth arrangements, when the diaphragm chamber is at the secondary pressure, the primary pressure flows to the output port after being reduced to a set pressure by the main valve. When the diaphragm chamber is at the primary pressure, the main valve is fully closed. The pressure control apparatus is applicable to an air blower, an air motor, etc.  
           [0022]    In the pressure control apparatus according to the seventh arrangement, an adapter is connected to either of two adapter connecting openings, and the other adapter connecting opening is closed with a blanking plate assembly. Because the adapter can be connected to a desired one of the two adapter connecting openings, the degree of freedom with which to assemble the apparatus increases.  
           [0023]    Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.  
           [0024]    The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a sectional side view showing first and second embodiments of the pressure control apparatus according to the present invention.  
         [0026]    [0026]FIG. 2 is a sectional view taken along the line X-X in FIG. 1, showing the first embodiment.  
         [0027]    [0027]FIG. 3 is a sectional view taken along the line X-X in FIG. 1, showing the second embodiment.  
         [0028]    [0028]FIG. 4A is an explanatory view of the first embodiment in which a directional control valve is shown by symbols.  
         [0029]    [0029]FIG. 4B is an explanatory view of the second embodiment in which a directional control valve is shown by symbols.  
         [0030]    [0030]FIG. 5 is a sectional side view of a conventional pressure control apparatus. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    [0031]FIGS. 1, 2 and  4 A show a first embodiment of the pressure control apparatus according to the present invention. It should be noted that FIG. 2 is a sectional view taken along the line X-X in FIG. 1, in which the illustration of a bonnet  7  is partly omitted because the interior of the bonnet  7  is the same as in FIG. 1. The pressure control apparatus has a pressure control apparatus body  1  in which a main valve  2  and a diaphragm  4  are provided. The pressure control apparatus further has a directional control valve  5  provided, for example, outside the pressure control apparatus body  1 . The pressure control apparatus body  1  comprises a body member  6  and a bonnet  7 . The outer peripheral portion of the diaphragm  4  is held between the body member  6  and the bonnet  7 . A supply port  8  and an output port  9  are formed to open on two side surfaces, respectively, of the body member  6 . The main valve  2  is disposed in a passage for communication between the supply port  8  and the output port  9 . The main valve  2  has a main valve element and a main valve seat. The main valve  2  is urged in a direction in which it is closed by a valve spring  10 . More specifically, the body member  6  has a center hole  12  formed therein to open on the top thereof. The center hole  12  is in communication with the supply port  8 . The main valve seat is positioned at the bottom of the center hole  12 . A valve guide assembly  13  is disposed in the center hole  12 . A cover  14  and a cap  15  are disposed at the upper side of the valve guide assembly  13 . The valve guide assembly  13  has a valve retainer. The valve spring  10  is fitted between the valve retainer and the main valve element of the main valve  2 . The valve spring  10  urges the main valve element in a direction in which it comes in contact with the main valve seat.  
         [0032]    A diaphragm chamber  16  is formed by the diaphragm  4 . In other words, the diaphragm chamber  16  is formed between the diaphragm  4  and a body partition wall  17 . An insertion hole is formed in the center of the body partition wall  17 . A central communicating passage  18  is formed between a part of the body partition wall  17  around the insertion hole and the main valve seat of the main valve  2 . The central communicating passage  18  is in communication with the output port  9 . The proximal end of a stem  19  is connected to the center of the main valve element of the main valve  2 . The lower end portion of the stem  19  is slidably inserted in the insertion hole of the body partition wall  17  in an airtight manner. A shell  3  is secured to the center of the diaphragm  4  (in FIGS.  1  to  4 A and  4 B, a discharge valve is shown by reference numeral  3 ; however, it is assumed that, in actuality, a disk is provided on the upper side of the diaphragm  4 , and a spring retainer is provided on the lower side of the diaphragm  4 ). The stem  19  is arranged such that the distal end thereof is capable of abutting against the shell  3 . The valve spring  10  urges the shell  3  in a direction in which it is depressed through the stem  19 .  
         [0033]    A pressure regulating spring chamber  20  is formed below the diaphragm  4  in the bonnet  7 . A nut  21  is inserted in a lower end position of the pressure regulating spring chamber  20  in such a manner as to be unrotatable but reciprocatable. A pressure regulating spring  22  is fitted between the spring retainer of the shell  3  and the nut  21 . The pressure regulating spring  22  urges the main valve  2  in a direction in which it opens through the stem  19 . A pressure regulating screw  23  is in engagement with the internal thread of the nut  21 . The lower end of the pressure regulating screw  23  is connected to a handle  24 . Rotating the handle  24 , together with the pressure regulating screw  23 , causes the nut  21  to move upward or downward, thereby allowing the urging force of the pressure regulating spring  22  to be adjusted. A discharge port  25  is formed in the side wall of the bonnet  7 . The discharge port  25  provides communication between the pressure regulating spring chamber  20  and the atmosphere.  
         [0034]    As shown in FIG. 2, adapter connecting openings  26 A and  26 B are formed in two side surfaces, respectively, of the body member  6  of the pressure control apparatus body  1  (at respective positions 90 degrees off the supply port  8  and the output port  9  in a top plan view). The adapter connecting openings  26 A and  26 B are multi-stepped holes. That is, the adapter connecting opening  26 A has, in order from the outer side thereof, a large-diameter hole  27 A, a medium-diameter hole  28 A and a small-diameter hole  29 A formed adjacent to each other. Similarly, the adapter connecting opening  26 B has, in order from the outer side thereof, a large-diameter hole  27 B, a medium-diameter hole  28 B and a small-diameter hole  29 B formed adjacent to each other. The small-diameter holes  29 A and  29 B are in communication with the central communicating passage  18  through respective pressure communicating passages  30 A and  30 B. The medium-diameter holes  28 A and  28 B are in communication with the diaphragm chamber  16  through respective pressure communicating passages  33 A and  33 B. A connecting projection  34  of an adapter  31  is fitted into the large-diameter hole  27 A of the adapter connecting opening  26 A in an airtight manner, whereby the adapter  31  is connected to the adapter connecting opening  26 A. A hermetically sealing projection  36  of a blanking plate assembly  35  is fitted into the large-diameter hole  27 B, the medium-diameter hole  28 B and the small-diameter hole  29 B of the adapter connecting opening  26 B in an airtight manner, whereby the adapter connecting opening  26 B is hermetically sealed. It should be noted that the connecting projection  34  of the adapter  31  may be fitted into the large-diameter hole  27 B of the adapter connecting opening  26 B in an airtight manner, whereby the adapter  31  is connected to the adapter connecting opening  26 B. In this case, the hermetically sealing projection  36  of the blanking plate assembly  35  is fitted into the large-diameter hole  27 A, the medium-diameter hole  28 A and the small-diameter hole  29 A of the adapter connecting opening  26 A in an airtight manner, whereby the adapter connecting opening  26 A is hermetically sealed.  
         [0035]    A directional control valve  5  and a pressure gauge  38  are mounted on the outside of the adapter  31 . A mounting projection  39  of the pressure gauge  38  is fitted into a mounting hole  40  of the adapter  31  with a seal interposed therebetween. The adapter  31  has a multi-stepped communicating hole  41  for introducing the secondary pressure (pressure at the output port  9 ) into both the pressure gauge  38  and the directional control valve  5  and for other purposes. The multi-stepped communicating hole  41  has, in order from the inner side thereof, a large-diameter hole  42 , a medium-diameter hole  43  and a small-diameter hole  44  provided adjacent to each other. A communicating pipe  45  is fitted in the multi-stepped communicating hole  41  of the adapter  31  and the adapter connecting opening  26 A of the body member  6 . The left end of the communicating pipe  45  is fitted in the medium-diameter hole  43  of the adapter  31  in a hermetically sealed manner. The right end of the communicating pipe  45  is fitted in the small-diameter hole  29 A of the body member  6  in a hermetically sealed manner. The secondary pressure is introduced from the output port  9  into the pressure gauge  38  through the central communicating passage  18 , the pressure communicating passage  30 A, the internal passage of the communicating pipe  45 , the small-diameter hole  44  of the adapter  31  and the internal passage of the mounting projection  39 .  
         [0036]    As shown in FIGS. 2 and 4A, the directional control valve  5  has small input ports  46  and  47  and a small output port  48 . When a spool  49  is in an OFF position I shown in the figure, the small input port  47  is closed, and the small input port  46  and the small output port  48  are allowed to communicate with each other. When the spool  49  is switched to an ON position II against the urging force of a spring by fluid pressure or the like (including human power, machine, pilot pressure, and an electromagnetic operating method; the same shall apply hereinafter), the small input port  46  is closed, and the small input port  47  and the small output port  48  are allowed to communicate with each other. The adapter  31  has input communicating passages  50  and  51  and an output communicating passage  52 . The first input communicating passage (input communicating passage  50 ) provides communication between the small input port  46  and the atmosphere. The input communicating passage  51  provides communication between the small input port  47  and the small-diameter hole  44 . The output communicating passage  52  provides communication between the small output port  48  and the large-diameter hole  42 . Accordingly, the small input port  47  of the directional control valve  5  is in communication with the output port  9  through the second A input communicating passage (input communicating passage  51  and small-diameter hole  44 ) of the adapter  31 , the internal passage of the communicating pipe  45  and the second B input communicating passage (pressure communicating passage  30 A and central communicating passage  18 ) of the body member  6 . The small output port  48  of the directional control valve  5  is in communication with the diaphragm chamber  16  through the first A output communicating passage (output communicating passage  52  and large-diameter hole  42 ) of the adapter  31  and the first B output communicating passage (large-diameter hole  27 A, medium-diameter hole  28 A and pressure communicating passage  33 A) of the body member  6 .  
         [0037]    Next, the operation of the first embodiment will be described. When the directional control valve  5  is in the OFF position I, the diaphragm chamber  16  is in communication with the atmosphere through the first B output communicating passage of the body member  6 , the first A output communicating passage of the adapter  31 , the small output port  48  and the small input port  46  of the directional control valve  5  and the first input communicating passage. Consequently, the pressure in the diaphragm chamber  16  is equal to the atmospheric pressure. The urging force of the pressure regulating spring  22  is much greater than the urging force of the valve spring  10 . Accordingly, the urging force of the pressure regulating spring  22  causes the diaphragm  4 , the shell  3 , the stem  19  and the valve element of the main valve  2  to move upward against the urging force of the valve spring  10 . Thus, the main valve  2  is fully open. When a pressure fluid is supplied to the supply port  8 , the fluid passes through the main valve  2 , which is fully open, the central communicating passage  18  and the output port  9  and flows into a fluid cylinder (not shown). Because the main valve  2  is fully open, the secondary pressure at the output port  9  is equal to the primary pressure at the supply port  8 .  
         [0038]    When the spool  49  of the directional control valve  5  is switched to the ON position II, the small input port  46  is closed, and the small input port  47  is allowed to communicate with the small output port  48 . The secondary pressure flows from the output port  9  to the small input port  47  of the directional control valve  5  through the second B input communicating passage of the body member  6 , the internal passage of the communicating pipe  45  and the second A input communicating passage of the adapter  31  and further flows to the diaphragm chamber  16  through the small output port  48  of the directional control valve  5 , the first A output communicating passage of the adapter  31  and the first B output communicating passage of the body member  6 . Accordingly, the pressure in the diaphragm chamber  16  becomes equal to the secondary pressure. Consequently, the secondary pressure acts on the upper side of the diaphragm  4  with a large area, thereby generating downward force counteracting the urging force of the pressure regulating spring  22 . Thus, the pressure fluid reduced in pressure by the main valve  2  flows to the output port  9 . When the secondary pressure becomes equal to a set pressure, the diaphragm  4  is depressed to close the main valve  2 . When the secondary pressure becomes lower than the set pressure, the main valve  2  opens. Consequently, the pressure fluid from the supply port  8  is reduced in pressure, and the fluid reduced in pressure flows to the output port  9 . Thus, the pressure fluid supplied to the supply port  8  is reduced-in pressure to a set pressure by the main valve  2 . The fluid reduced in pressure to the set pressure flows into the fluid cylinder (not shown) through the output port  9 . The pressure control apparatus according to the first embodiment is applicable to a caulking machine, an automatic door, etc.  
         [0039]    [0039]FIGS. 1, 3 and  4 B show a second embodiment of the pressure control apparatus according to the present invention. It should be noted that FIG. 3 is a sectional view taken along the line X-X in FIG. 1 (second embodiment), in which the illustration of a bonnet  7  is partly omitted because the interior of the bonnet  7  is the same as in FIG. 1. The second embodiment differs from the first embodiment in the arrangement of portions that are communicated with the small input ports  46  and  47  and the small output port  48  of the directional control valve  5 . In the other respects, the second embodiment is the same as the first embodiment. In FIGS. 3 and 4B, the same members as those in FIGS. 2 and 4A are denoted by the same reference numerals as those in FIGS. 2 and 4A, and a description of these members will be omitted or given only briefly.  
         [0040]    In FIGS. 3 and 4B, the adapter  31  has a communicating projection  57 , and the body member  6  has a communicating recess  58 . The communicating projection  57  is fitted into the communicating recess  58 . The communicating recess  58  is communicated with the center hole  12  through an input communicating passage  59 . An end portion of an input communicating passage  54  (described later) is formed in the communicating projection  57 . Consequently, the input communicating passage  59  and the input communicating passage  54  are in communication with each other. The directional control valve  5  has small input ports  46  and  47  and a small output port  48 . When the spool  49  is in an OFF position I shown in the figure, the small input port  46  is closed, and the small input port  47  and the small output port  48  are allowed to communicate with each other. When the spool  49  is switched to an ON position II by a fluid pressure or the like against the urging force of a spring, the small input port  47  is closed, and the small input port  46  and small output port  48  are allowed to communicate with each other. The adapter  31  has input communicating passages  54  and  55  and an output communicating passage  56 . The small input port  46  of the directional control valve  5  is in communication with the supply port  8  through the third A input communicating passage (input communicating passage  54 ) of the adapter  31  and the third B input communicating passage (input communicating passage  59  and center hole  12 ) of the body member  6 . The small input port  47  of the directional control valve  5  is in communication with the small-diameter hole  44  through the input communicating passage  55  of the adapter  31 . The small output port  48  of the directional control valve  5  is in communication with the large-diameter hole  42  through the output communicating passage  56 .  
         [0041]    Next, the operation of the pressure control apparatus according to the second embodiment will be described. When the directional control valve  5  is in the OFF position I, the diaphragm chamber  16  is in communication with the output port  9  through the first B output communicating passage (pressure communicating passage  33 A, medium-diameter hole  28 A and large-diameter hole  27 A) of the body member  6 , the first A output communicating passage (large-diameter hole  42  and output communicating passage  56 ) of the adapter  31 , the small output port  48  and the small input port  47  of the directional control valve  5 , the second A output communicating passage (input communicating passage  55  and small-diameter hole  44 ) of the adapter  31 , the internal passage of the communicating pipe  45  and the second B output communicating passage (pressure communicating passage  30 A and central communicating passage  18 ) of the body member  6 . When a pressure fluid is supplied to the supply port  8 , the secondary pressure is reduced to a set pressure in the same way as in a case where the directional control valve  5  in the first embodiment is set in the ON position II.  
         [0042]    When the directional control valve  5  is switched to the ON position II, the small input port  47  is closed, and the small input port  46  is allowed to communicate with the small output port  48 . The pressure fluid from the supply port  8  flows into the diaphragm chamber  16  through the third B input communicating passage of the body member  6 , the third A input communicating passage of the adapter  31 , the small input port  46  and the small output port  48  of the directional control valve  5 , the first A output communicating passage of the adapter  31  and the first B output communicating passage of the body member  6 . The primary pressure flowing into the diaphragm chamber  16  causes the diaphragm  4  to move downward. Consequently, the main valve  2  is fully closed by the valve spring  10 . The pressure control apparatus according to the second embodiment is applicable to an air blower, an air motor, etc.  
         [0043]    The first and second embodiments each use a single two-position, three-port directional control valve. In the first embodiment, the diaphragm chamber  16  is placed at the secondary pressure or the atmospheric pressure. In the second embodiment, the diaphragm chamber  16  is placed at the secondary pressure or the primary pressure. However, two series-connected two-position, three-port directional control valves may be used to allow the diaphragm chamber  16  to be placed at any one of the secondary pressure, the atmospheric pressure, and the primary pressure.