Abstract:
The drive device includes: a contact including a moving contact and a stationary contact; a rod connected to the moving contact; a piston which is connected to the rod and is slidably installed in a cylinder and which opens and closes the contact; a fluid pressure source for pressure-feeding a working fluid; and a control valve for driving the piston. In the drive device, the piston forms a partition between a supply pressure chamber communicated with the fluid pressure source and a small pressure-receiving area chamber which are on the moving contact side of the piston and a cylinder control chamber on the opposite side of the piston, and the control valve controls supplying and discharging the working fluid to and from the cylinder control chamber.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese Patent application serial no. 2013-172822, filed on Aug. 23, 2013, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a fluid-pressure drive device for a circuit breaker and, more particularly, to a fluid-pressure drive device for a circuit breaker suitable for suppressing a pressure rise when braking an opening operation. 
       BACKGROUND OF THE INVENTION 
       [0003]    An example of background technique in the field relevant to the present invention is disclosed in Japanese Unexamined Patent Publication No. 1989-279525. In the patent publication, it is stated that a liquid-pressure operation device for a circuit breaker is provided which offers sealing performance and component strength with reliability improved by removing a damper chamber contributing to a buffering effect toward the end of a piston stroke and by suppressing a pressure rise during a buffering action (see the abstract). 
         [0004]    In the liquid-pressure operation device disclosed in the patent literature, a flow control valve which is structured to decrease a flow-path cross-sectional area of high-pressure piping by moving a spool valve having a valve rod on each side thereof is installed in an intermediate portion of the high-pressure piping. Each valve rod extends to outside the spool valve casing via a packing, and one of the two valve rods is engaged, using a rotary lever, with a rod connected to a piston. In this structure, reducing the flow-path cross-sectional area requires use of another valve. Also, the device structure is complicated with the piston and one of the valve rods engaged with each other using the rotary lever. 
         [0005]    The present invention has been made in view of the above problems, and an object of the present invention is to provide a low-cost, high-reliability fluid-pressure drive device for a circuit breaker which has a simple structure for suppressing a pressure rise when the piston is braked toward the end of its stroke. 
       SUMMARY OF THE INVENTION 
       [0006]    To achieve the above object, the fluid-pressure drive device for a circuit breaker according to the present invention includes: a contact for passing and cutting off an electric current, the contact including a moving contact and a stationary contact; a rod connected to the moving contact; a piston which is connected to the rod and is slidably installed in a cylinder and which opens and closes the contact; a fluid pressure source for pressure-feeding a working fluid into the cylinder; and a control valve for driving the piston. In the fluid-pressure drive device: the piston forms a partition between a supply pressure chamber communicated with the fluid pressure source and a small pressure-receiving area chamber which are on the moving contact side of the piston and a cylinder control chamber on the opposite side of the piston; the control valve controls supplying and discharging the working fluid to and from the cylinder control chamber; and, when the piston starts opening operation, the cross-sectional area of a flow path formed, for the working fluid, between the supply pressure chamber and the small pressure-receiving area chamber increases compared with before the piston started moving and subsequently, when the piston is to be decelerated, decreases compared with immediately after the piston started moving. 
         [0007]    According to the present invention, a low-cost, high-reliability fluid-pressure drive device for a circuit breaker can be realized which has a simple structure for suppressing a pressure rise in a buffer chamber by changing the flow-path cross-sectional area according to movement of the piston. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a longitudinal sectional view of a fluid-pressure drive device for a circuit breaker according to a first embodiment of the present invention in a closed state. 
           [0009]      FIG. 2  is a longitudinal sectional view of the fluid-pressure drive device for a circuit breaker according to the first embodiment of the present invention in an initial stage of an opening operation. 
           [0010]      FIG. 3  is an enlarged view of a vicinity of the supply-side check valve shown in  FIG. 2 . 
           [0011]      FIG. 4  is a longitudinal sectional view of the fluid-pressure drive device for a circuit breaker according to the first embodiment of the present invention in a state during an opening operation. 
           [0012]      FIG. 5  is a longitudinal sectional view of the fluid-pressure drive device for a circuit breaker according to the first embodiment of the present invention in a final stage of an opening operation. 
           [0013]      FIG. 6  is a longitudinal sectional view of the fluid-pressure drive device for a circuit breaker according to the first embodiment of the present invention in an open state. 
           [0014]      FIG. 7  is a longitudinal sectional view of the fluid-pressure drive device for a circuit breaker according to the first embodiment of the present invention in a state during a closing operation. 
           [0015]      FIG. 8  is an enlarged view of a vicinity of the cylinder control chamber-side check valve shown in  FIG. 7 . 
           [0016]      FIG. 9  is a perspective view of a supply-side check valve included in the fluid-pressure drive device for a circuit breaker according to the first embodiment of the present invention. 
           [0017]      FIG. 10  is a longitudinal sectional view of a fluid-pressure drive device for a circuit breaker according to a second embodiment of the present invention in a closed state. 
           [0018]      FIG. 11  is a longitudinal sectional view of the fluid-pressure drive device for a circuit breaker according to the second embodiment of the present invention in a state during an opening operation. 
           [0019]      FIG. 12  is an enlarged view of a vicinity of the supply-side check valve shown in  FIG. 11 . 
           [0020]      FIG. 13  is a longitudinal sectional view of a fluid-pressure drive device for a circuit breaker according to a third embodiment of the present invention in a closed state. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Embodiments of the present invention will be described below with reference to drawings. Being described in the following are exemplary embodiments of the present invention, and they are not intended to limit the scope of the present invention. The present invention can be modified in various ways without departing from the scope of the invention as defined by the appended claims. 
       First Embodiment 
       [0022]    An embodiment of a fluid-pressure drive device for a circuit breaker  1  according to the present invention will be described below with reference to  FIGS. 1 to 9 .  FIG. 1  shows a fluid-pressure drive device for a circuit breaker in an energized closed state.  FIG. 2  shows the drive device in a stage of starting an opening operation.  FIG. 4  shows the drive device in an intermediate stage of an opening operation.  FIG. 5  shows the drive device in a final stage of an opening operation.  FIG. 6  shows the drive device in an open state.  FIG. 7  shows the drive device in an intermediate stage of a closing operation.  FIG. 9  is a perspective view of a check valve on the supply side. 
         [0023]    The fluid-pressure drive device for a circuit breaker  1  includes a rod  3  for opening/closing a contact  2 , a piston  10  connected to the rod  3 , a cylinder  11  in which the piston  10  slides, an accumulator  5  for accumulating a high-pressure working fluid, a fluid pressure source  4  for supplying a high-pressure fluid, and a directional control valve  6  serving as a control valve for pressure switching in the cylinder  11 . 
         [0024]    The piston  10  is slidable in the cylinder  11 , partitions the interior of the cylinder  11  into a small pressure-receiving area chamber  9  on the rod  3  side and a cylinder control chamber  17  on the opposite side, and is connected, via the rod  3 , to a moving contact  2   b  on the moving side of the contact  2 . The piston  10  has a projecting portion  10   b  on the cylinder control chamber  17  side thereof. The projecting portion  10   b  is shaped to be smaller in cross-sectional area toward the directional control valve  6 . 
         [0025]    The small pressure-receiving area chamber  9  is constantly subjected to the supply pressure of the working fluid released from the fluid pressure source  4  and accumulated in the accumulator  5 . The large pressure-receiving area side of the piston  10  making up the cylinder control chamber  17  is selectively connected, by the directional control valve  6 , to a high supply-pressure side or to a low-pressure return side connected to a reservoir  8 . 
         [0026]    The means of driving the directional control valve  6  is not particularly defined. The directional control valve  6  may be driven, for example, electromagnetically or using a pilot pressure. 
         [0027]    The reservoir  8  recovers and stores discharged fluid. In the present embodiment, pressure switching in the cylinder  11  is effected using the directional control valve  6 , but the pressure switching means is not limited to a directional control valve. An alternative structure may be adopted in which, for example, an opening control valve and a closing control valve are connected to the low-pressure return side and the high-pressure supply side, respectively. 
         [0028]    On the contact  2  side of the cylinder  11 , there is a supply-side guide member  12  which has a stepped convex portion including a small-diameter portion  12   g  and a medium-diameter portion  12   f.  The supply-side guide member  12  has, at its center, a through-hole  12   h  through which the rod  3  is inserted. The outer periphery of the medium-diameter portion  12   f  is fitted in the cylinder  11 . 
         [0029]    The through-hole  12   h  in the supply-side guide member  12  has a small-diameter portion  12   b,  a large-diameter portion  12   c , and a supply flow-path forming portion  12   d.  Of these portions: the small-diameter portion  12   b  is closest to the contact  2  and has the smallest diameter; the large-diameter portion  12   c  is on the cylinder control chamber  17  side of the small-diameter portion  12   b  and has the largest diameter; and the supply flow-path forming portion  12   d  is on the cylinder control chamber  17  side of the large-diameter portion  12   c  and has a diameter larger than that of the small-diameter portion  12   b  and smaller than that of the large-diameter portion  12   c . The supply flow-path forming portion  12   d  makes up a flow path for the fluid flowing from the supply pressure side toward the small pressure-receiving area chamber  9 . The large-diameter portion  12   c  makes up a supply pressure chamber  12   i  constantly supplied with a supply pressure. The small-diameter portion  12   b  makes up a sliding portion over which the rod  3  slides. 
         [0030]    The medium-diameter portion  12   f  of the supply-side guide member  12  has one supply through-hole  12   a  or more. The supply through-hole  12   a  is connected, via a cylinder supply path  11   a  formed in the cylinder  11 , to a supply path  7  through which the high-pressure working fluid from the accumulator  5  is supplied and is communicated with the supply pressure chamber  12   i . The supply pressure chamber  12   i  is kept at a high pressure equivalent to the pressure in the accumulator  5 . 
         [0031]    The rod  3  includes, from left to right as seen in  FIG. 1 , a contact-side sliding portion  3   d  having a uniform diameter, a decreasing-diameter portion  3   a  decreasing in diameter toward the piston  10 , a uniform small-diameter portion  3   c , and an increasing-diameter portion  3   b  gradually increasing in diameter toward the piston  10 . Each of the decreasing-diameter portion  3   a  and increasing-diameter portion  3   b  may be formed such that its diameter changes at a uniform rate or at stepped rates, i.e. stepped once or more or consecutively. 
         [0032]    The small-diameter portion  12   g  of the supply-side guide member  12  has one or more communication paths for check valve  12   e  allowing communication between inside and outside the small-diameter portion  12   g . A supply-side check valve  13  is provided between the outer periphery of the small-diameter portion  12   g  of the supply-side guide member  12  and the inner periphery of the cylinder  11  (see  FIG. 3 ). 
         [0033]    As shown in  FIG. 9 , the supply-side check valve  13  is cylindrically shaped and has, on the inner side thereof, a small-diameter portion  13   b  which is convex in a sectional view with an inner diameter smaller than that of the other portion. 
         [0034]    The outer diameter of the supply-side check valve  13  is smaller than the larger diameter of a step  11   b  formed on the contact  2  side of the cylinder  11  (i.e. the inner diameter of the cylinder portion on the contact  2  side of the step  11   b ). The supply-side check valve  13  also has plural communication paths  13   a  formed through a cylindrical portion thereof on one side (on the small pressure-receiving area chamber  9  side) of the small-diameter portion  13   b . The communication paths  13   a  allow communication between the inside and outside of the cylindrical portion of the supply-side check valve  13 . 
         [0035]    The supply-side check valve  13  is installed such that the communication path  13   a  side thereof comes on the step  11   b  side in the cylinder  11 . The inner diameter of the small-diameter portion  13   b  of the supply-side check valve  13  is larger than the outer diameter of the small-diameter portion  12   g  of the supply-side guide member  12  such that the small-diameter portion  13   b  is slidable without causing much fluid leakage. 
         [0036]    The supply-side check valve  13  is movable, by the pressure difference between the left and right sides thereof as seen in  FIG. 1 , in a stroke motion between an end portion of the medium-diameter portion  12   f  of the supply-side guide member  12  on the left side and the step  11   b  formed in the cylinder  11  on the right side. The small-diameter portion  13   b  of the supply-side check valve  13  is positioned such that the outside outlet of the communication path for check valve  12   e  is closer to the end portion of the medium-diameter portion  12   f  of the supply-side guide member  12  than the small-diameter portion  13   b  of the supply-side check valve  13 . 
         [0037]    In the above structure, the supply-side check valve  13  is moved by the pressure difference between the small pressure-receiving area chamber  9  side and the communication path for check valve  12   e  in the supply-side guide member  12 . When the pressure is higher on the communication path for check valve  12   e  side than on the small pressure-receiving area chamber  9  side, the pressure difference between the two sides moves the supply-side check valve  13  to the step  11   b  in the cylinder  11 . This causes the working fluid in the communication path for check valve  12   e  to flow into the small pressure-receiving area chamber  9  through between the end portion of the medium-diameter portion  12   f  of the supply-side guide member  12  and the supply-side check valve  13  and between the outer periphery of the supply-side check valve  13  and the inner periphery of the cylinder  11 , then through the communication paths  13   a  of the supply-side check valve  13  (see  FIG. 3 ). 
         [0038]    When the pressure is higher on the small pressure-receiving area chamber  9  side than on the communication path for check valve  12   e  side, the pressure difference between the two sides moves the supply-side check valve  13  until the valve is pressed against the medium-diameter portion  12   f  of the supply-side guide member  12 . As a result, the flow path between the end portion of the medium-diameter portion  12   f  of the supply-side guide member  12  and the supply-side check valve  13  is closed, so that no working fluid flows. Thus, the supply-side check valve  13  functions as a check valve to allow the working fluid to flow in one direction only. 
         [0039]    On the opposite side to the contact  2  of the cylinder  11 , there is a cylinder control chamber-side guide member  18  which has a two-stage convex portion including a small-diameter portion  18   c  and a medium-diameter portion  18   b . The cylinder control chamber-side guide member  18  has, at its center, a through hole  18   e  used as a flow path, and the outer periphery of the medium-diameter portion  18   b  is fitted into the cylinder  11 . 
         [0040]    On the piston  10  side of the through-hole  18   e  formed in the cylinder control chamber-side guide member  18 , a small-diameter through-hole portion  18   a  smaller in diameter than the through-hole  18   e  is provided. The through-hole  18   e  in the cylinder control chamber-side guide member  18  is communicated with outside the small-diameter portion  18   c  of the cylinder control chamber-side guide member  18  through a communication path for check valve  18   d.    
         [0041]    A cylinder control chamber-side check valve  15  is provided over the outer periphery of the small-diameter portion  18   c  of the cylinder control chamber-side guide member  18 . The cylinder control chamber-side check valve  15  is cylindrically shaped, and an end portion thereof is formed as a small-diameter portion  15   b  having an inner diameter smaller than that of the other portion. In a sectional view, the cylinder control chamber-side check valve  15  is, for example, L-shaped. The cylindrical portion of the cylinder control chamber-side check valve has plural communication paths  15   a  formed therethrough. The outer diameter of the cylinder control chamber-side check valve  15  is smaller than the larger diameter of a step  11   c  formed on the cylinder control chamber  17  side of the cylinder  11 . 
         [0042]    The inner diameter of the small-diameter portion  15   b  of the cylinder control chamber-side check valve  15  is larger than the outer diameter of the small-diameter portion  18   c  of the cylinder control chamber-side guide member  18  such that the small-diameter portion  15   b  is slidable without causing much fluid leakage. 
         [0043]    The cylinder control chamber-side check valve  15  is movable, by the pressure difference between the left and right sides thereof as seen in  FIG. 1 , in a stroke motion between the step  11   c  formed in the cylinder  11  on the left side and an end portion of the medium-diameter portion  18   b  of the cylinder control chamber-side guide member  18  on the right side. 
         [0044]    The cylinder control chamber-side check valve  15  is installed such that the small-diameter portion  15   b  thereof is positioned on the medium-diameter portion  18   b  side of the cylinder control chamber-side guide member  18 . When the cylinder control chamber-side check valve  15  is on the medium-diameter portion  18   b  side of the cylinder control chamber-side guide member  18 , the communication path for check valve  18   d  is blocked. 
         [0045]    In the above structure, the cylinder control chamber-side check valve  15  is moved by the pressure difference between the cylinder control chamber  17  side and the communication path for check valve  18   d . When the pressure is higher on the communication path for check valve  18   d  side than on the cylinder control chamber  17  side, the pressure difference between the two sides moves the cylinder control chamber-side check valve  15  to the step  11   c  formed in the cylinder  11 . This causes the working fluid in the communication path for check valve  18   d  to flow into the cylinder control chamber  17  through between the end portion of the medium-diameter portion  18   b  of the cylinder control chamber-side guide member  18  and the cylinder control chamber-side check valve  15  and between the outer periphery of the cylinder control chamber-side check valve  15  and the inner periphery of the cylinder  11 , then through the communication paths  15   a  of the cylinder control chamber-side check valve  15  (see  FIG. 8 ). 
         [0046]    When the pressure is higher on the cylinder control chamber  17  side than on the communication path for check valve  18   d  side, the pressure difference between the two sides moves the cylinder control chamber-side check valve  15  until the valve is pressed against the end portion of the medium-diameter portion  18   b  of the cylinder control chamber-side guide member  18 . As a result, the flow path between the end portion of the medium-diameter portion  18   b  of the cylinder control chamber-side guide member  18  and the cylinder control chamber-side check valve  15  is closed, so that no working fluid flows. Thus, the cylinder control chamber-side check valve  15  functions as a check valve to allow the working fluid to flow in one direction only. 
         [0047]    The operation of the fluid-pressure drive device for a circuit breaker according to the present embodiment will be described below. When, with the drive device in the closed state shown in  FIG. 1 , a command for opening operation is issued, the directional control valve  6  enters a state of opening operation in which the cylinder control chamber  17  is connected to the low-pressure reservoir  8  side as shown in  FIG. 2 . 
         [0048]    When the cylinder control chamber  17  is connected to the low-pressure side, the high pressure in the small pressure-receiving area chamber  9  causes the piston  10  to start moving in the direction for opening operation. This causes the pressure on the small pressure-receiving area chamber  9  side of the supply-side check valve  13  to lower and the supply-side check valve  13  to move toward the step  11   b  formed in the cylinder  11 . As a result, the working fluid is supplied from the supply pressure chamber  12   i  into the small pressure-receiving area chamber  9  through, as indicated by arrow  20  in  FIG. 3 , the communication path for check valve  12   e  of the supply-side guide member  12 , the outer periphery side of the supply-side check valve  13 , and the communication paths  13   a  of the supply-side check valve  13 . 
         [0049]    At the same time, the working fluid is also supplied from the supply pressure chamber  12   i  to the small pressure-receiving area chamber  9  through the flow path formed between the supply flow-path forming portion  12   d  of the supply-side guide member  12  and the increasing-diameter portion  3   b  of the rod  3 . This causes the piston  10  to be kept subjected to a driving force for movement in the direction for opening operation. 
         [0050]    Subsequently, when the piston  10  moves and the projecting portion  10   b  of the piston  10  enters, as shown in  FIG. 4 , the small-diameter through-hole portion  18   a  of the cylinder control chamber-side guide member  18 , a buffer chamber  17   b  surrounded by the outer periphery of the projecting portion  10   b , piston  10 , cylinder  11 , and control chamber-side guide member  18  is formed in the cylinder control chamber  17 . 
         [0051]    The diameter of the projecting portion  10   b  is gradually larger from the projection end portion toward the piston  10 . The diameter may change at a uniform rate or at stepped rates, i.e. stepped once or more or consecutively. 
         [0052]    The movement of the piston  10  causes a pressure difference between the cylinder control chamber  17  and the communication path for check valve  18   d , and the cylinder control chamber-side check valve  15  moves rightward as seen in  FIG. 4 . Thus, the flow path to the communication path for check valve  18   d  is closed. 
         [0053]    As a result, the buffer chamber  17   b  is closed except where a gap is formed between the projecting portion  10   b  and the small-diameter through-hole portion  18   a , and the pressure of the working fluid confined and compressed in the buffer chamber  17   b  starts rising, thereby generating a braking force against the piston  10 . The length of the projecting portion  10   b  is determined such that it approximately corresponds to the position where braking of the piston  10  is to be started. The diameter changing rate of the projecting portion  10   b  can be set so as to achieve a desired pressure increase. 
         [0054]    On the other hand, the decreasing-diameter portion  3   a  of the rod  3  gradually enters the supply flow-path forming portion  12   d  of the supply-side guide member  12 , so that the flow path formed between the supply flow-path forming portion  12   d  and the decreasing-diameter portion  3   a  is gradually narrowed. When the contact-side sliding portion  3   d  of the rod  3  subsequently enters the supply flow-path forming portion  12   d , the flow path is most narrowed. At the same time, with the communication path for check valve  12   e  of the supply-side guide member  12  communicated with the supply flow-path forming portion  12   d , the flow path leading to the supply-side check valve  13  is restricted. 
         [0055]    Thus, every flow path leading from the supply pressure chamber  12   i  to the small pressure-receiving area chamber  9  is narrowed. Since, in this state, the piston  10  is moving in the direction for opening operation, the pressure in the small pressure-receiving area chamber  9  greatly reduces compared with the pressure in the supply pressure chamber  12   i . Hence, the driving force applied to the piston  10  for movement in the direction for opening operation is greatly reduced. 
         [0056]    The pressure in the small pressure-receiving area chamber  9  and the deceleration of the piston  10  can be adjusted by adjusting the diameter changing rates for the decreasing-diameter portion  3   a  and the projecting portion  10   b . It is, therefore, possible to design the decreasing-diameter portion  3   a  and the projecting portion  10   b  such that the pressure in the small pressure-receiving area chamber  9  and the deceleration of the piston  10  fall within desired ranges, respectively. 
         [0057]    When it is desired to effect, with a reduced driving force, braking comparable to that effected with an unreduced driving force, the pressure increase required in the buffer chamber  17   b  can be suppressed. This allows the device to be made smaller and more reliable. In cases where a pressure increase is tolerable, the area required for braking, i.e. the pressure receiving area on the buffer chamber  17   b  side of the piston  10  can be reduced. In this way, design flexibility is increased. 
         [0058]    When, with the drive device in the open state shown in  FIG. 6 , a command for closing operation is issued, the directional control valve  6  enters a state of closing operation in which the cylinder control chamber  17  is connected to the high-pressure working fluid side as shown in  FIG. 7 . 
         [0059]    Subsequently, the pressure in the through-hole  18   e  in the cylinder control chamber-side guide member  18  rises, then the pressure in the communication path for check valve  18   d  in the cylinder control chamber-side guide member  18  rises. As a result, the cylinder control chamber-side check valve  15  moves to the step  11   c  side (leftward as seen in  FIG. 6 ) in the cylinder  11 . 
         [0060]    This causes the working fluid to flow into the cylinder control chamber  17  as indicated by arrow  21  in  FIG. 8 . At the same time, the working fluid also flows into the cylinder control chamber  17  through the flow path formed between the small-diameter through-hole portion  18   a  of the cylinder control chamber-side guide member  18  and the outer periphery of the projecting portion  10   b  of the piston  10 . This generates a driving force applied to the piston  10  for movement in the direction for closing operation. 
         [0061]    On the other hand, the working fluid in the small pressure-receiving area chamber  9  flows into the supply pressure chamber  12   i  through between the inner periphery of the supply flow-path forming portion  12   d  and the uniform small-diameter portion  3   c  of the rod  3 . The flow path used at this time puts up a resistance against the flowing working fluid. Since the closing operation is slow compared with the opening operation, the effect of the resistance is small, but it is necessary to secure a flow-path cross-sectional area large enough to achieve a prescribed closing operation speed. 
         [0062]    The supply-side check valve  13  is moved by the high pressure in the small pressure-receiving area chamber  9  to be pressed to the supply-side guide member  12  side, so that the flow of working fluid from the small pressure-receiving area chamber  9  to the supply pressure chamber  12   i  through the supply-side check valve  13  is blocked. When the piston  10  is further moved, the flow path formed between the inner periphery of the supply flow-path forming portion  12  and the increasing-diameter portion  3   b  starts being narrowed. As a result, in the small pressure-receiving area chamber  9 , a buffer chamber  9   b  is formed by the outer periphery of the increasing-diameter portion  3   b , the piston  10 , the inner periphery of the cylinder  11 , the supply-side guide member  12 , and the supply-side check valve  13 . 
         [0063]    The buffer chamber  9   b  is closed except where a gap is formed between the increasing-diameter portion  3   b  and the supply flow-path forming portion  12   d . As a result, the working fluid confined in the buffer chamber  9   b  is compressed, and the fluid pressure starts rising to generate a force for braking the piston  10 . The length of the increasing-diameter portion  3   b  is determined such that it approximately corresponds to the position where braking of the piston  10  is to be started. The diameter changing rate of the increasing-diameter portion  3   b  can be set so as to achieve a desired pressure increase. 
         [0064]    The above structure makes it possible to reduce, toward the end of an opening operation, the drive force applied to the piston  10  for movement in the direction for opening operation. This makes it possible to suppress rising of the pressure in the buffer chamber  17   b  formed in the cylinder control chamber  17 . Hence, the strength requirement for the fluid-pressure drive device can be lowered, the device can be made smaller, and the device reliability can be increased. 
       Second Embodiment 
       [0065]    A second embodiment of the present invention will be described below in which the flow path for high-pressure working fluid to the supply-side check valve differs from the flow path used in the first embodiment. 
         [0066]      FIG. 10  shows a fluid-pressure drive device for a circuit breaker  100  of the second embodiment. In the following, description will be omitted for structures and parts of the fluid-pressure drive device  100  which are denoted by reference numerals identical to those used in describing the fluid-pressure drive device for a circuit breaker  1  shown in  FIG. 1  or which have functions identical to those of the fluid-pressure drive device  1 . 
         [0067]    The second embodiment differs from the first embodiment in that the communication path for check valve  12   e  is open, at one end thereof, to the upstream side (the accumulator  5  side) of the supply through-hole  12   a  formed through the supply-side guide member  12  and in that the supply-side check valve  13  is structured differently from that of the first embodiment. 
         [0068]    The supply-side check valve  13  is cylindrically shaped and has, on the inner side of an end portion thereof, a small-diameter portion  13   f  having an inner diameter smaller than that of the other portion. In a sectional view, the supply-side check valve  13  is L-shaped. The cylindrical portion of the supply-side check valve  13  has plural communication paths  13   a  formed therethrough. The outer diameter of the supply-side check valve  13  is smaller than the larger diameter of a step  11   b  formed on the supply side of the cylinder  11 . 
         [0069]    The inner diameter of the small-diameter portion  13   f  of the supply-side check valve  13  is minutely larger than the outer diameter of the small-diameter portion  12   g  of the supply-side guide member  12  so as to make the supply-side check valve  13  slidable without causing much leakage. 
         [0070]    The supply-side check valve  13  is movable, by the pressure difference between the left and right sides, as seen in  FIG. 10 , in a stroke motion between the step  11   b  formed in the cylinder  11  on the left side and an end portion of the medium-diameter portion  12   f  of the supply-side guide member  12  on the right side. 
         [0071]    The supply-side check valve  13  is installed such that the small-diameter portion  13   f  thereof is closer, than the other portion thereof, to the end portion of the medium-diameter portion  12   f  of the supply-side guide member  12 . When the supply-side check valve  13  is positioned against the end portion of the medium-diameter portion  12   f  of the supply-side guide member  12 , the communication path for check valve  12   e  is blocked. 
         [0072]    The operation of the present embodiment will be described below. When, with the drive device in a closed state shown in  FIG. 10 , a command for opening operation is issued, the directional control valve  6  enters a state of opening operation in which the cylinder control chamber  17  is connected to the low-pressure reservoir  8  side as shown in  FIG. 11 . At this time, the supply-side check valve  13  is pressed, by the high pressure on the communication path for check valve  12   e  side, against the step  11   b  formed in the cylinder  11 . In this state, the high-pressure working fluid passes through the communication path for check valve  12   e , passes through between the supply-side check valve  13  and the end portion of the supply-side guide member  12 , passes over the outer periphery of the supply-side check valve  13 , and enters the small pressure-receiving area chamber  9  through the communication paths  13   a  of the supply-side check valve  13 . 
         [0073]    At this time, the working fluid is also supplied to the small pressure-receiving area chamber  9  from the supply flow-path forming portion  12   d . However, the flow path from the supply-side check valve  13  does not pass the supply chamber  12   i , so that it is possible to reduce, when starting the opening operation, the pressure loss caused between the cylinder supply path  11   a  and the small pressure-receiving area chamber  9 . The high-pressure working fluid continues to be supplied through the flow path indicated by arrow  21  in  FIG. 12  also toward the end of the opening operation. However, since the flow path cross-sectional area reduces in the supply flow-path forming portion  12   d , the total cross-sectional area of the flow paths formed on the supply-side check valve  13  side also reduces. A pressure loss can, therefore, be caused as in the first embodiment, making it possible to reduce the pressure on the small pressure-receiving area chamber  9  side. In other respects, the operation performed in the present embodiment is the same as the operation performed in the first embodiment, so that its description is omitted. 
         [0074]    According to the present embodiment, advantageous effects similar to those of the first embodiment can be achieved and, since the working fluid is supplied to the small pressure-receiving area chamber  9  through plural flow paths for opening operation, it is possible to reduce the pressure loss at the beginning of opening operation requiring a drive force. Hence, based on an equal drive force requirement, the fluid-pressure drive device can be made smaller. 
       Third Embodiment 
       [0075]    A third embodiment of the present invention will be described below in which the rod is shaped differently from the first embodiment. 
         [0076]      FIG. 13  shows a fluid-pressure drive device for a circuit breaker  200  of the third embodiment. In the following, description will be omitted for structures and parts of the fluid-pressure drive device  200  which are denoted by reference numerals identical to those used in describing the fluid-pressure drive device for a circuit breaker  1  shown in  FIG. 1  or which have functions identical to those of the fluid-pressure drive device  1 . 
         [0077]    The third embodiment differs from the first embodiment in that the rod  3  includes, from left to right as seen in  FIG. 13 , a contact-side sliding portion  3   d  which has a uniform diameter and slides against the supply-side guide member  12 , an increasing-diameter portion  3   e  increasing in diameter toward the piston  10 , a uniform large-diameter portion  3   f , a decreasing-diameter portion  3   g  decreasing in diameter toward the piston  10 , a uniform small-diameter portion  3   c  which is smallest in diameter, and an increasing-diameter portion  3   b  increasing in diameter toward the piston  10 . 
         [0078]    The operation according to the present embodiment is identical to the operation according to the first embodiment, so that its description is omitted. 
         [0079]    According to the present embodiment, the driving force generated is dependent on the pressure applied to the diameter difference between the maximum diameter of the piston  10  and the diameter of the contact-side sliding portion  3   d . Since the contact-side sliding portion  3   d  has a small diameter, a large driving force can be obtained. 
         [0080]    According to the present embodiment, advantageous effects similar to those of the first embodiment can be achieved. Even when the piston  10  is made smaller in diameter as compared with the piston  10  of the first embodiment, a driving force equivalent to that obtained according to the first embodiment can be obtained, so that designing flexibility is increased. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           1 ,  100 ,  200  Fluid-pressure drive device for circuit breaker 
           2  Contact 
           3  Rod 
           4  Fluid-pressure source 
           5  Accumulator 
           6  Directional control valve 
           7  Supply path 
           8  Reservoir 
           9  Small pressure-receiving area chamber 
           10  Piston 
           11  Cylinder 
           12  Supply-side guide member 
           13  Supply-side check valve 
           15  Cylinder control chamber-side check valve 
           18  Cylinder control chamber-side guide member