Abstract:
To protect a drive unit in a fluid valve assembly from impurities over a long term by preventing entrance of such impurities to the drive unit, a seal structure the sealing performance of which is hardly reduced is adopted around a moving part of the fluid valve assembly. The fluid valve assembly is provided with a valve member displaceably arranged in a flow passage through which a fluid is allowed to pass, a drive unit arranged outside the flow passage, a connecting member extending through a bore formed in a wall of the flow passage and connected at its end thereof to the valve member and at its opposite end to the drive unit, and a seal member arranged in gas- or water-tight relations with the wall of the flow passage and the connecting member and made of a flexible material such that the seal member does not interfere with a displacement of the valve member when the valve member is driven by the drive unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     a) Field of the Invention  
         [0002]     This invention relates to a fluid valve assembly for regulating the flow rate of a fluid or for a like purpose. Between a valve member and a drive unit for driving the valve member, the fluid valve assembly is provided with a seal structure that prevents entrance of the fluid from a flow passage, in which the valve member is arranged, to the side of the drive unit. In particular, the present invention is concerned with such a fluid valve assembly suitable for use in a valve unit of an exhaust gas recirculation system for recirculating to an intake side an exhaust gas emitted from an internal combustion engine such as an automotive vehicle.  
         [0003]     b) Description of the Related Art  
         [0004]     A fluid valve assembly which functions to regulate the flow rate of a fluid is provided with a drive unit for driving a valve member. Conventional fluid valve assemblies include those provided with drive units for driving their valve members, respectively. In each of such conventional fluid valve assemblies, there is a slide contact area between the valve member and a valve seat supporting the valve member thereon and defining an orifice of a flow passage therethrough. With the slide contact area serving as a boundary, its drive unit is arranged on an outer side of the flow passage. In some applications, it is desired to protect the drive unit from the fluid or from impurities contained in the fluid. Such fluid valve assemblies include, for example, exhaust gas recirculation control valves (hereinafter simply called “EGR (Exhaust Gas Recirculation) valves” for the sake of brevity).  
         [0005]     An EGR valve is used in an exhaust gas recirculation system for an internal combustion engine such as an automotive vehicle, and is arranged to inhibit the occurrence of harmful components in an exhaust gas by recirculating an appropriate amount of the exhaust gas to an intake side in accordance with a state of running of an automotive vehicle or the like.  
         [0006]     This EGR valve is effective not only for a gasoline engine but also for a diesel engine to lower NOx (nitrogen oxides) in an exhaust gas. In the case of a diesel engine, however, the setting of the amount of an exhaust gas, which is to be recirculated (hereinafter called “the amount of the EGR gas”), at an excessively large value induces an increase in the content of PM (particulate matter) in the exhaust gas, while the setting of the amount of the EGR gas at an unduly small value is unable to sufficiently lower the content of NOx in the exhaust gas.  
         [0007]     It is, therefore, necessary to perform an accurate control on the amount of an EGR gas (hereinafter called “EGR flow rate control”). This EGR flow rate control is generally performed by adjusting the opening of an EGR valve. As EGR valves, motor-driven EGR valves have been developed in recent years with a view to making improvements in the accuracy of control although pneumatic, pressure differential EGR valves making use of a positive pressure or a negative pressure are still in common use.  
         [0008]     A motor-driven EGR valve is high in the accuracy of control, and has made it possible to properly control the EGR flow rate. Nonetheless, the entrance of impurities such as SOx, unburned fuel components and soot to the interior of the motor develops inconveniences such as corrosion and sticking motor, and in the worst case, involves a potential problem of a stall of the motor.  
         [0009]     Described specifically, an EGR valve is arranged in an exhaust gas passage so that a poppet valve, butterfly valve or the like, which constitutes the EGR valve, is exposed to an exhaust gas. A motor which serves to drive the EGR valve is connected to such a poppet or butterfly valve or the like, and therefore, is in an environment where the exhaust gas can easily enter the interior of the motor. In particular, the exhaust gas is higher in pressure than the atmospheric pressure and is also high in temperature, so that the exhaust gas tends to enter the interior of the motor.  
         [0010]     There is, accordingly, a need for a structure that prevents the exhaust gas from entering the interior of the motor.  
         [0011]     As a conventional structure for preventing the entrance of an exhaust gas or the like to the side of a motor in a motor-driven EGR valve, there is, for example, a structure such as that illustrated in  FIG. 5 .  
         [0012]     As depicted in  FIG. 5 , this EGR valve is provided with an exhaust gas passage  11  as a part of an exhaust gas recirculation passage, a valve element  12  interposed as a valve member in the exhaust gas passage  11  at an intermediate location thereof, an electric motor  13  adapted as a drive unit to drive the valve element  12  in an axial direction via a valve shaft  14  of the valve element  12 , a bearing  15  supporting thereon the valve shaft  14  of the valve element  12 , and a casing  16  with the bearing  15  and valve shaft  14  accommodated therein. When the electric motor  13  is rotated, the valve element  12  is driven in the axial direction via a translation mechanism (not shown), which translates the rotation of the electric motor  13  to an axial motion, so that the exhaust gas passage  11  can be opened or closed or its opening can be adjusted.  
         [0013]     Between an inner circumferential wall of the casing  16  and an outer circumferential wall of the valve shaft  14 , a resin-made seal diaphragm  17  is interposed. This seal diaphragm  17  is in a substantially funnel-shaped form. Its outer circumferential portion is secured in a gas-tight fashion on the inner circumferential wall of the casing  16 , while its inner circumferential portion is maintained in sliding contact with the outer circumferential wall of the valve shaft  14 . Under a condition that the pressure of an EGR gas is higher than the atmospheric pressure, the seal diaphragm  17  is caused to expand like a kite by the pressure of the EGR gas as shown in  FIG. 5  so that its sealing performance is increased at a slide contact area between its inner circumferential portion and the outer circumferential wall of the valve shaft  14 .  
         [0014]     JP-A-5-187328, on the other hand, discloses a technique that provides an EGR valve with an impurities entrance blocking means, although the EGR valve is not of the motor-driven type but is of the pressure differential type.  
         [0015]     As illustrated in  FIG. 6 , this EGR valve is provided with an exhaust gas passage  101 , a valve element  102  interposed in the exhaust gas passage  101  at an intermediate location thereof, a diaphragm  103  to which the valve element  102  is connected, and a negative pressure chamber  104  for driving the valve element  102  in cooperation with the diaphragm  103 . By depressurizing the negative pressure chamber  104 , the valve element  102  is caused to move upward as viewed in  FIG. 6 , whereby a pintle  112  of the valve element  102  is separated from a valve seat  105  to open the exhaust gas passage  101 . By releasing the reduced pressure in the negative pressure chamber  104 , on the other hand, the valve element  102  is caused to move downwardly by a return spring  106  as viewed in  FIG. 6  so that the pintle  112  is brought into close contact with the valve seat  105  to close the exhaust gas passage  101 .  
         [0016]     A valve shaft  107  of the valve element  102 , which is a movable member, is provided with a shield plate  108 , and an impurities blocking member  110  is arranged between the valve shaft  107  and a guide member  109  through which the valve shaft  107  extends and slidingly reciprocates up and down.  
         [0017]     Opposing the guide member  109 , the shield plate  108  is fixedly secured on an approximately central part of the valve shaft  107  as viewed in the longitudinal direction. This shield plate  108  has an outer diameter dimensioned slightly smaller than an inner diameter of a cavity  111 , and can prevent an exhaust gas, which is passing through the exhaust gas passage  101 , from flowing to the side of the guide member  109  and can also reduce a radiation of heat from the exhaust gas toward the side of the guide member  109 .  
         [0018]     The impurities blocking member  110  has approximately U-shaped configurations as viewed in cross-section, and its inner and outer surfaces are formed with corrugations such that it is provided with high elasticity and is facilitated to undergo flexible deformations. As the material of the impurities blocking member  110 , a flexible, fluorinated resin is used for its excellent heat resistance and flexibility and its high abrasion resistance. The impurities blocking member  110  is provided at an upper circumferential edge thereof with a flange portion  110   a  molded integrally with the impurities blocking member  110 , and at a lower end portion thereof with an opening  110   c . The impurities blocking member  110  is disposed, with its flange portion  110   b  being fitted in an annular groove  111   a  formed adjacent the cavity  111  such that its upper surface is maintained in close contact with the guide member  109 , and with its lower end portion  110   b  being positioned close to the shield plate  108  and its opening  10   c  being fixedly secured on the outer circumferential wall of the valve shaft  107 .  
         [0019]     As readily appreciated from the foregoing description, the impurities blocking member  110  is arranged between the guide member  109  and the shield plate  108  and, even when the valve element  102  is in its open position, shields, in other words, covers the guide member  109 , including a slide contact area between an axial bore  109   a  and the valve shaft  107 , in a shielded state to maintain gas tightness, so that soot and the like in the exhaust gas are prevented from entering the side of the guide member  109 .  
         [0020]     However, the above-described conventional art is accompanied with problems as will be described next.  
         [0021]     In the case of the first conventional art depicted in  FIG. 5 , there is the slide contact area between the inner circumferential portion of the seal diaphragm  17  and the outer circumferential wall of the valve shaft  14 . As the use of the EGR valve goes on, the seal diaphragm  17  hence wears out at the slide contact area so that the sealing performance is progressively lowered.  
         [0022]     As a consequence, the exhaust gas which has entered through the slide contact area between the valve shaft  14  and the bearing  15  (see arrows A 1 ,A 2 ) is allowed to enter the side of the electric motor  13  through the slide contact area between the inner circumferential wall of the seal diaphragm  17  and the outer circumferential wall of the valve shaft  14  (see arrows A 3 ). As the EGR valve is used for a longer time, the abrasion wear of the seal diaphragm  17  increases, eventually resulting in a potential problem that the seal diaphragm  17  may fail to exhibit its sealing function and the motor  13  may be damaged accordingly. Even when an abrasion-resistant material such as polytetrafluoroethylene, for example, “TEFLON” (registered trademark) is used for the seal diaphragm  17 , it is still difficult to inhibit the reduction in sealing performance.  
         [0023]     In the case of the second prior art illustrated in  FIG. 6 , on the other hand, such a slide contact area is included at neither the inner circumference nor the outer circumference of the impurities blocking member  110 . Taking a look at the inner circumference of the impurities blocking member  110 , however, the lower end portion  110   b  is fixedly secured only at an edge portion of the opening  10   c  on the outer circumferential wall of the valve shaft  107 . There is, accordingly, a potential problem that the inner circumference of the impurities blocking member  110  may separate from the outer circumferential wall of the valve shaft  107  by the high-pressure, high-temperature EGR gas. Once such separation takes place, the impurities blocking member  110  can no longer exhibit its sealing function, leading to a potential problem that the motor may be damaged.  
         [0024]     These problems are not limited to EGR valves, but may occur on any of various valves insofar as they include in the proximity of a movable member a drive means which should be protected from a fluid or from impurities or the like in a fluid (a means corresponding to an EGR valve drive motor).  
       SUMMARY OF THE INVENTION  
       [0025]     The present invention has been created in view of the above-mentioned problems, and has as an object thereof the provision of a fluid valve assembly in which the entrance of impurities to a drive unit to be protected can be prevented over along term by adopting around a movable member a sealing structure the sealing performance of which is hardly reduced.  
         [0026]     In one aspect of the present invention, there is provided a fluid valve assembly comprising: 
        a valve member displaceably arranged in a flow passage through which a fluid is allowed to pass;     a drive unit arranged outside the flow passage;     a connecting member extending through a bore formed in a wall of the flow passage, and connected at an end thereof to the valve member and at an opposite end thereof to the drive unit; and     a seal member arranged in gas- or water-tight relations with the wall of the flow passage and the connecting member and made of a flexible material such that the seal member does not interfere with a displacement of the valve member when the valve member is driven by the drive unit.        
 
         [0031]     When the connecting member undergoes a displacement as a result of a displacement of the valve member driven by the drive unit, the fluid or impurities in the fluid may leak out from the side of the end (which may hereinafter be called “the lower end”) of the connecting member (i.e., the side of the valve member) to the side of the opposite end (which may hereinafter be called “the upper end”) of the connecting member (i.e., the side of the drive unit). This fluid or these impurities in the fluid are, however, prevented from entering the side of the drive unit by the seal member arranged in the gas- or water-tight relations with the wall of the flow passage and the connecting member and made of the flexible material. It is, therefore, possible to protect the drive unit from the fluid or the impurities in the fluid over a long term.  
         [0032]     Preferably, the fluid valve assembly may further comprise a communication hole formed such that an interior of the flow passage, said interior facing the seal member, is kept in communication with an exterior of the flow passage.  
         [0033]     The communication hole can be formed, for example, through the wall of the flow passage at a location between a bearing, on and along which the connecting member is slidably supported, and the seal member.  
         [0034]     For example, the flow passage can be an exhaust gas recirculation passage for reciculating an exhaust gas of an internal combustion engine to an intake side, the valve member can be an exhaust gas recirculation valve capable of adjusting a flow rate of the exhaust gas through the exhaust gas recirculation passage, and the drive unit can be an electric motor for displacing the exhaust gas recirculation valve.  
         [0035]     Preferably, the seal member can be held in place between a step portion of a large-diameter section arranged on an inner circumferential surface of the wall and an end face of a fitted member fitted in the large-diameter section. Alternatively, the seal member can be held in place between two ring-shaped members fitted in an annular groove formed in an inner circumferential surface of the wall.  
         [0036]     In another aspect of the present invention, there is also provided a method for manufacturing a fluid valve, which comprises: 
        displaceably arranging a valve member in a flow passage through which a fluid is allowed to pass;     arranging a drive unit outside the flow passage;     arranging a connecting member to extend through a bore formed in a wall of the flow passage, and connecting the connecting member at an end thereof to the valve member and at an opposite end thereof to the drive unit; and     arranging a seal member in gas- or water-tight relations with the wall of the flow passage and the connecting member such that the seal member does not interfere with a displacement of the valve member when the valve member is driven by the drive unit.        
 
         [0041]     According to the manufacturing method, it is possible to efficiently manufacture with high accuracy the above-described fluid valve assembly in which the drive unit can be protected from the fluid or impurities in the fluid over a long term.  
         [0042]     Preferably, the manufacturing method may further comprise forming a communication hole such that an interior of the flow passage, said interior facing the seal member, is kept in communication with an exterior of the flow passage.  
         [0043]     The communication hole can be formed, for example, through the wall of the flow passage at a location between a bearing, on and along which the connecting member is slidably supported, and the seal member.  
         [0044]     For example, the flow passage can be an exhaust gas recirculation passage for recirculating an exhaust gas of an internal combustion engine to an intake side, the valve member can be an exhaust gas recirculation valve capable of adjusting a flow rate of the exhaust gas through the exhaust gas recirculation passage, and the drive unit can be an electric motor for displacing the exhaust gas recirculation valve.  
         [0045]     Preferably, the manufacturing method may further comprise fitting a fitted member in a large-diameter section arranged on an inner circumferential surface of the wall such that the seal member is held in place between a step portion of the large-diameter section and an end face of the fitted member. Alternatively, the manufacturing method may further comprises fitting two ring-shaped members in an annular groove formed in an inner circumferential surface of the wall of the flow passage such that the seal member is held in place between the ring-shaped members. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]      FIG. 1  is a schematic cross-sectional view of an EGR valve assembly (fluid valve assembly) with impurities entrance preventing structure as one embodiment of the present invention;  
         [0047]      FIG. 2  is a schematic construction diagram of an automotive engine equipped with an EGR system in which the EGR valve assembly with the impurities entrance preventing structure as the one embodiment of the present invention can be used;  
         [0048]      FIG. 3  is a schematic fragmentary cross-sectional view of an EGR valve assembly (fluid valve assembly) with impurities entrance preventing structure as a first modification of the one embodiment of the present invention;  
         [0049]      FIG. 4  is a schematic fragmentary cross-sectional view of an EGR valve assembly (fluid valve assembly) with impurities entrance preventing structure as a second modification of the one embodiment of the present invention;  
         [0050]      FIG. 5  is a schematic cross-sectional view of an EGR valve assembly (fluid valve assembly) with impurities entrance preventing structure according to a first conventional technique; and  
         [0051]      FIG. 6  is a schematic cross-sectional view of an EGR valve assembly (fluid valve assembly) with impurities entrance preventing structure according to a second conventional technique. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0052]     With reference to  FIGS. 1 through 4 , a description will hereinafter be made about the one embodiment of the present invention and its modifications.  
         [0053]     In this embodiment, the fluid valve assembly will be described by taking an EGR valve as an example.  
         [0054]     As illustrated in  FIG. 2 , an exhaust gas recirculation control system (hereinafter simply called “EGR (Exhaust Gas Recirculation) system” for the sake of brevity) is arranged, for example, in an automotive engine or the like. This EGR system is constructed of an EGR passage (exhaust gas recirculation passage)  4  arranged between an exhaust passage  2  and an intake passage of an engine (internal combustion engine)  1 , an EGR valve (exhaust gas recirculation valve)  5  interposed in the EGR passage  4  at an intermediate location thereof, and an ECU (electronic control unit)  6  as a controller for adjusting the opening of the EGR valve  5 . As a result of combustion in individual combustion chambers of the engine  1 , an exhaust gas is produced, and is then emitted into the exhaust passage  2  from an exhaust manifold  7 . Owing to the arrangement of the EGR system, a portion of the exhaust gas is recirculated depending on the opening of the EGR valve  5  through the EGR passage  4 , and is fed back to the respective combustion chambers from the intake manifold  8 . As the EGR valve  5 , a motor-driven EGR valve is used from the viewpoint of improvements in the accuracy of control.  
         [0055]     As illustrated in  FIG. 1 , the EGR valve  5  is provided with an exhaust gas passage  11  as a part of the exhaust gas recirculation passage  4 , a valve element  12  composed of a pintle (valve member)  14   a  and a valve shaft  14  and interposed in the exhaust gas passage  11  at an intermediate location thereof, an electric motor  13  connected to the valve shaft  14  of the valve element  12  and adapted as a drive unit to drive the valve element  12  in an axial direction, a bearing  15  supporting thereon the valve shaft  14  of the valve element  12 , and a casing  16  with the bearing  15  and valve shaft  14  accommodated therein. Incidentally, the valve shaft  14  corresponds to the connecting member that connects the pintle (valve member)  14   a  and the electric motor  13  together, and is arranged extending through a bore (axial bore)  15   a  of the bearing  15 .  
         [0056]     The exhaust gas passage  11  is composed of an upstream passage section  11   a  and a downstream passage section  11   b , with an orifice  11   c  being formed therebetween such that the orifice  11   c  can be operatively opened or closed by the pintle  14   a.    
         [0057]     The valve element  12  is provided on an end (a lower end as viewed in  FIG. 1 ) of the valve shaft  14  with the pintle  14   a  as an acting part, and an intermediate portion of the valve shaft  14  is constructed as a slide contact portion  14   b  that is brought into sliding contact with the bearing  15  while being supported by the bearing  15 . To the opposite end (an upper end as viewed in  FIG. 1 ) of the valve shaft  14 , the electric motor  13  is connected. Incidentally, the pintle  14   a  is formed in mushroom-shaped configurations.  
         [0058]     The electric motor  13  is a conventional electric motor. Inside a motor casing  13   a , a coil  13   b  is arranged in an outer circumferential part and a metal core  13   c  is disposed as a rotor in an inner circumferential part. Internal threads are formed in a central portion of the metal core  13   c , while external threads are formed on the opposite, i.e., upper end portion of the valve shaft  14  or a shaft member coaxially connected to the opposite, i.e., upper end portion. The internal threads on the side of the metal core  13   c  and the external threads on the side of the valve shaft  14  are maintained in threaded engagement to make up a translation mechanism such that the valve shaft  14  is caused to move in the axial direction when the metal core  13   c  rotates. By energizing the coil  13   b  as needed, the metal core  13   c  is caused to rotate as much as needed in a predetermined direction such that the valve shaft  14  can be brought to a predetermined axial location.  
         [0059]     The pintle  14   a  and a portion of the valve shaft  14 , said portion being on the side of the lower end of the valve shaft  14  relative to the slide contact portion  14   b  of the valve shaft  14 , are inserted in the exhaust gas passage  11 . As the pressure of the exhaust gas in the exhaust gas passage  11  becomes higher than the atmospheric pressure, the exhaust gas may leak out along the slide contact portion  14   b  of the valve shaft  14  from the side of the lower end of the valve shaft  14  (the side of the exhaust gas passage  11 ) to the side of the upper end of the valve shaft  14  (the side of the electric motor  13 ) as indicated by arrows a 1 ,a 2 . However, a substantially disk-shaped, flexible seal member  21  which functions as a fluid entrance blocking member is arranged between the bearing  15  in the casing  16  (the wall of the passage  11 ) and the electric motor  13  such that the seal member  21  serves to prevent the exhaust gas, which has leaked out along the slide contact portion  14   b  and has entered a chamber  34 , from entering the side of the electric motor  13  through a chamber  35 .  
         [0060]     The seal member  21  is formed in the substantially disk-shaped configurations (is in the form of a circular sheet), is flexible, and is provided at a central part thereof with a hole through which the valve shaft  14  extends. Its outer circumferential edge portion  21   a  is connected to an inner circumferential wall of the casing  16 , while its inner circumferential edge portion  21   b  is connected to an outer circumferential wall of the valve shaft  14 . Especially at these connected parts, the seal member  21  is sandwiched in a gas-tight fashion (or gas-tight and liquid-tight fashion) from both sides thereof by sandwiching members  22 , 23 , 24 , 25 , respectively. In the case of the valve for liquids, a liquid-tight fashion is required instead of the gas-tight fashion.  
         [0061]     Described specifically, two tubular members (sandwiching members)  22 , 23  having a predetermined thickness are externally fitted on the outer circumferential wall of the valve shaft  14 . The inner circumferential edge portion  21   b  of the seal member  21  is sandwiched by (is held between) these two tubular members  22 , 23  such that the seal member  21  is connected in the gas-tight fashion to the outer circumferential wall of the valve shaft  14 . On the inner circumferential wall of the casing  16 , a first flange portion  24  is arranged extending inwardly as the sandwiching member. On a part of the casing  16 , said part being located higher than the first flange portion  24  and having an enlarged inner diameter, a cylindrical member  26  is internally fitted, with a second flange portion  25  being arranged extending inwardly as the sandwiching member. The outer circumferential edge portion  21   a  of the seal member  21  is sandwiched (held) between these two flange portions  24 , 25 , and is connected in a gas-tight fashion to the inner circumferential wall of the casing  16 .  
         [0062]     As illustrated in  FIG. 3 , the above-described first and second flange portions  24 , 25  may be omitted, and instead, a large-diameter section  28  which is provided with a step portion  27  may be formed on an inner circumferential wall of a casing  16 . The outer circumferential edge portion  21   a  of the seal member  21  may then be sandwiched (held) in a gas-tight fashion between the step portion (sandwiching member)  27  and an end face (sandwiching member)  29   a  of a flange-free, cylindrical member (internally-fitted member) internally fitted on the large-diameter section  28 .  
         [0063]     As an alternative, the gas-tight connection may also be assured as illustrated in  FIG. 4 , specifically by forming an annular groove  30  in an inner circumferential wall of a casing  16 , and fitting two broken ring members (sandwiching members)  31 , 32 , the diameters of which can be reduced through resilient deformations, in the annular groove  30  while keeping the seal member  21  sandwiched (held) at the outer circumferential edge portion  21   a  thereof between the two broken ring members (sandwiching members)  31 , 32 .  
         [0064]     As depicted in each of  FIGS. 1, 3  and  4 , a communication hole  33  is also formed through the casing  16  at a location between the bearing  15  and the seal member  21  to communicate the interior and exterior of the casing  16  with each other. When the exhaust gas enters the chamber  34  between the bearing  15  and the seal member  21  in the casing  16  through the clearance between the bearing  15  and the associated slide contact portion  14   b  of the valve shaft  14  and the internal pressure of the chamber  34  increases, the exhaust gas in the chamber  34  is automatically vented to the outside through the communication hole  33  to avoid such a situation that the exhaust gas would accumulate to high pressure in the chamber  34 .  
         [0065]     As the EGR valve assembly (fluid valve assembly) with the impurities entrance preventing structure according to the one embodiment of the present invention is constructed as described above, the EGR valve  5  is adjusted in opening as needed during an operation of the engine  1  to regulate the EGR flow rate such that the exhaust gas is rendered cleaner.  
         [0066]     At this time, the exhaust gas may enter the chamber  34  between the bearing  15  and the seal member  21  in the casing  16  through the clearance between the slide contact portion  14   b  and the bearing  15 . However, this chamber  34  is sealed by the seal member  21  relative to the chamber  35  on the side of the electric motor  13  so that the exhaust gas does not enter the side of the electric motor  13 .  
         [0067]     In particular, the seal member  21  is sandwiched (held) at the outer circumferential edge portion  21   a  and inner circumferential edge portion  21   b  thereof by means of the cylindrical member and step portion and the tubular members, respectively, and is connected to the inner circumferential wall of the casing  16  and the outer circumferential wall of the valve shaft  14  in a gas-tight fashion, respectively. The seal member  21 , therefore, does not include any sliding part and moreover, is free of the potential problem of separation as experienced in the case of the connection by mere fixing, so that the seal member  21  is facilitated to assure high sealing performance (gas tightness) at the connected parts over a long term. Even when the exhaust gas enters the chamber  34  along the slide contact portion  14   b  of the valve shaft  14  as shown in  FIG. 1  (see arrows a 1 ,a 2 ), this exhaust gas is blocked by the seal member  21  so that the electric motor  13 , the element to be protected, can be surely protected from the exhaust gas or from the impurities in the exhaust gas.  
         [0068]     The seal member  21  is required to undergo an elastic deformation by following each axial movement of the valve shaft  14  of the valve element  12 . As the seal member  21  includes no sliding part and is free of the potential problem of wearing, it is unnecessary to use a high-class slidable material having strong abrasion resistance (for example, “TEFLON” or the like). Any material can be used insofar as it is equipped with certain degrees of heat resistance and oil resistance and also with flexibility sufficient to avoid exerting a resistance to the movement of the valve shaft  14 . For example, general rubbers such as diaphragm materials (e.g., fluorosilicone rubber) employed in pneumatic EGR valves can be used.  
         [0069]     The communication hole  33  is formed through the casing  16  at the location between the bearing  15  and the seal member  21  to communicate the interior and exterior of the casing  16  with each other. Even when the exhaust gas enters the chamber  34  between the bearing  15  and the seal member  21  in the casing along the slide contact portion  14   b , the exhaust gas in the chamber  34  is therefore automatically vented to the outside through the communication hole  33  as the internal pressure of the chamber  34  increases. Accordingly, the exhaust gas does not accumulate to high pressure in the chamber  34 .  
         [0070]     The chamber  34  on the side of the slide contact portion  14   b , therefore, does not become significantly high in pressure relative to the chamber  35  on the side of the electric motor  13  with the seal member  21  being located therebetween in the casing  16 . A pressure difference, if any, is not so large as inducing the exhaust gas to enter the chamber  35  from the chamber  34  along the tightly-connected outer circumferential edge portion  21   a  and inner circumferential edge portion  21   b  of the seal member  21 . In this respect too, the electric motor  13  can be surely protected from the exhaust gas or the impurities in the exhaust gas.  
         [0071]     In addition, the seal member (fluid entrance blocking member)  21  is formed in the substantially disk-shaped configurations (is in the form of a circular sheet), that is, in planar, extremely simple configurations, thereby bringing about a still further merit that the production, assembly and management of the seal member  21  can be conducted with ease.  
         [0072]     The operation and advantageous effects of the EGR valve assembly (fluid valve assembly) with the impurities entrance preventing structure as the one embodiment of the present invention as shown in  FIG. 1  have been described in the above. These operation and advantageous effects equally apply to its modifications depicted in  FIGS. 3 and 4 , respectively.  
         [0073]     The one embodiment and its first and second modifications have been described in the above. It is, however, to be noted that the present invention is not limited to such embodiment and modifications but can be practiced by modifying them in various ways to extent not departing from the spirit of the present invention.  
         [0074]     For example, the configurations of the seal member (fluid entrance blocking member)  21  are not limited to the configurations in the above-described embodiment and modifications. To permit following the movement of the valve shaft  14  more easily, the seal member  21  can be formed into more three-dimensional configurations, for example, by providing it with bellows.  
         [0075]     The above-described embodiment and modifications are each provided with both of the feature (first feature) that the seal member (fluid entrance blocking member)  21  is sandwiched at both the outer circumferential edge portion  21   a  and the inner circumferential edge portion  21   b  by the sandwiching members and the feature (second feature) that the communication hole  33  is arranged to vent the exhaust gas from the chamber  34 . Even if only one of these features is equipped, the advantageous effects of the present invention can still be brought about to certain satisfactory extent.  
         [0076]     The embodiment and its modifications were described by taking the EGR valves as examples. The present invention is, however, not limited to such EGR valves. When applied to various fluid valve assemblies for gases or liquids each of which includes in the vicinity of a movable member a drive means (a means corresponding to the EGR valve drive motor) to be protected from a fluid or from impurities in a fluid, similar advantageous effects can be brought about as in the above-described embodiment and modifications.  
         [0077]     The present invention can be widely applied to fluid valve assemblies, led by EGR valves, of the type that a drive means (a means corresponding to the EGR valve drive motor) to be protected from a fluid or from impurities in a fluid exists in the vicinity of a valve element as a movable member, and can surely protect their drive means from a fluid or from impurities in a fluid.  
         [0078]     This application claims the priority of Japanese Patent Application 2003-317162 filed Sep. 9, 2003, which is incorporated herein by reference.