Abstract:
A solenoid valve includes a cylindrical bobbin provided with a coil thereon, a fixed core provided through an inner periphery of the bobbin, a case provided at an outer periphery of the bobbin, the case housing the fixed core, a plunger provided facing the fixed core and being movable by a magnetic attractive force, a shaft member extended to penetrate through the fixed core, the shaft member having a first end provided with the plunger in a way that the plunger is movable integrally with the shaft member, a valve member contacting a second end of the shaft member and driven by the shaft member, a biasing member biasing the valve member, the shaft member and the plunger in a counter direction of a moving direction by the magnetic attractive force, and a plurality of magnetically attracting portions provided at the plunger.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on and claims priority under 35 U.S.C. §119 with respect to Japanese Patent Application No. 2005-265612 filed on Sep. 13, 2005 the entire content of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a solenoid valve which is applied in a hydraulic circuit.  
       BACKGROUND  
       [0003]     A known solenoid valve includes a plunger which is made of ferrous material and driven by an electromagnetic coil and a spool member which is biased by a spring and contact the plunger via a shaft (e.g., described in JP09-053742A). According to the known solenoid valve described in JP09-053742A, a spool ( 13 ) slidably included in a sleeve ( 11 ) contacts a plunger ( 4 ) via a shaft ( 40 ) at one end thereof. A return spring ( 14 ), which applies a biasing force to the spool, is supported at the other end of the spool by means of a screw plug ( 15 ).  
         [0004]     A magnetically attracting portion is formed by the plunger and by a fixed core ( 2 ). A magnetic attractive force is primarily determined by an outer diameter of the fixed core. In order to increase the magnetic attractive force, the amount of energizing current supplied to a coil ( 3 ) may be increased. However, because supplying excessive electric current to the coil leads to a magnetic saturation and/or generating heat of the coil, increasing the amount of energizing current supplied to the coil is not an effective counter measurement.  
         [0005]     Generally, an outer diameter of a core is increased in order to increase magnetic attractive force. However, the increase of a diameter of the core leads to increase a diameter of a coil, and thus to increase a size of a solenoid valve per se.  
         [0006]     A need thus exists for a solenoid valve, which acquires greater magnetic attractive force without increasing a size of a diameter of a coil thereof.  
       SUMMARY OF THE INVENTION  
       [0007]     In light of the foregoing, the present invention provides a solenoid valve, which includes a cylindrical bobbin provided with a coil thereon, a fixed core provided through an inner periphery of the bobbin, a case provided at an outer periphery of the bobbin, the case housing the fixed core, a plunger provided facing the fixed core and being movable by a magnetic attractive force, a shaft member extended to penetrate through the fixed core, the shaft member having a first end provided with the plunger in a way that the plunger is movable integrally with the shaft member, a valve member contacting a second end of the shaft member and driven by the shaft member, a biasing member biasing the valve member, the shaft member and the plunger in a counter direction of a moving direction by the magnetic attractive force, and a plurality of magnetically attracting portions provided at the plunger. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a cross-sectional view in an axial direction of a solenoid valve according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0010]     One embodiment of the present invention will be explained with reference to illustrations of a drawing figure as follows. A solenoid valve according to the embodiment of the present invention is shown in  FIG. 1 . According to the embodiment, the solenoid valve corresponds to a linear solenoid valve for a hydraulic pressure control apparatus for feedback control.  
         [0011]     As shown in  FIG. 1 , a coil  14  which is wound around a bobbin  13  made of resin is included in a case  11 . A connector  12  made of resin is fixed at a first end of the case  11 . The connector  12  is electrically connected with the coil  14  and is integrally formed with a terminal  26  for electrifying the coil  14 . A stepped magnetically attracting portion  11   a  which is thin-walled gradually is formed at a second end of the case  11 .  
         [0012]     A cylindrical front yoke (i.e., serving as a fixed core)  16  having a flange  16   a  at a first end thereof at the terminal side  26  is included in the bobbin  13 . A ring shaped rear yoke  17  tapered at an external surface thereof and having an inner diameter which is approximately the same size with an outer diameter of the front yoke  16  is integrally press fitted to a second end of the front yoke  16  to support the coil  14 .  
         [0013]     The front yoke  16  includes a through-hole which penetrates through the center thereof. A bearing  19  is provided at the second end of the front yoke  16  and a bearing  29  is provided at the first end of the front yoke  16  to support a plunger  15  with which a shaft (i.e., serving as a shaft member)  18  is integrally press fitted to be slidable in an axial direction.  
         [0014]     A stopper  30  is integrally provided at a tip end of the shaft  18  in order to restrict the plunger  15  from moving in the axial direction.  
         [0015]     The plunger  15  includes an approximately cylindrical configuration having a bottom, and a tapered internal surface  15   d  having similar configuration with an external surface  17   a  of the rear yoke  17  and facing the rear yoke  17 . A first magnetically attracting portion  27  is constructed between the tapered external surface  17   a  of the rear yoke  17  and the plunger  15 . Further, at an end surface of the cylindrical portion of the plunger  15  which is arranged facing the case  11 , a thin-walled tapered external surface  15   a  of the plunger  15  is fitted into a thin-walled tapered internal surface (i.e., a second magnetically attracting portion)  11   a  which is formed stepwise at the case  11  thus to form a second magnetically attracting portion  28 . Upon a move of the plunger  15  against a biasing force, the tapered external surface  17   a  of the rear yoke  17  and the tapered internal surface  15   d  of the plunger  15  contact each other, and the thin-walled tapered internal surface  11   a  of the case  11  and the thin-walled tapered external surface  15   a  of the plunger  15  which faces the case  11  contact each other. The plunger  15  further includes plural through-holes  15   b  at the bottom and a through-hole  15   c  at cylindrical surface. External corner portions of the plunger  15  at opposite side relative to a surface which faces the rear yoke  17  are tapered and removed within a range which does not influence on magnetic path formation.  
         [0016]     By removing the external corner portions of the plunger  15  and forming the through-holes  15   b,    15   c,  mass of the plunger  15  is reduced and further, when the plunger  15  moves by means of magnetic attractive force, fluid in a void defined by the plunger  15 , the rear yoke  17 , and the front yoke  16  and fluid in a void defined by the plunger  15  and the bobbin  13  are discharged outside of the plunger  15  to improve an operational response of the plunger  15 .  
         [0017]     The plunger  15  is covered with a cylindrical cover  31  having a bottom keeping a predetermined clearance therebetween, and an opening portion of the cover  31  closely contact an external periphery of the case  11  to fluid-tightly seal from outside.  
         [0018]     An approximately cylindrical sleeve (i.e., serving as a valve member)  22  contacts the flange portion  16   a  of the front yoke  16 , and the flange portion  16   a  and the sleeve  22  are clinched with a thin-walled skirt portion of the case  11  to be integrally attached.  
         [0019]     An approximately cylindrical spool (i.e., serving as a valve member)  21  is slidably included in the sleeve  22  to contact a shaft  18 . A spring (i.e., serving as a biasing member)  23  is provided at a first end portion of the spool  21  which is an end portion opposite to the shaft  18  to bias the spool  21  towards the shaft  18  side.  
         [0020]     A first end of the spring  23  is connected to the spool  21  and a second end of the spring  23  is connected to an adjuster  24  which is threadedly engaged with the sleeve  22 .  
         [0021]     An operation of the solenoid valve according to the embodiment of the present invention will be explained as follows.  FIG. 1  shows a non-energized state. Under a non-energized state, although oil is supplied to a port Pi of the sleeve  22  from an oil pump, because the port Pi is closed by the spool  21 , oil supply to a controlled object is stopped. In this state, hydraulic pressure is not generated at the controlled object because a port Pc which is connected to the controlled object and a drain port Pd are in communication with each other through a communication groove  21   a.    
         [0022]     When electric current is supplied to the coil  14  through the terminal  26 , the plunger  15  is attracted in a direction that the external surface  17   a  of the tapered rear yoke  17  and the internal surface  15   d  of the tapered plunger  15  contact and in a direction that the external surface  15   a  of the tapered plunger  15  and the stepwise thin-walled internal surface  11   a  of the tapered case  11  contact against a biasing force of the spring  23  by means of a magnetic attractive force generated at the first magnetically attracting portion  27  constructed between the plunger  15  and the rear yoke  17  and at the second magnetically attracting portion  28  constructed between the plunger  15  and the case  11 , and the plunger  15  is moved downward in  FIG. 1 . Upon the move of the plunger  15 , the shaft  18  pushes the spool  21  in an axial direction (i.e., downward in  FIG. 1 ). Eventually, the plunger  15  contacts the front yoke  16  via a spacer  25  to stop moving.  
         [0023]     In a state where the plunger  15  contacts the front yoke  16  via the spacer  25 , the drain port Pd is closed by the spool  21 , and communication between the port Pi and the port Pc of the sleeve  22  is established through the communication groove  21   a.  Oil supplied to the port Pi is supplied to the controlled object from the port Pc of the sleeve  22  through the communication groove  21   a  to control the controlled object by hydraulic pressure.  
         [0024]     A part of oil supplied to the controlled object affects a port Pf through a hydraulic circuit to maintain an outputted hydraulic pressure at a desired level by balancing a magnetic attractive force and a biasing force of the spring as a feedback pressure.  
         [0025]     Namely, by applying electric current to the coil  14  through the terminal  26 , the solenoid valve (e.g., linear solenoid valve) serves as a hydraulic pressure control valve having a feedback control performance which supplies hydraulic pressure supplied from the port Pi to a controlled object through the port Pc.  
         [0026]     According to the embodiment of the present invention, because the plural magnetically attracting portions are provided at the plunger, magnetic attractive force is increased without increasing a diameter of a coil compared to known plungers which includes a magnetically attracting portion at a single position, and thus an operational response of the plunger is improved.  
         [0027]     According to the embodiment of the present invention, because the first magnetically attracting portion is provided between the plunger and the fixed core (e.g., the rear yoke) and the second magnetically attracting portion is provided between the plunger and the case, the plunger can be attracted in an axial direction.  
         [0028]     According to the embodiment of the present invention, because the through-hole which extends in a radial direction is formed through the cylindrical portion of the plunger having approximately cylindrical configuration with a bottom, fluid in the void defined by the plunger and the bobbin is likely to be discharged outside, and a response of the plunger is increased accordingly.  
         [0029]     According to the embodiment of the present invention, because at least a portion of the plunger at backside thereof relative to the fixed core (e.g., rear yoke), which does not affect the forming magnetic path, is removed, mass of the plunger is reduced, and an operational response by the magnetic attractive force can be improved.  
         [0030]     According to the embodiment of the present invention, because the valve member includes the spool which contacts the shaft member (e.g., shaft) and the sleeve which slidably includes the spool, and further because the biasing member (e.g., spring) provided in the sleeve biases the spool towards the shaft, a linear solenoid valve with a small diameter can be provided.  
         [0031]     According to the embodiment of the present invention, because the second magnetically attracting portion is formed between the open end of the cylindrical portion of the plunger configured in approximately cylindrical having a bottom and the open end at the plunger side, it is not required to increase a diameter of the plunger.  
         [0032]     According to the embodiment of the present invention, because the magnetically attracting portions are tapered, magnetically attracting force is likely to be constant relative to a stroke of the shaft.  
         [0033]     The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiment disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.