Abstract:
The solenoid valve ( 10 ) has an armature chamber ( 38 ) communicated with the outlets ( 86 ) of the valve to receive feed of fluid. The fluid in the chamber ( 38 ) is drained through drain passages ( 96 ) which are arranged to open into the chamber ( 38 ) at a location radially outwardly offset from the axis of the valve. The flow of fluid flowing into the chamber ( 38 ) is directly transferred to the drain passages ( 96 ) to thereby wash ferrous particles away from the magnetic gap to self-clean the gap each time the valve is opened. A spacer ( 62 ) made of a non-magnetizable material covers the upper end face of the armature ( 36 ) to prevent accumulation of ferrous particles.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a solenoid-operated flow control valve which is particularly suitable for use in a hydraulic system wherein a working fluid contaminated by and laden with minute particles of metallic materials is circulated. 
     2. Description of the Prior Art 
     Solenoid valves are widely used in various hydraulic systems to electronically control flow of a fluid. 
     As shown in FIG. 1A, a solenoid valve may typically include a movable valve member  1  connected to an armature or plunger  2 . The armature  2  is operated by a solenoid assembly comprised of a solenoid windings  3 , a magnetic pole piece  4 , and a yoke member  5 . The armature is biased downwards by a return coil spring  6  which is supported at its upper end by a spring retainer  7  which may be in the form of an adjusting screw adjustably screwed into the pole piece  4 . An annular spacer  8  made of a non-magnetizable material is fixed to the lower end of the pole piece  4  so as to limit the upward travel of the armature  2  to thereby space the armature at a given distance from the pole piece in the fully open position of the valve. 
     In the case where the solenoid valve is controlled by an electronic control system, it is customary to operate the solenoid valve on the duty cycle basis by cyclically energizing the solenoid windings with intermittent drive pulses having a frequency which may range, for example, from 200 to 300 cycles per second, the degree of opening of the valve being controlled by varying the width, or duty factor, of respective drive pulses. 
     Such an intermittent energization of the solenoid coil would result undesirable chattering of the valve. In order to suppress or subdue chattering of the valve that would result from the duty cycle operation of the solenoid, the armature chamber  9  receiving the armature  2  is filled with a fluid to thereby damp the vibratory movement of the armature  2 . To this end, the armature chamber  9  is communicated by an annular passage  10  with the outlet port  11  of the valve to admit the fluid at the outlet to flow into the armature chamber, the passage  10  being formed between the armature and the yoke member  5 . The fluid in the armature chamber is drained by a drain passage  12  which extends axially throughout the spring retainer  7 . 
     In certain applications of the solenoid valves, a hydraulic fluid is inevitably contaminated by fine particles of ferrous materials resulting from wear of machine parts. For example, in an automatic transmission system of a vehicle, an automatic transmission fluid is circulated through various metallic moving parts such as gear trains and clutch discs so that the fluid will become considerably contaminated by finely divided debris, fragments or particles of ferrous materials resulting from wearing of gears and other metallic parts. 
     The problem encountered with the solenoid valves as used to control a ferrous contaminant-laden fluid is that ferrous particles born in the hydraulic fluid are magnetically attracted and trapped in the magnetic gaps of the solenoid structure as the fluid is passed through the armature chamber. 
     More specifically, as shown in FIG. 1B wherein parts and members encircled by a circle in FIG. 1A are shown in an enlarged scale, ferrous particles are attracted to and deposit on the lower end face of the pole piece  4  as well as on the upper end face of the armature  2  as schematically shown at  12  and  13 . Metallic particles are also magnetically held at the radial gap between the armature and the yoke member as shown at  14 . Furthermore, particles are accumulated between the consecutive turns of the coil spring as shown at  15 . 
     The ferrous particles magnetically accumulated in this manner at the magnetic gaps of the solenoid will be oriented along the magnetic flux path in an acicular fashion to project from one surface toward the opposite surface of the magnetic gap, thereby giving rise to a situation in which the gap is somewhat bridged or short-circuited by chains of attracted particles. As a result, the magnetic permeability across the gap is inadvertently increased in response to a lapse of time so that the operating property of the solenoid valve, e.g., the current versus fluid pressure characteristics, is undesirably altered during the service life of the solenoid valve. 
     Accordingly, it is an object of the present invention to provide a solenoid valve which is suitable for use in controlling a fluid which is contaminated by and laden with minute particles of metallic materials. 
     Another object of the invention is to provide a solenoid valve which is capable of effectively preventing ferrous particles from accumulating at the magnetic gaps of the solenoid assembly. 
     A still another object of the invention is to provide a solenoid valve which exhibits a constant operating characteristics throughout the service life of the valve. 
     SUMMARY OF THE INVENTION 
     This invention provides a solenoid valve having a movable valve member operated by a solenoid actuator having an armature movably received in an armature chamber communicated by an annular passage with an outlet of the valve, a drain passage extending from the chamber to communicate the chamber with the outside of the valve. 
     The feature of the invention is that the drain passage is arranged to open into the armature chamber at a location radially outwardly offset from the axis of the chamber. 
     With this arrangement, a flow of fluid flowing from the annular passage into the armature chamber is directly transferred and delivered toward the drain passage without passing the central region of the chamber in which the fluid tends to stay stagnant. As a result, the flow of fluid that has entered into the armature chamber will continue to flow into the drain passage without loosing its velocity to any substantial degree. Accordingly, sludge of ferrous particles magnetically attracted at the magnetic gaps of the solenoid assembly is washed away by the flow of fluid so that the gaps are self-cleaned each time the solenoid valve is actuated. 
     Another advantage is that the flow of fluid entered into the armature chamber is allowed to leave the armature chamber without being brought into contact with the return coil spring which is arranged at the center of the chamber. This prevents the ferrous particles in the fluid from being attracted and adhering to the coil spring. 
     Preferably, the drain passage is formed between a magnetic pole piece of the solenoid structure and a spring retainer received in the central bore of the pole piece. 
     In a preferred embodiment of the invention, the solenoid assembly includes an annular spacer, of a non-magnetizable material, arranged between the armature and the magnetic pole piece. The spacer is mounted to the armature and covers an end face of the armature. With this arrangement, ferrous particles in the fluid is prevented from accessing the end face of the armature. 
     In an alternative embodiment, the spacer is fixed to the magnetic pole piece and is configured to cover an end face of the pole piece facing the armature in such a manner as to prevent ferrous particles from accessing the end face of the pole piece. 
     In another embodiment of the invention, the outer surface of the armature exposed in the armature chamber is coated with a coating of a non-magnetizable material, such as a fluorocarbon resin. The resin coating on the armature assists the ferrous particles magnetically held at the radial gap between the armature and the yoke member to be readily released and washed away from the opposite surfaces of the gap. 
     These features and advantages of the invention, as well as other features and advantages thereof, will become apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross-sectional view of the solenoid valve of the conventional design; 
     FIG. 1B is an enlarged cross-sectional view showing the part encircled by the dotted circle in FIG. 1A; 
     FIG. 2 is a cross-sectional view of the solenoid valve according to the first embodiment of the invention; 
     FIG. 3 is an enlarged cross-sectional view showing the part encircled by the dotted circle in FIG. 2; 
     FIG. 4 is a perspective view of the spacer shown in FIGS. 2 and 3; 
     FIG. 5 is a cross-sectional view taken along the line V—V of FIG. 2; 
     FIG. 6 is a cross-sectional view taken along the line VI—VI of FIG. 3; 
     FIG. 7 is a cross-sectional view of the solenoid valve according to the second embodiment of the invention; 
     FIG. 8 is a cross-sectional view in an enlarged scale of the spacer shown in FIG. 7; 
     FIGS. 9 and 10 show the modified forms of the spacer; and, 
     FIGS. 11 and 12 show the modified versions of the armature and valve assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 2-6, the solenoid valve according to the first embodiment of the invention will be described. As shown in FIG. 2, the solenoid valve  20  includes an upper solenoid section  22  and a lower valve section  24  coupled together to form a unitary structure. 
     The solenoid section  22  includes a solenoid windings  26  wound around a flanged tubular solenoid bobbin  28  made by injection molding of a plastic material. The solenoid windings  26  are connected by lead wires, not shown, to connecting pins  30  of an electric socket  32 . 
     The solenoid section  22  also includes a generally cylindrical magnetic pole piece  34  made of a ferromagnetic material and tightly fitted within the upper part of the central bore of the bobbin  28 . 
     The solenoid section  22  further includes a movable armature or plunger  36  which is movably received in an armature chamber  38  defined by the stepped lower part  68  of the central bore of the bobbin  28 . The armature  36  is downwardly biased by a return coil spring  40  having its lower end abutting against the bottom of a cylindrical recess formed in the armature  36 . The upper end of the spring  40  is adjustably supported by a spring retainer  42  which is in the form of an adjusting screw threadingly engaged in a threaded central bore  44  axially extending through the pole piece  34 . 
     The solenoid section  22  also has an annular lower yoke member  46  made of a ferromagnetic material. The yoke member  46  is insert molded in the bobbin  28  and has an axially extending tubular portion  46 A and a radially extending flange portion  46 B. A disc-shaped top plate or upper yoke member  48  made of a ferromagnetic material and having a central opening is mounted on an annular shoulder formed at the upper part of the pole piece  34 . 
     The central pole piece  34 , yoke member  46 , solenoid windings  26  and top plate  48  are surrounded by a tubular outer casing  50  made of a ferromagnetic material and having an inturned lower flange  52 . The top plate  48  is firmly held against the pole piece  34  by inwardly crimping the upper end of the outer casing  50 . 
     When the solenoid windings  26  is energized, a path of magnetic flux will be formed as shown by the dotted line  54  across the pole piece  34 , armature  36 , yoke member  46 , outer casing  50  and top plate  48 , to thereby attract the armature  36  toward the pole piece  34  against the bias of the return spring  40 . 
     To ensure that the magnetic attractive force acting on the armature  36  is as proportional as possible to the intensity of the electric current supplied to the solenoid windings, part of the lower end face  54  of the pole piece  34  and the upper end face  56  of the armature  36  are conically tapered upwardly as shown at  58  and  60 , respectively, as will be best understood from FIG.  3 . 
     An annular spacer  62  made of a non-magnetizable material such as stainless steel, copper, brass or plastics is mounted at the top of the armature  36  to limit the upward stroke of the armature  36 . When the armature  36  has fully stroked, the spacer  62  will abut against the lower end face  54  of the pole piece  34  so that the armature  36  is axially spaced away from the pole piece  34  for a given minimum distance to thereby leave an axial magnetic gap of a minimum value between the armature  36  and the pole piece  34 . The presence of the minimum axial magnetic gap is also favorable to ensure that the magnetic attractive force acting on the armature  36  is as proportional as possible to the intensity of the electric current supplied to the solenoid windings. 
     The spacer  62  has an upper portion  62 A having a conically tapered side wall  62 B which is generally in flush with the tapered end face portion  60  of the armature  36 . As shown in FIG. 4, the upper end of the spacer  62  is provided with a plurality of cutouts  62 C for reasons described later. The spacer  62  is provided at the lower part thereof with a reduced-diameter tubular portion  62 D which is press fitted within a stepped central bore of the armature  36 . 
     As best shown in FIG. 3, a small annular clearance  66  is held between the stepped central bore  68  of the bobbin  28  and the outer surface of the armature  36 . This clearance  66  functions as a radial gap between the armature  36  and the yoke member  46 . The annular clearance  66  also serves as a fluid passage as described later. 
     Referring again to FIG. 2, the valve section  24  has a generally tubular body  70  of plastics which is molded integrally with the solenoid bobbin  28 . The body  70  has an axial bore  72  in which a tubular valve seat insert  74  made of a non-magnetizable metallic material such as stainless steel is interference fitted. 
     The valve seat insert  74  comprises a large diameter upper part  74 A serving as a guide sleeve for the armature  36  and a small diameter lower part  74 B serving as a valve seat. The guide sleeve  74 A slidably and guidingly receives the lower part  36 A of the armature  36 . As will be understood from FIG. 5, the guide sleeve  74 A is closely fitted within the bobbin  28  and, therefore, is firmly supported by the latter in the radial direction. The valve seat insert  74  has an annular shoulder which abuts against an associated annular shoulder formed in the body  70  at the bottom of the armature chamber  36 . The valve seat insert  74  is held in place by crimping the lower end thereof outwardly against a metal ring  80  insert molded within the body  70 . 
     The inner periphery of the guide sleeve  74 A and the outer periphery of the lower part  36 A of the armature mating with each other are precision machined to axially precisely guide the armature  36 . As shown in FIG. 5, a plurality of axially extending grooves  76  are formed on the inner periphery of the bobbin  28  to communicate with the annular passage  66 . 
     Referring further to FIG. 2, the valve seat  74 B has an axial bore  74 C forming an inlet  78  for the valve section  24 . The upper part of the bore  74 C is precision machined and slidably and snugly receives a movable valve member  82  which, in the illustrated embodiment, is made integral with the armature  36 . 
     The valve member  82  is tubular in form and is provided with a pair of diametrically opposed control ports  84  which are closed and opened by the valve seat  74 B as the valve member  82  is axially displaced in response to the movement of the armature  36 . The control ports  84  are located such that, in the fully closed position of the valve, the outer surface of the valve member  82  and the inner wall of the valve seat  74 B is preferably overlapped for a predetermined axial length in order to minimize any fluid leakage. 
     The body  70  is provided with a plurality of radially extending outlets  86  which are open into an annular space  88  formed at the lower part of the armature chamber  36  between the body  70  and the guide sleeve  74 A. The guide sleeve  74 A is, in turn, provided with a plurality of openings  90  which communicate the annular space  88  with an annular space  92  defined between the guide sleeve  74 A and the lowermost part  36 B of the armature  36 . 
     Referring to FIGS. 2 and 6, the opposite sides of the spring retainer  42  which is in the form of an adjusting screw are chamfered along the entire length thereof to present flat side faces  94 . As a result of chamfering, a pair of diametrically opposed axial passages  96  of a lunate cross-section are formed between the threaded central bore  44  of the pole piece  34  and the spring retainer  42 . The passages  96  serve as the drain passages for the armature chamber  38 . It will be noted that the lower end of each drain passage  96  opens into the armature chamber  38  at a location which is radially outwardly offset from the central axis  98  of the solenoid valve. 
     In use, the solenoid valve  10  may be installed on a hydraulic system  100  by fluid tightly fitting the body  70  into a conduit  102  of the system, with an O-ring  104  being fitted in an annular groove of the body  70 . The solenoid valve  10  may be operated on the duty cycle basis by an electric control unit in the conventional manner. 
     Upon application of an electric current to the solenoid coil  26 , the armature  36  will be magnetically attracted toward the pole piece  34  causing the valve member  82  to move on its valve opening upward stroke to thereby open the control ports  84 . The travel of the valve member  82  and, hence, the opening of the control ports  84  is controlled by varying the duty factor of the drive pulses. 
     As the control ports  84  are opened, the fluid at the inlet  78  is allowed to pass through the annular space  92  defined between the guide sleeve  74 A and the lowermost part  36 B of the armature  36 , the openings  90  in the guide sleeve  74 A, and the annular space  88  between the body  70  and the guide sleeve  74 A to flow toward the outlets  86 . 
     The fluid pressure at the annular space  88  will force a small amount of fluid to flow through the axial grooves  76  and the annular passage  66  into the top of the armature chamber  38 . The flow of fluid entered into the top of the chamber  38  will be guided and directed by the tapered side wall  62 B of the spacer  62  to smoothly flow into the radially outwardly-offset drain passage  96 , without passing the central region of the armature chamber  38 . Accordingly, the fluid will flow along the tapered end faces  58  and  60  of the pole piece  34  and the armature  36  while substantially keeping its velocity. As a result, sludge of ferrous particles that may be magnetically attracted at the magnetic gap between the pole piece  34  and the armature  36  will be washed away so that the gap will be self-cleaned each time the solenoid valve is opened. 
     Furthermore, as the fluid flow entered into the armature chamber  38  is drained therefrom without being brought into contact with the return coil spring  40 , the risk of ferrous particles to adhere to and accumulate on the coil spring  40  is considerably reduced. 
     The upper end face of the armature  36  is free from deposit of ferrous particles as it is covered by the spacer  62  made of a non-magnetizable material. The cutouts  62 C formed in the spacer  62  ensure a fluid flow even when armature has fully stroked to bring the spacer  62  into abutment with the pole piece  34 . 
     FIGS. 7 and 8 illustrate the solenoid valve according to the second embodiment of the invention. Parts and members similar to those of the first embodiment are shown by like reference numerals and, therefore, need not be described again. 
     Referring to FIGS. 7 and 8, the second embodiment differs from the first embodiment in that the annular spacer  110 , made of a non-magnetizable material, is fixed to the magnetic pole piece  34  in such a manner as to cover substantially the entire lower end face thereof and that the spring retainer  112  which is in the form of a rod is press fitted within the unthreaded central bore of the pole piece  34 . 
     As shown enlarged in FIG. 8, the spacer  110  has an upper tubular mounting portion  110 A press-fitted within an axial bore  114  of the pole piece  34  and a lower skirt portion  110 B closely mating with the tapered end face  58  of the pole piece  34 . As in this embodiment substantially the entire lower end face of the magnetic pole piece  34  is covered by the non-magnetizable spacer  110  and is, therefore, intercepted from the fluid in the armature chamber  38 , there is no risk of ferrous particles being attracted to the end face of the pole piece  34 . 
     Similar to the first embodiment, the lateral sides of the spring retainer  112  are chamfered to form the drain passages  96 , only one of which is shown in FIG.  7 . 
     FIGS. 9 and 10 show the modified embodiments of the spacer  110  shown in FIGS. 7 and 8. In the embodiment shown in FIG. 9, the spacer  116  with a similarly tapered skirt portion  118  is designed to closely fit with the uppermost tapered end face portion  120  of the pole piece  34 . In another modified version shown in FIG. 10, the spacer  122  of non-magnetizable material which is similarly shaped to cover the entire end face of the pole piece is affixed to the pole piece  34 . The spacer  122  is provided with a plurality of grooves  124  circumferentially spaced apart from one another. As shown, each groove  124  extends along the juncture between the pole piece and the spacer to ensure that the fluid issuing from the radial gap between the yoke member  46  and the armature  36  is directly transferred toward the drain passages  96  as shown by the arrows. 
     FIG. 11 illustrates a modified form of the armature incorporated in the foregoing embodiments. In this embodiment, the outer surface of the armature  36  at the uppermost part and the lowermost part  36 B thereof is coated with a coating  130  of fluorocarbon resin such as polytetrafluoroethylene. The thickness of the coating  130  is preferably 10-20 micrometers. The mid part  36 A of the armature which is slidingly guided by the guide sleeve  74 A is uncoated and precision machined so as to guide the armature with a high degree of accuracy. The coating  130  of polytetrafluoroethylene may be provided at the uppermost part of the armature  36  as shown in FIG.  12 . 
     The coating  130  prevents ferrous particles from being magnetically attracted to the outer surfaces of the armature facing the radial and axial magnetic gaps wherein the magnetic flux is highly concentrated and assists the ferrous particles magnetically held at these gaps to be readily washed away in response to the fluid flow. 
     While the present invention has been described herein with reference to the specific embodiments thereof, it is contemplated that the present invention is not limited thereby and various changes and modifications may be made therein for those skilled in the art without departing from the scope of the invention.