Abstract:
An improved solenoid for providing linear actuation. The outer polepiece of the solenoid is provided with a plurality of radially-dispersed labyrinthine passageways in communication between the exterior of the solenoid and an interior region including the armature and the interior of the windings. The polepiece is further provided with at least one passageway in communication between the exterior of the solenoid and an outer periphery of the windings within the housing. The two sets of passageways impede particles and droplets of moisture from entering the solenoid, while simultaneously permitting moisture such as condensation within the solenoid to evaporate and escape.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application, Ser. No. 60/184,571, filed Feb. 24, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates to electric solenoids as used in mechanical linear actuators; more particularly, to such solenoids as may be required to operate in moist conditions or over a wide range of temperatures; most particularly to such a solenoid having vents which impede the entrance of particles of dirt, including droplets of liquids, into the interior, while simultaneously permitting the draining and evaporation of condensation. 
     BACKGROUND OF THE INVENTION 
     Electric solenoids are well known in electrical engineering and are widely used as actuating components in electromechanical linear actuators. A typical electric solenoid consists of a plurality of windings of an electric conductor about north and south polepieces. When current is passed through the windings, a characteristic toroidal magnetic field is produced having field lines at the axis which are parallel to the axis. A ferromagnetic armature is slidably disposed in an axial bore in the polepieces. An axial force is exerted by the magnetic field on the armature which tends to displace the armature axially. The strength of such force can be varied by varying the current flowing through the windings. Thus, by attaching the armature to a shaft, a solenoid may be adapted readily to provide linear mechanical actuation of a device to which it is attached. Solenoids are probably the commonest type of such actuators in use today. 
     Solenoids rely for efficiency on having very close tolerances among the various mechanical and electrical components, and they are therefore susceptible to failure if foreign particles, corrosive gases, corrosive solutions, or moisture are allowed to enter and remain within the solenoid housing. Some known solenoids are provided with housings which are hermetically sealed in an effort to prevent any contamination from ever entering. However, a sealed housing presents a dilemma. To the extent that such a housing succeeds in keeping contamination out, it also traps within itself any contaminant that is able to enter despite the housing design. Thus, contaminants, and especially water, may become trapped within a solenoid and may either short out electrically or corrode electrical and mechanical elements critical to the operation of the solenoid. 
     What is needed is an improved solenoid wherein particulates and droplets of moisture are impeded from entering directly, but wherein the interior of the solenoid is vented to the exterior to permit drainage and evaporation of moisture from within the solenoid. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved solenoid for providing linear actuation. The outer polepiece of the solenoid is provided with a plurality of radially-dispersed labyrinthine passageways in communication between the exterior of the solenoid and the interior region comprising the armature and the interior of the windings. The polepiece is further provided with at least one passageway in communication between the exterior of the solenoid and the outer periphery of the windings within the housing. The two sets of passageways impede particles and droplets of moisture from entering the solenoid, while simultaneously permitting moisture such as condensation within the solenoid to drain or to evaporate and escape. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention, as well as presently preferred embodiments thereof, will become more apparent from a reading of the following description in connection with the accompanying drawings, in which: 
     FIG. 1 is an elevational cross-sectional view of a prior art solenoid actuator; 
     FIG. 2 is a plan view of an improved outer polepiece for a solenoid actuator in accordance with the invention; 
     FIG. 3 is an elevational cross-sectional view of the polepiece shown in FIG. 2, taken along line  3 — 3  therein; and 
     FIG. 4 is an elevational cross-sectional view of a solenoid actuator in accordance with the invention operationally attached to an exhaust gas recirculation (EGR) valve mounted on an internal combustion engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The benefits afforded by the present invention will become more readily apparent by first considering a prior art solenoid actuator. Referring to FIG. 1, a prior art actuator  10  includes a housing  12  containing first and second pole pieces  14 , 16 , respectively, and a plurality of electrically-conductive windings  18  about the polepieces. A ferromagnetic armature  20  is slidably disposed within a stepped first axial bore  21  in the pole pieces. An actuating shaft  22  is axially disposed and retained within armature  20  and extends from housing  12  via a second axial bore  24  in polepiece  16  for connection to useful work. Step  26  in bore  21  receives a coil spring  28  disposed in compression between step  26  and a well  30  in armature  20  for biasing the armature into the solenoid. A generally cylindrical non-magnetic sleeve  32  surrounds armature  20  and spring  28  for slidably guiding and centering the armature axially of polepieces  14  and  16 . Typically, the sleeve is formed of a non-galling non-ferromagnetic material such as stainless steel or ceramic, and either the sleeve or the armature may be coated with any of various well-known dry lubricants. Typically, the armature is hard-nickel coated. 
     Referring to FIGS. 2 through 4, improved outer polepiece  16 ′ is similar in shape, and preferably is substitutable for, prior art polepiece  16 , having both a radial portion  36  and a tapered axial portion  38 . However, polepiece  16 ′ is provided with three important and novel features not present in polepiece  16 . 
     First, axial bore  24  is replaced by a new axial bore  24 ′ for receiving a new steady bearing  40  not found in prior art solenoid  10 . Bearing  40  has an axial bore  42  for receiving shaft  22  and for supporting and radially guiding the shaft during reciprocating actuation of the solenoid. Preferably, the diametral relationship between bore  42  and shaft  22  is as close as is practical without introducing significant drag on shaft  22  during actuation thereof. The presence of close-tolerance guidance of the shaft through the bearing permits a significant reduction in the size and axial extent of sleeve  32  which is replaced by a half-sleeve  32 ′ as shown in FIG.  4 . Preferably, bearing  40  is press-fit into bore  24 ′. 
     Second, at least one radial passageway  44  is provided in the floor  46  of well  48  formed within axial portion  38 , preferably a plurality of such passageways, for example, three radially disposed at 120° as shown in FIG.  2 . Passageways  44  extend from floor  46  through polepiece  16 ′ to the exterior of the solenoid. The elimination of prior art axial bore  24 , which is loose-fitting of shaft  22 , and its replacement by bore  24 ′ containing press-fit bearing  40  and close-tolerance bore  42  create a drainage problem within well  48 , in that condensation forming within the central region of the solenoid will be trapped, which is highly undesirable and can lead to mechanical and/or electrical failure of the solenoid. Thus, central region passageways  44  provide ready drainage of well  48 . Provision of drainage passageways  44  represents a recognition that an actuator having a reciprocable shaft extending therefrom cannot be, as a practical matter, hermetically sealed against entrance of moisture, especially when the actuator may be called upon to function over a very large range of temperatures and ambient humidities, for example, in an EGR valve assembly such as assembly  50  shown in FIG.  4 . Therefore, it is preferable to accept the fact that internal condensation will occur, to configure the solenoid as open to the atmosphere, and to provide for ventilation and drainage from the solenoid. Note that passageways  44  are preferably labyrinthine, having two offset 90° bends after installation of bearing  40 , to impede ingress of particulates and moisture droplets by direct impingement, without also impeding drainage and ventilation. 
     Third, at least one peripheral drainage and ventilation passageway  52  is provided through polepiece  16 ′ for draining condensation from regions of the solenoid outside of well  48 . Preferably, the windings and/or housing of a solenoid incorporating polepiece  16 ′ also are modified to provide an annular gap  54  between the outer surface of the windings and the inner surface of the housing  12 , as shown in FIG.  4 . Gap  54  provides air circulation around the windings via passageway  52  for cooling of the windings, evaporation of local condensation, and collection of non-evaporated condensation for drainage through one or more peripheral passageways  52 . 
     In the preferred embodiment, polepiece  16 ′ is further provided with one or more depressions  56  in upper surface  58  of radial portion  36 , preferably three such areas radially disposed 120° apart, as shown in FIG.  2 . Preferably, each depression includes a large central angle of about, for example, 90°, thereby leaving a relatively small percentage of surface  58  for contacting and supporting windings spool  60 . Many actuators, including those used in EGR valve assemblies, must operate at significantly elevated ambient temperatures which can increase the resistance in windings  18  and proportionately reduce the strength of the solenoid. Depressions  56  and gap  54  serve to partially insulate the windings from heat rising from the valve  62  itself, thereby lowering the operating temperature of the solenoid with respect to the valve. 
     EGR valve assembly  50  comprises an improved solenoid actuator  10 ′ including an improved outer polepiece  16 ′ and the concomitant features, bearing  40 , passageways  44  and  52 , and depressions  56 , all as just described. In use, such an assembly may be connected as by bolts  64  to the exhaust manifold  66  and the intake manifold  68  of an internal combustion engine  69  for metering the flow of exhaust gas  70  therebetween. 
     The foregoing description of the preferred embodiment of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive nor is it intended to limit the invention to the precise form disclosed. It will be apparent to those skilled in the art that the disclosed embodiments may be modified in light of the above teachings. The embodiments described are chosen to provide an illustration of principles of the invention and its practical application to enable thereby one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, the foregoing description is to be considered exemplary, rather than limiting, and the true scope of the invention is that described in the following claims.