Abstract: There is provided structure for use in adjusting certain design parameters of a relay during initial assembly of the relay. The structure includes an adjustment member insertable between a bobbin and a core. The adjustment member is preferably U-shaped having arms straddling the core and a backspan which acts as a stop to define the travel of an armature. The adjustment member is driven between the bobbin and core and permanently fixed in place when the desired parameters have been attained. The armature is forced against the motor assembly until the desired design parameters are obtained and the motor assembly is then permanently fixed in place within the relay. An armature is forced against the core moving it within the bore of the bobbin until the desired parameters have been obtained. Thereafter, the core is permanently fixed in position within the bore of the bobbin.

Abstract: A stator component is described which comprises first and second limbs defining respective pole faces. Each limb includes an outer surface which is provided with a pair of elongate recesses, each extending in a direction angled to the pole faces. The recesses conveniently intersect one another, and are conveniently formed using a plunge cutting technique.

Abstract: A rotary electromagnetic actuator comprises a rotatable shaft carrying a rotor which is rotatable relative to a stator. A magnetic circuit is set up which comprises the stator and the rotor, the reluctance of the magnetic circuit being dependent on the relative rotational orientation of the rotor and stator and decreasing in a particular direction of rotation such that the reluctance is a minimum at an equilibrium position to which the rotor is therefore biased. The rotor is selectively impelled to advance away from the equilibrium position, subsequently again becoming part of a magnetic circuit, the reluctance of which decreases in the direction of rotation to a minimum at an equilibrium position to which the rotor becomes biased. The actuator provides undirectional rotation for successive actuations at substantially uniform torque and is suitable for use in actuating rotary devices such as rotary valves.

Abstract: A proportional solenoid-driven valve control assembly contains a moveable, magnetic armature positioned adjacent to one end of a fixed pole piece that protrudes from a solenoid bore and is supported by means of a compact suspension spring arrangement adjacent to the one end of the bore. The movable armature is configured to provide two radial magnetic flux paths. One flux path traverses a fixed radial shunt gap at one end of fixed magnetic pole piece that is located in and protrudes from the one end of the solenoid bore. The other flux path traverses a variable geometry reluctance gap defined between a radially projecting, tapered rim portion of the armature and an inwardly projecting tapered portion of the solenoid assembly housing.

Abstract: An electromagnetically actuated valve for an automotive vehicle includes a pole assembly having a coil situated therein and a combination valve element and armature, including a tubular axially moveable valve spool for controlling the flow through at least one inlet port and at least one outlet port. A magnetic head having a first end attached to an end of the valve spool and a second end nested within a relieved area provided in the pole assembly functions to move the tubular valve spool in response to magnetic flux induced by the coil.

Abstract: An electromagnetically actuated friction clutch includes a rotatably driven input gear. A rotatable armature is connected to the input gear and includes a friction face and nitrocarburized outer surfaces. A pole piece has a pole face for engaging the friction face of the armature and nitrocarburized outer surfaces. An electromagnet generates an electromagnetic field to shift the armature from a disengaged position to an engaged position wherein the friction face of the armature and the pole face of the pole piece are frictionally engaged. An output shaft is driven by the input gear when the armature is in the engaged position. The nitrocarburized outer surfaces of the armature and the pole piece resist wear, while interior portions of the armature and pole piece direct magnetic flux flow along a desired path.

Abstract: A sealing element is fitted between the outside periphery of the magnet armature and the inside periphery of the valve housing and, parallel to the sealing element, an orifice is fitted in the armature, thereby establishing a permanent pressure fluid connection between the pressure fluid chamber disposed inside the magnet armature and the pressure fluid chamber disposed outside the armature.

Abstract: An electromagnetically operated pressure switching valve has a valve housing, a magnet coil arranged inside the valve housing, a magnet core associated with the magnet coil and having a passage for a pressure medium provided with a valve seat, a magnet armature, a valve closing element arranged in the valve housing for controlling a flow of the pressure medium. The magnet armature has a portion extending in the magnet coil to an area of a greater magnetic flux density. The portion of the magnet armature cooperates with the valve seat of the passage and is limited by a ring-shaped circular flange-like edge extending outside of the magnet coil.

Abstract: The metering valve is controlled by an electromagnet having a fixed core, a oil, and an armature. The core is formed by pressing and subsequently sintering a mixture of powdered ferrous material and an epoxy binder; and presents a low magnetic hysteresis and low parasitic currents, so that, for a given energizing current, a greater magnetic force is achieved and more rapidly, and, for a given magnetic force or maximum operating frequency, the core and/or coil may be made smaller.

Abstract: A low-noise solenoid has an armature with a projecting ridge receivable within a groove in a pole piece. As the ridge enters the groove, the magnetic force which opposes a return spring decreases until there is equilibrium whereat the armature remains suspended without impacting upon the pole piece.

Abstract: A solenoid actuator, adapted for use in a fuel injector, has a pole member, an armature associated therewith, and an energizable winding associated with the pole member. The solenoid actuator is latchable by residual magnetism between the pole member and the armature without the aid of a permanent magnet. The pole member and the armature are composed of steel having a carbon content of between 0.8% and 1.2% and an RC hardness of between 40 and 60. The armature is reciprocable between a first position in which the armature makes physical contact with the pole member and a second position in which the armature is spaced from the pole member. The armature occupies the first position when the winding is energized and the second position when the winding is deenergized. The armature is held in the first position by residual magnetism between the pole member and the armature when the winding is deenergized after being initially energized.

Abstract: An electromagnetic valve opens and closes a fuel return passage (23, 24) of a fuel injection pump, to control a time of fuel return when fuel compressed by a plunger (12) of the fuel injection pump is overflowed from a high-pressure fuel chamber (19) to a low-pressure fuel chamber (11). A valve needle (36) causes a reciprocative movement in an axial direction thereof to open or close the fuel return passage (23, 24). A terminal (46), supplying electric current to the electromagnetic valve, is disposed along the axial direction of the valve needle (36). A plate-like armature (43), connected to one end of the valve needle (36), has a through hole (43a) allowing the terminal (46) to enter therein and causing a reciprocative movement together with the valve needle (36). The outer periphery of the armature (43) extends radially further than the terminal (46).

Abstract: The invention concerns a bistable magnetic drive with an assembled armature 3, 4 consisting of a high-grade, disk-shaped permanent magnet 3 with two appropriately shaped pole pieces 4,4' and displaceable sideways off the coil 1 transversely to its axis inside an electromagnetic annular pole-piece Rp between two electromagnetic stop pole-pieces 5, 5'. The invention describes illustrative embodiments and applications of this drive as an excursion and oscillating magnet, further a circuit for DC operation. A capacitor circuit furthermore allows mono-stable operation. The advantages are a lightweight armature, high efficiency, bilateral operation, versatility and simplicity.

Abstract: A print head of a wire-dot printer and a production method thereof, in which superhard chips are attached to contact surfaces of an armature and a core by brazing. The superhard chips can be readily secured to the contact surfaces of the armature and the core and are hardly worn by repeated impacts between the armature and the core which makes it possible to maintain the initial print quality for a long time and to obtain excellent abrasion resistance and strength of the contact surfaces in comparison with a conventional superhard film. It is not necessary to perform a grinding processing of the superhard chips with high accuracy after deposition thereof, and thus the thickness of the superhard chips can be readily controlled. As a result, production cost of the print head can be reduced.

Abstract: A compressor having an electromagnetically actuated valve is disclosed. The movement of the compressor's piston is controlled by an electromagnetically actuated valve. The valve includes an electromagnetic element having a core and a coil, and an armature. The core has a surface and an opening at the surface extending through the core. The surface also includes a continuous channel extending around the opening. The channel has a top portion of frustroconical cross-section and a bottom portion. The coil is disposed in the bottom portion. The armature element has a raised portion dimensioned to be received within the top portion of the channel. The valve shaft is disposed within the opening of the electromagnetic element and is connected to the armature. The compressor piston is connected to one end of the shaft. A support spring is disposed within the opening of the electromagnetic element, and extends from the retaining bar to the upper surface of the cylinder.

Abstract: A length of wire is toroidally wound, and then the toroid is encapsulated and cut in two along a plane that is perpendicular to the toroidal axis. The resulting portions are used as stator and/or armature of a solenoid, and in the solenoid the cut face of each forms one side of the solenoid's working gap so that the faces move toward and away from each other as the solenoid operates.

Abstract: A high-speed impact hammer type solenoid capable of operating at high speed such as 500 to 1000 strokes per second and with a relatively long stroke length. It is housed in a cylindrical shell and includes two separate coils and magnetic circuits operating on a single short plunger. The plunger is driven magnetically in both directions. The short plunger has reduced mass. This is made possible by elongated backstops extending through the coils to the plunger and by a flux washer extending from the shell to the plunger. Close control of the plunger stroke is achieved by a non-magnetic sleeve surrounding the plunger and spacing the backstops which are screw threaded into the shell. Resilient anti-residual pads on each side of the plunger absorb the impact of the plunger with the backstop, start the plunger in the opposite direction and speed up the decay of the magnetic field.

Abstract: A d-c electromagnetic solenoid designed to open or close circuit breakers, for operating valves, and any other applications which a relatively large force is applied to a member which moves a relatively short distance through the conversion of electrical energy into optimum mechanical work from magnetic attraction.

Abstract: A driving apparatus for imparting motion to an image sensor is disclosed that includes a frame structure, a support spring coupled to the frame structure, a support structure coupled to the support spring and a foundation structure, wherein the support structure keeps the support spring suspended above the foundation structure, a first drive solenoid coupled to a first side of the frame structure, and a second drive solenoid coupled to a second side of the frame structure that is adjacent to the first side of the frame structure wherein at least one drive solenoid includes a solenoid body, a magnetic coil, and a magnetically deflectable solenoid beam.

Abstract: A solenoid having a bobbin with a rectangular core hole in which is mounted an armature having a rectangular cross section, and a pole piece having a rectangular cross section. The bobbin has a magnet wire coil wound around a bobbin tubular body which is rectangular in cross section. The bobbin has an upper flange and a lower flange that encloses the ends of the coil. A tubular can is seated around the bobbin. A top flux plate is seated on the upper can surface and a bottom flux plate is seated under the lower can surface. The flux plates are secured to the bobbin flanges. A sealing, electrical insulating heat dissipating material is disposed inside of the tubular can which seals the coil and insulates the coil from the flux plates, tubular can and solenoid mounting screws.

Abstract: A solenoid has a coil energizable to displace an armature in one direction and a spring urging return of the armature in the other direction to a rest position in which an end of the armature engages a stop. The end of the armature has a conical projection machined thereon which projects from the end thereof. The stop is a resilient grommet having conical bore therein so that the rest position of the armature is defined by the spring urging the armature to the rest position with the conical projection of the armature resting in the conical bore of the stop to thereby provide noise-free and rattle-free positioning of the armature at the rest position.

Abstract: An improved electrically actuated solenoid is used to regulate the function of a hydraulic flow control valve. The solenoid directs a control valve to generate a predetermined output pressure as a function of the solenoid input current. The working air gap surface area is increased by locating it at a greater circumferential radius than the pole piece to increase the air gap permeance, thereby increasing the output force obtainable from a solenoid of relatively small physical dimensions and low cost.

Abstract: A solenoid plunger magnet system is disclosed, preferably to be used as print hammer in a print hammer device. Known solenoid plunger magnets have in the exciting coil a first interferric gap as working interferric gap and a second interferric gap outside the exciting coil as loss interferric gap. The magnetic lines of force at the second gap are lost as moving forces for the solenoid plunger. The purpose of the invention is to increase the magnetic force of solenoid plunger magnets by using the second gap as well for generating forces, without affecting the generation of forces at the inner gap. For this purpose, a male taper control is arranged at the outer gap, which has a cylindrical shape and the usual length of loss gaps. A considerable increase of the magnetic force is thus achieved.

Abstract: A solenoid apparatus includes a plurality of plate members of which at least some are electromagnets, each of the latter including at least one toroidal coil of conductive wire. Guiding device is connected to the plate members so as to keep them in alignment, and so as to allow the plate members to separate from each other and approach each other while preventing the separation between any adjacent pair of electromagnets from exceeding a given maximum. The device includes means allowing electrical energization of the coils of conductive wire.

Abstract: An electromagnetic switch device for a starter motor has a plunger 21 defining a cylindrical body 22 slidably fitted in a sleeve 8, and a trumpet shaped intermediate plate 23 which generally extends in a plane perpendicular to the central axis of the cylindrical body. At least part of the cylindrical body has a thin-walled portion which extends in a direction opposite the extension direction of a cylindrical rod 24 driven by the plunger, from a junction 21a to the radially outer portion of the intermediate plate.

Abstract: A solenoid actuator that includes a ferromagnetic armature body mounted for movement through a defined stroke in the direction of its central axis. A stator includes an electrical coil surrounding the axis of the armature, and a ferromagnetic stator body having a first portion axially opposed to the armature body and a second portion radially surrounding and spaced from the armature body. A ferromagnetic ring is positioned on the armature body radially adjacent and opposed to the second portion of the stator body. The opposed surfaces that define the air gap between the armature ring and the stator body are of identical conical construction and overlap each other as viewed in the axial direction.

Abstract: A solenoid operated hydraulic control valve which has a specially ported spool valve to statically balance spool force in the opened and closed positions, and a specially designed solenoid operator which includes very small ratios of gaps to cross-sectional areas for the working and non-working gaps between the armature and stator.

Abstract: An electromagnetic shear lock comprising an electromagnet having an E-shaped cross section with a coil positioned about the middle leg of the E is mounted to a doorway and an armature is mounted to the door, where in one form the middle leg of the E is shortened to define a recess to receive an elongated armature member and provide mechanical reinforcement against shearing when the electromagnet is energized. The armature is constructed of at least two telescoping members where the outer armature member is U-shaped with legs which contact the outer legs of the electromagnet and the inner member is attracted into the defined recess and will engage the inner surfaces of the outer legs of the electromagnet. Even if the outer member is pryed down from the electromagnet, at least the inner armature members will still be in a magnetic circuit with the inner electromagnet armature member in the defined recess and providing mechanical reinforcement in engaging the inner surfaces of the outer legs.

Abstract: A composite elastomer-tipped tubular metallic device for armature and check valve applications has a hollow tubular metal body and a generally spherical elastomer ball fitted into an end of the body so as to project outwardly beyond the body end. Also, the body of the device has a seat structure provided internally at the end thereof which is adapted to capture and retain the ball in the body end. In several embodiments of the device, the seat structure is defined by a pair of axially spaced annular ribs formed internally in the body at the end thereof, with the ribs projecting radially inwardly toward a central axis of the tubular body. In one embodiment, one of the ribs may be formed as a split ring received in an annular groove in an interior surface of the hollow body.

Abstract: A torque motor including a frame of soft magnetic material, such as soft iron, spaced first end portions on the frame, an armature which is a magnet, spaced second end portions on the armature, a mounting member for mounting the armature on the frame for allowing motion of the armature relative to the frame, each of the end portions of the armature including a north pole and a south pole, air gaps between the end portions of the frame and the end portions of the armature, a coil mounted on the frame, and the armature being in the form of a laminate with a central layer being a rare earth magnet between outer layers of magnetic material.

Abstract: An electromagnetic clutch according to the invention includes a first rotatable member having an axial end plate of magnetic material and a second rotatable member. The second rotatable member includes a drive shaft, and a hub mounted on an end of the shaft. An annular armature plate is disposed on an outer peripheral portion of the hub facing the axial end plate of the first rotatable member with an axial air gap therebetween. An electromagnetic device is disposed within the first rotatable member for attracting the armature plate to the axial end surface of the first rotatable member. The armature plate is elastically connected with the hub by a plurality of leaf springs and is made of materials with a low Curie point. Meltable elements are disposed on the armature plate to define the axial air gap between the first rotatable member and the armature plate.

Abstract: Solenoid-actuated valves in a four-speed automatic transmission which are direct-acting and have a non-close clearance design to prevent dirt or other contaminants from interfering with the operation of said valves. The solenoid-actuated valves also include armature having an offset portion extending toward electromagnetic means to be offset toward the center inside the electromagnetic means for efficient magnetic flow.

Abstract: An electromagnetic acutator device for selecting needles in a knitting machine comprises two windings (3,4) positioned on opposite sides relative to an armature (5) in a support structure (2) made from a ferromagnetic material. The armature (5), which is also made from a ferromagnetic material, is hinged onto the structure (2), according to an axis substantially perpendicular to the axis of the windings (3,4) and is subject to the action of the magnetic filed generated by the windings (3,4) in order to be moved between two angular positions. The armature is hinged at one end engages a selection lever (11) at its other end, and has a configuration with two transversal portions (19, 20) protruding from opposite sides, and penetrating inside the interior of the windings (3,4).

Abstract: A solenoid has a coil with an outer core and an inner core. The cores provide the magnetic circuit around the coil. The inner core includes a rotor which has radially extending armatures. The armatures are positioned in gaps between radially extending poles of the outer core so as to form air gaps between the poles and the armatures. When the coil is energized, the armatures are attracted to the poles and cause the rotor to rotate, thereby closing the air gaps. The shaft is coupled to the rotor through a drive mechanism so as to rotate with the rotor. The shaft is made of a material with a relatively high magnetic permeability, as are the inner and outer cores, so that the shaft is magnetically coupled to the inner core rotor. When the rotor stops rotating, the shaft also stops. A return spring returns the rotor to its deenergized position when the coil is deenergized, recreating the air gaps. The drive mechanism allows the shaft to remain stationary while the rotors return to the deenergized position.

Abstract: A high-speed solenoid valve is described having a polymer film lubricant located in the radial magnetic gap between the bore of the stator housing and the solenoid armature. The film forms a lubricating barrier during axial movement of the armature in the stator bore.

Abstract: An electromagnet, for valves, comprises thin-walled axially symmetrical parts and is adapted to different uses by special air gap arrangements and armature suspensions; the electromagnet generates high magnet forces at low armature mass and is employable, in particular, for actuation of fuel injection valves for internal combustion engines and in general hydraulics; the electromagnet possesses a high mechanical stability and eddy current depletion combined with simple armature suspension and low cost of manufacture.

Abstract: An electromagnet includes a lightweight movable body in the form of a hollow cylindrical body. The resistances of circulation paths for eddy currents flowing in the movable body due to electromagnetic induction are increased to suppress the eddy currents. Excitation coils are connected parallel to each other to reduce the coil inductance for increasing the magnetic forces to attract the movable body and the response thereof.

Abstract: An electromagnetic contactor is taught having an E-shaped movable magnetic armature and a complementary E-shaped permanent magnet. The outboard legs of the E-shaped members abut at closure. Each of the central legs of the complementary magnetic members have significant magnetic material removed from the face thereof leaving complementary abutting nipples or protrusions which mate or make with each other as the outer legs make or mate with each other. The total reluctance of the magnetic system when in the abutted state is significantly lower than if the air-gap material had not been removed. This allows the kickout spring to operate effectively to separate the magnets at an appropriate time. The lower the magnetic reluctance the higher the residual magnetism and the more difficult it is for the kickout spring to separate the magnetic members and thus the contacts of the contactor during an opening operation.

Abstract: An electromagnetic actuator comprises a stationary element made of soft magnetic material which element has a plurality of magnetic poles, a magnet one magnetic pole of which is secured to the stationary element, a movable element made of soft magnetic material which element is faced with the magnetic poles of the stationary element and the other magnetic pole of the magnet through narrow gaps so as to form a magnetic circuit arranged in parallel to the direction of magnetic flux generated by the magnet, and a coil element wound around the stationary element which coil is so arranged as to energize the magnetic circuit in series. When electric current is flowed through the coil, the balance of magnetic force between these magnetic fluxes loses and therefore, the movable element is reciprocally moved with respect to the stationary element. This device can be actuated with a large moving force generated by a fine current.

Abstract: A winding method for winding the coils of a stator structure which has one or more intermediate coils and a pair of end coils having a reduced number of turns. The coil or coils may be directly connected to supply terminals and the coils are connected in series. The method involves using a dummy connector to form a connection point between the two end coils, all the coils being wound from a single length of wire with the dummy connector forming the start and finish point for the winding operation.

Abstract: An electromagnet or solenoid includes an armature in the form of a piston guided for sliding axially in a magnet housing surrounding it. The armature has at least one bearing ring providing the slide bearing of the piston. The bearing ring is held by engagement in at least one groove in the outer surface of the piston. The groove extends axially on the piston and is open to one piston end surface so that the bearing ring and the groove are engaged without great deformation of the bearing ring and the bearing ring can be closed. The bearing ring has an inwardly projecting curvature engaging the groove. The outside edges of the groove are rounded along a length corresponding to the width of the bearing ring forming an edge recess which receives the curvature and holds the bearing ring against axial thrust.

Abstract: An electromagnet which serves particularly to control a fuel injection valve for fuel injection systems in internal combustion engines. The electromagnet includes an inner core of soft-magnetic material, which is surrounded by a magnetic coil. An outer core at least partly surrounds the magnetic coil and has an outer pole located in the same plane as an inner pole of the inner core. On one side of the magnetic coil, between the inner core and the outer core, there is a first annularly embodied and radially magnetized permanent magnet, and on the other side of the magnetic coil there is a second annularly embodied and radially magnetized permanent magnet. Facing the poles, there is an armature, which at one end is joined to a valve needle that has a valve body cooperating with a valve seat, and on its other end forms a first working air gap with the outer pole and a second working air gap with the inner pole.

Abstract: An electromagnetic device for use in, for example, the diaphragm control of a camera. The electromagnetic device includes an electromagnet, an armature, an armature shaft and a restricting member for inclining the armature with respect to the armature shaft, whereby even if the armature shaft is not parallel with the attraction face of the electromagnet, holding the armature attraction by the electromagnet is certain.

Abstract: An actuating magnet comprises a magnet body (10), an exciting winding (15) rranged in the said magnet body (10) and a radially movable armature (17) having a pre-determined first radial cross-sectional surface. One end of the armature (17) having a second radial cross-sectional surface (19) substantially larger than the said first cross-sectional surface (20) forms a pole piece (18). Between the said pole piece (18) and a wall (11) of the said magnet body (10) which covers in the radial direction at least part of one end face of the said exciting winding (15) there is provided an air gap (20).In order to increase the actuating force of the magnet without changing its outer dimensions and to provide a closed system, the wall (11) covers the said end face of the exciting winding (15) and shields it magnetically, the pole piece (18) is arranged opposite to the inside of the said wall (11) and the air gap (20) extends in the radial direction (FIGS. 1a+b).

Abstract: A DC current electromagnet is provided developing a high pulling force with reduced current consumption. This electromagnet has the form of a cylindrical pot and comprises a mobile armature which slides along a yoke piece through a closure air gap --e, S--, whose reluctance is low with respect to the reluctance of the working air gap --E, s-- when this latter is open and substantially constant during movement.

Abstract: In order to define more precisely the pivot line (25) on which the armature (17) swings about the magnet yoke (12) at its line of contact therewith, the armature restoring spring (28) is made of spring wire (29) bent into meander shape. One end of it is attached to the magnet yoke and the other to an extension (31) of the armature that extends across the pivot line. A spring clip (34) is snapped over the restoring spring and onto the sides of the magnet yoke pushes the restoring spring against the magnet yoke and opposes lengthwise shifting of the armature across the edge (32a) on which it is pivoted.

Abstract: A polarized electromagnetic relay has a movable block which includes two U-shaped magnetic plates, each of which has first and second ends. The plates are fixed on individually associated and opposite poles of a permanent magnet to oppose each other. A core has one end placed between the two magnetic plates. A yoke is magnetically connected to the core, the yoke having a fork on one end thereof. The fork is placed outside each of the two magnetic plates. A spool with a coil mounted thereon has a center hole through which the core is inserted. The movable block is supported in a manner which enables it to move in a direction which is parallel to the magnetic axis of the permanent magnet.

Abstract: A relay is disclosed which is sufficiently miniaturized for use in a DIP socket. It employs tapered switching contacts for obtaining maximum deflection with a given force. Novel adjustable fixed contacts are employed. The solenoid portion is designed to avoid offset adjustment problems and maximize the efficiency of magnetic coupling.

Abstract: A proportional solenoid consisting of a stationary pole piece of ferromagnetic material which has a radially externally facing frusto-conical section surrounding a cylindrical recess in the stationary pole piece. Both the stationary pole piece and armature are fitted into the bore of a guide tube. Thus, the bore of the guide tube provides the required concentricity between the movable armature and the stationary pole piece. The movable armature is provided with an integral reduced diameter cylindrical nose that is complimentary to the cylindrical recess of the stationary pole piece.

Abstract: A proportional solenoid has a fixed pole piece 39 and a movable armature 45, both fitted into the bore 52 of a guide tube 36 that provides the required concentricity between the fixed and movable pole pieces 39 and 45. One of the pole pieces has a cylindrical recess 56 and the other pole piece has a reduced diameter cylindrical nose 62 that is complementary to cylindrical recess 56. A radially inwardly facing frusto-conical surface 95 is formed in cylindrical nose 62 to be disposed within recess 56 of the other pole piece and provide a frustoconical pole piece section producing a linear forcestroke curve.