US3792390A

US3792390A - Magnetic actuator device

Info

Publication number: US3792390A
Application number: US00365024A
Authority: US
Inventors: D Boyd
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1973-05-29
Filing date: 1973-05-31
Publication date: 1974-02-19
Anticipated expiration: 1991-02-19

Abstract

There is provided in accordance with an embodiment of the invention a magnetic actuator device, such a flux cancelling, transferring or shifting type trip device comprising an annular permanent magnet, and a first main pole piece of magnetic material engaging one axial end of the permanent magnet and a threaded magnetic hollow stud member integral with the first main pole piece and extending axially upwardly through the central opening of the annular-shaped permanent magnet. One end of an annular magnetic shunting pole piece engages the opposite axial end of the permanent magnet. An annular magnetic sleeve engages the opposite axial end of the annular shunting pole piece. A plastic bobbin on which is wound an electrical trip coil is positioned radially inwardly of the annular magnetic sleeve. A second magnetic main pole piece of annular-shape engages the axially outer end of the magnetic sleeve. The second main pole piece has an integral hollow threaded magnetic stud member extending axially inwardly of the assembly. A hollow cylindrical magnetic armature is axially movable against a spring force through the opening of the annular second main pole piece and in the communicating hollow interior of the stud member carried by the second main pole piece. The armature is normally held in magnetically latched position by the magnetic field of the permanent magnet against the spring force and in contact with the axially inner end of the stud member carried by the first main pole piece. When a signal pulse is applied to the coil, a substantial amount of the permanent magnet flux is diverted through the shunting pole piece, permitting the biasing spring force to move the armature to a tripped position. An important feature of the construction is the fact that the plastic bobbin on which the trip coil is wound is threadedly engaged with the threaded studs carried by the oppositely disposed first and second main pole pieces whereby to hold the assembly securely together without the use of bolts or other fastening means. A further feature of the construction is the provision of a ''''built-in'''' non-magnetic gap on the movable armature member in the form of a plated coating of a non-magnetic material on the surface of the armature which engages the stud member of the first pole piece in the latched position of the armature. This ''''built-in'''' gap, insures that the movable armature and cooperating stationary magnetic structure operate at an optimum point or region on the curve of magnetic force exerted on the armature vs. gap between the armature and the axially inner end of the stud member carried by the first main pole piece.

Description

United States Patent [191 Boyd [451 Feb. 12, 1974 MAGNETIC ACTUATOR DEVICE [75] Inventor: Donald R. Boyd, Waukesha, Wis.

[73] Assignee: Allis-Chalmers Corporation,

Milwaukee, Wis.

[22] Filed: May 29, 1973 [21] Appl. No.: 365,024

[52] U.S. Cl 335/229, 335/174, 335/234 [51] Int. Cl. H0lf 7/08 [58] Field of Search.... 335/174, 229, 230, 234, 279

[56] References Cited UNITED STATES PATENTS 3,022,450 2/1962 Chase 235/229 X 3,693,122 9/1972 Willard 335/174 3,755,766 8/1973 Read 335/229 FOREIGN PATENTS OR APPLICATIONS 395,271 12/1965 Switzerland 335/229 Primary Examiner-George Harris Attorney, Agent, or FirmRobert C. Sullivan [57] ABSTRACT There is provided in accordance with an embodiment of the invention a magnetic actuator device, such a flux cancelling, transferring or shifting type trip device comprising an annular permanent magnet, and a first main pole piece of magnetic material engaging one axial end of the permanent magnet and a threaded magnetic hollow stud member integral with the first main pole piece and extending axially upwardly through the central opening of the annular-shaped permanent magnet. One end of an annular magnetic shunting pole piece engages the opposite axial end of the permanent magnet. A riannular magnetic sleeve engages the opposite axial end of the annular shunting pole piece. A plastic bobbin on which is wound an electrical trip coil is positioned radially inwardly of the annular magnetic sleeve. A second magnetic main pole piece of annular-shape engages the axially outer end of the magnetic sleeve. The second main pole piece has an integral hollow threaded magnetic stud member extending axially inwardly of the assembly. A hollow cylindrical magnetic armature is axially movable against a spring force through the opening of the annular second main pole piece and in the communicating hollow interior of the stud member carried by the second main pole piece. The armature is normally held in magnetically latched position by the magnetic field of the permanent magnet against the spring force and in contact with the axially inner end of the stud member carried by the first main pole piece. When a signal pulse is applied to the coil, a substantial amount of the permanent magnet flux is diverted through the shunting pole piece, permitting the biasing spring force to move the armature to a tripped position. An important feature of the construction is the fact that the plastic bobbin on which the trip coil is wound is threadedly engaged with the threaded studs carried by the oppositely disposed first and second main pole pieces whereby to hold the assembly securely together without the use of bolts or other fastening means. A further feature of the construction is the provision of a built-in non-magnetic gap on the movable armature member in the form of a plated coating of a nonmagnetic material on the surface of the armature which engages the stud member of the first pole piece in the latched position of the armature. This built-in gap, insures that the movable armature and cooperating stationary magnetic structure operate at an optimum point or region on the curve of magnetic force exerted on the armature vs. gap between the armature and the axially inner end of the stud member carried by the first main pole piece.

15 Claims, 3 Drawing Figures 26 QPERMANENT MAGNET MAGNETIC ACTUATOR DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to magnetically operated actuator devices such as a magnetic flux cancelling, transferring or shifting trip device which may be used as a tripping device for electrical breakers or the like. However, the linear mechanical movement produced during the operation of the device of the present invention may be utilized in other environments than in the tripping of electrical circuit breakers, although it will be described as embodied in such an environment.

2. Description of the Prior Art Devices of the general type to which the present invention relates are well known per se in the art and are shown, for example, in the US. Pat. No. 3,693,122 issued to Henry G. Willard on Sept. 19, 1972, and in the patents referred to in the specification of the foregoing patent.

These devices in general operate upon the principle that an armature formed of magnetic material is retained in a magnitically latched position by a permanent magnet which forms part of the structure, but that upon transmission of an electrical pulse to an electrical winding forming part of the device, the magnetic effect of the permanent magnet upon the armature is counteracted in such manner as to permit a biasing spring to linearly move the armature to a tripped position. The movement of the armature to the tripped position upon the receipt of the electrical pulse applied to the electrical winding may be utilized to trip a circuit breaker or to perform other switching operations or the like.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetically operated actuator device such as a flux cancelling, transferring or shifting trip device, which has an improved assembly arrangement as compared to prior art devices of this type.

It is a further object of the invention to provide a magnetically operated actuator device of the flux cancelling, transferring or shifting type, such as a magnetic trip device of this type, which may be assembled without the use of bolts or other fasteners.

It is an object of a further feature of the invention to provide a magnetically operated actuator device such as a flux cancelling, transferring or shifting trip device, in which the movable armature member is provided with a built-in non-magnetic gap between the armature and the stationary part of the magnetic circuit, whereby to permit operation of the device on an optimum portion of the curve of magnetic force exerted on the armature by the permanent magnet field of the device vs. dimension of gap between the movable armature and the stationary part of the magnetic circuit toward which the armature moves.

In achievement of these objectives, there is provided in accordance with an embodiment of the invention a magnetic actuator device, such as a flux cancelling, transferring or shifting type trip device comprising an annular permanent magnet and a first main pole piece ofmagnetic material engaging one axial end of the permanent magnet and a threaded hollow magnetic stud member integral with the first main pole piece and extending axially upwardly through the central opening of the annular-shaped permanent magnet. One end of an annular magnetic shunting pole piece engages the opposite axial end of the permanent magnet. An annular magnetic sleeve engages the opposite axial end of the annular shunting pole piece. A plastic bobbin on which is wound an electrical trip coil is positioned radially inwardly of the annular magnetic sleeve. A second magnetic main pole piece of annular-shape engages the axially outer end of the magnetic sleeve. The second main pole piece has an integral hollow threaded magnetic stud member extending axially inwardly of the assembly. A hollow cylindrical magnetic armature is axially movable against a spring force through the opening of the annular second main pole piece and in the communicating hollow interior of the stud member carried by the second main pole piece. The armature is normally held in magnetically latched position by the magnetic field of the permanent magnet against the spring force and in contact with the axially inner end of the stud member carried by the first main pole piece. When a signal pulse is applied to the coil, a substantial amount of the permanent magnet flux is diverted through the shunting pole piece, permitting the biasing spring force to move the armature to a tripped position. An important feature of the construction is the fact that the plastic bobbin on which the trip coil is wound is threadedly engaged with the threaded studs carried by the oppositely disposed first and second main pole pieces, whereby to hold the assembly securely together without the use of bolts or other fastening means. A further feature of the construction is the provision of a builtin non-magnetic gap on the movable armature member in the form of a plated coating of a non-magnetic material on the surface of the armature which engages the stud member of the first pole piece in the latched position of the armature. This built-in gap insures that the movable armature and cooperating stationary magnetic structure operate at an optimum point or region on the curve of magnetic force exerted on the armature vs. gap between the armature and the axially inner end of the stud member carried by the first main pole piece.

Further objects and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a view partially in elevation and partially in vertical section of a device in accordance with the invention, the device being shown in magnetically latched position;

FIG. 2 is an exploded perspective or isometric view of the device of FIG. 1; and

FIG. 3 is a curve showing the typical relation between the magnetic force exerted on the movable armature by the magnetic field of the permanent magnet as a function of the gap between the movable armature and the stationary stud 16 to which the armature is magnetically attracted by the field of the permanent magnet.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, the actuator device of the invention is generally indicated at 10 and includes what might be termed a base member generally indicated at 12 including a cylindrical base portion or main pole piece 14 from the opposite surface of which projects an upstanding centrally located hollow cylindrical stud or leg 16. The member 12 including pole piece 14 and stud 16 is formed of a magnetic material such as soft iron or soft steel. The stud 16 has a hollow interior 18. The upper end of stud 16 is open and the lower end of the hollow interior 18 of stud 16 is defined by surface 20 which is flush with the upper surface 22 of the cylindrical pole piece 14 of member 12 which lies radially outwardly of stud 16. Stud 16 is externally threaded for approximately the upper half of the height thereof as indicated at 24.

A permanently magnetized annular-shaped permanent magnet 26 which has the same outer diameter as the outer diameter of the cylindrical pole piece 14 is seated on upper surface 22 of member 12. THe inner diameter of permanent magnet 26 is substantially greater than the outer diameter of stud 16, whereby to define a core window 28 between the inner diameter of permanent magnet 26 and the outer diameter of stud 16.

The annular permanent magnet 26 may be made of any suitable permanent magnet material, but is preferably made of Alnico V, which is well known in the art.

A second or shunting pole piece or member 30 of annular shape and having the same outer diameter as pole piece 14 of base number 12 and as permanent magnet 26 is seated on the upper surface (relative to FIG. 1) of permanent magnet 26. In the illustrated embodiment, the upper surface of shunting pole piece 30, when seated in position, is flush with the upper surface of stud 16. Pole piece 30, like member 12, is formed of a magnetic material such as soft iron or soft steel. The inner diameter of shunting pole piece 30 is approximately one-eighth inch greater, for example, than the outer diameter of the upper threaded end of stud 16, thereby defining an annular air gap 31 between the upper end of stud 16 and pole piece 30.

A bobbin generally indicated at 32 of a suitable nonmagnetic material which is also an electrical insulator, such as nylon, for example, is adapted to be mounted above shunting pole piece 30 in a manner which will now be described. Bobbin 32 includes a axially extending hollow cylindrical hub portion 34 and oppositely disposed radially extending

annular flanges

36 and 38. Bobbin 32 is also provided with a short hollow cylindrical flange-like extension 40 which extends in an axial direction beyond the lower radial flange 36 and which, in substance, is an axial extension of'hub portion 34 of the bobbin. It will be noted that the flange-like extension 40 of bobbin 32 serves as a guide means interposed between the upper end of stud l6 and the inner periphery of shunting pole piece 30 which insures concentric coaxial relation of pole piece 30 relative to the rest of the assembly.

An electrical winding or coil 42 which may have, for example, 1,000 turns, is positioned on the outer surface of hub portion 34 of bobbin 32 between

radial flanges

36 and 38 of the bobbin. A suitable insulating covering or wrapping 44 covers the radially outer periphery of coil 42. The internal periphery of hollow hub 34 of bobbin 32 including the internal periphery of the axially extending flange-like extension 40 of the bobbin is threaded. The internal thread of bobbin hub 34 including extension 40 is adapted to threadedly engage the threaded upper portion of stud 16 of base member 12,

and also to threadedly engage stud 54 of the upper main pole piece 50, as will be described.

An annular sleeve 46 of magnetic material such as soft iron or soft steel and having the same outer diameter as pole piece 14 and also the same outer diameter as permanent magnet 26 and shunting pole piece 30 is adapted to seat on the upper surface (relative to FIG. 1) of pole piece 30 in concentric coaxial relation with respect to bobbin 32. The outer diameter of

radial flanges

36 and 38 of the bobbin is just slightly less than the inner diameter of the annular magnetic sleeve member 46 whereby the bobbin 32 serves to properly locate the magnetic sleeve 46 in concentric coaxial relation to the rest of the assembly 10.

It will be noted that magnetic sleeve member 46 is provided with a passage 47 through which connection leads 49 of coil 42 may be brought out for connection to the source of the electrical control signal applied to coil 42.

What will be referred to as end member generally indicated at 48 is provided and includes an annular main pole piece 50 having the same outer diameter as the

member

14, 26, 30 and 46 and an inner diameter somewhat greater than the outer diameter of the slidable armature to be hereinafter described, and adapted to receive non-magnetic bushing 56 (to be described) in which armature 60 is slidably movable. The inner diameter of the annular pole piece 50 bounds a central opening indicated at 52. A hollow cylindrical stud member generally indicated at 54 is integral with and projects downwardly from the undersurface (relative to FIG. 1) of annular portion 50 of member 48. The passage through stud 54 has the same diameter as and is in axial registry with the central opening 52 of annular pole piece 50. Stud member 54 is of such outer diameter that it may be received within the upper end of threaded hollow hub 34 of bobbin 32. The outer surface of stud 54 is threaded and adapted to threadedly engage the thread on the inner periphery of hollow'hub member 34 of bobbin 32.

The member generally indicated at 48, including the annular main pole piece 50 thereof and also the integral stud 54 thereof, is made of the same magnetic material, such as soft iron or soft steel, of which the

members

12, 30 and 46 are made.

An armature member of a magnetic material such as soft iron or soft steel generally indicated at 60 is adapted to be received in axial sliding engagement iwth the inner periphery of a bushing 56 of a suitable nonmagnetic material such as brass which lines the inner periphery of the axial passage through main pole piece 50 and through stud 54 which extends axially inwardly from member 50. The use of non-magnetic bushing 56 maintains armature 60 in coaxial relation to the passage through the pole piece 50 and through stud member 54, whereas in the absence of such a non-magnetic bushing 56, the armature, under the influence of magnetic forces acting thereon, would be pulled into an eccentric relation to the passage in which it is received. Armature 60 is of cylindrical outer periphery, has a hollow interior or recess indicated at 61, and is open at the lower end thereof relative to the view in FIG. 1.

In accordance with a further feature of the construction, the armature 60 is plated on the entire outer surface thereof with a plating of a suitable non-magnetic material such as nickel phosphate to a coating thickness of, for example, 4 mils (0.004 inch). The purpose of this is to provide the bottom surface 63 of armature 60 with a built-in non-magnetic shim which will space the magnetic material of armature 60 contiguous the bottom surface of the armature in its magnetically latched position as seen in FIG. 1 at a distance from the facing upper surface 17 of magnetic stud 16 which will cause the armature 60 to operate at an optimum point or region on the curve of magnetic force on the armature vs. gap between the armature and the facing surface of stud 16.

Refer to the curve of FIG. 3, which is a graphical example of a typical relationship between force exerted on the armature by the field of the permanent magnet vs. gap in mils between the facing surfaces 63 and 17 of the armature and stud 16. It can be seen that this curve is very steep in the portion of the curve lying be tween and 3 mils, for example. Thus, for example, at 0 mils spacing, the force on the armature might be 75 pounds, while at 3 mils spacing, the force on the arma ture might be 35 pounds, a difference of 40 pounds between 0 mils spacing and 3 mils spacing. However, after about 3 mils spacing, the curve of FIG. 3 flattens out considerably (or is less steep) so that the difference in force exerted on the armature between 3 mils and say mils is much less than between 0 mils and 2 mils. Since it is inevitable that particles or a layer of dust having an axial thickness of, for example, 1 mil, will get between .the surfaces 63 and 17 of the armature 60 and stud 16, it can be seen that if no intentional nonmagnetic gap were provided, such as the plated nonmagnetic coating of the present construction, then the assumed 1 mil thick or greater layer of dust just mentioned would be a variable unknown which would have a pronounced effect on what part of the steep portion of the curve of FIG. 3 the armature 60 would be operating. In fact, if no nonmagnetic gap were provided and the device were operating on the steep portion of the curve between 0 and 3 mils, a deposit of dust of say I mil thickness could simulate the effect of a trip signal received on coil 42 in causing a lessened magnetic attraction exerted on armature 60, and cause a false tripping of the armature 60. However, by providing a built-in gap of non-magnetic material in the form of a plated coating of non-magnetic material of, for example, 4 mils thickness, on the external surface of armature 60, as in the construction of the present invention, it can be insured that the armature operates on the flatter portion of the curve of FIG. 3 which starts, for example, at approximately 3 mils gap. Thus, in accordance with the construction of the present invention, any additional thickness of gap caused by a layer of dust between armature 60 and stud 16 will not cause a significant change in the force exerted on armature 60 by the field of the permanent magnet in the latched position of the armature. This permits a spring 84 to be selected which will be capable of operating in the relatively narrow range of force exerted in the flat portion of the curve of FIG. 3 in which armature will operate. In other words, the magnetic force shown in the curve of FIG. 2 which the spring 84, aided by the opposing or negative magnetic force set up by the electric signal on coil 42, must overcome, varies over a narrower range when the built-in nonmagnetic shim of the type just described is used. Consequently, the magnetic actuator device 10 is more sensitive because of the use of the plated nonmagnetic coating just described.

It will be understood, of course, that the gap value of 3 mils as the point at which the curve begins to flatten out is given only by way of example, and this gap thick ness might vary in different constructions. It might be pointed out that it is not broadly new to use nonmagnetic shims for the purpose just described. However, it is believed that the use of a plated non-magnetic coating on the armature to achieve this result is new.

A rod or stem member generally indicated at 66 which is formed of a nonmagnetic material such as aluminum extends through a passage 68 in the upper end wall 70 of armature 60. Stem or rod 66 is suitably rigidly secured as by pinning or the like to end wall 70 of the armature so that the stem or rod 66 and the armature 60 move linearly as a unit. Rod 66 projects through end wall 70 of armature 60 and through the hollow interior 61 of the armature, thence through the hollow interior 18 to stud 16 of base 12, and thence through a passage 74 in the pole piece 14 of member 12, stem or rod 66 being of sufficient length to project several inches or more beneath pole piece 14 as seen in FIG. 1. A portion of stem or rod 66 indicated at 76 projects above upper end wall 70 of armature 60. The lower portion of stem or rod 66 is externally threaded as indicated at 78 and a nut member 80 is threadedly engaged with threaded stem 66 at a predetermined adjusted position to serve as a stop member which limits the upward motion of the armature 60 and of the connected stem or rod 66 under the influence of the spring biasing means as will be described.

a nut member 83 is threadedly engaged with rod 66 contiguous the lower end of rod 66. Nut member 83 is adjusted to engage a mechanical latch on a circuit breaker when armature 60 and stem 66 move upwardly relative to FIG. 1 upon the receipt of the trip signal by control coil 42, to thereby trip to open position the associated circuit breaker, as will be explained in more detail hereinafter.

A spring generally indicated at 84 is positioned coaxially about rod or stem 66 in such manner that the upper end of the spring relative to FIG. 1 bears against the under surface of the wall 70 of armature 60, while the opposite end of spring 84 bears against the surface 20 which defines the bottom of the hollow interior 18 of stud 16 which forms a part of base 12. In the latched position to armature 60 as shown in FIG. I, the spring is under its maximum compression.

In assembling the device of FIG. 1, and with the parts in the relative relation shown in FIG. 2, base portion 12 is rotated relative to bobbin 32, and the opposite end 48 is rotated relative to bobbin 32 in such manner as to threadedly engagethe thread on stud 16 of member 12 with the internal thread at one end (or lower end relative to FIG. 1) of the bobbin and to threadedly engage the thread on stud 54 with the thread at the opposite end (or upper end relative to FIG. I) of the bobbin whereby to tighten the assembly of parts into tightly assembled relation with respect to each other without use of bolts or other fasteners.

Throughout the specification, any use of the terms upper or lower etc., is with reference to the view shown in FIG. 1, and these terms are only used for convenience in description.

DESCRIPTION OF OPERATION Assume that the device of FIGS. 1 and 2 is intended for use in tripping and associated electrical circuit breaker. During the tripping. operation of the associated circuit breaker, which was initiated by the upward movement (relative to FIG. 1) of armature 60 and nut 83, as will be explained, a suitable mechanism on the circuit breaker will engage end wall 70 of armature 60 and will push the armature 60' axially downwardly relative to FIG. 1 against the force of spring 84 to approach the position shown in FIG. 1 in which the lower or open end of the armature bbuts against the upper surface. of stud 16, to thereby reset the device of FIG.

1 for the next tripping operation. The mechanical push- I tion shown in FIG. 1 by the magnetic field of permanent magnet 26. In the closed or latched position of the armature shown in FIG. 1, most of the magnetic flux of permanent magnet 26 will pass from permanent magnet 26 upwardly (relative to FIG. 1) axially through pole piece 30, through magnetic sleeve 46, thence radially inwardly across pole piece 50 and into the armature 60, thence axially downwardly through armature 60 into stud l6, thence radially outwardly across the lower pole piece 14 and back to permanent magnet 26. The magnetic force of the magnetic flux just described will hold'armature 60 in the latched position ofFIG. 1 against the force of the compressed spring 84.

Assume now thata control signal, which typically might have a duration of 10 milliseconds, is received through the conductors 49 and transmitted to coil 42.

. This. control signal calls for tripping of the associated circuit breaker. The winding of coil 42 will set up a magnetomotive force (MMF) with associated magnetic flux which will oppose the magnetic field of permanent magnet 26 in such manner that a substantial amount of the magnetic flux of the permanent magnet instead of passing upwardly from pole piece 30 into magnetic sleeve 46, pole piece 50 and armature 60 as previously described will instead pass from pole piece 30 across the air gap 31 into the stud 16, thence downwardly relative to FIG. 1 through the stud 16 into the pole piece 14 and thence back to permanent magnet 26, thereby diverting (or short circuiting) the path of a substantial amount of the magnetic flux from permanent magnet 26 which had previously flowed through armature 60. The diversion of the magnetic flux path away from armature 60 due to the signal pulse applied to coil 42 will cause armature 60 under the influence of spring 84 to practically instantly release from contact with stud 16 and the armature will move very rapidly upwardly until stop member 80 on stem 66 abuts against the lower surface, relative to FIG. 1, of pole piece 14. During the travel of stem 66, which travel typically might be about seven-sixteenths, the nut or abutment 83 on stem 66 will engage tripping mechanism on the associated circuit breaker to cause tripping of the circuit breaker. As previously mentioned, the armature 60 is rest to its magnetically latched position as seen in FIG. 1 by mechanism on the breaker during the tripping operation of the breaker.

It will be noted that inthe assembled position of FIG. 1, the lower end of magnetic stud 54 is spaced from the upper end or surface 17 of stud 16 by a distance which typically might be about one-eighth inch, thereby defining an air gap 19 between the lower end of upper stud 54 and the upper end 17 of lower stud 16. Thus, when armature 60 is in magnetically latched position as seen in FIG. 1, practically all of the magnetic flux passing from pole piece 50 will pass through armature 60 to lower stud 16, and practically no magnetic flux will pass through upper stud 54 to lower stud 16.

'However, when the device of FIG. 1 is actuated to tripped position by the action of trip coil 42, as previously described, armature 60 moves upwardly through a travel of typically about seven-sixteenths inch until nut abuts against the under surface of lower pole piece 14. Thus, in the tripped position of armature 60,

the lower end (i.e., the open end) of armature 60 will be spaced about 7/16 inch (i.e., the travel of armature 60) from the upper end 17.0f lower stud 16. During most of the tripping movement of armature 16'just described, practically all magnetic flux flowing from upper pole piece 50 to lower stud 16(this will normally now be a relatively minor portion of the total magnetic flux) will flow through upper stud 54 to lower stud 16, rather than through armature 60 to stud 16. This is because during most of the tripping stroke of the armature 60, the airgap between armature 60 and the upper end 17 of stud 16 is considerably greater than the air gap between the lower end of upper stud 54 and the upper end 17 of lower stud 16, thus causing the shuntingpath from upper stud 54 to lower stud 16 to have a lower reluctance then the path from the upwardly moving armature 60-(during the tripping movement of the armature) to stud l6. Shunting the magnetic flux away from armature 60 and through stud 54 during the upward tripping movement of armature 60 as just described reduces the magnetic force pulling armature 60 toward stud 16, thus making more efficient use of the stored energy of spring 84 during the tripping operation. Under the tripping or tripped condition just described, a substantial part of the magnetic flux will flow through shunting pole piece 30, and across air gap 31 to lower stud 16, and thence through lower pole piece 14 back to the permanent magnet 26.

From the foregoing detailed description of the invention it has been shown how the objects of the invention have been obtained in a preferred manner. However, modifications and equivalents of the disclosed concepts such as readily occur to those skilled in the art are intended to be included with the scope of this invention.

The embodiments of theinvention in which an exclusive property or privilege is claimed are defined as follows: 1

l. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of said assembly, a magnetic first pole piece positioned axially on one side of said permanent magnet and located at one axial end of said assembly, a magnetic first stud member carried by said first pole piece and extending axially inwardly of said assembly, said first stud member being threaded, a second magnetic pole piece positioned axially on an opposite side of said permanent magnet and located at an opposite axial end of said assembly, a magnetic second stud member carried by said second pole piece and extending axially inwardly of said assembly and toward said first stud member, said second stud member being threaded, at least one of said stud members being hollow, a magnetic armature member slidably movable in the hollow interior of said one stud member, a nonmagnetic bobbin axially interposed between said first magnetic pole piece and said second magnetic pole piece, and coaxially positioned about said first stud member and said second stud member, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, spring means normally biasing said armature away from its magnetically latched position, an electrical signal on said winding being effective to counteract the magnetic effect of said permanent magnet whereby to permit said spring to move said armature to released position, said bobbin being threaded and being in threaded engagement with said threaded first and second stud members whereby to hold said assembly in assembled relation.

2. A magnetic acutator device as defined in claim 1 in which each of said stud members is externally threaded and said bobbin has a hub member which is internally threaded, said threaded stud members being in threaded engagement with said threaded hub memher.

3. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece and adapted to receive magnetic flux from said permanent magnet, said second magnetic pole piece having a central opening extending axially of said assembly, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to said central opening of said secondmagnetic pole piece, said second stud member being threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and towards the facing axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin axially interposed between said first magnetic pole piece and said second magnetic polepiece, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position against said facing axial end of said first stud member by magnetic flux from said permanent magnet flowing through said armature and through said first stud member, an electrical signal on said winding being effective to counteract the magnetic effect of said permanent magnet on said armature whereby to permit said spring to move said armature to tripped position, said bobbin having threaded means thereon in threaded engagement with said first and second stud members whereby to hole said assembly in assembled relation.

4. A magnetic actuator device assembly as defined in claim 3 in which each of said stud members is externally threaded and said bobbin has a hub member which is internally threaded, said threaded stud members being in threaded engagement with said threaded hub member.

5. A magnetic actuator assembly as defined in claim 3 in which the end of said second stud member is positioned at a predetermined axial distance from said facing axial end of said first stud member whereby to define a predetermined air gap between said facing ends of said first and second stud members, said predetermined air gap being less than the axial distance between the end of said armature and said first-stud member during most of the tripping travel of said armature,

through which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded, one axial end of a shunting magnetic pole piece engaging an axial end of said permanent magnet opposite said one end of said permanent magnet, said shunting pole piece being radially spaced outwardly of said first stud member to define an air gap between said shunting pole piece and said first stud member, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece, means defining a magnetic path from said shunting pole piece to said second magnetic pole piece, said second magnetic pole piece having a central opening extending in a direction axially of said assembly, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to said central opening of said second magnetic pole piece, said second stud member being threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin axially interposed between said shunting pole piece and said second magnetic pole piece, said bobbin being bounded radially outwardly thereof by said means defining a magnetic path from said shunting pole piece to said second magnetic pole piece, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said coil being effective to cause mangetic flux from said permanent magnet to bypass said armature through said shunting pole piece and said air gap to permit said spring to move said armature to released position, said bobbin having threaded means thereon in threaded engagement with said first and second stud members whereby to hold said assembly in assembled relation.

7. A magnetic acutator device assembly as defined in claim 6 in which each of said stud members is externally threaded and said bobbin has a hub member which is internally threaded said stud member being in threaded engagement with said hub member.

8. A magnetic actuator device assembly as defined in claim 6 in which at least the axial end of said armature which faces the axial end of said first stud member has a nonmagnetic plating thereon to define a shim which spaces the magnetic material of said armature a predetermined optinum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.

9. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member 'carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded, one axial end of a shunting magnetic pole piece engaging an axial end of said permanent magnet opposite said one end of said permanent magnet, said shunting pole piece being radially spaced outwardly of said first stud member to define an air gap between said shunting pole piece and said first stud member, one axial end of a magnetic sleeve engaging an axial end of said shunting pole piece opposite said one end of said shunting pole piece, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece, said second magnetic pole piece engaging the axial end of said magnetic sleeve opposite said one end of said magnetic sleeve, said second magnetic pole piece having a central opening, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to the central opening of said second magnetic pole piece, said second stud member being threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin bounded radially outwardly thereof'by said magnetic sleeve, said bobbin being coaxial with said first and second stud members, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said coil causing magnetic flux from said permanent magnet to by-pass said armature through said shunting pole and said air gap to permit said spring to move said armature to released position, said bobbin having threaded means thereon in threaded engagement with said threaded first and second stud members whereby to hold said assembly in assembled relation.

10. A magnetic actuator device assembly as defined in claim 9 in which each of said stud members is externally threaded and said bobbin has a hub member which is internally threaded, said stud members being in threaded engagement with said hub member.

11. A magnetic actuator device assembly as defined in claim 9 in which at least the axial end of said armature which faces the axial end of aid first stud member which spaces the magnetic material of said armature a predetermined optimum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.

12. A magnetic actuator device assembly comprising an annular permanent magnet having a central opening therethrough which extends in a direction axially of said assembly, a cylindrical magnetic first pole piece engaging one axial end of said permanent magnet, a cylindrical magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded on the external surface thereof, one axial end of a shunting magnetic pole piece of annular shape engaging an axial end of said permanent magnet opposite said one end of said permanent magnet, said shunting pole piece being radially spaced outwardly of said first stud member to define an annular air gap between said shunting pole piece and said first stud member, one axial end of an annular magnetic sleeve engaging an axial end of said shunting pole piece opposite said one end of said shunting pole piece, an annular second magnetic pole piece engaging the axial end of said annular sleeve opposite said one end of said magnetic sleeve, said annular second mag netic pole piece having a central opening extending in a direction axially of said assembly, a hollow cylindrical axially inwardly extending second magnetic stud member carried by said second magnetic pole piece in coaxial relation to the central opening of said second magnetic pole piece, said cylindrical second stud member being externally threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second cyindrical stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin bounded radially outwardly thereof by said magnetic sleeve, said bobbin being coaxially positioned about said first and second stud members, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said coil causing magnetic flux from said permanent magnet to by-pass said armature through said shunting pole and said air gap to permit said spring to move said armature to released position, said bobbin having a hollow internally threaded hub member, said threaded hub member being in threaded engagement with said externally threaded first and second stud members whereby to hold said assembly in assembled relation.

13. A magnetic actuator device assembly as defined in claim 12 in ,which at least the axial end of said armature which faces the axial end of said first stud member has a nonmagnetic plating thereon to define a shim which spaces the magnetic material of said armature a predetermined optimum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.

14. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent 13 magnet, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece and adapted to receive magnetic flux from said permanent magnet, said second magnetic pole piece having a central opening extending in a direction axially of said assembly, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to said central opening of said second magnetic pole piece, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin axially interposed between said first magnetic pole piece and said second magnetic pole piece, an electrical winding carried by said bobbin, said armature normally being held in mangetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said winding being effective to counteract the magnetic effect of said permanent magnet on said armature whereby to permit said spring to move said armature to released position, at least the axial end of said armature which faces the axial endof said first stud member having a nonmagnetic plating thereon to define a shim which spaces the magnetic material of said armature a predetermined optimum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.

15. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, one axial end of a shunting magnetic pole piece engaging an axial end of said permanent magnet opposite said one end of said permanent magnet, said shunting pole piece being radially spaced outwardly of said first stud member to define an air gap between said shunting pole piece and said first stud member, one axial end of a magnetic sleeve engaging an axial end of said shunting pole piece opposite said one end of said shunting pole piece, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece, said second magnetic pole piece engaging the axial end of said magnetic sleeve opposite said one end of said magnetic sleeve, said second magnetic pole piece having a central opening which extends in a direction axially of the assembly, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to the central opening of said second magnetic pole piece, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin bounded radially outwardly thereof by said magnetic sleeve, said bobbin being coaxial with said first and second stud members, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said coil causing magnetic flux from said permanent magnet to by-pass said armature through said shunting pole and said air gap to permit said spring to move said armature to released position, at least the axial end of said armature which faces the axial end of said first stud member having a nonmagnetic plating thereon to define a shim which spaces the magnetic material of said armature a predetermined optimum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.

Claims

1. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of said assembly, a magnetic first pole piece positioned axially on one side of said permanent magnet and located at one axial end of said assembly, a magnetic first stud member carried by said first pole piece and extending axially inwardly of said assembly, said first stud member being threaded, a second magnetic pole piece positioned axially on an opposite side of said permanent magnet and located at an opposite axial end of said assembly, a magnetic second stud member carried by said second pole piece and extending axially inwardly of said assembly and toward said first stud member, said second stud member being threaded, at least one of said stud members being hollow, a magnetic armature member slidably movable in the hollow interior of said one stud member, a nonmagnetic bobbin axially interposed between said first magnetic pole piece and said second magnetic pole piece, and coaxially positioned about said first stud member and said second stud member, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, spring means normally biasing said armature away from its magnetically latched position, an electrical signal on said winding being effective to counteract the magnetic effect of said permanent magnet whereby to permit said spring to move said armature to released position, said bobbin being threaded and being in threaded engagement with said threaded first and second stud members whereby to hold said assembly in assembled relation.

2. A magnetic acutator device as defined in claim 1 in which each of said stud members is externally threaded and said bobbin has a hub member which is internally threaded, sAid threaded stud members being in threaded engagement with said threaded hub member.

3. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece and adapted to receive magnetic flux from said permanent magnet, said second magnetic pole piece having a central opening extending axially of said assembly, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to said central opening of said second magnetic pole piece, said second stud member being threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and towards the facing axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin axially interposed between said first magnetic pole piece and said second magnetic pole piece, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position against said facing axial end of said first stud member by magnetic flux from said permanent magnet flowing through said armature and through said first stud member, an electrical signal on said winding being effective to counteract the magnetic effect of said permanent magnet on said armature whereby to permit said spring to move said armature to tripped position, said bobbin having threaded means thereon in threaded engagement with said first and second stud members whereby to hole said assembly in assembled relation.

5. A magnetic actuator assembly as defined in claim 3 in which the end of said second stud member is positioned at a predetermined axial distance from said facing axial end of said first stud member whereby to define a predetermined air gap between said facing ends of said first and second stud members, said predetermined air gap being less than the axial distance between the end of said armature and said first stud member during most of the tripping travel of said armature, whereby to provide a lower reluctance shunt path for magnetic flux through said second stud member to said first stud member than through said armature to said first stud member during most of the tripping travel of said armature, thus reducing magnetic force acting on said armature during its tripping travel.

6. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded, one axial end of a shunting magnetic pole piece engaging an axial end of said permanent magnet opposite said one end of said permanent magnet, said shunting pole piece being radially spaced outwardly of said first stud member to define an air gap between said shunting pole piece and said first stud member, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece, means defining A magnetic path from said shunting pole piece to said second magnetic pole piece, said second magnetic pole piece having a central opening extending in a direction axially of said assembly, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to said central opening of said second magnetic pole piece, said second stud member being threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin axially interposed between said shunting pole piece and said second magnetic pole piece, said bobbin being bounded radially outwardly thereof by said means defining a magnetic path from said shunting pole piece to said second magnetic pole piece, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said coil being effective to cause mangetic flux from said permanent magnet to bypass said armature through said shunting pole piece and said air gap to permit said spring to move said armature to released position, said bobbin having threaded means thereon in threaded engagement with said first and second stud members whereby to hold said assembly in assembled relation.

9. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded, one axial end of a shunting magnetic pole piece engaging an axial end of said permanent magnet opposite said one end of said permanent magnet, said shunting pole piece being radially spaced outwardly of said first stud member to define an air gap between said shunting pole piece and said first stud member, one axial end of a magnetic sleeve engaging an axial end of said shunting pole piece opposite said one end of said shunting pole piece, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece, said second magnetic pole piece engaging the axial end of said magnetic sleeve opposite said one end of said magnetic sleeve, said second magnetic pole piece having a central opening, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to the central opening of said second magnetic pole piece, said second stud member being threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin bounded radially outwardly thereof by said magnetic sleeve, said bobbin being coaxial with said first and second stud members, an electRical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said coil causing magnetic flux from said permanent magnet to by-pass said armature through said shunting pole and said air gap to permit said spring to move said armature to released position, said bobbin having threaded means thereon in threaded engagement with said threaded first and second stud members whereby to hold said assembly in assembled relation.

11. A magnetic actuator device assembly as defined in claim 9 in which at least the axial end of said armature which faces the axial end of aid first stud member has a nonmagnetic plating thereon to define a shim which spaces the magnetic material of said armature a predetermined optimum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.

12. A magnetic actuator device assembly comprising an annular permanent magnet having a central opening therethrough which extends in a direction axially of said assembly, a cylindrical magnetic first pole piece engaging one axial end of said permanent magnet, a cylindrical magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, said first stud member being threaded on the external surface thereof, one axial end of a shunting magnetic pole piece of annular shape engaging an axial end of said permanent magnet opposite said one end of said permanent magnet, said shunting pole piece being radially spaced outwardly of said first stud member to define an annular air gap between said shunting pole piece and said first stud member, one axial end of an annular magnetic sleeve engaging an axial end of said shunting pole piece opposite said one end of said shunting pole piece, an annular second magnetic pole piece engaging the axial end of said annular sleeve opposite said one end of said magnetic sleeve, said annular second magnetic pole piece having a central opening extending in a direction axially of said assembly, a hollow cylindrical axially inwardly extending second magnetic stud member carried by said second magnetic pole piece in coaxial relation to the central opening of said second magnetic pole piece, said cylindrical second stud member being externally threaded, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second cyindrical stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin bounded radially outwardly thereof by said magnetic sleeve, said bobbin being coaxially positioned about said first and second stud members, an electrical winding carried by said bobbin, said armature normally being held in magnetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said coil causing magnetic flux from said permanent magnet to by-pass said armature through said shunting pole and said air gap to permit said spring to move said armature to released position, said bobbin having a hollow internally threaded hub member, said threaded hub member being in threaded engagement with said externally threaded first and second stud members whereby to hold said assembly in assembled relation.

13. A magnetic actuator device assembly as defined in claim 12 in which at least the axial end of said armature which faces the axial end of said first stud member has a nonmagnetic plating thereon to define a shim which spaces the magnetic material of said armatUre a predetermined optimum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.

14. A magnetic actuator device assembly comprising a permanent magnet having a central opening therethrough which extends in a direction axially of the assembly, a magnetic first pole piece located at one axial end of said assembly and engaging one axial end of said permanent magnet, a magnetic first stud member carried by said first pole piece and extending axially inwardly through said central opening of said permanent magnet, a second magnetic pole piece positioned at an opposite axial end of said assembly from said first pole piece and adapted to receive magnetic flux from said permanent magnet, said second magnetic pole piece having a central opening extending in a direction axially of said assembly, a hollow axially inwardly extending second stud member carried by said second magnetic pole piece in coaxial relation to said central opening of said second magnetic pole piece, a magnetic armature member slidably movable through said central opening of said second pole piece and in the hollow interior of said second stud member and toward the axial end of said first stud member, spring means biasing said armature away from said first stud member, a nonmagnetic bobbin axially interposed between said first magnetic pole piece and said second magnetic pole piece, an electrical winding carried by said bobbin, said armature normally being held in mangetically latched position by magnetic flux from said permanent magnet flowing through said armature, an electrical signal on said winding being effective to counteract the magnetic effect of said permanent magnet on said armature whereby to permit said spring to move said armature to released position, at least the axial end of said armature which faces the axial end of said first stud member having a nonmagnetic plating thereon to define a shim which spaces the magnetic material of said armature a predetermined optimum axial distance from said axial end of said first stud member when said armature is in magnetically latched position.