US3831729A

US3831729A - Solenoid having increased throw capability

Info

Publication number: US3831729A
Application number: US00322298A
Authority: US
Inventors: R Howard
Original assignee: Centronics Data Computer Corp
Current assignee: Genicom Corp
Priority date: 1971-11-30
Filing date: 1973-01-10
Publication date: 1974-08-27
Anticipated expiration: 1991-08-27
Also published as: FR2213568A2; CH612281A5; GB1426655A; DE2362972A1

Abstract

A solenoid for actuating a print wire normally biased against the impact direction by novel spiral spring means. Energization of the solenoid coil rapidly moves the print wire in the impact direction against the bias of the spring. Release of the energy supplied to the solenoid coil causes very rapid return of the print wire to the non-printing (i.e., quiescent) position. The novel spiral spring structure experiences a substantially linear spring force upon deflection and provides significantly increased deflection as compared with conventional structures to allow for substantially rapid initial acceleration of the print wire in the impact direction while providing for rapid return of the print wire to the quiescent condition. The mounting of said spring reduces bouncing on overshooting.

Description

United States Patent 1 Howard [451 Aug. 27, 1974 [54] SOLENOID HAVING INCREASED THROW 3,729,079 4/1973 Zenner et a1. 197/1 CAPABILITY P E R b E P If rimary xaminer-- o ert u rey [75] Inventor. Robert Howard, Roslyn, N.Y. Assistant Examiner R' T Rader [73] Assignee: Centronics Data Computer Corp., Attorney, Agent, or Firm-Ostrolenk, Faber, (jerh &

Hudson, NH. Soffen [22] Filed: Jan. 10, 1973 [57] ABSTRACT [21] P 322,298 A solenoid for actuating a print wire normally biased Related us Application Data against the impact direction by novel spiral spring [63] Continuation-impart of Ser No 203 230 Nov 30 Imeam: Qfthe 5 coil raPidly mOYeS 197] abandoned the print wire 1n the impact direction against the bias of the spring. Release of the energy supplied to the so- 52] Us. CL I I 197 R 33 5 I27 4 lenoid coil causes very rapid return of the print wire to [51] Int Cl 3 3/10 the non-printing (i.e., quiescent) position. The novel [58] Field /274 spiral spring structure experiences a substantially lin- 25l/129 car spring force upon deflection and provides significantly. increased deflection as compared with conven- 56] References Cited tional structures to allow for substantially rapid initial acceleration of the print wire in the impact direction UNITED STATES PATENTS while providing for rapid return of the print wire to 3,521.854 7/1970 LClbel' Cl 81 197/1 UX the quiescent ondition The mounting of aid pring $584,575 6/1971 Distl 197/1 X reduces bouncing on overshooting; 3,661,178 5/1972 Wichmann 335/274 X 0 3,690,431 9/1972 Howard 197/1 14 Claims, 5 Drawing Figures Z] a m 1 1 w /l r 15 /4 7////, tji s A D JZCQ /j 25 a; Z 736 l Q i T I Pmmeuwc w 3.881.729

SHEH EN 2 This application is a continuation-in-part of US. Pat. Application Ser. No. 203,230 filed 30 Nov. 1971 by Robert Howard entitled Improved Solenoid Having lncreased Throw Capacity and assigned to the assignee of the present case, which application has been abandoned.

The present invention relates to solenoid assemblies and more particularly to a novel solenoid assembly especially advantageous for use in dot matrix impact printers and the like due to its capability of providing rapid acceleration in the impact direction and rapid return to its normal position preparatory to subsequent energization and further providing increased throw as compared with conventional designs.

I BACKGROUND OF THE lNVENTlON Solenoid assemblies of the general category described herein are extremely advantageous for use in high speed dot matrix printers of the impact type such as, for example, that described in US. Pat. No.. 3,703,949 issued Nov. 28, 1971 to Robert Howard entitled High Speed Printer and assigned to the assignee of the present case. The printer described therein is capable of forming characters or other symbols by selectively impacting one or more of the slender print wires against an inking ribbon so as to print dots" upon the surface of the paper document. Since printing speed is of the essence in such devices. it becomes quite important to provide an apparatus for rapidly impacting the print wires and rapidly returning the print wires to the rest position. The printing speed in the above-mentioned printing apparatus is of the order of 50 to 165 characters per second depending on the gap in the solenoid which could be from 0.015 to 0.018. To obtain such speeds, it is important to provide solenoid assemblies which are capable of moving the slender print wires from the rest position to the impact position and to return the print wires to the rest position within a time interval of the order of l to 3 milliseconds while at the same time providing an impact of sufficient force to print a clearly legible dot upon the paper document or other printing surface. Whereas solenoid assemblies of the type described in the abovementioned copending application discloses such a solenoid assembly which provides an extremely long useful operating life, it has been found that certain applicationsrequire solenoid assemblies having the capability of moving a print wire over a significantly greater distance (i.e.. throw") than has heretofore been attainable. For example, with solenoid assemblies of the type described in the above-identified copending application, the print wires are caused to move a distance of the order of 0.1 inch between the rest position and the impact position. There exists a number of applications in which a significantly increased throw (i.e., total linear movement) of the solenoid print wire is required. In such applications it has been found that the spring element described in the aforementioned copending application is incapable of providing such an increased throw since deflection beyond its present capabilities causes either the spring to deflect beyond its elastic limit whereby a permanent deformation takes place and the spring member is no longer capable of resuming its normal undeflected condition or its fatigue failure limit is exceeded and fracturing occurs.

BRIEF DESCRlPTION OF THE INVENTION The present invention is characterized by providing a solenoid assembly which incorporates a novel spring element employed to achieve the characteristics described hereinabove. The solenoid assembly of the present invention comprises a case for housing the coil and armature. A stem portion threadedly engages the forward end of the housing and is provided with an axially aligned elongated opening for receiving and reciprocally mounting a slender elongated print wire. The rearward end of the print wire is fixedly secured to the solenoid armature at the forward end of the armature. The rearward end of the armature is fixedly secured to a spring member of the spiral type at its central portion. The peripheral portion of the spring rests against an annular-shaped spacer which abuts a shoulder provided therefor within the solenoid housing. The rearward portion of the armature assembly abuts the projection of a cap member which threadedly engages and seals the rearward end of the housing. The cap member is adjustable to regulate the amount of stress or tension (i.e., preloading) imposed upon the spring member and also provides for half cycle snubbing as anti-bounce device. A tubular wire guide is positioned within the solenoid assembly stem portion and is appropriately lubricated so to reduce wearing of the reciprocating print wire extending therethroughf The spiral spring means is formed from a llat sheet of prehardened tempered spring steel or any other suitable material exhibiting similar characteristics. The

spring has a centrally located hub portion for joinder with the rearward end of the armature. The spring may be compared with an elongated leaf spring which has been constructed in the form of a tight spiral with the entire length of the leaf spring lying within an imaginary plane. The particular spiral design employed may be either an Acrhimedean spiral or an involute ofa line wherein separate interleaving spirals are cut or otherwise formed within the flat spring steel disc. This provides for a relatively long active beam over which the deflection stresses can be absorbed.

The spring maintains the slender print wire of its solenoid in the quiescent condition when the solenoid coil is deenergized. Upon energization of the solenoid coil, the electromagnetic field urges the armature toward the forward end of the solenoid assembly. This force overcomes the spring constant of the spring member to deflect the center portion of the spring member secured to the armature relative to the outer periphery which rests against the aforementioned shoulder. The spring constant of the spring is substantially linear with increasing deflection over the entire range which is substantially increased by as much as 3 to 5 times the throw of conventional solenoid assemblies. This spring design permits significantly increased throw of the sole noid print wire without causing the spring member to be deflected beyond its elastic limit while at the same time providing a spring structure having an extremely long useful operating life.

OBJECTS OF THE lNVENTlON It is therefore one object of the present invention to provide a novel solenoid assembly for use in high speed dot matrix printers of the impact type having an increased throw as compared with conventional structures.

Another object of the present invention is to provide a novel spring member of the leaf spring type in the form of a circular disc member having spiral grooves cut therein, which is extremely advantageous for use in printers of the dot matrix type.

BRIEF DESCRIPTION OF THE FIGURES FIG. 3 is a plan view showing another spring design embodying the principles of the present invention; and

FIG. 4 shows another alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE FIGURES The solenoid assembly I0, shown best in FIG. I, is comprised ofa cylindrical shaped case 11 which is also described in detail in U.S. Pat. 3,690,431 issued Sept. 12, 1972 to Robert Howard entitled Print Head Assembly Containing Solenoids" and assigned to the assignee of the present case (see FIG. la thereof) which is incorporated herein by reference thereto. The case is a hollow cylindrical member provided with a recessed shoulder llu spaced inwardly from its righthand end. The right-hand end is threaded at 11C for receiving an adjustable threaded closing cap 23. The forward end of casing 11 is further provided with a slot 110 through which the leads 17a and 17b of the solenoid extend so as to couple the solenoid to the peripheral driving circuitry. not shown herein for purposes of simplicity.

Case 11 receives a stem member 12 having a threaded portion 12:! threadedly engaging a tapped mounting hole provided in a print head assembly such as. for example. that described in copending U.S. Pat. application Ser. No. 179,457 filed Sept. 10, 1971 by Robert Howard entitled "Printer Head Assembly and assigned to the assignee of the present application. Lock nut 13 threadedly engages threaded portion 12a to firmly secure the solenoid assembly to the print head assembly in a manner shown in detail in the abovementioned copending application.

The stem is provided with a circular-shaped flange portion 12b which is received within the forward end of case 11 whereby the right-hand peripheral edge of flange 12h may nominally abut against shoulder Ila. The rearward most portion 121' of stem 12 has a diameter which is less than both flange 12b and threaded portion l2u with the extreme right-hand end portion 12b being of still further reduced diameter so as to form a shoulder 12c positioned between sections 120 and 1211.

Stem 12 is provided with an axially aligned opening which is comprised of portion 12f of a first diameter and portion 12g of slightly enlarged diameter extending therethrough. The extreme left-hand end portion 1211 is tapered to form a conical entrance portion to facilitate the insertion of a hollow tubular elongated wire guide 14 which is positioned within opening portion 12g so that its right-hand end abuts against shoulder l2j positioned between portions 12f and 12g of the axially aligned opening. The wire guide is preferably forcefitted within opening 12g so as to experience no linear movement relative to stem 12 when the solenoid is operating. The interior of wire guide 14 is preferably coated with a suitable lubricant to reduce friction losses and wearing of print wire 15 which is mounted for reciprocating movement therethrough.

A solenoid core member 16 having a tubular shaped portion 16a, and an outwardly extending flange portion 16b, is telescoped upon the right-hand end of stem 12 whereby the left-hand end of hollow cylindrical portion 160 abuts against shoulder 122 in the manner shown. The hollow cylindrical portion 16a is preferably forcefitted upon the right-hand portion 12d of stem 12. The flanges 12b and 16b, the stem portion 12c, the tubular portion 16a and the interior surface of case 11 define a hollow interior space which is provided for receiving the solenoid coil 17 whose turns or windings are wound about stem portion and tubular portion 16a with the axial length of the windings being defined and physically limited by flanges 12b and 16b. The two end terminals of solenoid coil 17 are wound so as to extend to slot lle provided in case 11. The coil end terminals 17a and 17b are shown as extending outwardly through slot lle. A predetermined length of each of the leads 17a and 17b is mounted with an associated insulating sleeve 17c and 1711. respectively.

As was described hereinabove, the slender elongated print wire 15 is slidably received by the interior of wire guide 14 and extends still further to the right so as to be slidably received by the portion l2fof the stem axial opening and to protrude therebeyond by a predetermined distance. The right-hand end portion of print wire 15 is mounted within an axially aligned opening 18a provided within the solenoid armature 18. Armature 18 is substantially cylindrical in shape and is further provided with a right-hand portion of reduced diameter relative to the main body portion, forming an annular shoulder. The right-hand portion of armature 18 is further provided with an axially aligned opening for receiving a fastening member to secure the armature to spring member 19. Aforementioned U.S. Pat. No. 3,690,431 shows a detailed view of the armature member in FIG. lb thereof.

A solenoid spring 19 is mounted upon armature 18 so that its central opening (to be more fully described in connection with FIGS. 2 and 3) receives the reduced diameter portion of armature l8 and rests against the aforementioned annular shoulder. A stress relief washer 20 is positioned upon spring 19 so that its central opening receives the reduced diameter portion of armature 18. A fastening member 21 has a shaft portion (not shown) passing through the openings in washer 20, spring 19 and armature opening 18d so as to firmly secure washer 20 and spring 19 to the righthand end of armature 18 with a minimum of stress concentration applied to the center section of the spring.

Spring member 19 has a substantially circular-shaped outer periphery, as will be described in detail hereinbelow. A ring-shaped spacer member 22'has its left-hand surface bearing against shoulder 11b of solenoid case 11 and has its right-hand surface bearing against the periphery of spring 19. The spacer 22, spring 19 and armature 18 are maintained in position within case 11 by means of cap 23 which is provided with a threaded portion 23a which threadedly engages tapped portion 23b whose left-hand surface bears against the head of fastener 21. An axially aligned opening 230 is provided for adjusting the preloading of spring 19 in a manner to be more fully described. A linear groove 23d is provided in the left-hand surface of cap 23 to facilitate insertion of an adjusting tool such as, for example, a screwdriver head. By rotating cap 23 within casing 11, the amount of preloading of spring 19 may be easily and accurately controlled.

As is directed in detail in aforementioned US. Pat. No. 3,690,431 the various components of the solenoid assembly are adjustable so as to facilitate fine adjustment thereof. Once the spring has been appropriately preloaded by the rotation of end cap 23 within the sole noid assembly, an epoxy 25 may be deposited in the region shown in FIG. 1 to rigidly maintain cap 23 in the desired position. From the foregoing, it can be seen that the initial assembly of the solenoid can be performed in a simple, straightforward fashion, since relatively little concern need be given at the time of initial assembly to the final adjustment thereof. The final and accurate adjustment of the solenoid assembly may then be performed, at which time epoxy is deposited at the forward and rearward portions of the housing in the manner shown in FIG. 1 to retain the components in their desired alignment. For example, the amount of preloading exerted upon spring member 19 may be measured by the insertion of a probe (not shown) through axial opening 23C. After precise adjustment of the preloading, the epoxy may be deposited at the position 25 as shown in FIG. 1.

FIG. 2 is a detailed plan view of one preferred embodiment of spring 19 shown in FIG. 1 which is comprised of a central portion 19a having a central opening 191: for receiving the reduced diameter portion of armature 18. As was mentioned hereinabove, spring member 19 is preferably formed from a flat sheet of spring steel so as to have a circular periphery 19c. In addition thereto, a pair of spirals 1911 and 190 are cut or otherwise formed within the disc-shaped spring member with the spirals defining an involute of a line. The inner ends 19f and 19g of the

spirals

19d and 19a,

respectively, can be seen to lie opposite one another along a diameter D, of the disc-shaped spring. The outer ends 1911 and 191' of spirals 19 and 190, respectively, also lie opposite one another and along a diameter D of the disc. It can be seen that the region be- I tween the two separately generated involutes constitutes an elongated leaf spring wound in a right spiral fashion. The characteristics of this design cause the spring member 19, even though it is of a spiraldesign, to function in a manner substantially similar to that in which an elongated straight leaf spring functions. FIG. 2a shows the spring characteristics of spring 19 wherein spring deflection d is plotted along the X-axis, while spring force], is plotted along the Y-axis. The slope C relates deflection to spring force. From the point of zero deflection (i.e., with the spring in its normal undeflected state) the spring which is preloaded follows a constant spring rate slope. Low stress springs of this design having high length to deflection ratios have proven highly successful for use in solenoids where the length of travel reaches a maximum of 0.085 inch whereas springs of aforementioned US. Pat. No. 3,690.43] have been found to reach their elastic limit and to become permanently deformed after deviation distances of the order of 0.015 inch.

The positioning of the cap 23 with respect to the neutral position of the spring 19 (preloading in direction of forward stroke) prohibits reverse bending hence eliminates bounce, i.e.,: rearward kinetic energy is dissipated in impacting the cap 23. The caps absorbtion characteristics are enhanced by the selection of a suitable plastic material.

FIG. 3 shows a modified spring member 19' having a central portion 19a provided with a central opening 191) for receiving the fastening member which joins it to armature 18.

The spring member is provided with a circular periphery 19c and is further provided with two

spiralshaped slots

19d and 19e whose inner ends 19f and 19g respectively, lie along a first diameter D, and whose outer ends 1911 and l9j lie opposite one another along a diameter D The spiral shaped

elongated slots

19d and 19e' are of slightly greater width W than the width W of the spiral-shaped slots provided in the spring 19 of FIG. 2. The spirals 19d and of FIG. 2 represent curves lying in a plane which are generated by a point moving away from or toward a fixed point at a constant rate while the radius vector from the flxed'point rotates at a constant rate. Such a spiral is known is an Archimedean spiral. As was the case with the spring member 19 of FIG. 2 the pair of spiralshaped slots 19d and 190 can be said to define an elongated leaf type spring which is wound in a tight spiral manner so that the entire leaf spring lies within a plane. The characteristics of the spring force for the spring 19' of FIG. 3 are substantially similar to those shown in FIG. 2a.

In one preferred embodiment of the present invention, the print wires are moved a distance of the order of 0.075 inch from the rest position to the impact position. Relationship of force plotted against deflection distance is a similar linear relationship as compared with the logarithmic relationship obtained from the spring design of the spring shown in FIG. 2 of the aforementioned U.S. Pat. No. 3,690,431. The linear spring force characteristic greatly simplifies the determination of the required driving force provided by the solenoid coil 17 while the design further provides a greatly increased amount of deflection without causing the spring to be deflected beyond its elastic limit. Thus, the spring 19 (or 19) provides a substantially linear relationship between spring force and deflection distance over the entire throw of the print wire 15.

When the solenoid coil is deenergized (i.e. when'impact against a paper document has occurred) the magnitude of the spring force is substantially at its maximum value, thereby placing the print wire exclusively under the control of spring 19. The force imposed upon the armature by spring 19 at this time causes a rapid return of the armature toward the rest position.

FIG. 4 shows an alternative arrangement for the spring of FIG. 3, for example, wherein cutout portions 19k and 19/ have been provided behind the end portions 19] and 1911 of the slots formed in the spring to relieve the torsional stresses which are developed at points 20a and 20b, respectively. Likewise, slots 20c by radially aligned integral joining portions 200 and f which are transversely aligned to the spiral spring portions. This structure significantly improves the useful operating life of the spring by markedly reducing torsional stresses which are otherwise present at the inner and outer extremities of the spiral spring portions.

It can therefore be seen that the foregoing invention provides a novel spring design enabling substantially increased throw for a slender print wire not heretofore attainable in conventional solenoid driving assemblies.

Although there has been described a preferred em-' bodiment of this novel invention, many variations and modificationswill now be apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.

What is claimed is: l. A solenoid assembly comprising: a housing; an elongated substantially cylindrical stem having an axial opening of small diameter extending the length of said stem; said stem being secured to the forward end of said housing; an annular shaped coil positioned within said housing and surrounding a portion of said stem; a closure cap threadedly engaging the rearward end of said housing; an armature positioned within said housing and along the axis ofsaid housing, at least the forward portion of said armature extending to a position relative to said coil; whereby said armature is activated and moved by the magnetic field generated by the coil when energized; V slender elongated wire positioned within the axial opening of said stem and having its forward end extending beyond the forward end of said stem and its rearward end secured to the forward end of said armature; circular shaped spring means having a circular outer periphery and having its central portion joined to the rearward portion of said armature; said housing having an annular shoulder engaging the periphery of said spring means; said spring means being formed of a resilient metallic sheet and having a pair of spiral shaped slots encircling said central portion and interleaving one another; each of said spiral shaped slots having an inner end portion near said central portion and an outer end portion near said spring outer periphery and encircling said central portion at least once. 2. The solenoid assembly of claim 1 wherein the inner ends of said slots lie on opposite sides ofsaid central portion.

3. The solenoid assembly of claim 2 wherein the inner ends of said slots lie along a common diameter of said spring.

4. The solenoid assembly of claim 1 wherein the outer ends of said slots lie on opposite sides of said central portion.

5. The solenoid assembly of claim 4 wherein the outer ends of said slots lie along a common diameter of said spring.

6. The solenoid assembly of claim I wherein said spring is formed from a sheet of spring steel.

7. The solenoid assembly of claim I wherein said spring is adapted to exert a rearwardly directed force on said armature to normally hold said print wire in the non-impact position when said solenoid coil is deenergized, said coil being adapted to overcome the rearward directed force of said spring when energized to rapidly move said print wire toward the impact position, and thereby flex said spring to cause said print wire to return rapidly to the non-impact position when said coil is deenergized; said spring being adapted to engage said closure cap in returning to the non-impact position, which closure cap eliminates bounce.

8. The solenoid assembly of claim 1 wherein each of said spiral slots represents an Archimedian spiral.

9. The solenoid assembly of claim 1 wherein each of said spiral shaped slots represents an involute of a line.

10. A solenoid assembly comprising:

a hollow substantially cylindrical housing;

an elongated substantially cylindrical stem having an axial opening of small diameter extending the length of said stem;

said stem being secured to the forward end of said housing;

an annular shaped coil positioned within said housing and surrounding a portion of said stem;

a closure cap threadedly engaging the rearward end of said housing;

an armature positioned within said housing and along the axis of said housing. at least the forward position of said armature extending to a position relative to said coil; whereby said armature is activated and moved by the magnetic field generated by the coil when energized;

a slender elongated wire positioned within the axial opening of said stem and having its forward end extending beyond the forward end of said stem and its rearward end secured to the forward end of said armature;

circular shaped spring means having its central portion joined to the rearward portion of said armature;

said housing having an annular shoulder engaging the periphery of said spring means;

said spring means being formed of a resilient metallic sheet and comprising an inner ring and a concentric outer ring;

a pair of spiral-shaped spring portions each encircling the inner ring at least once and having their inner ends adjacent the inner ring being integrally joined to said inner ring and having their outer ends adjacent said outer ring being integrally joined to the outer ring.

1 l. The assembly of claim 10 whereby the outer ends of said spiral-shaped spring portions are diametrically opposite one another.

of the spring means.

14. The assembly of claim 10 wherein the free ends of the inner ends of said spiral-shaped portions are transversely aligned to their spring spiral-shaped portions, thereby forming an integral joining section between said spring spiral portion and said inner ring to significantly reduce torsional stresses during deflection of the spring means.

Claims

1. A solenoid assembly comprising: a housing; an elongated substantially cylindrical stem having an axial opening of small diameter extending the length of said stem; said stem being secured to the forward end of said housing; an annular shaped coil positioned within said housing and surrounding a portion of said stem; a closure cap threadedly engaging the rearward end of said housing; an armature positioned within said housing and along the axis of said housing, at least the forward portion of said armature extending to a position relative to said coil; whereby said armature is activated and moved by the magnetic field generated by the coil when energized; a slender elongated wire positioned within the axial opening of said stem and having its forward end extending beyond the forward end of said stem and its rearward end secured to the forward end of said armature; circular shaped spring means having a circular outer periphery and having its central portion joined to the rearward portion of said armature; said housing having an annular shoulder engaging the periphery of said spring means; said spring means being formed of a resilient metallic sheet and having a pair of spiral shaped slots encircling said central portion and interleaving one another; each of said spiral shaped slots having an inner end portion near said central portion and an outer end portion near said spring outer periphery and encircling said central portion at least once.

2. The solenoid assembly of claim 1 wherein the inner ends of said slots lie on opposite sides of said central portion.

6. The solenoid assembly of claim 1 wherein said spring is formed from a sheet of spring steel.

7. The solenoid assembly of claim 1 wherein said spring is adapted to exert a rearwardly directed force on said armature to normally hold said print wire in the non-impact position when said solenoid coil is deenergized, said coil being adapted to overcome the rearward directed force of said spring when energized to rapidly move said print wire toward the impact position, and thereby flex said spring to cause said print wire to return rapidly to the non-impact position when said coil is deenergized; said spring being adapted to engage said closure cap in returning to the non-impact position, which closure cap eliminates bounce.

10. A solenoid assembly comprising: a hollow substantially cylindrical housing; an elongated substantially cylindrical stem having an axial opening of small diameter extending the length of said stem; said stem being secured to the forward end of said housing; an annular shaped coil positioned within said housing and surrounding a portion of said stem; a closure cap threadedly engaging the rearward end of said housing; an armature positioned within said housing and along the axis of said housing, at least the forward position of said armature extending to a position relative to said coil; whereby said armature is activated and moved by the magnetic field generated by the coil when energized; a slender elongated wire positioned within the axial opening of said stem and having its forwarD end extending beyond the forward end of said stem and its rearward end secured to the forward end of said armature; circular shaped spring means having its central portion joined to the rearward portion of said armature; said housing having an annular shoulder engaging the periphery of said spring means; said spring means being formed of a resilient metallic sheet and comprising an inner ring and a concentric outer ring; a pair of spiral-shaped spring portions each encircling the inner ring at least once and having their inner ends adjacent the inner ring being integrally joined to said inner ring and having their outer ends adjacent said outer ring being integrally joined to the outer ring.

11. The assembly of claim 10 whereby the outer ends of said spiral-shaped spring portions are diametrically opposite one another.

12. The assembly of claim 10 wherein the inner ends of said spiral-shaped spring portions are diametrically opposite one another.

13. The assembly of claim 10 wherein the free ends of the outer extremities of said spiral-shaped spring portions are transversely aligned to their spiral portions, whereby forming an integral joining section between said spiral-shaped portions and said outer ring to significantly reduce torsional stresses during deflection of the spring means.