US2570281A - Snap switch mechanism - Google Patents

Description

Oct. 9, 1951 M, N, RUSSELL ErAL '2,570,281

SNAP SWITCH MECHANISM 2 Sheets-Sheet 1 Filed March l, 1948 2 Sheets-Sheet 2 M. N. RUSSELL ETAL SNAP SWITCH MECHANISM Oct. 9, 195.1

Filed March 1, 1948 Patented Oct. 9, 1951 UN IT ED PATENT Vilfl-l(1f-l?.

astanti 4sNArsvif11-oH MECHANISM Mark N. aussen and william F. Zimmerman,

Syracuse, N. Y., assignors to Pass '& Seymour,

Inc.-, Syracuse, N. Y., aicorporationof'NewiYork ApplicmmMarch 1, 1948,1seriai No. .12.216 y is claims. (ertoe-:svi

.l This inventionrelates to electric "circuit inakers 'and breakers and more particularly 'to what lare commonly known as snap switches of the type used `principally for 'the control of lighting circuits `in residences, stores, offices, factories and the like. p

j It 'is a rgeneral object of the "present invention to provide a novel type of snap rswitch anda unique principle of operation Afor Such switches. More particularly it is yan object of the present invention to develop `a switch of the snap 'type based on the engagement between a rigid oscillatable member and `aresilient prestressed contact in such a. manner that rebound is sbstantiallly prevented. f y AAn important object of the lpresent invention consists in arranging the resilient contact so that it is preloadedand capable of rapidly transferring the preload to the rigid contacting member without changing the stresspattern .in the preloaded portion thereof whereby .rebound and consequent arcing ofthe contacts is prevented. Another object of the inventionconsists in the construction and arrangement of the Vresilient Contact unit and the switch casing `whereby.the assembly thereof automatically applies prelcad to,r a designated portion of the unit.

- Still another object of the present `invention consists in the construction `and arr-angerr'ient of an integral .stationary contact `uni-t for A.srxap switches from resilient sheet metal or spring hardness to comprise a relatively large terminal area, a rigid foot or blade portion and an elongated intermediate or connecting section which isfadapted to be torsionally stressed at alltinies by suitable deflection `of said foot `and in .which the intermediate and blade portions are Vformed to remain in the plane of the `original metal stock until assembled into theswlitch.

`A further object of the present invention con sists in the construction .and arrangement of an integral molded casing for a snap switch ofthe type xdescribedin which recesses are ,provided to receive and immobilize-the terminal portion of the stationary contact runit and to ralorum the heel of the blade uthf` reof and V.in which .an inclined abutment engages the blade beyond the heel and deflects `it .about the iulcrum as the contact smit is moved into position into said ,recessesand arbplies torsional Apre-load to the ysection of the unit between the terminal and blade portions. l

One of the features of the :snap switch of `the present invention -is the arrangement of the 'casing thereof with Slots and recesses to receives.

`stationaryVcontact unit and position it ivltliot the use of fastening 5means `other than th'ecasing cover,A 'and with surfaces cooperating with the unit during and after insertion to prelcad the nnit in such `arnannerthat `the preload is transierred to Vanfeng'aging contart without changing the ex'ire pattern in ,the 1stationary contact.

`Another 'feature "involvingr vfth'e elimination of contact rebouni `on c`i1`-ci1it closing is based-on the torsional stress imparted to a 'nat spring-like con#- nettingy portion of "the Contact nnit *extending between an immobilized v"terminal porti-on `and a laterally projecting frigid blade by its ve'ngaglement `"with portion-s of thecasifrig so :arranged that the moving 'contact can jquickly `assnine this preload and `increase the 'torsion in said connecting por`- tlonwithout changing the ``stress pattern therein. f Other and further objects and features of the invention. including consti-notional arrangements `and 'cooperation of the various 'parts thereof will `be more apparent to those skilled in the art noon -a consideration `ci? the laccorn'pan'ying drawings and following specification rwherein is disclosed but a embodiment of the invention, V4with the understanding tha-tfsuch modifications may be made therein as fall within the scope of appendeclaim's. l

russie drawings:

Fi'g. 1 is a longitudinal central section on enlarged `scale through a ysingle 'pole switch con structed according to the present invention and with the parts arranged `in the on yposition;

Fig. '2 `a fraeinentaryv topplan View of the switch Lcasing and assembled stationary contact and their terminate;

Fig. 3 is fa sectional View` taken parallel to the top and bottom ac'es of the in the plane of line B--B of Fig. land showing the amonnt of pre'flexure of the stationary contacts as `effected assembling them int-he housing;

Fig. 1l lis a transversesection taken on the plane oi line l--I of, Fig. 1l, showing only a .single stationary contact in position: v Fig. 5 is arfragmentary view :similar to Fig. 3 but. showing the bridgmg engaging stationary contacts;

.6 is a top man view .of `the stationary contacts in relaxed position;

Fig.. 7 isza rside v.elevatitm of the contact of Stand Fig. :8 is a front elevation thereof viewed in plane .of the blade and `connecting portion. A

Snap `switches of `the type commonly recessed in walls and 4used 4iirincipnlly for the control of illumination .in residences, pinces. stores and like, operate under the serious handicaps of small size, easy operation, low cost, heavy loads and lamp-load surges. Type C Mazda lamps prevail in most installations and are particularly troublesome because of their characteristic rapid current rise to a peak of approximately ten times that of thesteady-state value when the circuit is first closed. `@This `is' als'the interval when conf tact vibration occurs in most switches and results in arcing and burning which rapidly de stroys the contacts. n Y

Snap switches operate by engaging'ione ormore substantially stationary contacts by a 4 den engagement with the stationary contacts, or with either of the yoke stops 24 and 25.

Figs. 2, 3 and 5 show the switch provided, in the single-pole type, with a pair of relatively stationary contact units 32 and. 34, spaced apart transversely near one end of the housing and symmetrically and oppositely disposed. Their `blade portions, 'which'.shall be morerfully described later, bear, intermediate their lengths. on the vertical edges of the yoke stop 25 and their forward flared ends extend beyond it toward the'center of the casing to receive between spring snapped movable contact.. They have been constructed in manyways .toendeavor to eliminate the above difculties. type of switch preloading of the movable `contacts ll v `In the blade was a satisfactory solution of the problem. as Uy taught by Patent 2,3l8,(ll5, granted May 4, 1943, toARussell and Keilien.

In the'more popular type of snap switch in which laterally 'spaced stationary contacts are bridged by the rigid'sn'apfmember or by a contact thereon unsuccessful attempts have been made'to apply preloading, but vibration was not damped properly since by their construction the stationary contacts were stressed by bending `and the impact from the bridging member of necessityV occurred at some ydistance from the point of application of the preload and hence the bridging` memberdidnot immediately'assume the preload and vibration occurred.

The switch of the present invention is of the popular snap-yoke-bridging type arranged vfor wall box mounting' by 'means of a top strap I0, seen in Fig. 1,. which serves also to retain the insulation cover t2 overthe open upper or front face ofthe switchcasing I4 which is of unitary molded constructionclosed at bottom, ends and sides. Y

A Ur-shapedyoke I5 hastrunnionsjournalled innotches I6 in the side walls I1 of the casing* beneath the notches 1 8 forming journals for the integral trunnionsI I9V on themolded handle Unsinn- 5 This handle unit includes depending `post 2l having a hemispherical end providing va sliding fit with the smaller diameter top` turn of helical spring 22. ,'Ihe Vpost and spring Vpass between the armsrof the yoke, and the reduced lovverend of the lspring fits seat 23 integral with the lower yoke portion. This construction provides snap action tothrow the yoke from the position shown in Fig. l against stop 24,190 the corresponding opposite .position against stop- 25. Lateral shoulders `21 on the handle unit engage beneath the insulating cover I '2 and form limiting stops for the handle, the manipulative portion of which extends out through Va, slotv 2 8 4in the insulating cover andin the strap. To insure positive kickoff, of the yoke in the eventfit should not be ini-.-

' mediately loosened bythe spring-the lower ,ex-

tensions 3 01of the .shoulders 21 are arranged to engagelthe upper ends ofthe yoke armsat 3l f them the transverse member of the yoke. Fig. billustrates this rigid yoke element l5 engaged between theblades of the stationary contacts and restingnalso against the stop 25, which is closed circuit position of the switch.

yThe, contact units as briey referred to are integral stampings formed from flat sheet metal such, as .bronze of spring hardness. They are formed tothe shape illustrated in Figs. 6, '1 and 8 with those vonopposite sides of the Vyoke stop of opposite hand. Referring now to the three figures mentioned, it will be seen that each contact unit includes a relatively large, substantially'squara flat area 35 having an upwardly open slot 35 therein for a purpose which shall be subsequently defined.

Extending at substantially a right angle to the plane of area 35 and from the mid portion of the lower edge thereof is offset section 36, the side edges` of which extend at substantially right angles to the plane of 35. Depending from the remote edge ofthe offset portionand at substantially a right angle thereto is connecting sec-v tion 31 merged with the remote edge by means of Va flange 38. This remote edge is not parallel to the junction of 35 and 36 but extends at a small angle thereto, as clearly seen in Fig. 6, whereby the parts 35 and 31 are not in parallel planes although the Vertical axes of the two planes are parallel. The flange 38 and the connecting portion 31 are arranged to, merge with a slight extension 39 in the offset portion whereby a rigid,V corner is formed at 40 toy prevent twisting of the junction of 36 and 31. The right hand edge of 31, as viewed in Fig. 7, is not substantially out of alignment with"the corresponding edge of the terminal area35.

'The part` 31 is relativelynarrow and of sube stantially uniform width. springing from its left edgenear and `at the'lower end thereof is the transverse blade 4l widened in a vertical direction at 42 to provide adequate contacting area remote from the connecting portion'and having a canted orared tip 44 providing for entrance of the yoke between thetwo blades with effective guidance. The linel45 of junction between the blade 4l and thecanted tip is at such an angle as will be formed by thefplane of the yoke when'passing between thetipsd. -From the junction of the line 45 with its bottom-,edge substantiallyto the heel 46 of the blade and connecting portion, there extends flange 141' at right angles to the plane of the blade and-substantially parallel tothe offset portion 36. Inplan this flange, as shown in'y dotted linesin Fig. 6, is of substantiallyuniform width with rounded corners. Y

-The contact unit will be seen to provide a flat area which canbe immobilized in the casing, an offset portionto position the connecting portion away from 'the wall of the casing, and a blade or foot lextendingfrom the connecting portion and having a heeL'unencumbered by the ange which providesstiffness for the remainder of the bladeA avatar to substantially prevent bending `cf the same undery any lateral stress imparted to it. The

unencumbered heel nts in a slot in the wall of the casing, which acts as a fulcrum so that when contact loading is effected by displacing 'the blade by lateral pressure Von its inner face a substantialiy pure torsional stress is imparted to the connecting or spring portion 31 of the contact unit, the pattern of which does notchange when the ioadis varied. With the only really nex-ible portion of the contact `un-it always under substantial stress, as will next be described, and so arranged that the fiexure pattern therein remains the same whether or not the movable contact is in engagement therewith, it will be seen that a constant damping is applied to the spring and vibration is inhibited, particularly when the circult is being closed, which is the time of greatest potential dilerence between the stationary and movable contacts.

i By stress pattern is meant the disposition of lines and curves of stress in the material of the contact unit such as might be illustrated in a transparent replica under polarized illumination. A- change in stress magnitude such as is elected when the preload is transferred to the movable contact only changes the intensity of the stress pattern lines without changing their shapes or positions on the contact unit, hence the pattern is the same.

In order that the stationary contact unit may eifectively cooperate with the various engaging surfaces of .the casing to be held therein without fastening and be properly preloaded., it is essen tial that it be accurately shaped to conform to the shape of the casing surfaces, which themselves present no diiiculty vbecause of the uniformity of results achieved in molding. Metal of spring hardness is difficult to bend to small angles to exact pattern, and for this reason the connecting portion 31 and the blade 4| remain in the 'same plane as that of the original stock `from which the unit is stamped, whereby no inaccuracies occur in their relative dispositions, until the unit is assembled in the casing. i

Preloading of the stationary contact `units is effected automatically by their mere insertion into the proper places in the casingbecause of cooperative surfaces on the two parts. To immobilize the terminal portion 35 of each contact unit it has attached thereto by means of terminal screw 50 a rectangular block 5I of rigid metal of `substantially the same dimensions as the part 3.5. The vslot permits assembly of these two parts Without removing the screw which `is threaded into the block. This composite `terminal is received in close iitting, facing vertical grooves 5'2 inthe side walls of the casing, as seen in Figs. 2 and 4. The olTset portion 3B being narrower than the terminal portion permits the latter to ntin these grooves, as seen in Fig. 1, with the upper edge of the terminal substantially iiush with the top of the housing where 'it will be engaged by the cover I2 and rmly positioned. The slight variation from 90 of the angle between the terminal and offset portions allows the latter to have resilient engagement with the edge 53 (Fig. 4) at the wall extending between the bottoms of the slots to insure against looseness ci' the terminal in its grooves. The side wall is cut away to permit access to the terminal screw 50 for attaching a conductor thereto.

i Fig. 4 shows how the offset portion 36 Lof one of the stationary contact units rests on the side wail of. the casing and holds the connecting portion 81 clear of the casing walls. It also shows' at the left, where the contact has been omitted.' the notch 55 which forms the fulcrum for-the heel of the blade. This notch is formed between two abutments -51 land 51' on the end wall 56 of the casing. The 'latter abutment is best seen .in Fig. 2, and has walls which slope in two directions, thus forming a funnel-#like entrance and insuring properpositionin-g of the heel 45 in the notch 55 when the contact unit is slid into position.

While the heel 'of the unit is sliding into the previously mentioned funnel-like entrance to the notch 55, the intermediate portion of the inner face of the blade engages the sloping side wall 58er the `yoke stop 25. les it moves down over the wall a portion of the angularity of blade to plane oi terminal area 35 is deected out of it, as seen in Fig. 3, where the dotted lines represent the blade as formed and the solid lines the deflected position of the blade Whenfthecone tact unit has been fully pressed home into the casing. This gure also shows the aring of the inner or unused `face 50 of the notch 55 to clear the heel of the blade and eliminate any chance of blade bending.- It is only the shoulder Si against which the blade i'ulcrums, either under the action of the inclined edge 58 ofV the yoke stop as it is being inserted or when contacted by the yoke. It will be noticed that this inclined edge 58 becomes straight at 6-3 where the blade comes to rest, in order to engage over most of the vertical width of the same.

A-sseen in many of the iig-uros, the yoke stops 24 and 25 whose inclined lateral edges provide the movement to preload the contacts, extend up integral from the bottom wall of the casing and are secured to the end walls respectively by Webs such as 2t', 25'., to increase their rigidity.

The yswitch `illustrated is of the single pole), single throw type and connection is `made by the yoke bridging the blades to close the circuit. In the event that a double throw switch is desired the contact 'assembly shown at the right hand end of Fig. 2 is duplicated at the other end of the casing `and `the added contacts are engaged by the same yoke when in its opposite position. For a three-way switch this same arrangement of `duplicate `sets of contacts is provided and the Contact terminals on one side are connected togather by a bridging strap.

When each stationary contact unit is inserted in the casing the lower edge of its terminal `area 35 and connected terminal block 5| is first reA oeived in the upper ends of the appropriate facing slots 52 .prior to engagement of the blade against the corresponding yoke stop. This then insures against any `rotation of the terminal area about a verticai axis as the heel ofthe blade enters the slot 55 and the intermediate portion of the blade engages the sloping surface 58.1at the edge of `the yoke stop member, deiiecting the blade and preloading the connecting portion 31. Thus when the units are pressed .home the blades are assured of being properly positioned and spaced to receive the entering portion of the yoke which contacts them.

By carefully determining the relative positions of the slot surface 6| and the sloping edge 58 of the yoke stop, the degree of preload given the contacts is `readily controlled. Because `of the careful spacing of the terminal receiving slots si `in respect to the heer slots 55 ne bending iexure of the connecting portion 31 of the contact is el-ected.A Substantially all of thepreload abreast is concentratedI in the spring-likesection 3l in pure torsional stress.A if

`The blades extend forsomewhat less than onehalf of their length inward beyond the stop 25 to receive the yoke, as shown in Fig. 5. The rigidity of the blades resulting from flanges.41 prevents any substantial flexure thereof sov that when the yoke enters between the outer ends of the blades it quickly assumes the load which the stressed contacts placedon the yoke stop by lifting them clear of its edges. When theyoke engages the blades only very little additional latin from andtO a position bridging Said blades saidconductor vWhenbridging so engagingwthe:

blades as to increase the torsional stress on'esaid stress is given to the sections 31 because of the additional deflection imparted to each blade byy the yoke is wholly transferred from-the blade to connecting section 31 where it merely increases the torsionin this strip. Thus there is no change in the pattern of the stress in the contact and no tendency toward Vibration. The stationary contacts being always under'atleast a predetermined load are fully damped against the type of vibration resulting from thesnap closing ofthe circuit and thus superior switch action is achieved, which results in extremely long life under the most adverse conditions. We claim:

l.l A switch vof the type described having in combination, a` pair of terminals, spaced contact units one connected to each terminal, a snapactuated member having a rigid connector movable into and out of bridging relation with said contact units, a casing supporting and coordinatingsaid parts, each of said contact units including, a mounting portion, a relatively narrow connecting portion, and a rigid foot-like blade at the remote end of the connecting portion, means in said casing engaging each contact unit to irnmobilize the mounting portion and engage the Vblade portion on the outside near the connecting portion and on the inside intermediate its length to apply initial torsional stress only, to said connecting portion, said blade portions being arranged side by side with the blades divergent for entrance of said rigidconductor, certain of said means being so related to said blades that the resultant increase in. divergence increases the said torsional stresses, said last mentioned means Dreventing the application of bending stress to said connecting portions. .l

2: A snap switch Yof the type described `having in combination, at least one pair of symmetrically disposed spaced' contact units each including a mounting portion, a relatively narrow resilient connecting portion extending therefrom and a rigid blade projecting laterally from Vthe connecting portion at the free end thereof and having a heel at the rear edge of the connecting portion, a casing receiving said units and immobilizing the mounting portions thereof, fulcrum means in said casing engaging the heel of each blade portion, means normally engaging a side of each blade portion intermediate its ends to impartin'itial torsional stress to the connecting portion, a rigid conductor arranged in .said casing for oscilconnecting portions by moving the blades farther about theirfulcrums in the casing. e 4.

3. A snapswitch of the type described having in combination, at least one pair of symmetrically disposed spaced contact units each including a mounting portion, a Vrelatively narrow resilient connecting portion extending therefrom and a rigid blade projecting laterally from the` connecting portion at the free end thereof and haying a heel at the rear edge of the connecting portion, a casing receiving said units and immobilizing the mounting portions thereof with the blades spaced side by side, fulcrum `means in saidcasing engaging the heel of each blade portion, means in said .casing fitting between the blades and en gagingtheir sides intermediate the ends to slightly rotate them about their fulcrums and impart initial torsional stress to each connecting portion, a-,rigid conductor arranged in said casing for os cillation from and to a position between andengaging said blades, said conductor being of such length as Vto increase the said torsional stress by rotating the blades farther about their fulcrums.

4. The' switch as defined in Vclaim 3 in which the means in the casing fitting between the blades and prestressing the connecting Yportions is a stop for the said conductor. Y. y

5. A snap switch of the type described having in combination, a pair of oppositely disposed, sheet metal contact units each including ya mounting portion, a relatively narrow resilient connecting portionV extending therefrom and` 'a foot-like blade projecting from the free endof the Vconnecting portion, a casing receiving said units, means immobilizing the 'mountingV portions so as to space the blades substantially parallel to each other, fulcrum slots in said casing receiving the heels of the blade portions, means between the blade portions imparting torsional stress to Veachwcontact unit is formed from a sheet .of

metal, the blade and connecting portion remain-i ing in the sameV planey of the metal sheet until stressedin the casing. y '7. A contact unit for snap switches comprising in combination, a single piece of sheet metal of spring hardness `formed toprovide a relatively large flat terminal area for immobilization by a. switch casing; an offset portion extending laterially of the plane of said area from one edge thereof; a flange extending from the remote edge of the offset portion and oppositely to said area;- a narrow connecting portion forming a prolongation of a portion of the width of said flange; a blade in the plane of saidflange and connecting portion and extending from a side of the latter; a flange on the longest'edge of the blade to lstiffen the same; a guide tab at the free end of the blade deviating from the plane thereof and a heel at the opposite end of the blade for cooperation with a suitable fixed fulcrum. 8. The contact unit as defined in claim y7 in which the connected edges of the offset portion are Ainclined toward each other to direct the 9 planes of thebl'adeeand terminal area toward inthe connecting portion. i

9. A contact unit for snap switches comprising in combination, a single piece of sheet metal of spring hardness formed to provide a terminal area adapted to be immobilized, by suitable housing parts; an offset portion eatendingsfrorn `and at a substantially right angle to-one edgeo-f'saiid area; a narrow connecting :portion extending .from one edge of the onset portiorrand ina plane at a large angle to the plane thereof and adapted to be torsionally stressed in use; a blade longer than the width of said portion projecting from one long edge of the connecting portion and adapted to be engaged adjacent its junction with the connecting portion and also intermediate its length by suitable housing parts to stress the connecting portion; said connecting portion and said blade being in the original plane of the sheet metalA stock.

10. A snap switch having in combination a housing, a relatively stationary resilient contact, means on said housing to engage said contact in three spaced areas to preload said contact by pure torsional ilexing of the same to a predetermined stress pattern, a rigid contact, means to oscillate said rigid contact into such engagement with the rst contact as to lift it from one of the engaging areas and assume the load from the preload means, said stationary contact being so mounted as to prevent deflection thereof in the direction of movement of the rigid contact and constructed and arranged as to retain the original stress pattern upon such load assumption.

11. A snap switch of the type described having in combination, at least one pair of symmetrically disposed spaced contact units each including a mounting portion, a relatively narrow resilient connecting portion extending therefrom and a rigid blade projecting laterally from the connecting portion at the free end thereof and having a heel at the rear edge of the connecting portion, a casing receiving said units and immobilizing the mounting portions thereof, fulcrum means in said casing engaging the heel of each blade portion, said blades being so positioned in respect to the mounting portions as to initially toe in, means in the casing between said blades to engage them intermediate their ends to reduce the toe in and impart initial torsional stress to the connecting portions, a rigid conductor arranged in said casing for oscillation from and to a position between and engaging said blades nearer their free ends than said casing means, said conductor being of such length as to increase the torsional stress in said connecting portions but allowing them to close behind it to prevent rebound.

12. A snap switch of the type described having in combination, a pair of oppositely disposed spaced contact units each including a mounting portion, a relatively narrow resilient connecting portion extending therefrom and an elongated rigid blade projecting laterally from the connecting portion at the free edge thereof and having a heel at the rear edge of the connecting portion, an insulating casing supporting said units and having means immobilizing the said mounting portions, a fulcrum in said casing engaging the heel of each blade portion, means on the casing between said blades to engage them intermediate their ends to slightly rotate them apart about their fulcra to impart initial pure torsional stress ltersecton in a line parallel .to the long axisof i0 to the connecting portions, a rigid conductor 4arranged for snap oscillation in said casing from Vand to a. position between and engaging the por,- tions of said4 blades extending beyond said stress imparting means to increase the torsional stress. 13.` Aswitchof the type described having in combination, a pair of terminals, spaced contact units one connected to each terminal, a snapactuated memberahavinga rigid connector movable into andfoutcf bridging relation with said contact units, a casing supporting and coordinating said parts, each of said contact units including an elongated rigid blade portion, a mounting portion immobilized by the casing and a narrow connecting portion meeting the blade at right angles, means forming a part of said casing engaging each contact unit at longitudinally spaced positions thereon to apply initial torsional stress to said connecting portion, said blade portions being localized by said casing means so as to be each engaged by said bridging connector near one of said casing means to increase said torsional stresses, said casing means and contact units being so constructed and positioned relative to each other as to apply zero bending stresses to the units at all times.

14. A switch of the type described having in combination, a pair of contact units each including a mounting portion, an elongated rigid blade portion and a narrow intermediate connecting portion extending normal to the axis of the blade portion, a casing supporting said units with the blade portions spaced apart and confronting each other, said casing being constructed and arranged to engage and immobilize the said mounting portions and having means to engage each of the blade `portions at two axially spaced positions to apply initial pure torisonal stress to its connecting portion, and a rigid conductor mounted in said casing for oscillation from and to a position between and bridging said blades, said conductor in bridging the blades so engaging and moving them each from one of said casing engaging means as to increase the torsionalstress in the connecting portion thereof.

15. A switch of the type described having in combination, a pair of contact units each including a mounting portion, an intermediate connecting portion extending longitudinally therefrom and an extended rigid blade projecting normal from the end of said portion, a casing supporting said units with the blade portions spaced apart, said casing being constructed and arranged to engage and immobilize the said mounting portions and having means to engage each blade portion on opposite sides to apply initial torsional stress to its connecting portion, and a relatively inelastic conductor mounted for oscillation from and to a position between and bridging said blades, said conductor in bridging the blades so engaging and moving them each about one of the casing means as to increase said initial torsional stress in the connecting porti-on while maintaining the stress pattern therein unchanged.

16. A snap switch of the type described comprising in combination, a resilient stationary contact unit having a mounting portion, an intermediate flexing portion and a rigid blade portion normal to the flexing portion, a rigid Contact oscillatable to and from engagement with the said blade portion, a casing for said unit and Contact and arranged to immobilize said unit mounting portion, means in said casing fulcruming one end of said blade, a portion in said casing engaging said blade remote from the fuli u Y orum to apply ,a prelosadingV stress ot'piire torsion to the flexing portion of said unit, said unit. house `ing and Vrigid contact being so oonstruosed` and related that' the' blade v1s iifted from the prejection and the preload is assumed by'the rigid 5 vcontact'. while maintaining the initial stressgpat- 'tern unchanged in the exing portion of the limit".A V MARK N.,RUSSELL.

WILLIAM F. ZIMMERMAN. "xo

. l2 REFERENCES CITED The following references are of record .in the le of this patent:

UNITED STATES PATENTS

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