US2927171A - Snap acting switch - Google Patents

Description

March 1, 1960 w. E. RHODES 2,927,171

SNAP ACTING SWITCH Filed Nov. 29, 1957 2 Sheets-Sheet 1 0 0 w w R m n Ill-l 5 mm m M A m i .4233 W (P Q W/ y a 2 Q 1 I w W .1 v n a u u n h i m/ 4 a a L'tllnvk l I I w 4:4 2/4; r E RH E mm v z ||lV1.l.\l1||| H M I HH. Y V/ 1 w w L a 5% w W w ATTORNEY March 1, 1960 w. E. RHODES 2,927,171

SNAP ACTING SWITCH Filed Nov. 29, 1957 2 Sheets-Sheet 2 INVENTOR ATTORNEY 6-.0 44 .08 J? ./a .20 24 .26 .32 I a SNAP ACTING SWITCH William E. Rhodes, Columbus, Ohio, assignor to Robertshaw-Fulton Controls Company, Richmond, Va., a corporation of Delaware Application November 29, 1957, Serial No. 699,719

8 Claims. (Cl. 200-67) The present invention relates to a snap acting device, and more particularly to a new improved snap acting electrical switch incorporating a novel improved type of arcuate operating spring of special configuration.

United States Letters Patent to E. F. Kohl Nos. 2,237,705, 2,558,219 and 2,558,258 disclose snap acting switches incorporating an arcuate operating spring of rectangular outline. The aforesaid Kohl patents disclose that the arcuate operating spring for such switches can be of normally flat thin rectangular resilient metal strip bent into arcuate form when assembled in the switch, as well as preformed. However, it was found in practice that such a normally fiat rectangular spring was usually not as good as the alternative preformed arcuate spring. The reason for this is that when such an arcuate operating spring is bent from normally flat rectangular strip material upon assembly in the switch, the end loading on the spring generally causes a sharper curvature at the center of the spring are than at its sides. This sharper curvature at the center of the arcuate spring provides a localized high stress concentration similar to a notch effect, which sometimes causes the spring to fracture in use, and generally shortens the operating life of the spring and switch in any event. Moreover, although snap switches with such normally flat rectangular springs are fully satisfactory for some applications, the switch operating characteristics such as contact pressure and operating force are not as precise or controllable for high tolerance applications, and deteriorate relatively rapidly even if initially satisfactory, compared to a like switch having a carefully made preformed arcuate spring.

Accordingly, in making switches like those disclosed in the aforementioned Kohl patents, it has been the usual commercial practice to preform the arcuate springs in such switches rather than use a normally flat spring. In making these arcuate springs in large quantities, the arcuate shape is obtained by bending the spring strip around a roller to give it a permanent deformation of desired arcuate contour, or by an appropriate stamping operation. However, in mass production of such preformed arcuate springs, it is difiicult to eliminate slight variations in the length of chord between the mounting ends of the spring. Variations in chord length are also introduced by heat treatment of springs heretofore thus preformed, as amplified below. All other conditions being equal, these variations in chord length cause a slight variation from switch to switch in such characteristics as operating force, click point (i.e. operating snapover point), release force, and contact pressure. Furthermore, preforming of the arcuate springs and production control and inspection thereof are relatively costly compared to the over-all cost of making Kohl type switches. For example, for close-tolerance precision switch applications, each preformed arcuate spring is tested with a spring tester to measure its force thus increasing switch production cost significantly.

In mass production, there is frequently a slight varianited States Patent tion from switch to switch in the spacing between the spring supporting shoulders of the actuating blade and the actuated contact blade, due to tolerances in these blade members and the switch stops which -control their operating positions. For some critical close-tolerance switch applications, these variations may introduce significant changes in operating characteristics and control thereof when a preformed spring is used, because they have an efiect similar to variations in the chofd length of the arcuate operating springs as discussed above.

The present invention provides a new improved, lower cost precision snap acting switch combination incorporating a novel normally flat arcuate spring of special design and configuration, which effectively solves the abovediscussed problems in prior arcuate rolling spring switches.

It is an object of the present invention to provide a i new improved snap switch which has greatly improved operating characteristics.

It is another object of the present invention to provide an improved snap acting switch incorporating a normally fiat arcuate operating spring of novel design which makes it unnecessary to use preformed arcuate springs, even for close-tolerance precision switch uses. It is a related object to provide such a switch with such a novel normally fiat operating spring which eliminates variations in switch operating characteristics and, control thereof heretofore encountered with preformed arcuate springs due to variations in their chord length. It is still an other related object of this invention to provide for such snap acting switches a novel operating spring which renders insignificant variations in compression of the spring during operation, thereby greatly reducing the adverse effect on operating characteristics of production variations, compared to prior switches having preformed arcuate springs.

It is also an object of the present invention to provide a new improved snap acting switch including a novel operating spring of flat tempered strip material, which assumes an arcuate shape upon assembly into the switch, and has a special configuration such that the flexural stress of assembled arcuate spring is substantially the same throughout its length (mounting and effects being ignored), and does not significantly exceed the elastic limit of the spring material. It is a related object of this invention to provide such a novel normally flat oper- V ating spring which will eXert the desired snapover force on the actuated contact member at the critical snapover position.

it is a further object of the present invention to provide a new improved snap acting switch incorporating a novel normally fiat operating spring of special configuration which achieves higher contact pressures andhas better anti-vibration characteristics compared with a similar switch having a preformed spring of rectangular outline with substantially the same operating pressure and travel of the actuating member.

It is still another object of the present invention to provide a new improved snap acting switch and a novel normally flat operating spring of special configuration which can be blanked directly from high tensile strength material, thereby obviating the steps of preforming the springs from low tensile strength material and heat treating them to raise their tensile strength, as has been heretofore done in making preformed arcuate springs. It is performance and uniformity of characteristics with a substantial savings in production cost that may run as high as of the over-all cost of the switch in lower priced,

'more competitive precision switch lines, whereby the present invention affords important competitive advantages;

The above-discussed and other objects and advantages of the present invention will be apparent from the following description and appended claims with reference to the drawings wherein:

Figure 1 is a plan view showing a so-calied closed type snap acting switch embodying the present invention, 7

with parts broken away to show underlying structure;

Figure 2 is a cross-sectional view along line 2-2 in Figure l; a

F e 3 s a de i plan iew sho ing the actuating,

flat switch operating arcuate spring of this invention,

illustrating the difierence between (a) the total displacement of the spring mounting ends when assembled into a switch and (b) the displacement of said ends during overcenter switch operation;

operatively associated with the actuating and actuated members 34 and 36 in such a manner that the spring bears upon the abutments or shoulders '42 and 44 during switch operation as hereinafter more fully described. Spring 52 is made of a thin flat piece of resilient metal which is cut to the desired length and has a main portiont54 of novel special configuration according to thepresent invention as hereinafter amplified. The spring 54 has at its ends a pair of mounting cars 56 provided with apertures 58 which loosely receive the tongues 38 and 40 when the spring is compressed into substantially semicircular shape and assembled in the switch 20 with the ears 56 bearing upon the abutments or shoulders 42 and 44 respectively.

Suitable contacts 60 and 62 are arranged opposite the double movable contact 50 on the opposite sides of the actuated blade 36. The contact 60 is mounted on a suitable electrically conductive arm 64 which is'secured to the sw h base 221W, and in eleet icalcontact with, bushing 66 which is provided at its lower end with a terminal screw 68., The contact 62 is mounted inlike manner on "a suitable electrically conductive arm 70 which is mountedon the base 22 by, and in electrical contact with, a bushing 72 similar-to bushing'66 and pro Figure 6 is a schematic free body diagram illustrating I a normally flat rolling spring under the action of loading imposed on itwhen used in a switch such as shown in Figure 1;

Figure 7 is a cross-sectional view taken along line 77 in Figure 6;

Figure 10 is a perspective view showing a modified form of actuated and actuating blade members for use in the switch of Figures 1 and 2; and

Figure 11 is an enlarged plan view showing a modified,

normally flat, constant stress rolling spring according to' this invention, incorporating the same principles of design configuration as the spring inrthe embodiment of Figures 1-8, with modified endmounting tongues, for use with the actuating and actuated blade'members shown in Figure 10. a

Referring particularly to Figures l4,'there is shown at 20 a closed type snap acting switch embodying the present'inventi'on. Switch 20 comprises a base 22 of dielectric material, which is provided adjacent one end with a bore 24 in which there'is seated a metal sleeve 26 having an internally threaded bore 28'; A screw 30 is threaded into the upper end of bore 28 to secure to the upper side 32 of the switch base 22 an elongated actuating blade 34 which extends within a rectangular opening in an actuated blade 36. Each of the actuating and actuated switch blades 34 and 36 has a tongue or projection 38 and 40, respectively, which extends toward the other. Each of said actuating and actuated blades 34 and 36also has a pair of shoulders or abutments 42 and 44, respectively, in opposed spaced relation. The actuating and actuated blades may be formed as a single vstamping from a sheet of flexible metal, which is provided a with a hole 46 through which the mounting screw 30 extends. Alternatively, the actuating and actuated blades 34 and 36 may be formed separately if desired, with each having suitable mounting hole like aperture 46 atthe mounting end thereof, in a manner obvious to those in the art. In the stamping operation, an aperture 48 is provided at the free end of the actuated blade member 36 for securing a movable contact 50 thereon.

the assembled switch 20, an arcuate spring 52 is V ated member 6 applies a three or thrust, action to' id Figure 8 is a'free body diagram of a section of the roll- 3 vided with a like terminal screw, not shown, A terminal screw. 74 is similarly mounted'in the lower end of bushing 26 which is electricallyconnectcd to the actuating blade 34. Thus, electrical current can flow from terminal screw 74 through the actuated blade 36 and movable contact 50 to either of the fixed contacts 60 or 62 and thence to the corresponding terminal screw at the latter contacts. As is well known in the art, if desired to make and break only one circuit,the contact 60 may be omitted and the arm 64 utilized as a stop.

.Switch 20 is providedwith a cover 76, having sides 7 S which are received on shoulders'80 extending around the periphery of switch base 22. Cover 26 is provided with a bore'82 in which there is slidably mounted a reciprocable operating, plunger 84, the lower end of which engages the actuating blade' member34.

As illustrated in dotted line in Figure 2, when the plunger 84 is depressed, the actuating'lever 34 is moved downward, causing the spring 52 to move with a rolling action untilv the actuating lever 34 reaches a point at which the apertures 58 in spring 52 are substantially in alignment with the free end of actuated member 36, and the ends 56 of the Spring are substantially 'at right angles to the actuated member 36,. ,At, this point, the actuating member 34 and the spring 52 are overcen'ter critical position, and upon further downward movement ofthe actuating m mber 34, the pring end 56 engaging the actumember 36 'andfsnaps it upward in Figure 2, thus substantially instantaneously opening the contacts 50, 62 and closing the contacts 50, 60. Slots 58 have a somewhat greater width than the thickness of the tongues 38 and 40 to provide a non-interfering connection, The abovedescribed switch operation may occur through engagement of the tongues 38 and '40 with the sides of the cooperating apertures 58 in the ends of spring 52 and/or through engagement of the sides of the spring cars 56 against the shoulders 42 and 44 on the actuating and actuated member's, respectively. It will be noted that in the overcenter critical position, the distance between 6a the spring supporting abutments 42 and 44 is less than the distance between thosejabutments in the contact positions, so that the spring is compressed during switch operation. V

The upper surface 32 of switch base 22 is slanted to permit the above-described displacement of the actuating blade 34. The actuating blade 3.4 is biased so that when the operating plunger 8.4, is, released, blade 34 returns to its full-line normal position, causing the operating spring 52 to roll on the abutments 42 andj44 in the opposite direction and snap the actuated member 36 back to normal position thereby closing contacts 50, 62- and opening contacts 50, 60.

As discussed in the intrc duction hereof, the arcuate operating spring in switches of this kind was heretofore generally preformed to the desired arc, and in some instances might be made of normally fiat rectangular strip spring material bent into arcuate shape upon assembly; but such prior springs had certain disadvantages as noted above. In the present invention, however, the novel improved normally fiat arcuate operating spring 52 now disclosed in detail provides an improved snap acting switch combination with better precision snap acting characteristics which may be more precisely controlled than heretofore.

This improved normally flat operating spring 52 has a novel configuration whereby the plan contour, and

hence the cross section, of the spring is varied so thatwhen the spring is bent into arcuate form upon assembly in the, switch, (1) the stress in the spring is substantially uniform at all points along its length, and (2) the stress in no part of the spring substantially exceeds. the

elastic limit of the material in either installation or use.

It has been developed that there is an optimum mathe-,

matical relationship between the width at any point along the length of the spring 52 as a function of (a) the spring length, ([2) spring thickness, and (0) maximum width at the longitudinal center of the spring. And it is possible to mathematically develop and compute the optimum plan configuration of this new normally fiat spring for a given material and size.

The principal limiting conditions on the arcuate spring for snap-acting switches such as here involved, include the following:

The following notations are used in developing the formulas for determining the optimum plan configuration of rolling spring 52 so that the maximum stress is substantially uniform throughout thelength of. the bent spring when assembled in the switch, the end efiects of ears 56 being ignored:

SFlexural stress M-Bending moment tThickness of spring (see Figure 7) b-Width of spring (see Figure 7) 1) ,-width of spring at (i=3 6 Position parameter (see Figure 6) R-Radius of spring (see Figure 6) PLoad on spring (see Figure 6) a IArea moment of inertia of spring, EModulus of elasticity ;Figure 6 illustrates the configuration of the subject rolling spring under the action of loading denoted by P. The differential equation of the elastic curve of this spring is defined by the equation, I

fl'here is essentially no stress concentration due, to

curvature because the, thickness, 1, is so small incomparison with the radius of curvature, R, of the spring. Accordingly,-the'fiexural stress at any section location defined by 0 canbe computed from the well-known elementary equation, r

Simultaneous solution of Equations 1 and 2 by elimination of the factor yields the equation,

The factor dy 2 an (dx is the radius of the elastic curve of the spring at any point defined by the position parameter 0. For any spring under consideration in this application, the factors E and t are both constant. Thus, for the fiexural stress, S, to remain constant along the spring at any value of 9, it is evident from Equation 3 that the radius of curvature must remain constant. This means that the elastic curve of the spring is a perfect circle. Stated another way, if a spring with constant B and tis forced to assume a circular configuration, then the maximum fiexural stress will remain constant at all cross sections of the spring.

It is now necessary to compute the proper width contour to cause the spring to assume a circular configuration under the action of the loading shown in Figure 1.

Since (from Figure 6) M=PR sin a and Q! dz 1 dy 3,. R nil then Equation 1 can be rewritten as sin 0 (6) If I is known at some position, say at then from Equation 6,

. PR l9.-l'. E Q (7) Equation 6 can now be expressed as I=Il sin 6 (8) Since bt and t is constant, then Equation 8 can be expressed as b=lv| sin 6 (9) 2 Equation 9, then, defines the required width contour.

From the foregoing, the following formula relationships are applicable to design the optimum plan configuration for operating spring '52 according to the present invention:

(1 The maximum cessive flexural stresses'so that the elastic limit is no exceeded can be computed from the equation,

where S is the yield point stress of the material. If a a -I. r is known or assumed. a a

(3) Finally, if desired, the resulting actuating force P can be computed from Equation 7.

Following is an example calculation for thedctailed ing to the present invention: Data;

,sm,.,=s00,000 p.s.i. R I :0.200 inches ,plan design of a typical normally fiat spring 52 accord- E 30,000,000 p.s.i. b =0.250 inches 2SR 2 300,000X0.200 (1) E 30,000,000 9 2 b=b a ,xsin a=0.250 sin!) Computational schedule Developed V 0 8 Length, sin@ b Degrees Radians R 1 Inches Inches 0 0 0 0 0 10 17453 0349 17304 0434 20 349000 0098 34202 0856 30 52359 1047 50000 1250 40 698120 1090 04279 1007 50 872650 1745 70004 1915 00 1. 0471s 2094 86603 2105 70 1 22171 .2443 .93909 .2349 so 1.39024 .2792 .98481 .2402 90 1. 47077 3142 1. 00000 2500 3 I} .=b i =L3553 inches Y 3O 1OX1. 3333X10' P-Ellg l.0()0 pound R2 The completed design is shown companying drawings.

In designing the novel constant. stress, special corifiguration operating spring 52 ofthis invention, the elastic'limit or ,proportional elastic limit is taken for the value of S to compute the varying width and cross sectional area along the length of the spring by .the

above-disclosed formulas. This eliminates or minimizes setin the spring 52 when it is..assembled in the switch, so

' that it would return to normally flat shape if it were in Figure 4 oi -the acthickness'of spring to, avoid ex V springs may have set within limits which will provide desired uniformity between springs with respect to'uniform loading on the actuating and actuated members (34 and 36) of the switch. The limit to such set without objectionally adverse'elfects can be determined'for given switches by routine experimentation. The spring thickness and/or material (which determines S can be appropriately changed in the light of the above-discussed formulas to avoid excessive set.

Referring to the schematic drawing in Figure 5, a normally flat spring according tothe present invention is indicated at 52c. The notation total travel designates displacement of the mounting ends of the spring 52c from flat position to minimum chord in the overcenter critical position" during switch operation, The displacement of said spring mounting ends with respect to each other in contact position (e.g., as in Figure 2) to minimum chord in said overcenter critical position is designated as "operating travel. It will be noted that the ratio of operating travel to total travel? is quite small in the case of the normally flat, constant stress spring 52 of this invention. However, in prior switches using a-preformed arcuate spring, the ratio of operating travel to displacement of the mounting ends of the preformed spring when assembled into the switch (hereinafter called assembly travel) is large. I Assembly trave is a major determinant of switch. operating characteristics when a preformed arcuate spring is used, so that a considerable variation in switch operating characteristics results from a small varation in operating trave e.g., due to tolerance variations in stamping of actuating and/or actuated members, 34.and 36, or the positioningof stops or contacts. .Thus, the new normally flat, special configuration, constant stress spring 52 renders variations in operating travel relatively insignificant and. thereby greatly reduces the adverse efli'ects of such variations on precision switch characteristics.

Similarly, in prior switches incorporating preformed arcuate springs, small production variations in the chord between the mounting portions of the spring (corresponding to the slotted portions 56 of spring 52), introduce versely afiects operating characteristics. As an alternasubstantial diiferences in switch characteristicsfsuch as contact pressure, operating force, and resistance to .contact chatter under vibration. Likewise, fora given spring chord, substantial differences in operating characteristics are introduced by small variations in the spacing between the spring supporting abutments 42 and 44 due to production tolerances, stamping of the actuating and actuated blades, and in the positioning-of blade stops, In the newimproved switch of the present invention embodying the above-described novel constant stress, normally flat spring 52, these shortcomings are substantially eliminated due to the small ratio of the size of such variations to the total travel of the spring ends.

' With prior preformed arcuate springs, as the ratio and snap acting thrust exerted on the actuated blade are both increased. Therefore, an increase in' this ratio has sometimes been resorted to in order to'increase the contact pressure, snap acting spring thrust, ,and vibration resistance in prior arcuate spring switches of a given size. However, this expedient increases the stress at the center of the .spring during switch operation and frequently results in a bend in the spring which adtive, therefore, designers sometimes resorted to a thicker spring in which this chord ratio was smaller. However,

' in practice it is found that use of a heavier spring precise control required for close tolerance precision.

creates problems because its'force is not subject to the switches, excepting when the spring operates between very close limits. Another advantage of the novelstress-eontrol, normally flat, operating spring 52 of this invention is that it is possible to provide an increased amount of fiexure, with resulting lower operating force and higher contact pressure, without over-stressing the spring and thereby avoid adversely afiecting operating characteristics.

Higher forces are achieved with material of the same thickness for constant stress springs of this invention as compared to the preformedor chorded. This is because all of the spring is worked at high stress while the preformed or chorded spring has high stress only in localized areas thus much of the spring is not fully utilized. From an energy point of view, more energy is stored in the constant stress spring of similar operating force because it has been compressed through a larger distance. Likewise more energy is available to propel switch parts since the force does not fall off as rapidly with expansion as for the preformed or chorded spring.

This invention thus achieves a-wider range of control of switch characteristics than was possible with prior preformed arcuate springs.

It has been found that the contour of the novel controlled stress, arcuate spring 52 of this invention (i.e., the width at given points along the spring for a given material, spring length, and thickness) can be varied slightly from the optimum dimensions determined by the above-disclosed formulas, while still retaining the essential features and advantages of the above-disclosed invention. The degree that the width at each point along the length of the spring can be varied from the optimum width computed according to the above formulas, and more especially Equation Number 9, can be determined by routine experimentation and is governed to large extent by the factors discussed above in connection with slight permissible set.

In commercial practice, the springs, 52 are stamped to their special outline with dies of corresponding contour. In order to use lower cost dies, the spring configuration plotted according to the above formulas, especially Equation 9, is substantially reproduced as closely as possible by a composite curve made up of a series of circular arcs substantially coinciding with the plan contour of said spring according to Equation 9; The stamping dies thus can be cut on a radius, according to practices known in the die making art.

It will be apparent that the ears 56 provided for mounting the spring in the switch necessitate some departure from the optimum spring configuration according to the above-discussed formulas. To a certain extent, end effects due to provision of these ears may be ignored. The size of ear dimensions designated at and y in Figure 9 is important, however, especially in small size springs, e.g., a ear on a ,3 maximum width spring has a much greater effect on switch characteristics causing. more of a departure from the theoretical optimum than a ;i j""ear' on a spring of .375 inch maximum width. The maximum size of the ear can be determined by empirical tests. In most switches the size of the car can be designed as the minimum to satisfy mechanical requirements based on the dimensions of tongue receiving slots 58 (Figure 4).

In Figures and 11 there is shown another embodiment of this invention which illustrates that the desired variation of spring width, and hence spring cross section, along the spring length, may be obtained by specific means difiering from that employed in the abovedescribed preferred embodiment of Figures 1-9, but utilizing the same principle. For clarity, parts of this embodiment corresponding to like parts in the embodiment of Figures 1-9 are identified with like numerals plus the subscript a.

Referring especially to Figure 11, there is shown a spring of rectangular outline 52a with a central portion removed leaving an aperture with curved sides having a configuration plotted according to Formula No. 9 above,

As will be apparent from Figure 11, for each value-oil R9 (proceeding from the spring ends towards its center) a distance equal to according to Formula 9 is plotted from the sides of the spring towards the center, to develop the side curves of the central cut-out portion. The end mounting portions 56a are provided similarly to mounting ears 56 in the Figures 1-9 embodiment, and have end mounting notches 58a. The distance x from the center line of notches 58a and the mounting ears 56a of spring 52a is determined by routine experimentation according to the principles discussed above in connection with thesize of the dimensions at for ears 56 in the embodiment of Figures 1-9.

It will also be apparent from Figures 10 and 11 that: the new and improved snap acting switch and spring; according to this invention need not use a tongue and groove connection such as in the switch embodiment of Figures 1-9. Thus, for example, referring to Figures 10 and 11, the spring 52a is provided at its mounting ends 56a with a plurality of notches 58a extending in- Heretofore, preformed arcuate springs for switches of this type have been formed to the desired arcuate shape from a low tensile strength material of say 100,000 p.s.i., and then heat-treated to raise the tensile of the spring to approximately 180,000 p.s.i. in a typical design. This heat treatment varies the spring chord and thus the forces the spring will exert on the actuated and actuating members in an assembled switch, thereby causing variations in operating forces and characteristics, from spring to spring in a given heat-treating batch, and from batch to batch. This method of spring production has been used heretofore because it has not been practical to preform arcuate spring directly from high tensile strength material to the desired chord. However, the novel configuration, constant stress, normally fiat arcuate spring of this ,invention is blanked directly'from high tensile strength material such as 180,000 p.s.i. beryllium copper or 300,-

000 p.s.i. Elgiloy. Thus, besides providing more uniform switch characteristics, which is the primary aim, this invention results in substantial production savings due to these process simplifications alone.

The novel special configuration, stress-controlled, arcuate spring of this invention provides substantial savings in spring production costs which cut the over-all cost of an entire switch unit as much as 10%, especially in lower priced, more competitive switches.

The springs 52 or 52a of this invention are bent into arcuate form upon assembly into the switch and special tools may be provided for doing this.

It will be apparent from the foregoing that the present invention provides a new improved rolling spring snap switch incorporating a novel improved, constant stress, normally fiat, arcuate spring of special plan configuration and predetermined cross sectional variation along its length; which achieves new improved results and advantages in precision switch operating characteristics and control thereof over prior arcuate rolling spring switches; and that the same solve important shortcomings of such prior switches, and achieve other objects and advantages as discussed in the introduction of this specification, and otherwise developed herein.

The invention may be embodied in other specific forms 11 without departing from'the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative andnot re strictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. a v a What is claimed and desired to be secured by United States Letters Patent is: V r 1 1 A snap acting switch comprising: a switch body; an actuating member on said body and having tongue means on'one end; an actuated memberton'said body,

portion being larger than and loosely receiving the tongue; means of said actuating and actuated member whereby movement of said actuating memberfcauses said spring to move with a rolling action and whereby upon actuation ofsaid actuating member through critical position in substantial alignment with said actuatedmember, the spring snaps said actuating member and the movable contact thereon in a direction opposite to the movement of the actuating member to efiect switch operation; said spring, being unformed prior to assembly in the switch and having a plan contour such that the width of spring material progressively decreases along its length from a maximum at the center thereof so that'no part of the arcuate spring substantially exceeds the elastic limit of the spring material and said spring has asubstantially constant bending stress throughout its length, end effects where 't is'the thickness of the spring, S is the yield point stress, of the spring material, R istthe radius describing the arcuate form of said operating spring, and E is the modulus of elasticity of the spring material. 7

3. A snap acting switch as defined in claim 1, wherein I the plan contour is such that the width at each point along the, length of the spring excepting at said mounting 12 portions is substantially the value of b calculated according to the equation,

and where 0 is the angle defining the position parameter of said operating spring. 1 i

4. A snap acting switch as defined in 'claim 3, wherein the central portion of said, spring has a double convex plan outline with end mounting ears.

5. A snap acting switch as defined in claim 3 wherein the central portion of said spring is cut out providing an aperture of double concave plan outline.

'6. A snap action device for a switch comprising an actuating member, an actuated member, a flexible operating spring having a semi-circular portion operably disposed between said members and mounting end portions,

abutment means operativelyinterconnecting, each mounting end portion with adjacent ends ofsaid actuating and actuated members whereby movement of actuating member causes said spring to move with a rolling action, means for moving said actuating member through a critical position relative to saidactuated member whereby said spring causes said actuated member to move with a snap action, the semi-circular portion of said/spring having a plan contour of varying width which decreases from a maximum at its center to a minimum at the end portions to produce a substantially constant bending stress in the semi-circular portion at each point along its length, and the width of the semi-circular portion varies according to the equation 1 r b.=b sin 0, Where b T is the maximum width of the spring, where b is the width at a position parameter along the semi-circular portion and 0 is the angle measured from one of the end portions to such position parameter. v 7

7. A snap action device as defined in claim 6 wherein the semi-circular portion of said spring has aldouble convexplan outline.

8. A snap action device as defined in claim 6 wherein the semi-circular portion of said spring "is cut out providing an aperture-of double concave plan outline;

References Cited in the file of this patent UNITED STATES PATENTS Great Britain ,Mar. 3. 1947

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