US3808390A

US3808390A - Switch with swaged leaf-spring contact

Info

Publication number: US3808390A
Application number: US00350298A
Authority: US
Inventors: K Hammell; N Graeff
Original assignee: AMP Inc
Current assignee: TE Connectivity Corp
Priority date: 1973-04-11
Filing date: 1973-04-11
Publication date: 1974-04-30
Anticipated expiration: 1991-04-30

Abstract

This invention relates to a switch device having a flexible arm and at least one co-operating contact. More particularly, the invention relates to the modification of the movable arm by swaging, and the method for accomplishing the same, so as to give improved spring characteristics to the movable arm, including reduced metal fatigue.

Description

United States Patent [191 Hammell et al.

SWITCH WITH SWAGED LEAF -SPRING CONTACT Inventors: Kemper Martel Hammell; Norwood Claude Graefi, both of Harrisburg, Pa.

Assignee: AMP Incorporated, Harrisburg, Pa.

Filed: Apr. 11, 1973 Appl. No.: 350,298

us. CL, .,....200 246, 200/283, 2 9/630 c, 29/1310. 1:; 1m. (:1. 110111 1/26 Field of Search 200/166 J, 1 A, 166 C;

29/630 C, 630 G, DIG. 18, 622

References Cited UNITED STATES PATENTS 10/1962 Dal Bianco et a]. "335/154 [451 Apr. 30, 1974 3,258,557 6/1966 Scheepstra et al 335/154 3,015,876 1/1962 Hutt 29/630 C 3,314,028 4/1967 Wyland et a1 200/166 c x Primary ExaminerRobert K. Schaefer Assistant Examiner-William J. Smith Attorney, Agent, or Firm-Allan B. Osborne [57] ABSTRACT This invention relates to a switch device having a flexible arm and at least one co-operating contact. More particularly, the invention relates to the modification. of the movable arm by swaging, and the method for accomplishing the same, so as to give improved spring characteristics to the movable arm, including reduced metal fatigue.

18 Claims, 8 Drawing Figures PATENTEDAPRBO I914 51808390 SHEET 2 [IF 4 P nmao an (808,390

sum 3 or a Q' COMPRESSIVE TEYNSILE RESIDUAL STR ES -COMPRESSIVE I BENDING STRESS COMPRESSIVE TENSILE 'i' RESIDUAL PLUS BENDING STRESS PATENTEDAPR 30 mm sum u UF 4 TJMILH.VII..U 2

SWITCH WITH SWAGED LEAF-SPRING CONTACT BACKGROUND OF THE INVENTION Moving-arm electrical switches formed from spring tempered conductive sheet metal are notoriously old in the electrical arts. However such switch arms have always been subject to metal fatigue, which has become an increasingly important factor as the trend to miniaturization progressed due the the premium placed on space, weight, and cost requirements by the requirements of the space exploration and the computer age.

A correlary to the foregoing is the object of reducing the complexity and number of parts of the electrical device and of its assembly procedures.

In addition, in another aspect of this invention, it is an object to form all, or a major portion, of the metal components of the electrical switch device from the stamping and forming of a uniform strip of conductive metal, even though the metal characteristics required for various metallic parts of the device differ.

These and other objects and advantages have been realized by the present invention which contemplates swaging the movable contact arm to give a thinned zone across the arm sufficient to concentrate the major portion of the flexing of the arm along the zone. Applicant has discovered that this swaging rather than weakening the contact arm (such as would occur if the arm were just nicked); on the contrary increases its longevity by decreasing the metal fatigue which results from repeated cycling of the'arm in making and breaking electrical contact.

Having achieved this improvement, applicants now theorize (without wishing necessarily to be restricted to this theory), that the decrease in metal fatigue is due to the introduction of residual compressive stresses in the underlying areas immediately adjacent to the swaged surfaces (which include the areas of said swaged surfaces). Apparently a cantilever beam fixed at one end can better withstand in said outer areas compressive stresses than tensile stresses. The effect of this precompression stressing will bediscussed further below with regard to the drawings.

Various practical applications of this invention have been disclosed and claimed in related copending applications of ourselves and our co-worke-rs: Ser. Nos. 306,112; 306,113; and 306,114; all filed on Nov. 13, 1972, (the disclosure of which is incorporated herein by reference).

It has been known in the past to deform movable contact arms in electrical switches. However, these apparently were restricted to snap action devices where the deforming was restricted to corregating or thinning only a portion of the contact arm blade, often accompanied by aperturing the blade in the vicinity of the localized deformation. Examples of this prior art are to be found in U.S. Pat. Nos. 2,361,202; 2,604,316; and 2,814,685. The purpose and structure of these foregoing devices are clearly differentiable from the present invention as disclosed and claimed herein.

In this specification and the accompanying drawings applicant have shown and described preferred embodiments of their invention and have suggested various alternatives and modifications thereof; but it is to be understood that these are not intended to be exhaustive and that many other changes and modifications can be made within the scope of the invention. These suggestions herein are selected and included for purposes of illustration in order that others skilled in the art will more fully understand the invention and the principles thereof and will thus be enabled to modify it and embody it in a variety of forms, each as may be best suited to the conditions of a particular use.

In the accompanying drawings:

FIG. 1 is an exploded perspective view of a simplified specific application of a preferred embodiment of the present invention (more particularly described in copending application Ser. No. 306,113);

FIG. 2 is a simplified side elevation of the swaged contact arm and the co-operating swaging dies;

FIG. 3 is a simplified perspective of one of the swaging dies;

FIG. 4 is a theoretical graphic representation of residual stresses typically induced by equal working of opposite surfaces of a beam;

FIG. 5 is similar to FIG. 4 showing the theoretical internal stresses resulting from bending a beam without any prior residual internal stresses.

FIG. 6 is a graphic representation illustrating the addition of the residual stresses of FIG. 4 to the bending stresses of FIG. 5, showing a net reduction in the amplitude of the more destructive tensile stresses; and

FIGS. 7 and 8 are perspective views illustrating the fabrication from uniformly thick sheet metal of another electrical device embodying the present invention (described in more detail in copending application Ser. No. 306,112).

The switch device 10 illustrated in F IG. 1 is a simple mechanical cam-actuated single pole single throw switch having a first contact member 12 with a fixed contact area 14 and a contact member 16 having a movable contact arm 18. The contact arm 18 has a swaged zone 20. The upright portions of

members

12 and 16 are secured in

corresponding slots

22 and 24 of the switch base member 26 with switch cam 28 rotatably mounted in base member 26. The cam lobe 30 can be rotated to lift the cam arm 18 away from spring contact with fixed contact 14. In this particular embodiment the spring arm 18 has a dimpled portion 32 which acts as a cam follower.

The thinning of the swaged or compressed zone 20 can be more clearly seen by reference to FIG. 2. In this preferred embodiment, the swaging dies 34 are of identical shape and strike the contact arm 18 equally on both sides thus symmetrically swaging the arm 18 and introducing symmetrically oriented internal residual compressive forces, substantially as envisioned by FIG. 4. The striking surface of the dies 34 are polished to give a polished impression to the surface of the swaged zone 20 thereby further reducing the likelihood of surface cracks and thus reducing the susceptibility of the movable contact arm'to metal fatigue. The arrows in FIG. 3 indicate the'direction ofgrinding and polishing to enchance this characteristic.

FIGS. 7 and 8 particularly illustrate how a number of complexly shaped, densely populated, minute components can be stamped and formed from a single piece of sheet metal 36 of uniform thickness. The individual contact members l2, 16, 38, and 40, are initially formed with an expendable webbing 42 to form a frame It can be appreciated that the thickness of the sheet metal 36 must be reasonably thick to form sturdy posts 46 for plugging into printed circuitry or for wire wrapping and sufficiently strong and ductile to accept right angle bends during the forming operation'without failure; all of which gives a thickness and characteristics which form a beam too stiff to make a good contact spring arm 20.

Therefore the swaging of the movable contact arm 20 has the advantage of giving the thinness required for good flexibility, enhancing this flexibility by work hardening, and enhancing the formation of the particular device illustrated in FIG. 7 by extending the length of the contact arm 20 so that its free end 48, the contact end, is extended between the

cooperating contact areas

14 and 50 of respective first and second contact members I2 and 38. In this particular embodiment the sheet metal thickness can be 0.008 inches with a number 4 hardness, is thinned in the swaged area 20 by 50 percent, and thereby worked to approximately a number 10 hardness. This swaging gives the desired softer.

spring enabling the use of physically smaller electromagnets with less current demands to actuate such springs and yet, with the work hardening, the yield strength increases thus permitting greater deflection of the spring without causing it to take a set.

The area, contour, and depth of swaging required will vary according to the physical characteristics of the metal of the contact arm and its physical dimensions. For example swaging beyond a 50 percent reduction can be used, but should not be so deep as to cause rupturing of the metal. Similarly the transition face 35 relative to the flat face 37 of the swaging die can be at more than the illustrated angle of the embodiment shown in FIG. 2, but must not be so abrupt as to fracture the metal or introduce a point of weakness. Thus with the surfaces of the dies 34 substantially flat in their center portions and being slightly .chamfered or angled on their sides as shown; the forming dies having this configuration will produce a cross-section in the switch arm 18 as shown in FIG. 2. It is interesting to note that during the swaging operation, the longitudinal extrusion of the metal from between the dies tend to give to the transition area 39 of the area 18 a lesser angle than that of the die face 35.

Referring to FIG. 7, it will be noted that the free end 48 of the contact arm 18 is illustrated as having an atrest position in contact against the fixed contact area 14 of the first contact member 12. This can be achieved by an uneven swaging causing the free end 48 to tend to assume an angled position with respect to the remainder of the contact member 16 on the other side of the swaged area 20. More commonly, this positioning is determined independent of the swaging. The at-rest position could alternatively be free of either fixed

contact

12 or 50, or could be up against contact 50. Both of the foregoing can be combined to give an atrest set against one fixed contact and higher residual internal compression stresses on the sideof the arm 18 away from which the arm 18 is moved to break the atrest contact.

As stated previously, the swaging operation produces residual compressive stresses on the upper and lower surfaces of the switch arm and by virtue of these stresses, the tendency towards fatigue failures is substantially reduced. FIGS. 4-6 illustrate the manner in which these residual compressive stresses on the upper and lower surfaces bring about this improvement in fatigue life.

At the outset, it should be remembered that fatigue failures in beams are a result of the repeated compressive and tensile stresses which are imposed when the beam is repeatedly flexed in opposite directions. The tensile stresses produce extremely high unit stresses at surface imperfections such as cracks or other surface flaws. The cracks are propogated as a result of these high unit stresses and failure eventually results. It follows that any reduction in the magnitude of the tensile stresses produced during repeated flexure of a beam in opposite directions will reduce the possibility of fatigue failure and/or improve the fatigue life of the beam.

FIG. 4 diagramat ically illustrates the residual stresses produced in the swaged zone of the spring arm 48 as a result of the swaging operation. As shown in FIG. 4, compressive stresses are produced on the upper and lower surfaces and the magnitude of these compressive stresses is reduced as the neutral axis or center of the beam is approached. In fact, low level tensile stresses are present in the center of the switch arm. However, such internal tensile stresses are of relatively little significance, since they are not acting on surface flaws and therefore do not originate nor aggravate such origins of failure.

FIG. 5 shows the straight line stress distribution induced by bending a beam which has not been swaged in accordance with the instant invention (and therefore is without the residual stresses shown in FIG. 4). The dotted line in FIG. 5 indicates relatively low level tensile stresses on the upper side and relatively low level compressive stresses on the lower side, a condition which would exist when the blade or switch arm end 48 is in the pre-set condition against the lower contact shown in FIG. 7. If the contact arm 18 is flexed from its normal position upwardly against the upper fixed contact 50, a high level of tensile stress will be induced on one side of the arm and a high level of compressive stresses on the other side as shown by the solid line in FIG. 5. It is, of course, this high level tensile stress which causes fatigue failure.

By addition, it is possible to find the resultant stresses when the normal bending stresses of FIG. 5 are applied to a switch arm having residual stresses as shown in FIG; 4. It will be noticed that the resultant maximum tensile stress on the upper side of the switch arm is considerably reduced by virture of the addition of the residual compressive stress to the applied tensile stress. It will also be noted that the bending stress on the lower side of the switch arm is increased (by the addition of the residual compressive stress) but failure ordinarily does not take place on a compressively stressed side of the switch arm and these compressive stresses are therefore comparatively unobjectionable. The reduction of the tensile stress, on the other hand, does produce a significant improvement in fatigue life.

It is emphasized that the stress distribution curves shown in FIGS. 4-6 are diagramatic and do not represent observed data. In fact, where the parts of the switch arm are extremely small, it is virtually impossible to measure the stresses induced by flexure thereof. However, an analysis of data on switch arms in accordance with the invention clearly shows a substantial improvement in fatigue life.

We claim: v

1. An electrical switch comprising a first contact, a movable contact arm formed of substantially flat conductive metal stock, said arm having a free end and a fixed end, said free end being positioned to be capable of being moved into electrical contact with at least said first contact, said arm being having been swaged to give a zone of spring quality transversely thinned across said arm which zone acts as a pivot area for the deflection of said arm.

2. A device according to claim 1 comprising a second contact which is positioned relative said first contact with the free end of said arm movable therebetween, and said two contacts are fixed in position.

3. A device according to claim 2 wherein said arm is elongated and formed from sheet metal having physical characteristics of thickness, hardness and ductility which are unsuited, if unmodified, to form a spring contact arm.

4. A device according to claim 1 wherein said swaged zone is flat and therefore of uniform thickness and is positioned adjacent said fixed end.

5. A device according to claim 4 wherein said arm has been swaged to a degree which work hardens the metal and sets up compressive stresses in the underlying areas adjacent the swaged surfaces, which compressive stresses are substantially uniform transversely across the zone and in substantial excess of any compressive forces in the unswaged stockand which have longitudinal components which reduce fatiguefrom repeated transverse flexing of said arm by significantly reducing or eliminating the tensile stresses in said areas normally appearing upon deflection of said arm in making or breaking electrical connection with said contact.

6. A device according to claim 5 wherein said arm is asymmetrically swaged more on one surface than on the opposing surface to give a permanent angular set to said arm.

7. A device according to claim 5 wherein the underlying areas, adjacent the swaged surface which faces away from the only direction in which said arm moves from its normal at-rest position, have higher compensating residual compressive forces than are present in the areas underlying the opposite swaged surface.

8. A device according to claim 7 wherein the portion of said arm adjacent said fixed end is angled in the atrest position relative to the portion of said arm adjacent said free end.

9-. A device according to claim 3 wherein said arm has a normal at-rest position in electrical engagement with one of said-contacts.

10. A device according to claim 3 wherein the swaging of said arm has extended its length.

11. A device according to claim 1 wherein the thickness of said arm is of the order of magnitude of 0.01 inches.

12. A device according to claim 11 wherein the swaged zone is reduced in thickness by an order of magnitude of about one half the mean thickness of said arm.'

13. A device according to claim 12 wherein the swaged zone is substantially flat and of uniform cross-' section with a gradual transition at either end having an order of magnitude of about 14. A device according to claim 12 wherein the conductive metal is a spring tempered phospher bronze or berillium copper.

15. An electrical switchcomprising a pair of spacedapart fixed switch contacts, a sheet metal cantilever switch arm, said switch arm having a free end disposed between said contacts and being selectively engageable with said contacts, said switch arm having a compressed zone'intermediate the ends thereof which is thinned and formed in such a manner as to limit the majority of the flexing to said zone and to introduce residual compressive stresses in and under the surfaces thereof, suchthat upon movement of said free end between said contacts said compressive stresses are increased within tolerable limits on the one side of the zone which faces the direction of movement and said compressive stresses are decreased on the other side of said zone without the development of the high tensile stresses which typically lead to early metal fatigue, said compressive stresses being substantially evenly distributed'transversely across the compressed zone of said switch arm.

16. In a switch or the like having first and second fixed contacts and having a movable contact arm, said movable contact arm comprising a sheet metal stamped and formed arm having a fixed end and a free end, said arm having a swaged zone adjacent to said fixed end, said zone having residual internal compressive stresses which are relatively high adjacent to the surface one side of said swaged zone and are relatively low adjacent to the other surface side of said swaged zone, a first portion of said arm between said swaged zone and said fixed end defining a reference plane, a second portion of said arm extendingfrom said swaged zone to said free end normally extending, with respect to said reference plane, in an oblique direction divergently away from said swaged zone on said other side, said free end normally being in contact with said first fixed contact, said second portion of said arm being pivotally movable with respect to said swaged zone to a position in which said arm extends obliquely divergently away from said swaged zone on said one side so that said free end is in contact with said second fixed contact, saidrelative ly high compressive stresses on said one side being increased and said relatively low compressive stresses being decreased during movement of said arm from said first position to said second position.

17. Method of forming an electrical switch having a movable cantilever contact arm and a corresponding fixed contact by progressive stamping and forming from conductive sheet metal of a substantially uniform thickness and of physical characteristics sufficient to withstand the forming procedure but too thick and without the proper spring characteristics required for said contact arm, comprising the step of swaging said arm in a zone extending over an area entirely transversely across said arm and to a depth sufficient to thin and work harden said zone to attain the necessary spring characteristics, to limit the majority of the flexing thereto, and to introduce residual compressive stresses in the general underlying area of the surfaces thereof to optimize the resistance to metal fatigue in said arm by counteracting at least a substantial portion of the high tensile forces otherwise encountered in said areas in switching said arm absent such swaging.

18. Method according to claim 17 wherein said swaging step is longitudinally unrestricted and said arm is elongated by the swaging to overlap said fixed contact.

Claims

5. A device according to claim 4 wherein said arm has been swaged to a degree which work hardens the metal and sets up compressive stresses in the underlying areas adjacent the swaged surfaces, which compressive stresses are substantially uniform transversely across the zone and in substantial excess of any compressive forces in the unswaged stock and which have longitudinal components which reduce fatigue from repeated transverse flexing of said arm by significantly reducing or eliminating the tensile stresses in said areas normally appearing upon deflection of said arm in making or breaking electrical connection with said contact.

8. A device according to claim 7 wherein the portion of said arm adjacent said fixed end is angled in the at-rest position relative to the portion of said arm adjacent said free end.

9. A device according to claim 3 wherein said arm has a normal at-rest position in electrical engagement with one of said contacts.

12. A device according to claim 11 wherein the swaged zone is reduced in thickness by an order of magnitude of about one half the mean thickness of said arm.

13. A device according to claim 12 wherein the swaged zone is substantially flat and of uniform cross-section with a gradual transition at either end having an order of magnitude of about 20*.

14. A device according to claim 12 wherein the conductive metal is a spring tempered phospher bronze or berillium copper.

15. An electrical switch comprising a pair of spaced-apart fixed switch contacts, a sheet metal cantilever switch arm, said switch arm having a free end disposed between said contacts and being selectively engageable with said contacts, said switch arm having a compressed zone intermediate the ends thereof which is thinned and formed in such a manner as to limit the majority of the flexing to said zone and to introduce residual compressive stresses in and under the surfaces thereof, such that upon movement of said free end between said contacts said compressive stresses are increased within tolerable limits on the one side of the zone which faces the direction of movement and said compressive stresses are decreased on the other side of said zone without the development of the high tensile stresses which typically lead to early metal fatigue, said compressive stresses being substantially evenly distributed transversely across the compressed zone of said switch arm.

16. In a switch or the like having first and second fixed contacts and having a movable contact arm, said movable contact arm comprising a sheet metal stamped and formed arm having a fixed end and a free end, said arm having a swaged zone adjacent to said fixed end, said zone having residual internal compressive stresses which are relatively high adjacent to the surface one side of said swaged zone and are relatively low adjacent to the other surface side of said swaged zone, a first portion of said arm between said swaged zone and said fixed end defining a reference plane, a second portion of said arm extending from said swaged zone to said free end normally extending, with respect to said reference plane, in an oblique direction divergently away from said swaged zone on said other side, said free end normally being in contact with said first fixed contact, said second portion of said arm being pivotally movable with respect to said swaged zone to a position in which said arm extends obliquely divergently away from said swaged zone on said one side so that said free end is in contact with said second fixed contact, said relatively high compressive stresses on said one side being increased and said relatively low compressive stresses being decreased during movement of said arm from said first positIon to said second position.