Snap-over center spring and method of making

BACKGROUND OF THE INVENTION Field of the invention 25

This invention is a snap-over center spring pertaining as a mechanical member to spring devices and as of plastic construction to the miscellaneous sub-class thereof. As a possible member for use in electrical switches having a double snap action this plastic snap-over center spring has the longevity required for such an application.

In particular the process of making the switch includes the general class of plastic forming and deforming with the steps of forming and reforming defining a sub class. A particular method further includes the step of severing the article from sheet material by a cutting action simultaneously on all areas being cut. A further method step includes forming a portion of the snap-over spring with an incomplete penetration of the material being removed. The hinge portion of the spring may also be made by a ⁴⁰positively applied heating or cooling step being performed on the plastic member.

Description of the prior art

The snap-over center spring of this invention uses the ⁴⁵well known principle of having a supporting beam portion of determined length carry at its ends a leaf which is longer than the supporting beam portion. The snap-over center springs commonly used and known are metal and the entire leaf is treated for bowing. This treatment often is not completely satisfactory and is often expensive. The life cycles are often less than expected.

Snap-action springs are found in prior art showings, as for example, U.S. Patent 2,604,316 to Obrien et al. of July 22, 1952, in which his disc spring is made with deformed portions in order to provide a spring having four legs or at least a multiplicity of leaves, which spring incidentally is made out of metal with no decrease in thickness and no hinge portions. A leaf type spring in which the leaf is extended and bent at one end is shown in U.S. θθ Patent 3,076,070 to Millerwise of Jan. 29, 1963. Other patents showing the state of the art are U.S. Patent 1,668,974; U.S. Patent 2,487,268 and U.S. Patent 2,700,079. The use of metal in a snap-action spring usually results in the leaf portion developing an embrittle- θ⁵ment which of course determines the eventual length or life of the snap-action spring.

The spring of this invention can be made from quite inexpensive plastic material and with the formation of the hinge being a simple mechanical action or a mechanical ‘ and heat-applied action results in a hinge portion being formed with an increase in strength providing for an operation life expectancy literally in the millions of flexings. This invention thus provides a solution to one of the several problems in the use of a snap-action spring. The high versatility provided by this snap-action spring which may be of a very small size or large size while still providing a determined amount of travel results from a method of production which is of extreme simplicity and low cost.

Summary of the invention

This invention pertains to a snap-over center spring preferably formed from sheet plastic such as polypropylene and others of like characteristics in which the spring is blanked from a strip of material as it is advanced in the way of a die. Usually at the time of blanking a leaf portion of the spring is reduced in thickness at one or both ends thereof to form a hinge portion having a life expectancy of several million flexings. In the formation of the hinge at one end of the leaf the leaf is caused to be increased in a determined amount length wherbey it becomes bowed in relation to attached and supporting beam members adjacent thereto. The product of this method is a snap-over center spring of extreme simplicity, low cost, and which in addition to having a tremendous life expectancy is also in its contemplated plastic composition non-conductive of electrical current so that it may be safely used as a portion of an electrical switch without providing a precise insulation therefor.

It is therefore an object of this invention to provide a method for inexpensively producing a snap-over center spring, which spring is made of plastic and the like and in which the leaf or bowed portion is provided with at least one hinged portion made by deforming the thickness of the material to cause an increase in length of the leaf and at the same time to provide a hinge having a flexing life of many thousands of times. The product of this method is a hinge of low cost, of high life expectancy and having natural characteristics of insulation or a non-conductor of electrical properties.

It is a further object of this invention to provide a snap-over center spring blanked from a sheet or molded into a substantially flat blank with a leaf portion extending between ends of a beam portion and at least one juncture of the leaf to the beam a hinge portion of a reduced thickness and easy flexure which during its formation causes a determined amount of material to flow toward the leaf to increase the length of the leaf a determined amount to cause it, the leaf, to bow out of the plane of the beam.

It is a further object of this invention to provide a snapover center spring blanked from a sheet of plastic such as polypropylene and with a leaf portion extending between ends of a beam portion and a hinge formed at at least one of the junctures of the leaf and beam, said hinge being a cold formed displacement of the material to provide a reduced thickness portion of the sheet and the displacement of the material causing a substantial portion to flow to the leaf to increase its length a determined amount and to cause the leaf to bow out of the plane of the beam.

There has been outlined rather broadly the most important features of the method and article of the present invention in order that the detailed description to follow may be understood and the improvement to the present art may be better appreciated. Those persons skilled in the art will appreciate that the conception on which the present disclosure is based may be utilized as the basis for designing other structures for carrying out the several purposes of this invention.

A specific method of making the snap-over center spring and three embodiments have been chosen for purposes of illustration and description, and is shown in the accompanying drawing forming a part of the specification wherein:

3,389 3

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a plan view of a method of forming a snap-over center spring of a strip of plastic wherein a progressive die provides separating slits to provide a _ leaf, and in a later stage of the operation forms a hinge ⁰at that portion of the leaf which is connected to the beam portion of the spring and at yet another stage of operation is severed from the strip to provide the completed spring;

FIG. 2 represents a sectional view taken on the line ¹⁰2—2 of FIG. 1 and showing the relationship of the leaf to the adjacent beam portions;

FIG. 3 represents a sectional view taken on the line 3—3 of FIG. 1 and showing the configuration of the hinge portion at the juncture of the leaf .and beam portions;

FIG. 4 represents a sectional view taken on the line 4—4 of FIG. 1 and showing a transverse section through the leaf and adjacent beam portions to show the relation- ₂θ ship of the leaf to the beam portions in one extreme of its, the leaf, movement;

FIG. 5 represents an isometric view showing a completed snap-over center spring representative of the method of FIG. 1; _2g

FIG. 6 represents an isometric view of an alternate snap-over center spring construction in which there is one leaf and one beam portion, said beam portion being sufficiently rigid to overcome the bending moment developed by the bowed length of the leaf; and θθ

FIG. 7 represents yet another alternate snap-over center spring construction in which there are two span-action leaves adjacent an intermediate beam member.

DESCRIPTION OF THE PREFERRED EMBODIMENTS ³⁵

Referring now in particular to the drawings in which like numbers refer to like members and in particular to FIG. 1 there is shown the steps of the manufacture of a snap-action spring in which a plastic strip 10 is advanced 40 in the way of a die whereby punches, not shown, form slots 12 and 14 in the strip. The leaf 16 portion between slots 12 and 14 is at this stage of the die or at a subsequent station lengthened by the forming of a hinge portion 18. This hinge is made by reducing the thickness of 45 the strip 10 at juncture 18 as by providing die embossing members both above and below the leaf 16 wherein the die embossing members arranged both above and below the surface of the strip 10 are brought towards each other to cause a displacement of material and thinner 50 hinge section as at 18 seen in cross-section in FIG. 3. The bringing together of the two spaced-apart and opposed die portions causes the polypropylene to flow thereby causing the leaf portion 16 to become longer than the adjacent side pieces of strip 10 thereby causing the por- 55 tion 16 to bow out of the plane of the strip 10. The final step of forming the spring is the severing from the strip as at point 20 and the completed spring now identified as 21 is shown in FIG. 5.

Referring now in particular to FIG. 2, it is to be noted ⁶⁰that after the punches in the die have formed slots 12 and 14 in the strip, the stock 10 forming the intermediate tongue 16 still retains its original, as formed, thickness.

Referring next to FIG. 3, it is to be noted that the 65 spaced-apart and opposed die portions in forming the hinge 18 enter the stock 10 to displace and cause the material to flow and resulting in a reduced thickness portion and hinge. The reduction caused by the depth of the hinge formation is usually at least half the total thick- 70 ness of the stock while the width of the deformation is usually at least as much as the thickness of the stock. The width of the slots 12 and 14 is preferably the same as the thickness of the stock so that there is no interference in the flexing of the leaf portion 16 and adjacent 75

902 portions as it is moved from one snap-action limit to the other limit.

Referring next to FIG. 4 there is shown the leaf 16 in one limit of movement after the deformation at hinge 18 has been completed. The leaf 16 has moved out of the plane of the stock 18 an amount approximately equal to the thickness of the stock. The amount of displacement of material effected at the hinge 18 determines the amount of displacement of the leaf from the plane of the stock.

Referring particularly to FIG. 5 it is to be noted the intermediate portion of leaf 16 has a smooth bow which is a result of method of making and in the fact that the amount of expansion force is equal in all of the increased length of the leaf. The size and rigidity of side beams 22 and 24 determine whether a minor bowing occurs in them in response to the lengthening of the leaf. When the spring is mounted for example at end 25 the opposite end will move counterflow and in response to the moving of the leaf to the opposite limit of snap action.

Referring next to FIG. 6 there is shown a spring having instead of side members 22 and 24 as in FIG. 5 a side or beam 30 which is approximatediy double the width of the leaf 16. This width and strength of beam 30 is made to accommodate the forces developed by the lengthening of leaf 16.

Referring finally to FIG. 7 there is shown yet another alternate snap-action spring wherein a center beam portion 40 is provided to support and substantially overcome the bowing force of leaves 42 and 44. The overall length of beam 40 is of a sufficient extent to allow the leaves 42 and 44 to be formed and the deformation points 18 forming the hinges determining the amount of bow developed in the snap action.

USE AND OPERATION

In the use and application of the snap-over center spring the amount of bow of the leaf or the extent of the two positions of repose of the end-attached frame often requires that the leaf be shorter or longer. This adjustment of length is accomplished by making the hinge portion shallower or deeper, by making the hinge wider or narrower or by adding a second hinge portion at the other end of the leaf. When the leaf is hinged at both ends, the activation of the spring is best served by applications of force to the leaf itself. In the application of the snap-over center spring to a particular installation it may be desirable to attach one end or one or more beam members. Holes for mounting may be provided in the spring during the punching operation with punches disposed in a die as required.

A common application is the attachment of end 25 (FIG. 5) to a frame or support whereupon the application of movement to the other end of the spring will cause the leaf to snap to one limit or the other. With a small amount of reactant bow being induced into the beam members 22 and 24 by the bowing of leaf 16, as the leaf is snapped first to one limit and then the other the beam members likewise bow causing end 26 to move to one of two determined positions in air.

The use of snap-over center springs for holding doors in predetermined positions, in snap-action electric switches and many other such commercial applications show a need for an inexpensive snap-action spring of high reliability and low cost. The flowed hinge portion 18 provides a sure and inexpensive hinge usable for millions of cycles.

In the formation of the hinge 18 the term “cold formed” is applied to the displacing of the material by one or more dies at less than melting or welding temperature. This cold formation of the hinge or hinges is usually accomplished with either the plastic sheet, the dies or both being heated to a temperature discernably above room temperature and substantially below the melting point of the plastic material. In practice this temperature may be midway between room temperature and the melting point of the plastic used. Of course the sheet and dies may be at

3,389,902 ⁵ room temperature during the cold forming but, as in the case of polypropylene, the hinge may be weakened to reduce its life expectancy.

METHOD OF MAKING A SNAP-OVER CENTER SPRING

The making of the snap-over center spring of this invention includes the preferred steps of:

Advancing a strip of material in the way of a slitting means;

Forming at least one slot in the strip to provide a leaf portion attached to and carried by a beam or support portion;

Shaping at least one hinge portion in the leaf by reducing the thickness of a portion of the leaf thereby causing a flow of material toward the leaf to increase its length and causing the leaf to be bowed out of the plane of the beam; and

Severing the spring from the strip.

The die contemplated for the forming of slots 12 and 14 may also carry opposed and spaced punches for forming hinge 18. A cut-off die portion for making the shearing cut 20 may also be incorporated into the die so that in one stroke of the die the spring of FIG. 5 may be made.

Descriptive terms such as “up,” “down,” “in,” “out,” and similar terms are applicable to the method, embodiments and the use of the springs as described in conjunction with the accompanying drawing and such terms are merely for the purpose of description and do not necessarily apply to a position in which the spring may be constructed or used.

The invention in its broader aspects is not limited to the specific embodiments or method shown and described but departures therefrom may be made within the scope of the accompanying claims and without sacrificing its chief advantages and protection is sought to the broadest extent the prior art allows.