GB2103028A - Composite electrical contact - Google Patents

Abstract

A composite electrical contact made of two or more pieces of wire (1, 2) cold welded at their free ends, comprises a shank portion 1' of non- precious metal and a contact portion 2' made of precious metal or an alloy thereof. The ratio between the diameters of the shank portion and the contact portion is from 1:1 to less than 1:1.5 and the force bonding the abutting surfaces is more than about 8 Kg/mm<2> against shearing force. At least one of the free ends of the abutting wires has a reduced diameter projection, before cold welding. <IMAGE>

Description

SPECIFICATION Composite electrical contact This invention relates to composite electrical contacts manufactured by cold welding.

It is known to obtain, by a cold welding process and by subsequent shaping when needed, composite electrical contacts, viz., bimetallic or trimetallic contacts, base material or shank portion of which is made from non-precious metals such as copper and alloys thereof, and the contact surface or contact portion of which is made from precious metals such as silver and silver alloys and is provided at one end or both ends of said shank portion. in this process which is disclosed, for example, U.S. Patents Nos.

3 460 735 and 3 600 794, actively sheared end surfaces of a short wire material as the shank portion and of other short wire materials as the contact portion which are to be cold welded, and abutted to each other and subjected under pressure of their axial direction so as to form, by their plastic deformation, expanded portions about their abutting surfaces and to be welded thereby.

Conventional manufacturing methods of composite electrical contacts such as briefly explained above have the following drawbacks.

A: Unless expanded portions, diameters of which are larger than the diameters of the employed short wire materials are made about their corresponding abutting surfaces, force sufficient enough for bonding said abutting surfaces can hardly be obtained. Experience the present inventor has and the experiments he has made shown that said bonding force is obtainable only when the diameter of said expanded portions or beads are preferably twice those of the corresponding short wires or at least 1.5 times greater. When the beads have a diameter of less than 1.5 times said short wires, bonded surfaces are weak and can hardly stand practical use as electrical contacts. These facts are shown in Figs.

1 and 2, each of which shows the contact and shank portions before welding and shaping. To wit, as illustrated in Fig. 1, numerical values in which and throughout the drawings are in millimeters, a bimettalic electrical contact of the rivet type has generally a contact portion 2' of a diameter of more than twice the diameter of a shank portion 1' or short wire material 1. Only when they have such proportions, their bonded surface 3 formed between the copper shank portion 1' and the silver contact portion 2' can have an efficient bonding force, viz., about 235 Kg or 12 Kg/mm2 at its shearing force.This means that the ratio between the diameter of the contact material 2' of contact portion C and that of the shank portion 1' or shank material 1 is 2:1, and when said ratio is made smaller than the above value, it is impossible to provide an efficient sharing force or stress to their bonded surface 3.

For example, when a bimetallic electrical contact is made as illustrated in Figure 1 but to have a ratio between the contact portion 1' and the shank portion 2' as 1.3:1 or to have its contact portion C comparatively smaller than the shank portion S, the bonding force between said two portions is only about 24 Kg or 3 Kg/mm2 at its shearing force, and cannot stand practical use. It is known, therefore, that for making a composite electrical contact having a practically acceptable bonded surface, the ratio between the diameter of a short wire material 1 which forms the shank 1' as well as a part of the contact portion C, and the finally obtained diameter of a short silver wire 2' has to be preferably 1:2 or at least 1 :1.5.

B: While a bimetallic composite electrical contact made by conventional methods comes to have, as aforementioned in the above paragraph A, a contact portion, the diameter of which is about twice that of shank portion, a trimetallic or double-contact rivet made by conventional methods comes to have inevitably, as illustrated in Figure 4, such shank which is thicker and shorter than its contact portion.To wit, with reference to Figure 4, when a double-contact rivet is made from a silver wire material 2-2, copper wire material 1, and another silver wire material 2-1 respectively of the diameters same to the cut wires 1 and 2 of Figure 1 so as to have its contact portion C, the diameter of which is same to that of Figure 1 and so as to provide the bonded surfaces 4 and 3 with shearing forces sufficient for practical use, its configuration and dimensions become inevitably as indicated in Figure 4. The bonding force at the surfaces 4 and 5 are respectively about 15 Kg (10 Kg/mm2) and about 235 Kg (12 Kg/mm2).

Thus, when Figure 1 and Figure 4 are compared, it shall be found that conventional cold welding methods can produce bimetallic composite contacts of only such configuration in which the diameters of its contact portion and shank portion are under a limited ratio, and also that a trimetallic or double-contact rivet having a contact and shank portion of such diameters which correspond to those of a specific bimetallic composite contact cannot be produced by the conventional methods. This is very troublesome for employing them to electrical appliances for example. There are many occasions that bimetallic or single-contact rivets and trimetallic or double-contact rivets are fitted to a same chassis via holes provided thereto.In this instance, as single-contact and double-contact rivets which are made by conventional cold welding methods can hardly have contact and shank portions which are equivalent to each other in their diameters, there arise many undesirable requirements in design and manufacture such as lack of unity of holes provided to the chassis.

C: As mentioned above repeatedly, in conventional cold welding methods, the expansion of materials for shank and contact portions have to be made about their abutting surfaces at about twice their original diameters for securing high bonding force between them.

This means that their contact portion C is made of a partial expansion of copper shank material and of expansion of silver contact material. In other words, said contact portion as illustrated in Figure 1 always consists of copper and silver.

D: Conventinal welding methods which require at least 1.5 times of expansion of wire materials in their diameter, need room necessary to allow them for such expansion. In order to achieve this, the wire materials have to be placed within a free space at their abutting ends and portions adjacent thereto, and subjected to pressure exerting along their axial directions. This often results in sliding and slipping of the abutting ends at their contact surfaces, whereby their firm bonding is lost. This tendency becomes larger when short wire materials are small in diameter.

According to this invention, there is provided a composite electrical contact made from a plurality of short metallic wires which are cold welded at their abutting surfaces comprising as said short metallic wires, a shank portion of nonprecious metals and a contact portion or precious metal which is cold welded to an end of said shank portion, the ratio between the diameters of said shank portion and of said contact portion being 1:1 to less than 1:1.5 and the force bonding the abutting surfaces being more than about 8 Kg/mm2 against shearing force; and in which said ratio is the value obtained after cold welding and/or shaping without employing any machining for the purpose of reducing the diameters of the shank or contact portion.

In conventional methods also such as disclosed in U.S. Patent No. 4 703 425, particularly at page 1 thereof, and lines 16 to 25, it is possible to reduce the diameter of contact portion by cutting it off circumferentially. However, these conventional methods cannot bear comparison with this invention, because of loss of precious materials and because of complexity of steps and processes in the former methods.

In contrast to the drawbacks set out in paragraph C, with this invention, said contact portion C can be, as illustrated in Figure 7 described later, solely of silver. This leads not only to a reduction of consumption of silver, but also to reduction of height of the contact portion C, whereby the contact can be made economically as well as compact.

With this invention in contrast to the drawbacks set out in paragraph D in which cold welding is made without the diametrical expansion of wire materials per se, the wire materials as a whole and including their free ends may be contained in a cavity of a diameter the same as those of the wire materials, whereby the sliding or slipping and bending of their contacting surfaces and their adjoining portions is prevented.

In addition to this noticeable advantage, as the abutting surfaces of wire materials which are to be cold welded are smaller than the diameter of cut wire materials, as explained in detail hereinunder, with this invention, the pressure applied to said wire materials along their axial directions is effectively concentrated to the abutting surfaces for producing excellent binding forces thereabout.

Preferred examples of embodiments of this invention will now be described, with reference to the accompanying drawings, in which:~ Figures 1, 2, and 4 are schematic views showing the cold welding of composite electrical contacts in accordance with conventional methods; Figures 3, 5, 6, 7, 8, and 9 are also schematic views showing the cold welding of various shapes of composite electrical contact in accordance with this invention, and Figures 10 to 16 show other examples of projections provied at free ends of short wire materials.

Example 1 (The production of a bimetallic or singlecontact material or rivet having a contact portion, diameter of which is about 1 to 1.5 times of the diameter of shank portion). A bimetallic or single-contact electrical contact of the dimensions as illustrated in Figure 3 (in which the diameter of contact surface 2' and that of shank portion 1' are equal, viz., 1:1) and as cold welded and the rivet produced therefrom by shaping it after cold welding (the dimetrical ratio between the contact portion C and the shank portion S is 1:1.3) are not producible by conventional methods, as explained in the above with reference to Figures 1 and 2.

With reference to Figure 3, to a sheared or free end of a short copper wire material 1 having the dimensions as indicated in the drawing, there is formed a projection 1 a of the illustrated dimensions, while to the free end of silver piece 2 which confronts to the projection 1 a, there is also provided a projection 2a. These wire pieces 1 and 2 are then put into a die 5 having a cavity of the diameter of 2.5 mm, so that their projections 1 a and 2a abut against each other at their forward surfaces. Then, the wires are pressed along their axial directions until the projections are sufficiently expanded and fill the vacant room of the cavity. The composite electrical contact thus obtained consists of a shank piece 1' and a contact piece 2' of the equal diameter, and has its bonded surface 3 of shearing force of about 59 Kg (12 Kg/mm2). The contact piece 2' is then shaped in a manner known to the art. Though the single contact rivet thus obtained is same to the one obtained in Figure 2 in their dimensions, the binded surface 3 of the former is about 96 Kg (12 Kg/mm2) which is far superior to about 24 Kg (3 Kg/mm2) and comparable to that of Figure 1.

Example 2 (A double-contact rivet having a shank portion, diameter of which is smaller than a contact portion and length of which is longer, compared to those which are producible by conventional methods, and another doublecontact rivet having a shank portion, diameter of which is smaller and length of which is longer than those which are obtainable by conventional methods and also having a contact portion, diameter of which is smaller than the conventional ones). The production of the former rivet is illustrated in Figure 5, while the latter rivet is illustrated in Figure 6. Both the former and the latter have such configurations and dimensions which are not attainable by conventional methods if their binded surfaces are not neglected. In Figures 5 and 6, those parts which are identical to those of Figure 4, are represented by same numerals.Their processes of cold welding and shaping are same to what are explained with reference to Figure 3. In Figure 5, however, the bonding between the wire 1 and the other wire 2-1 shall not necessarily be made within a die cavity 5, as said bonding is made as preliminary bonding.

The binding force of the bonded surface 3 in Figure 5 is, when expressed by shearing force, about 235 Kg (12 Kg/mm2), that of the bonded surface 4 in the same Figure 5 about 59 Kg (12 Kg/mm2), while that of the bonded surface 3 in Figure 6 is about 96 Kg (12 Kg/mm2) and that of the bonded surface 4 in Figure 6 about 59 Kg (12 Kg/m m2).

Example 3 (Bimetallic or single-contact rivet having a contact portion solely made from desired precious metals, which are not producible by conventional methods). As repeatedly described and explained above, conventional cold welding methods require the diametrical expansion of wire materials to about two times of their original diameters along their welding surfaces in order to have sufficient bonding force. This requirement inevitably produces the expansion or bead which consists both wire materials. Whereas, in this invention as illustrated in Figure 7, no bead is produced, and the contact portion can be made solely from silver. Numerals identical to those in Figure 3 are used in Figure 7. The shaping process itself is conventional.

Example 4 This example which is shown in Fig. 8 is essentially same to the one explained in the foregoing with reference to Figure 3. There is a difference, however, that either one of short wires 1 or 2 which is to be cold welded, cannot be provided with a projection 1 a or 2a, because of its volume being too small. The projection 1 a provided to the short wire 1 and another short wire 2 have configurations, dimensions, and functions which are same to those 1 a and 2a which are explained in Figure 3. The parts in this Figure 8 which are correspondent to those of Figure 3 are represented by same numerals.

Numeral 6 is a punch, and numeral 7 a pin.

Manufacturing steps of this example are the same as those which are described with reference to Figure 3.

Figure 9 shows another combination of short wires, with which this example 4 may start.

Besides Figure 3 and Figures 5 to 9, other examples of projections which can be provided at the free end of a cut wire piece in accordance with this invention are illustrated in Figures 10 to 16.

These projections, viz., those illustrated in the accompanying drawing can be made at their free ends by different methods such as cutting, stamping, forging, extrusion pressing, rolling and so on. However, in view of saving of raw materials, especially of precious metals, the methods other than cutting-off would be preferable. When raw wire materials are small in diameter, projections for such small diametered wires can be made by expanding the parts of wire other than the part to be made as a projection.

This formation is, of course, within the scope of this invention. Though the projections employable in this invention thus include various shapes and volumes, it might be the easiest way for designing and for production to make the projections which are formed to different wire pieces and which are to be pressed and expanded to make binded surfaces, equal to each other in their diameter and height, viz., dimensions which are one half of the diameter of employed wire materials.

Attention is directed to Application No.

80 37576 from which this application was divided.

Claims (3)

Claims

1. A composite electrical contact made from a plurality of short metallic wires which are cold welded at their abutting surfaces comprising as said short metallic wires, a shank portion of nonprecious metals and a contact portion of precious metal which is cold welded to an end of said shank portion, the ratio between the diameters of said shank portion and of said contact portion being 1:1 to less than 1:1.5 and the force bonding the abutting surfaces being more than about 8 Kg/mm2 against shearing force; and in which said ratio is the value obtained after cold welding and/or shaping without employing any machining for the purpose of reducing the diameters of the shank or contact portion.

2. A composite electrical contact as claimed in claim 1, in which the contact portion is made solely from precious metal.

3. A composite electrical contact substantially as hereinbefore described with reference to Figures 3 and 5 to 1 6 of the accompanying drawings.