ELECTRICAL CONTACTS

Abstract: An electrical contact is formed from a BeCu leaf spring (2) to which is welded a short length of silver wire (3) by means of a series of laser spot-welds (4). The spot welds are formed in the acute angle (5) between the leaf spring (2) and the wire. The wire (3) extends over the edges of the spring (2).

Description: Electrical Contacts The present invention relates to electrical contact assemblies of the type having a contact member carried by a conductive support member, and in particular, but not exclusively, to such contact assemblies wherein the contact member is formed of silver. The term "silver" used hereinafter is intended to include both fine silver and silver alloys produced to alter the properties for specific applications. At the present time, a large number of silver contact constructions are known. Silver is particularly preferred as a contact material in view of its excellent electrical and thermal conductivity and because during use, the oxide coating formed on the contact is also electrically conductive. Fine silver (99.9% pure) is usually used to produce electrical contact faces since this grade of purity gives optimum properties of low electrical interface resistance, wear resistance, long life, current rating, minimum contact pressure, small size, formability etc. for a given cost. This grade of silver is the preferred alloy used in most applications . To give a harder contact member or one with improved weld resistance, particularly at higher temperatures, other materials such as nickel, copper or cadmium oxide may be alloyed with the silver. Initially silver contact members were formed from silver wire formed into a contact head and shank, the shank being used to rivet the contact member on to a conductive strip member. However this led to unacceptable high usage of silver with high cost implications. This led to the development of composite contact members which comprised a silver contact facing welded or diffusion bonded onto a copper backing and rivet shank. The contact member was then riveted onto a conductive strip member, as in the earlier construction. This type of contact member is widely used today, but is not as competitively priced as before in view of increased assembly costs relative to the price of silver. It has also been proposed by the present applicant to press fit a slug of silver wire directly into a hole provided in a leaf member formed of beryllium copper (BeCu) . In this process the hole in the leaf member has an upstanding rim and is designed to provide an interference fit with the slug which has a slightly larger diameter than the hole. Subsequently, the plug is coined in a die to form the contact member. However, although this process makes more efficient use of silver than the more conventional rivetting method, it still uses more silver than necessary to account for contact wear. Furthermore, the dies have a limited life and must be replaced at regular intervals. It has also been proposed to weld silver contact members directly on to a strip member. It is difficult to weld silver to certain strip members, such as beryllium copper leaf members, since BeCu gives a brittle, crystalline weld. Furthermore, since both silver and BeCu are very highly conductive, an extremely high current is required if they are to be electrically welded. Further, the welding electrodes of spot-welding apparatus are normally of a material similar to BeCu which leads to the problem that the electrodes may actually bond to the components rather than the components bonding to each other. One proposal to overcome this problem has been to use a trimetallic tape of silver with nickel and stainless steel backing layers which is punched and trimmed and then fed automatically to be welded in place. However this tape and the machinery required to produce this type of contact is very expensive, and production is comparatively slow, with a maximum rate of less than 30 contacts per minute. A further problem is that it is necessary to stop production at frequent intervals in order to clean the electrodes. According to the invention there is provided an electrical contact assembly comprising an electrically conductive support member and a contact member laser- welded thereto. The invention also extends to a method of forming an electrical contact assembly comprising the steps of providing an electrically conductive support member and laser welding a contact member thereto. Thus, by means of the present invention, the bond between the contact member and the support member is formed by laser welding. This allows high conductivity materials such as silver and beryllium copper to be used without the problems associated with the known processes using electric welding. In particular, there is no physical contact between the welding apparatus and the assembly. Moreover, the wasteful use of expensive contact material (eg. silver) to form a mechanical connection as in the prior art riveting processes is avoided. Thus, a simple, single-metal contact member using the minimum quantity of material may be employed. As a result, it is possible to form contact assemblies by means of a simple process which makes efficient use of materials. Although a wide variety of materials can be used to form the contact assembly, the contact member material preferably has a very low electrical resistance (eg. silver, silver alloy or gold) , and the support member is a good conductor. Where a spring-contact is required, the support member preferably also has resilient properties (eg. BeCu alloy) . The welding is preferably carried out using known techniques in which a spot of laser light is directed at the assembly. Although a single weld could be employed, it is preferred that a plurality of spot welds be used. These may be formed using a single laser source arranged to provide pulsed laser beams having optically focused spots of for example about 0.25-0.35 mm, eg. 0.28 mm in diameter. Between pulses, the beam may be re-directed optically or in some other known manner. Alternatively, the contact assembly may be moved relative to the laser source. The assembly may be arranged to move in a series of steps, with each step aligning the assembly for one or more welds. However, since the laser pulses will generally be very short (in the order of microseconds) , the movement may be continuous. Thus, by moving the assembly past the laser source (s) at a constant speed, regularly pulsing laser source (s) will provide evenly spaced spot welds. Although it might be possible to weld the contact member through the support member, this is not preferred. This is because the support member will usually be highly thermally conductive, so requiring a very large energy input to effect a weld. Accordingly, therefore, the weld is preferably formed by the laser being directed at the contact/support interface. One problem that may occur when certain, particularly reflective materials, such as silver, are used is that the laser beam may be reflected without a significant proportion of its energy being absorbed. To overcome this problem, the contact assembly is preferably arranged such that an acute angle is defined at the interface between the support member and the contact member and the weld(s) is/are formed within this acute angle. In this way, a "wedge" is formed into which the beam may be directed. As a result, the beam reflects back and forth within the wedge, and a proportion of its energy is absorbed each time. Thus, although only a small proportion of the energy of the beam is absorbed at each reflection, by means of these multiple reflections, the beam's energy is largely absorbed. This leads to rapid heating of the joint, resulting in an efficient weld. The contact member or the support member may be specially shaped, eg. by chamfering, to obtain the acute angle, but preferably the contact member is formed from a material having a curved cross-section. Thus, where the contact member meets the support at a tangent, an acute angle is formed. Preferably the contact member is circular in section, being formed for example, of wire cropped to length. The invention is applicable to a wide variety of contact assemblies. For example, the squat-cylinder shaped contact member which is common in "rivetted" prior art may be laser-welded to a leaf spring contact. However, since the contact member no longer needs to form a "rivet" in order to secure itself to the support member, more economical forms of contact may be used. One particularly preferred form is for the contact member to be elongate and to extend across the surface of the support. Thus, if the contact assembly comprises a leaf-spring, the contact member lies in a plane parallel to the plane of the spring. This is very useful when the contact assembly is designed to co¬ operate with a further contact whose position is not precisely confined. If the elongate dimension of the contact member is aligned in the direction in which the other contact may move, then it is easy to accommodate such movement with the minimum quantity of contact material. Generally, where a leaf-spring is used, this requirement will dictate that the contact be roughly perpendicular to the elongate dimension of the leaf- spring. Such a contact assembly is, in itself believed to be novel and inventive, and therefore, from another. aspect, the invention provides an elongate electrically conductive support member and an elongate contact member mounted on the face of the support such that the longitudinal axis of the contact member is generally parallel to the face of the support. Preferably, the contact member extends substantially across the face of the support. In fact, the contact member may even extend beyond the edges of the support. As discussed above, the contact member will frequently be generally perpendicular to the longitudinal direction of the support. However, other arrangements may be used, depending on the application. For example, if a pair of contacts is to be formed using elongate contact members mounted on respective supports, it may be preferred to mount one of the contact members parallel to the axis of its support and the other perpendicular. In this way, the contact members will be mutually orthogonal, thereby facilitating alignment of the contacts and accommodating relative movement between the contacts. Suitable elongate members can be easily and cheaply obtained by cutting a suitable length from a wire. This approach also facilitates automation of the process since a roll of wire may be used to feed an assembly line. If the wire is cut only just before, or even just after the welding occurs, then material handling is greatly simplified. It will be appreciated that, although wire having virtually any cross-section may be used, if normal, round-section wire is used, then the acute angle referred to above will be formed between the contact member and the support member. Thus, a very simple and economical assembly is formed from just a support member, such as a leaf-spring, and a length of wire. Although the use of round wire directly from a roll will generally be satisfactory, in some high current applications, a round-section contact member may not provide a sufficiently large contact area for electrical conduction between the contact member and a further contact with which it co-operates. Accordingly a length of wire may be formed into a generally flattened section, eg. by rolling. In this way the contact surface area is increased, whilst avoiding an increase in material usage and retaining the advantageous acute angle of the interface between the components to facilitate welding. Furthermore, the area of contact between the contact member and the support member is also significantly increased. The precise way in which the components are welded together will depend on the final application of the contacts . The welded connection to the support may be formed along one side of the contact member only. This generally provides a satisfactory connection and allows for a comparatively simple production line because it is only necessary to arrange the laser (s) to direct beam(s) to one side of the assembly. However, a stronger result is achieved if at least opposite sides of the contact member are welded. Apart from allowing twice the area of welding, this also provides a further advantage when a flexible support member is used. This is that, as the assembly cools after being welded, the contact member will be pulled into greater contact with the contact member, thereby improving the contact between them. Although the invention has been described above with reference to leaf-spring support members, as may be used in movable contacts, the invention may also be readily applied to other forms of contacts for example fixed contacts in which the support member may be rigid. For example, a rigid contact pin may have a contact member laser welded to its end. One application fcr such pins is in cordless kettles where rigid pins are formed on the base of the kettle for co-operation with the contacts of a base unit. In such applications there is inevitably a significant variation in the precise position in which the kettle is located on the base by the user. As a result, an elongate contact member, as discussed above, is preferred which is laser welded along the end of the pin. It is particularly preferred for this to be used in combination with another contact assembly having a further elongate contact member (eg. on a leaf-spring, also as discussed above) , tne contact members being arranged substantially perpendicular to each other. In this way, variation in position of the contacts in two orthogonal directions can be accommodated. Certain embodiments of the invention will now be described, with reference to the accompanying drawings: - Fig. 1 is a plan view of a leaf-spring contact assembly according to the invention; Fig. 2 is a side elevation of the assembly of Fig. 1/ Fig. 3 is a plan view of a contact pin assembly according to the invention; Fig. 4 is a side elevation of the assembly of Fig. 3 ; and Fig. 5 is a schematic view corresponding to Figure 1 onto which an end elevation of the contact pin assembly of Figure 3 has been super-imposed in phantom. The contact assembly 1 illustrated in Figs. 1 and 2 comprises a BeCu leaf spring 2 to which is welded a short length of silver wire 3 by means of a series of laser spot welds 4. As may be seen from Fig. 2, the spot welds are formed m the acute angle 5 between the leaf spring 2 and the wire 3. The wire 3 extends over the edges of the spring 2. In order to form such an assembly, the leaf-spring 2 is cut to size and then a suitable length of silver wire is cut from a roll. The two components are then held together by a jig before being welded with a series of laser spot welds. The laser welds are formed using a pair of similar 50W laser sources on opposite sides of the assembly. The focused heads of the sources are located 50-60 mm away from the assembly and focused to provide a 0.28 mm spot. They are arranged such that the beam from the laser is at an angle of 55° to the leaf-spring 2 to allow access to the wedge-shaped interface 5 between the components . The jig holding the assembly 1 is aligned with the laser sources ready to form the first weld. Then the laser emits a 1.5 ms pulse having a peak power of 1.9 kW . The assembly 1 is then moved into position for the next weld 4 and the process is repeated until all the welds have been completed. Using a standard 50W laser, about 17 welds per second may be made on each side. Thus, with an efficient indexing arrangement, the welding for a single assembly may easily be completed in less than a second. Figs. 3 and 4 show a contact pin assembly 10 formed from a rigid terminal pin 12 and a flattened silver wire 13 mounted on its distal end. This is formed in virtually the same way as the leaf-spring arrangement discussed above except that the wire 13 is rolled before being applied to the pin 12. Thus, the wire 13 is located against the end of pin 12 and a series of spot welds 14 are formed along either side thereof . The oval section of the flattened wire 13 gives an increased area of contact between itself and the support 12 giving improved thermal and electrical contact. It will be appreciated that the embodiments of Figs. 1 and 2 and Figs. 3 and 4 could be combined in a cordless electrical connector, the contact assembly 1 being arranged in a base connector and the pin contact assembly being provided in the base of an electrical appliance. Fig. 5 shows such an arrangement in which the longitudinal axes of the spring 2 and pin 12 are arranged at right angles to each other to allow electrical contact to be made between the pin and spring with some misalignment of the components. This avoids the need to, say, provide traditional inverted contacts on the leaf-spring which would otherwise have to be substantially of the same diameter as the length cf the pin, representing a considerable saving in material costs. Such an arrangement may be employed in a cordless kettle in which three leaf-spring assembles 1 are located in the base unit to co-operate with three pins 10 forming the terminals in the base of the kettle.

Claims: Claims 1. An electrical contact assembly comprising an electrically conductive support member and a contact member laser-welded thereto. 2. An electrical contact assembly as claimed in claim 1, comprising a plurality of spot welds. 3. An electrical contact assembly as claimed in claim 1 or 2 , wherein the weld(s) are formed at the interface between the contact member and support member. 4. An electrical contact assembly as claimed in claim 3, wherein an acute angle is defined at the interface. 5. An electrical contact assembly as claimed in any preceding claim, wherein welds are formed on opposite sides of the contact member. 6. An electrical contact assembly as claimed in any preceding claim, wherein the contact member is elongate. 7. An electrical contact assembly as claimed in claim 6, wherein the contact member comprises a length of wire . 8. An electrical contact assembly as claimed in claim 7, wherein said wire is circular in section. 9. An electrical contact assembly as claimed in claim 7, wherein the wire is flattened into an oval section. 10. An electrical contact assembly as claimed in any of claims 6 to 8 , wherein the contact member is generally perpendicular to the longitudinal axis of support member. 11. An electrical contact assembly as claimed in any preceding claim, wherein the contact member is formed of silver or silver alloy. 12. An electrical contact assembly as claimed in any preceding claim, wherein the support member is formed of copper or copper alloy. 13. An electrical contact assembly as claimed in any preceding claim, wherein the support member is a leaf- spring. 14. An electrical contact assembly as claimed in any of claims 1 to 11, wherein the support member is a rigid pin. 15. An electrical contact assembly as claimed in any preceding claim, in co-operation with a further electrical contact assembly also as claimed in any preceding claim, the contact members of the contact assemblies each being elongate and being generally orthogonal to each other. 16. A method of forming an electrical contact assembly comprising the steps of providing a conductive support member and laser welding a contact member thereto. 17. A method as claimed in claim 16, wherein the welding comprises forming a plurality of spot welds. 18. A welding method as claimed in claim 16 or 17, wherein the contact member is located against the support member so that an acute angle is formed therebetween, and the laser welds are formed withm the angle. 19. An electrical contact assembly comprising an elongate conductive support member and an elongate contact member mounted on the face of the support such that the longitudinal axis of the contact member is parallel to the face of the support. 20. An electrical contact assembly as claimed in claim 19, wherein the contact member extends substantially across the face of the support. 21. An electrical contact assembly as claimed in claim 19 or 20, wherein the contact member is generally perpendicular to the conductive support member. 22. An electrical contact assembly substantially as hereinbefore described with reference to the accompanying drawings. 23. A method of forming an electrical contact assembly substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: TAYLOR JOHN CRAWSHAW, TAYLOR, JOHN, CRAWSHAW
Applicants: STRIX LTD, TAYLOR JOHN C, STRIX LIMITED, TAYLOR, JOHN, CRAWSHAW
Application year: 1997
Main classification: B23K26
IPC codes: B23K26, H01H11, H01R4, H01R13, H01R43