The Design of Light Duty Electrical Contacts ECONOMICS OF MANUFACTURING AND ASSEMBLY METHODS By J. G. Want Industrial Division, Johnson Matthey & Co. Limited

The performance of any type of electrical freedom from the type of oxide or sulphide equipment can be greatly affected by the care films which, if present, would lead to high given to the specification and design of the values of contact resistance. (It must be contacts. Many metals and alloys are available, understood, however, that slight surface films together with an increasing number and may still be present even on the most corro- variety of methods of contact manufacture sion-resistant of the platinum metals, these and assembly, but economic factors as well films consisting generally of adsorbed gases as metallurgical and mechanical considerations or of dust or organic matter.) must naturally be taken into account in arriving at the optimum design. The Platinum Metals This article is concerned only with the as Contact Materials range of electrical contacts broadly described Apart from rhodium, which is only used as operating on light duty. Concise definition as an electrodeposit-and then mainly for of this type of loading is not a simple matter, sliding rather than for make-and-break but in general it may be taken to cover contacts-the two platinum metals used contacts handling currents measured in either unalloyed or as bases for alloys with milliamperes rather than amperes, at voltages other metals are platinum and palladium. ranging up to 250. Such conditions obtain, of Platinum is tarnish-free at all tempera- course, in a wide variety of relays, signalling tures up to its melting point. In the form devices and electrical instruments in which normally used for contacts its hardness of operation is sensitive and possibly infre- 65 VPN (annealed) compares favourably with quent, but reliability is of first importance. In silver which is widely used for contacts, designing such contacts-particularly for loads particularly in the medium duty range. It is at the lower end of the range-the principal ductile and malleable and lends itself to the requirement is the maintenance of a low and manufacture of practically all of the required stable contact resistance. Generally speaking, contact forms. the extent to which the contacts have to with- Palladium, which has a hardness of 40 stand electrical or mechanical wear is of VPN (annealed), may be considered as secondary importance (although in some cases tarnish-free at temperatures up to 400°C; at of very high frequency of operation the wear this temperature it starts to form a brown problem does arise) while the low power to be oxide, which later dissociates at about 800°C. handled makes electrical or thermal conducti- This is seldom of consequence from the point vity of little significance. It is in this field that of view of contact behaviour, although it must the platinium metals and certain of their be taken into account if the method of manu- alloys, together with some alloys of gold, facture or assembly of the contacts demands comprise the only suitable contact materials a brazing operation. Apart from this, palla- for efficient operation on account of their dium lends itself to manufacture of contact

Platinum Metals Rev., 1961, 5, (2), 42-50 42 Electrical contacts that are required to control smut1 currents at lozv voltages-as for example in many small relays, sensitive switches and measuring instruments-must gener- ally be designed in one of the plati- num metals or their allays in order to ensure the maintenance of a low and stable contnct resistance. This article discusses the design of such contacts to give optimum perform- ancewhileensuring that the costs of the ussenably are kept us lolv as possible.

parts as readily as platinum. However, under The properties of the principal light duty corrosive conditions it behaves less satis- contact materials, together with details of the factorily than platinum and there is some forms in which they are available, are set evidence of its tarnishing in the presence of out in the table on page 44. high concentrations of sulphur compounds. It is not, however, the purpose of this Its lower price (roughly one-fifth that of article to discuss the selection of one of these platinum for a given volume) is still a positive materials to meet a particular set of electrical incentive towards its use, but because of the conditions; this aspect of contact specification risk of slight tarnish it is generally avoided in has been dealt with in a number of papers and very sensitive applications. articles (see, for example, Hunt, Platinum The hardness of platinum can be increased Metals Review, 1957, I, (3), 74-81). The by alloying-for example with iridium, present article seeks rather to draw attention ruthenium or nickel. The iridium-platinum to the factors involved in the design of alloys remain tarnish-free at high tempera- electrical contacts for economical manufac- tures; the 4 per cent ruthenium-platinum ture, and to give some little guidance in alloy is tarnish-free, although the 10 per cent arriving at the optimum combination of ruthenium-platinum and the 8 per cent material and assembly method for large-scale nickel-platinum alloys oxidise at higher production. temperatures. Copper, silver and ruthenium are used as The Design of Contacts alloying metals with palladium. The 40 per One characteristic of contacts in the light cent copper-palladium alloy has a hardness duty range is that the mechanical force avail- (annealed) of 145 VPN, while 40 per cent able between the contacts is usually small, silver-palladium has a hardness of- 95 VPN. while the limited overall size of the apparatus These additions-which considerably reduce usually makes it practically impossible to cost-do not materially affect the tarnish- introduce much wiping action on closure of the resistance of palladium in normal atmo- contacts. Thus mechanical breakdown of any spheric conditions, although they reduce its surface films present is generally not possible. resistance to attack under corrosive conditions. The choice of the correct configuration of Less common is 10 per cent ruthenium- contact head can, however, often help to palladium with a hardness of 160 VPN. overcome this problem. For light duty

Platinum Metals Rev., 1961, 5, (2) 43 I Properties and Availability of Light Duty Contact Materials FORMS IN WHICH MATERIAL CAN BE Electrical MANUFACTURED Vickers conducti- - ipecific hardness vity Material Zravity :annealed) per cent Coinposi te IACS -leaded Turned Contact ilectro. contacts bi-metal leposit! I rivets rivets or screws __ Platinum 21.3 65 15 X X X X X

10% Iridium-platinum 21.6 I20 7 X X X X

20% Iridium-platinum 21.7 200 5.7 X X

25% Iridium-platinum 21.7 240 5.4 X X

4% Ruthenium-platinum 20.8 I30 5.7 X X X

8% Nickel-platinum 19.3 I75 6.4 X X X

Iridium-ruthenium- platinum 20.8 310 4.4 X

Palladium 11.9 40 16 X X X X X

40% Copper-palladium 10.4 I45 4.9 X X X X

40% Silver-palladium 11.0 95 4.0 X X X X

10% Ruthenium-palladium 12.0 I60 4.8 X X X ------contacts the three combinations of contact of such contacts is severely limited to a few shape shown below are recommended. milliamps only. The conicaliflat combination (A) is recom- Where contact force is not so limited, the mended when the available contact force choice lies between the hemispherical/ is particularly low, since the extremely small hemispherical and the hemispherical/flat area of contact ensures the highest possible combinations, (B) and (C). In either case the contact pressure. Attention should be paid to radius of spherical curvature is extremely the radius at the apex of the cone, and the important and experience shows that this included angle of the cone should be at least should be between half and one times the head 120 degrees. The current-carrying capacity diameter, as shown below. Of the two, the

Recommended head configurations and proportions for light duty contacts

Platinum Metals Rev., 1961, 5, (2) 44 hemispherical hemispherical combination is drop. There is therefore a broad relationship to be preferred since it provides the highest between the current carried and the minimum contact pressure. If the overall design doesnot contact force required. permit sufficiently accurate contact alignment Where it is necessary to use the conical/flat in the closed position, it is better to employ combination the device can seldom be de- the hemispherical/flat combination. On no signed to give a specified contact force; rather account should two flat contacts be used to- advantage must be taken of whatever limited gether. contact force is available and the rating of the The use of two hemispherical contacts has contacts decided accordingly-generally em- commercial advantages; stock-keeping is pirically. Where either the hemispherical,’ simplified and larger numbers in one type hemispherical or the hemisphericaljflat com- can be ordered. bination is employed the contact force must Head Diameter be specified according to the required current- carrying capacity of the contacts. The mini- Where available contact force is so small mum contact force for platinum metal con- that it is necessary to employ the conical/flat tacts carrying up to one amp is also shown in combination, a head diameter between 0.040 the curve above. It is stressed that the inch and 0.062 inch will be adequate. Where values of contact force given by the curve the hemisphericaljhemispherical combination represent minima. They can, with advantage or the hemisphericaljflat combination is em- in terms of reliability, be increased by a factor ployed the head diameter should be within of two or three. Excessive force must the hatched area of the diagram above - that be avoided since this will cause undue is between 0.062 inch and 0.125 inch in dia- mechanical wear and so curtail the life of the meter according to the current handled. A contacts. larger diameter can of course be employed, but this may lead to unnecessary usage of Forms of Contact and noble metal. Contact Assembly Contact Force The small contact rivet which is the most For any pair of contacts with a film-free widely used form of light duty contact is most surface, the effect of increasing the contact economically manufactured by heading or force, within certain limits, is to reduce cold forging on high-speed machines. In this the contact resistance and so increase the process a short length of wire of the required current that can be carried for a given IR shank diameter is cropped off and one end of

Platinum Metals Rev., 1961, 5, (2) 45 general use as contact materials are suitable for heading, although it is essential to keep the proportions of the rivet within certain limits, depending to a large extent on the metallurgical properties of the contact material specified. Rivets having the proportions shown on page 44 are ideal for heading and can be produced from all the contact materials shown as headable in the table. These recommended proportions also offer a good compromise between the sometimes conflicting require- ments of economy of noble metal, electrical efficiency and mechanical strength. Heading offers two economic An example of the use of 40 per cent sihr-palladitcm rivets advantages: not only is the heading found in this duty relay by Waymouth Gauges & is light process fast but it produces no Instruments Ltd scrap. Manufacturers of contacts the cropped wire is then cold forged into the generally offer standard ranges of headed contact head. Rivets made this way have a contacts from stock; where non-standard hard head and a comparatively soft shank, contacts are required it is usually necessary which is an ideal combination from the point to order a minimum number (typically 5,000) of view of long life and ease of riveting. if they are to be made by heading. As can be seen from the table, the majority If smaller numbers of non-standard rivets of the platinum and palladium alloys in are required, or if the proportions of the con-

Platinum Metals Rev., 1961, 5, (2) 46 tact material rule out heading, the rivets must applications, particularly where linear adjust- be produced by turning. In comparison with ment of position of one contact is necessary. heading, turning is slow and produces swarf They comprise a base metal screw carrying the which must be returned for refining. More- noble metal contact at one end. Such contact over, unlike the headed contact, the hardness screws can be designed for manufacture by of the head of a turned rivet is the same as that force fitting (of rivets or cropped wires), by of the shank. welding or by brazing. An electrodeposit of The use of composite rivets,in which a disc silver is sometimes specified on the body of the of noble metal is brazed to a base metal back- screw to improve its contact with the assembly ing, is not always justified economically for into which it is fitted. The manufacture of contacts of 0.125 inch diameter and below, contact screws generally demands a high unless a very long shank is required. This is order of precision which is reflected in their particularly true with palladium and palladium cost. alloys because of their lower intrinsic cost. Welded and Brazed Assemblies Such construction is, however, essential if Many of the platinum metals lend them- the contact material is too hard to be riveted, selves to direct resistance welding on to as for example with more complex alloys such contact supports. A variety of techniques is as iridium-ruthenium-platinum, or is of very employed: for example, discs of platinum or high cost. The backing material employed iridium-platinum can be resistance welded on for composite rivets is normally copper, and to certain steels by forming a projection either the backing should preferably be designed for on the reverse side of the disc or on the production by heading. Here again contact support. In another technique, employed by manufacturers often have available a standard some large-scale manufacturers of telephone range of composite rivets with platinum and relays, platinum wire is taken from its reel, palladium alloy heads. Turned backings will butt-welded on to the contact support and of course be necessary for special shapes, but cropped, leaving the support with a small will result in an increase in cost. cylinder of platinum attached to it at go Contact screws are required for certain degrees. A second operation in a hand

In this regulator thermostat, manufactured by the Rheo- static Company Limited and used in relating the water temperature of central heat- ing systems to the outside air temperature, double rivets of 10 per cent iridium-platinum move between two base metal screwsjtted with contact discs in the same alloy

Platinum Metals Rev., 1961, 5, (2) 47 press forms the wire into a contact head. considerable care should be taken in this Discs of contact material can also be operation as poor riveting can lead to pre- attached to the contact support by low tem- mature failure, either electrical or mechani- perature brazing, using a silver brazing alloy cal, of the assembly. The hole into which the such as Easy-flo (BS 1845 Type 3) which has contact is to be riveted should be given a a melting range of 620 to 630°C. Unfortu- diameter 0.004 inch larger than the nominal nately, this brazing temperature softens the diameter of the shank, and should be burr- support so that if a material is used which free on the side against which the contact head will not respond to heat treatment (for is to fit, since the presence of a burr can example, nickel silver or phosphor bronze) prevent the back of the head from making the designer must expect the support materials contact with the support. A slight countersink to be in the soft condition. With heat- on the same side may also be provided. If the treatable support materials such as beryllium- rivet is to be double-headed, that is, if a copper it is possible to harden the support second head is to be formed from the shank after the brazing operation, provided that on the reverse side of the support, both sides care has been taken during brazing to prevent must of course be burr-free. irreparable metallurgical damage to the spring During the riveting operation the head alloy. should be supported in a die which must be recessed to match accurately the profile of the Assembly Techniques head. Failure to take this step will cause A number of different methods are available indentations in the head after assembly. for attaching contacts to their supports. A variety of forms of riveting punch can Suitable techniques for riveting, force fitting be used, some of which are shown below. and brazing are briefly outlined below, but If the head is required on only one side of the in practice these may be varied extensively blade the snap-heading punch or the staking to meet special requirements. punch can be employed, the staking punch being of advantage when the length of shank Riveting available for riveting is short. A spring-loaded By far the most widely used and the sleeve, shown in sketch A, is strongly recom- cheapest method of assembly is the riveting mended with either type of punch in order of a contact to its blade. Despite its simplicity to press the blade firmly against the rivet

Platinum Metals Rev., 1961, 5, (2) 48 before closing and to correct errors in the Shank angle of presentation of the blade to the punch. Recommended Lengths for SingIe Headed Assemblies A toggle press is generally used for this type of work. A spin/hammer riveter is recom- Thickness of Recommended mended with thin supports if the fixing hole contact support shank length is near the edge of the blade, because of the (inches) (inches) stress set up in the material by conventional riveting. uptQ 0.008 0.025 If a double head is required, a reverse- 0.009-0.0 I3 0.035 heading punch, shown in sketch C, must be O.OIC0.018 0.040 used, having a recess with the precise head form required for the reverse side. A siight 0.019-0.023 0.050 taper must be provided on any parallel sur- 0.024-0.033 0.060 faces to prevent the newly-formed head from 0.034-0.038 0.070 sticking in the punch. With thin blades a spin/hammer riveter or some similar machine 0.039-0.048 0.080 is essential, although orthodox press tech- niques can generally by employed with thicker against which the underside of the contact blades. head is to rest should be free from machining It is essential that the shank of the rivet burrs. A punch designed to match the con- swells in the hole during riveting and so tours of the rivet head should be used, with grips the support firmly; it is not sufficient to a conventional toggle press. rely on the “formed over” metal on the An important point arises in the manu- riveted side. With the shank correctly swollen facture of screws to be force-fitted with there is no risk of lateral movement of the headed rivets. If the overall length is sub- rivet in operation and sound electrical ject to a wide tolerance, it becomes impossible connection with the support is ensured. to employ a toggle press working on a set The length of shank is important. For stroke. Instead a hydraulic press or a toggle single heading the recommended shank press with a spring-loaded attachment must lengths for blades of various thicknesses are be used. Either method leads to an increase given in the table. For double heading, the in the cost of assembly. length of shank must be calculated from the volume of the required head. In practice Brazing a shank slightly longer than the theoretical A widely used technique for making length is normally required to allow for the composite contact assemblies, such as fitted metal used to fill the hole as the rivet swells. screws, is the low temperature brazing of a disc of contact material to its base metal Force Fitting backing. For mild steel, brass or copper, Contact screws can be produced by force- Easy-flo silver brazing alloy is normally fitting a rivet into a suitable hole in a copper recommended, with a conventional low tem- or brass screw. For economy and reliability perature brazing flux. in this type of assembly the drilled holes that The surface of the support to which the are to receive the contact shank should be held contact facing is to be attached must be free within + o -0.0005 inch of the nominal from burrs and turning pips, but too high a diameter and should be not less in depth than finish must be avoided. For small runs torch 14 times the shank diameter plus about brazing is generally used, with a billet of 0.030 inch. A slight lead-in at the mouth of brazing alloy whose volume is such as to the hole is desirable, and the face of the screw produce a joint thickness after assembly of

Platinum Metals Rev., 1961, 5, (2) 49 attention should be paid to the following points: (a) The material chosen should be the lowest in intrinsic value that will operate satisfactorilyunder the specified condi- tions. In comparing costs, account must be taken of density as well as of price per unit weight. (b) The contact should not be larger than is necessary for the specified electrical and mechanical conditions, and unnecessary thickness of contact material should be avoided. (c) Where rivets are required, “headable” materials should always be specified in preference to “unheadable” materials, and the proportions of the rivet should be such as to permit easy heading. (d) Tolerances on contact dimensions should not be so close as to demand special manufacturing techniques, but The I0 per cent iridium-platinum alloy is used in this electric dial gauge by Thomas Mercer (Air on the other hand the tolerance on Gauges) Limited. The moving contarts consist qf backings should not be so wide as to rivets fitted into bruss arms: the $fixed contacts reduce the speed of assembly or to comprise ,fitted screws demand special assembly techniques. about 0.002 inch. For larger runs the assem- (e) The use of two hemispherical contacts blies can be furnace brazed with pre-placed in each pair is to be preferred on grounds discs of solder, generally 0.003 inch thick and of economy to one hemispherical and having the same diameter as the contact discs, one flat. Not only are hemispherical which must be located on the support during contacts slightly cheaper than equivalent the brazing operation by means of graphite flat contacts but larger numbers in one jigs. Induction brazing can also be used, but size can be ordered, thus additionally with light duty contacts it demands special reducing the cost per contact. Where techniques because of the small component relatively small numbers are required size. the use of one type of contact instead Machining of brazed assemblies of this type of two may make it economic to manu- may be necessary if the overall tolerances on facture the contacts by heading instead the assembly are close because of the possi- of by turning. bility of slight eccentricity or tilt of the disc (f) In many cases more than one method in relation to the screw. of assembly should be considered. When comparing two or more such methods Economics of Contact Design consideration should be given not only The platinum metals are costly, so also to the cost of making prototype quanti- are certain of the techniques for manufactur- ties but to the economics of production ing contacts from them, particularly those quantities. The cheapest method of that produce scrap which must be refined. If, making prototypes is sometimes the therefore, the cost of a light duty contact most expensive method of making large assembly is to be kept as low as possible, numbers of components.

Platinum Metals Rev., 1961, 5, (2) 50