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The Design of Light Duty Electrical Contacts ECONOMICS of MANUFACTURING and ASSEMBLY METHODS by J

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The Design of Light Duty Electrical Contacts ECONOMICS of MANUFACTURING and ASSEMBLY METHODS by J 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.
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