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Aviation History
1958
1958 - 0106.PDF
108 FLIGHT, 24 January 1958 * Improving Commutator Performance USES OF THE RHODIUM-PLATING PROCESS IN AIRCRAFT ELECTRICAL EQUIPMENT ELECTRICAL machinery has frequently been limited in performanceby poor commutation; and this has been particularly critical in modern aircraft. Reduced operating current, lower brush wear and tempera-tures, increased life and less radio interference are among the advantages claimed for the rhodium-plating process described here. The matter is ofparticular interest in the light of the opinions expressed on pp. 116-117. THE main cause of commutator inefficiency is the copper-oxide film which forms on the working contact face. Thisfilm can form during storage and certainly does so during operation of the motor or generator. Copper oxide not only has ahigh specific resistance, creating heat-developing wattage loss, but is continuously formed at raised temperatures from the inside faceof the oxide film. This oxide film breaks away from the com- mutator in small particles, arcing occurs, and electrical noise andradio interference result. A practical solution of the problem is to deposit on the coppercommutator a film of non-oxidizing metal. This film must be hard wearing, have a low coefficient of friction and must be non-porous,so that oxidization of the copper below the film cannot occur. The protective film must be deposited after the armature/commutatorassembly has been balanced and all work completed. Rhodium has a low electrical resistance, is non-oxidizing, andhas a hardness of 500/600 Brinell and a coefficient of friction much lower than that of copper oxide. It is therefore a most suitablematerial to be used for this purpose. Rhodium can be deposited by bath techniques, but plating of a complete, wound commutator/armature assembly presents obvious difficulties. A method of avoiding them is the use of the technique known as the Dalicprocess of localized electrodeposition (Metachemical Processes, Ltd., Crawley, Sussex). The process requires equipment comprising a powerpack givinga D.C. output, a type of brush anode or stylus and special solutions carrying the metals it is required to deposit. No immersion insolutions is necessary, the process is rapid, and it can be controlled within extremely fine limits. The negative lead from the powerpack is connected to the work-piece and the positive lead to the brush anode or stylus. The stylus is formed with an absorbentmaterial, dipped in the plating solution and brushed over the area to be plated. The deposit is stated to have excellent adhesion to the basemetal and to be non-porous. Its thickness depends upon plating time and current, which is indicated precisely by a finely graduatedampere-hour meter on the power pack. Deposits of any reasonable thickness may be obtained with an accuracy of ± 5 per cent. The weight/power ratio of electric motors used in aircraft isalmost always an important factor; but quite frequently, because of commutator impotencies, motors and generators have to bede-rated, with a consequent weight/power ratio increase. In one particular instance, the electric fuel-pump motor on a famous intercepter aircraft was subject to delays in manufacture, owing todifficulties in obtaining the required fuel delivery with the maxi- mum current ordained; the working temperatuje was high, brushwear excessive, contact resistance was high and commutator life extremely short. After a fully loaded run of some 120-130 hours,commutator wear was noticeable and, owing to the rapid formation of a copper-oxide film, the current required to drive the motor waavery high. Similar commutators were rhodium-plated by Dalic to a thickness of 0.00005in and put on test pumps for trial. Vastimprovements in commutation were noted at once. As indicated in the table below, the rhodium plating gave the required pumpperformance with a lower current than that previously obtained and the current drain on the batteries was reduced by some 12 to13 per cent. After over 600 hours' operation, it is stated, the current required was still at the lower figure, no loss of rhodiumwas observed and the working face of the commutator remained unchanged. Temperature of the rhodium-plated commutators wasmuch lower than that of the unplated examples, and brush wear was negligible. Pump No. 1 Pump No. 2 Pump No. 3 Pump No. 4 Unplated Plated Unplated PlatedUnplated PlatedPlated Current (amp) Normal r.p.m. 15.6 15.0 16.8 15.5 15.3 15.516.0 High r.p.m. 25.9 22.8 26.0 22.826.0 23.523.6 In another instance a rotary convenor on an airliner causedheavy radio interference after some 300 hours' operation. When the commutator had been rhodium plated no further interferenceoccurred, even after some 2,000 hours' operation. It was con- sidered that the commutator would still operate in a static-freecondition after several thousand hours. To ensure that no damage could occur to commutator insula-tions through being in contact with Dalic solutions, certain tests were carried out. Two commutators were rhodium-plated and theplating solution left on one of them, while the other was swilled in clean water. After some three months the commutators wereexamined for corrosion. Staining was apparent, but was easily removed; no corrosion occurred, and the insulation value was notreduced. The value of the Dalic rhodium deposit on commutators is not,of course, limited to motors deriving their power from batteries, but it would appear that in this field lie the more urgent andimmediate applications. Development work is continuing, with the particular objective of the elimination of electrical noise on manyaircraft components without the use of suppressors. SEATS FOR BRITANNIA 253 SHOWN here in prototype form with three alternative types of upholsteryis the lightweight seating, designed and built by Short Bros, and Harland, Ltd., which is to be installed in the Bristol Britannia 253s whichthe Belfast firm is building. The backs and legs are detachable for stowage and the units can be installed either port or starboard. High-density seating, either forward- or aft-facing, can be providedwithout loss of comfort because the reclining back can be adjusted while the seat-cushion remains fixed, thus maintaining adequate leg-room. Officially specified safety-features include leather "ears" on the headreststo keep the occupant's head in position in the event of a forced landing, and a heel-restraint system to prevent ankle injuries. The seat, which is designed to withstand loads of up to 9g, is built almostentirely of pressed light-alloy section. Upholstery is in foam plastic, with dark blue hide covering and metal protective fittings. Normally positionedat 12 deg, the back can be adjusted through a further 32 deg in eight stages. The table, when not in use, is stowed in a pocket attached to the back ofthe seat. The seats to be produced for the Britannia 253 comprise 1,350 passengerplaces, made up of 60 double and 410 triple units. They are being made at Short's general engineering division at Newtownards, near Belfast, whereseats for B.E.A. Viscounts are already in production. The latter seat, for the V.802 and V.806, was illustrated in our issue of September 21, 1956.
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