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Aviation History
1963
1963 - 0177.PDF
PLIGHT International, 31 January 1963 Production Standards To deliver products representative of prototypes clearly demands hifther production standards than those applied on the test machines. Assembly variables are limited on prototypes; to ensure that pro duction machines comply with the original specification, a tighter regime is needed on the production line. Some of these follow naturally by the use of production tooling, by the employment of techniques of accurate repeatability—such as pressure die-castings, mouldings, and extrusions—but others must be brought about by selective assembly, or by restriction on original design tolerances. Table 1 attempts to show some variables affecting brushwear, indicating those which are the responsibility of Design and which Production. It is appreciated that all factors affecting brushwear TABLE I Component Commutator Brush Brush box Brush spring Armature bearing Property Diameter (nomina!) (tolerance) Separator undercut Hardness of copper bars (after soldering) Surface finish Composition Dimension (nominal) (tolerance) Brush box-to-com mutator clearance Dimension Surface finish Materia! Loading Configuration Material temper Dimensions Spacing to brush Shaft and housing fit Radial clearance Symptom Peripheral speed Permissible brush tipping in box Wear life Abrasion Abrasion Conductivity, lubricity Current density Freedom in box (causing rocking and wedging) Wedging of brush Freedom (causing rocking and wedg ing) Stiction due to broach marks Increased brush clearance as ma chine attains work ing temperature Maximum contact pressure with mini mum wear Constant loading during brush life Loading Loading Loading Brush vibration Brush vibration Responsibility Design Production Production Production Production Design Design Production Production Production Production Design Design Design Production Production Production Production Production are initially the responsibility of the Design department. However, it will be seen from the last column that a consistent brush-life depends largely on factors directly under the control of Production and Inspection functions. The table concerns itself with conditions under a given standard environment, and is not exhaustive. Maintenance of Reliability Of the two major amis of reliability—safety and economy—the former is more closely associated with design and manufacture than with the post-delivery functions of servicing and maintenance. It is true that progressive deterioration in installations involving the safety of the aircraft can be detected and rectified by regular maintenance, but a larger number of mishaps to aircraft stem from instantaneous component failures which show no prior evidence; Tor example, spring failures, cracked castings and sheared quill shafts. Adequate factoring of stressed members, avoidance of stress-raisers, correct heat treatment, suitable electrical clearances around "live" components and ample ratings of semiconductors are obvious ways of excluding such defects. Aircraft operators, constructors and service departments should be encouraged to provide statistics of items needing regular atten tion, following scheduled removal, and—especially—reports on persistent causes for unscheduled removals. This may show a trend in certain installations which'no amount of laboratory testing is likely to reveal. Removal of such units, particularly if fitted in inaccessible locations, may prove very costly in terms of labour and loss of revenue; whereas the unit may only require re-lubrication, or replacement of brushes or seals 165 Fig 5 Actuator ram damaged through faulty stop-setting. Right, cushion drive fitted to obviate this risk For the guidance of the user's service department a number of documents covering every aspect of the equipment are made avail able by the supplier. The basis for all production and inspection activities are the manufacturing drawings. Each is the undisputed birth certificate of the part it represents and may, in most instances, be changed only by official modification action. The change-note fulfils a similar function for the supplier's organization as does the service bulletin for the user. Attention of all departments concerned (purchasing, planning, production, assembly, testing, inspection) is drawn to the introduction of every modification, which may vary in urgency from Immediate and Retrospective to the fitting of modified items when stocks are exhausted or tools need replace ment. Where such a modified part is not fully interchangeable, the introduction of all parts affected by the change must be accurately phased and the procedure laid down in the service bulletin. To enable servicing departments to maintain equipment correctly, all relevant information not actually given on manufacturing drawings is contained in a maintenance manual. Herein will be found leading dimensions and performance data, a description of the equipment, its function, its dismantling and reassembly, special tools, checks, maximum permissible wear allowances and other details. To ensure that each unit fulfils the basic requirements, it is tested to a production acceptance testing (PAT) schedule before despatch. This test must be satisfied by new and overhauled equipment alike. To maintain a high standard of workmanship, units selected from the production line after a fixed interval, or after a fixed number of units have been made, are subjected to the more detailed and searching production quality test (PQT). The interval is normally six months or 50 units, whichever is the smaller. It is significant that before any new design is accepted for deve lopment flying it must pass through flight clearance tests for periods ranging normally from 25 to 200hr. In accordance with Av.P.25, Technical Procedure Requirements for Service aircraft and their equipment, such machines are then issued with Air Form 100D which qualifies it to be used during flight trials. For a more extensive flight programme, Experimental Flight Approval must be sought for new designs. Such EFA tests nor mally contain all those grouped under Full Type Approval testing; storage testing to DTD.1085C is omitted and endurance and vibra tion tests are reduced to one-fifth of the total cycle. Once equipment passes from the development to the production stage one or more units are selected to undergo the Type Approval test, to prove its fitness to meet the Declaration of Design and Performance (DDP), which lays down all the requirements of the Fig 6 Terminal post fitting as modified to overcome risk of loosening through over heating
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