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MODERN AIDS IN THE DESIGN OF RAILWAY BOGIES By A. W. Manser, B.Sc. (Eng.) (Member) The paper illustrates work carried out on motored and non-motored underground train bogies and the experiments carried out in the use of light-alloy bogies and vehicles. INTRODUCTION SMALL though the London underground system may appear in comparison with main line railways, the intensity of the service run and the physical characteristics of the system impose exceptionally severe conditions on many of the rolling stock components. This is particularly true of the bogies, where the high rate of acceleration and braking, frequent incidence of stopping, severity and number of curves, together with the unusually high load/tare ratio, combine to produce as exacting a problem as may be found in the railway world, apart from the further almost unique disadvantage-in the case of the Tube lines, where the tunnel diameter is only about 12 ft-of a critical space limitation. Of the two types of bogie, motored and non-motored, the former obviously presents by far the more difficult problem in design, and with the continuing trend toward higher rates of acceleration, the motored bogie has become the more numerous. This paper, therefore, deals primarily with the motored bogie, although many of the conclusions apply also to the non-motored bogie. HISTORY OF BOGIE EXPERIENCE The history of bogie experience on the London Transport system may be summarized by saying that, until recent times, only those bogies of exceptionally robust (and therefore heavy) construction had a record reasonably free from trouble in respect of main structural defects. Even in some of the heavier designs, freedom from structural troubles was achieved only after extensive and costly modification of what had started as basically a main line design of proven reliability. On a system where, by reason of station spacing, quite apart from signal checks, the requirement is to stop every two-thirds of a mile on average, there can be hardly anything more wasteful than to be pushing around unnecessary mass. It is obvious that, with the high present-day costs of energy and materials, such as tyres and brake blocks, and rates of interest and depreciation, the capitalized value of weight of rolling stock on an urban service is no mean consideration. With this in mind, it has been the aim of designers for many years to reduce weight, and it is principally from this consideration that the production of light alloy vehicles for the London Transport service has stemmed. Many of the earlier attempts at weight reduction of the bogies cannot be regarded as an unqualified success, as structural cracks developed in the course of time at many points. With the growth of welding technique between 1930 and 1939, opportunity was seen to achieve a major step forward in reduction of weight by adopting an all-welded design of tube-stock motor bogie (Fig. 1) incorporating some radical departures from former practice. Experience with this design was unhappy, as frame cracks appeared in a number of positions after relatively few years in service. Fig. 2 shows the positions at which cracks were, in the course of time, found to initiate. From the easy position of being wise after the event, it is now clear that the troubles resulted from a combination of the following disadvantageous conditions : (1)The design left something to be desired in that it was more like one of riveted construction, with the riveted knees and brackets omitted, and the main members welded together at their points of abutment. (2) The welding technique applied was inferior by present-day standards. (3) The structure was not stress-relieved after fabrication. (4) The design was of inadequate strength having regard to the necessity to withstand certain forces that arise in service, the severity of which had not been appreciated. Although much could be said on the subject of the first three points mentioned above, it is with the fourth point that the author wishes to deal in this paper, because the experience in carrying out investigations into the stressing of bogie frames in service shows how extremely valuable can be the contribution to design considerations that can be made by strain gauge measurements taken under service conditions. ATTEMPT TO USE THE TECHNIQUE OF RUBBER MODELS A technique has been developed in recent years in which rubber models are used to study the type of deformation produced by a system of forces which are known as to their direction of operation. This technique has been described in a recent paper presented at Nottingham University*. This technique seemed so admirable in revealing the manner of deformation under load and, in consequence, the most promising means of reinforcing the structure, that an attempt has been made to apply the technique to bogie frame design. Unfortunately, it is found that, for a structure of this type, a material which is sufficiently flexible to deform to an obvious extent under additional lateral loading, sags under normal gravity loading to an excessive degree. An alternative method of applying the same technique is being tried out by making the structure out of thin spring steel, strong enough to withstand vertical loading without deformation but readily deflected in a horizontal plane.Experiments with models made in this way have not yet been progressed to any real conclusion, but Fig. 10 shows a model of this type entering a curve. The points of flexure in the frame are remarkably coincident with the areas in which bogie frame fractures are commonly known to develop. The severity of loading produced by running into curves is of particular importance in the case of the London Transport system, where the original system, constructed for the most part during the first decade of the century, may quite aptly be described as a few short lengths of straight track joined together by a number of curves. Furthermore, it should be noted that these old tunnels were driven without transitions between straight and curved portions, so that not only is the lateral acceleration suddenly applied but the civil engineer has no opportunity to apply the correct cant at the beginning of the curve by reason of the limited rate at which the cant may be applied. The system may, therefore, be regarded as an endurance test track for bogies and it is not greatly surprising that, in endeavouring to lighten construction, much trouble has been encountered in the past. Some confidence can, however, now be felt that, with these modern aids to design, progress to lighter construction can be made with material used to good effect where it is required, in the knowledge that it is being sensibly loaded but not overloaded.