CONSIDERATIONS ON BOGIEDESIGN WITH PARTICULAR REFERENCETO ELECTRICRAILWAYS Paper presented to the Institution of Mechanical Engineers by W. S. GRAFF-BAKER* and reprinted in the Journd by kind permission of their Council. PAPER No. 513 SYNOPSIS An examination is made of the dynamic characteristics of wheel sets and bogies, and of the various forces which act upon a bogie under service conditions. The fundamentals of bogie design are considered, and particular mention is made of recent developments in Ifiethods of body suspension. Problems of frame construction, braking, and power transmission are also considered. The paper concludes with a survey of the development of bogie design on the railways of London Transport Executive and elsewhere, and a restatement of the basic problems in the relation of bogie to track. INTRODUCTION Railway passenger rolling stock was originally constructed as four-wheeled vehicles of limited length carried on two ayles. The requirement of a longer body in due course led to the introL :tion of a third axle fitted intermediately between the other two, and this construction, with detailed variations, was extended to the point where it could not be taken further. The demand for still longer bodies, however, persisted and this requirement could only be met by the provision of pivoted bogies under the ends of the car, each bogie in the first instance being * Late Chief Mechanical Engineer (Railways) , London Transport Executiv?, Past President, Institution of 1,ocomotive Engineers. 306 BOGIE DESIGN WITH REFERENCE TO ELEC*RIC RAILWAYS 307 provided with two axles. By this means a long body could be provided, able to negotiate the curves normally found on a railway, with the wheels separated by a limited rigid wheelbase. The functions of a bogie can, therefore, be described as providing running gear for a long-bodied vehicle while' keeping the rigid wheelbase within relatively small limits and providing comfortable riding for the passengers. CONDITIONS OF OPERATION Before considering the design of bogies calculated to provide comfortable riding, the factors governing their construction and operation must be considered in some detail. These can be summarized simply as :- (a) Use of " coned " wheels. (b) Use of wheels mounted solid on the axles. (c) Irregularities of the track on which the vehicle runs, divided into vertical and lateral irregularities. (d) Provision of means of acceleration. (e) Provision of means of braking. (f) Minimum maintenance cost. (a) Coned Wheels. The normal practice on railways is to use wheels coned on the tread.at 1 in 20 so that the tendency of the wheel set shall be to take a mid-position on a straight track-or such position as will equalise the tread diameters in contact-and so that the wheel set, if deflected owing to centrifugal force on a curve, will run on two different diameters of tread, thereby compensating to such extent as is practicable for the curvature (Fig. 1). It has been demonstrated that such a set of coned wheels will take a sinusoidal course when travelling along a straight track; that is to say, it will inherently travel from side to side when progressing along a track. All studies of this fact have necessarily been based upon wheels and track both in new condition. In practice, neither wheels nor track are more than instantaneously in new condition and, while the sinusoidal motion is fundamental, owing to variations in the practical condition the motion is far more irregular and extensive than is indicated by theory. It might be possible to predict with any given wheel set and rail condition the path which would be followed but, owing to the large number of probable variables, all that can be said is that in practice there is an effective tendency of the wheel set to move from side to side irregularly rather than regularly. So much so that experiments have been made with cylindrical-tread wheels with a view to checking the side move- ment. This is at the expense of the somewhat equivocal benefit of the compensation for CUN~S by coning. As, however, it is relatively difficult in the type of lathe used to turn accurately alike such large diameters, ccning of 1 in 100 had to be adopted in practice and has been used on a considerable scale. Whilst this results in improved running, the treads require more frequent turning to maintain the effect. 308 JOURNAL OF THE INST. OF LOCO. ENGINEERS (b) Rigid Wheel Sets. What has been said about the sinusoidal travel of a wheel set is related to sets in which the wheels are rigidly attached to the axles, and it can be assumed that the path followed by a set having wheels free on the axles could well be substantially different from that of the rigid wheel set. The question is, however, only of academic interest since, for reasons of braking, it is essential that the wheels should be rigidly attached to the axle. This requirement arises from the consideration that maximum braking is required often in service and inevitably under emergency conditions. Such braking is commonly achieved by pressing brake blockson to the wheel tyres at the maximum pressure which will avoid the wheels skidding on the rails. OUTER WHEEL INNER WHEEL xD. Circumfesplce of inner wheel at contact diameter. x(D+ 0.0688). Circumference of outer wheel at contact diameter 0.216 in. Circumferential difference at contact diameters. FIG.1. CONED WHEELS: WHEEL DIAMETERDIFFERENTIAL ON CURVEDTRACK To obtain the maximum braking, it is necessary, or at least convenient, that the pressures applied to each block on both wheels of the set shall be equal, as well as maximum. Under conditions of railway operation, it is improbable that the superimposed weight on the axle will be distributed equally on the two wheels of a set .at any one moment; for example, in going, round a curve, the centrifugal force will cause the reaction on the outer wheel to be greater than that on the inner. In this case, if the wheels were free on the axle and the maximum force (based on equal bearing) were applied to the brake shoes on the inner wheel, where the weight has been relieved by centrifugal action, this wheel would inevitably “pick up” and skid on BOGIE IIESIGN WITH REFERENCE TO ELECTRIC RAILWAYS 309 the rail. Conversely the weight on the outer wheel would be sufficient to permit even higher brake-block pressure on that wheel. The attachment of the two wheels rigidly to the axle ensures that the total friction force of the brake shoes is applied to the total weight carried by the wheels on the rails, without reference to the distribution of the weight on the two wheels. If wheels free on the axle were employed it would be necessary to limit the braking on both wheels to a figure which would prevent either wheel from picking up under the worst conditions of lateral weight transfer likely to be met in service owing to centrifugal or other effects. The result would be inadequate braking. (c) Truck Irregularities. Track irregularities may be divided into tno kinds, vertical and horizontal. Vertical irregularities are constituted by the conditions arising from wear of the rail head, unevenness at rail joints and the vertical variations in the travel of the wheel caused when traversing points and crossings. Lateral’ irregularities arise from side cutting of the track due to lateral oscillation of the wheel sets, whereby the flange of the wheel is caused to act as a cutter on the rail, since its reaction on the rail is of a sliding, rather than a rolling, character. In any case, play has to be allowed between the flange of the wheel and the rail, to provide working clearance and to permit coning to be effective in adjusting the contact diameters of the wheels to suit the curvature of the track or to equalise the diameters in contact on straight track, and this has been shown to cause side movement. (d) Acceleration. On electric railways, with motors mounted in the bogies, it is required that a motored bogie should not only carry , the motors but also be able to transmit the forces developed to accelerate the train and maintain it in motion, this force being derived from the adhesion of the wheel on the rail when the axle is caused to revolve by means of the motor. (e) Braking. Braking is an extension to all bogies of the requirement indicated for acceleration, in that the braking force for the whole weight of the train must be provided on the bogies up to the limit of the adhesion between the wheel and the rail. Railways are, generally speaking, electrified to provide a more intense service of trains. Obviously, the requirements of acceleration and braking become of paramount importance in the provision of services of high intensity, since not only do high acceleration and braking provide a greater capacity for the line, in trains per hour, but also they permit the operation of a given schedule most economically, in regard to energy consumption, by avoiding the need to run at high speed. In some cases, it has been the practice to motor every axle on a train, not only to provide the maximum acceleration but also to enable the use of electro-dynamic braking and so to avoid the use of brake shoes, which, after all, may seem to constitute a somewhat crude, if simple, method of dissipating the kinetic energy stored in a moving train. 310 JOURNAL OF THE INST. OF LOCO. ENGINEERS Electric braking can be regenerative or rheostatic. The kinetic energy of the moving train is reconverted to electric energy, in the former for use elsewhere, and in the latter to be dissipated as heat in resistances.