161 Introduction
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LIVE AXLES FOR COMMERClAL VEHICLES. 161 LIVE AXLES FOR COMMERCIAL VEHICLES. BY GEO. W. WATSON (MEMBEROF COUNCIL). INTRODUCTION. THE purpose of this paper is to present a rhm8 of up-to-date practice relating to the design of live back axles for commercid vehicles for load capacities of two tons and upwards. The paper admittedly does not go so fully into certain details as it might, but this has been chiefly due to unforeseen circumstances which have left the author all too little time to complete its preparation in the manner he had at first intended; he trusts, however, that it may suffice as a basis for discussion with a view to the improvement of so vital a unit in the anatomy of a commeroial vehicle. Any information that is obtainable in regard to experience with the thousands of vehicles with the Expeditionary Force would appear to show that no small percentage of the troubles experi- enced has related to back axles. British designers and con- structors will be pleased to learn that comparatively few failures have occurred with axles entirely of home design and manufacture. Before proceeding with details it should be made clear that in this paper it is not intended to deal with road wheels, bearing springs, cardan shafts and joints or gear-shaft brakes. The first-named, so far as forged stelel wheels are concerned, were ably dealt with by Mr. T. Clarkson, in a paper read last session,* while bearing springs have been dealt with in a paper which Mr. G. H. Baillie read in May, 1913.t The other points, it is hoped, may be fully discussed at a later date. The author has divided his subject into a number of headings, and it8 far as possible has avoided the overlapping of subject- * See Proo. I. A. E., Vol. IX.,p. 105. t See Proo. I. A. E., Vol. VII., p. 451. WATSON. L Downloaded from pau.sagepub.com at NANYANG TECH UNIV LIBRARY on June 4, 2016 162 THE 1KS 1 ITUTION OF AUTOMOI5ILE ESGIXHERS. mattcr, but this has not been possible in every case. The designer of a live axle ha4 to be guided very largely by proved practice. This may seoin ail unscientific method of treatment €or so im- portant a piece of machinery, but apart from the calculation of known loads, which is straightforward work, it is not possible to tbeorise with any degree of certainty, or even probability, con- cerning the momentary stresses due to road shocks of unknown intensity. In all calculations for axles the author takes as a basis the limit of adhesion between plain solid tyres and the surface of the road. He accepts this limit as 0’4 of the imposed weight on tho back tyres (including the wheels themselves), and as this is about the worst condition to be met with on any macadamised road, it has been found to meet the case better than working either from the power end of the transmission, or from the braking effort. A factor of safety of not lem than sevcn on thle yield point of the material should be allowed for the load-bearing parts, and not less than four on the transmission shafts and gears. It is, of course, desirable to keep down the unsprung weight as much as possible, but on no account should this consideration be allowed to outweigh others, as it has still to be proved that unsprung weight, as weight, has any real effect upon the wear of either tyre or road. Col. Crompton has stated tkat it is speed which counts most in these respects. L)UTIES OF A LIVE AXLE. A live axle has four important duties to perform: firstly, it has to carry a large part of the chassis load, plus a still larger part of the weight of the body and tho useful load; secondly, it has to transmit the engine power to the wheel rims and tyres; thirdly, it lias to transmit the tractive eBort, or push the vehicle along the road; and fourthly, the containing casing, or forging, has to resist the torquc due to the drive, in some axlos this casing has also to resist the braking load. The simplest form of live axle is, of course, that used on the majority of steam wagons. This consists, primarily, of a straight shaft of steel running in axle boxes which carry the rear bearing springs; one of the road wheels is keyed, or otherwise secured to one end of the shaft whilst the other wheel, freely mounted on tlic end of the shaft, is driven through a sleeve from a differential gearing. A greatly improved form of such an axle is shown in Fig. 1. .This siniple form of live axle, hmver, has its limitations; Downloaded from pau.sagepub.com at NANYANG TECH UNIV LIBRARY on June 4, 2016 LIVE AXLES FOR COMMEHCTAL VEHICLES. 163 whikt being the cheapest to sonstruct, it does not easily lend itself to my but a single-hain or spur-gear type of drive, and w the main shaft must perform all the three duties named, it follows that the direction of stress in the material will be con- tinually changing ,dlthe time the vehide is running. Conse- quently, it is neither practicable nor convenient to employ thi8 form of axle for other than oomparatively slow moving vehicles. On the majority of petrol vehicles the type of live axle is such that for the purpose of detailed oonsid,eration the complete unit can be divided into two dishct parts: (a) the outer casing, or forging, which carries the imposed load, and may be likened to a beam supported in the hubs of the road whleels {andloaded at the two FIQ.1 .-Live Axle used on Allchin Steam Wagons. spring flats; (b) the driving shafts, through whioh the power is transmitted to the wheels. There is another type used on certain vehicles which are partly or wholly of Amrioan origin, of which tho Sheldon axle may be taken as an oxample. In this the casing acts as B housing for all the bearings and gears, while the differ- ential shafts aarry the bending stresses due to the lodand tmtive effort, in addition to the torsional stmesses due to the drive, but &Q the differential gear oage is mounted in bearings within the main casing, the differential shafts are not called upon to take the bendiag load due to the worm drive. British builders of live axle commercial vehicles gen,erally have adopted the first named type of axle. L2 Downloaded from pau.sagepub.com at NANYANG TECH UNIV LIBRARY on June 4, 2016 Downloaded from pau.sagepub.com at NANYANG TECH UNIV LIBRARY on June 4, 2016 LIVE AXLES FOR COMMERCIAL VEHICLES. 165 FLOATINGAXLES. There appeaxs to be much diversity of opinion as to the meaning of the term ‘’ floating,” ‘‘ semi-floating,” and ‘‘ non-floating,” as applied to live axles. Them terms, which the author believea me of American origin, axe much used in impressive tones by many mleamen and others who have obviously no concaption of their meaning; they often couple them with varying arrangements of spring mounting, torque and thrust rods, etc., in a most perplexing manner. The interpretations which the author proposes to use throughout this paper, are as follows:- Floating Axle.-An axle in which the wheel-driving, or dif- ferential, shafts transmit only torsional effort. (Fig. 2.) Semi-Floating Axle.-An axle in which the wheel-driping sbafta I FIG.3.-An American-built Axle : The Sheldon. partly, or whlolly,snxpport the bending stresses due to the gear drive plus the torsional stresses, but which are relieved oP all bending stresses due to the imposed lload and the tradve resistanoe. (Fig. lo.) Non-Floating Axle.-An axle in which the differential shafts wholly or partly take the combined bending stresses due to the hposed load and the tnactive reaistanoe, plus the torsional stresses due to the transmission of power to the wheels. (Fig. 3.) TYPESOF AXLES. For the purpose of further identification it is also proposed to describe the preeent two standard methods of construction of the load-bearing axle proper as the solid-forging ” and the “ built- Downloaded from pau.sagepub.com at NANYANG TECH UNIV LIBRARY on June 4, 2016 166 THE INSTITUTION OF AUTOMOBILE ENGINEERS. up ” types. By ‘‘ solid-forging ” type is meant that class of axle in which the road wheels are jourdled upon the two ends of a solid forging mhich carries the imposed load, and on which the driving gearing and shafts are mounted. The “ built-up ” type includes all axleis which are built up of steel castings, tubes, eta., which not only take the imposed loads, but also act as housings for +he drking gears and shafts. The ‘‘ Solid-Forging ” Axle -The author will not venture to make any assertion as to who was the first to use an axle of tkis type. The Leyland Company, however, have long used one form on their petrol vehicles. In their early ales the road wheels were journalled upon hollowed, or tubular, extensions from a cradled forging, in the dip of which the housing for the driving and differential gearing was mounted. The wheel-driving shafts, projecting from each side of the housing passed through the tubu- lar extensions of the forging, to couplings on the wheel hubs. There were oertain objections to this form, arising from the dip, as any deflection of the axle was acoompmied by a corresponding end load on the wheel-driving shafts, with consequent momentary excessive side loads upon the ball bearings within the driving gear housing.