CHAIN TRANSMISSION VERSUS SHAFT DRIVE.
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Will the Chain Come Once More Into Favour for Vehicles Designed for Lighter Loads ? A Claim for Greater Efficiency.
By a Chain Enthusiast.
THE form-of final drive to be adopted has been the
subject of debate and experiment at some time or other on all types of vehicle, from the humble bicycle to the steam wagon._ It has been answered in most eases in favour of the chain on both of these extremes_ Chain transmission is universal on motor-. cycles, and it is significant that this form of drive is always embodied in attempts to produce a really cheap car. On the heavier petroland steam-driven vehicles—that is to say, on those designed to haul loads greater than four. tons—the final drive is by chain in the majority of cases. The reasons for the choice are, in the former case, cheapness, lightness and accessibility, and in the latter the difficulties experienced in the designing and manufacturing of a shaft-driven live axle which will stand up to the loads and usage.
Will Motor Manufacturers Turn Once Again to Chain Drive?
Experience apparently teaches that shaft drive is suitable for use over a limited range only ; signs are not wanting to show that this range may be still further decreased in the future and that makers in search of simplicity and robustness in axle construction will turn again to the chain as the solution of the problem. Those makers who do so, and are willing to take advantage of the chain manufacturers' advice and experience, will find that, owing to improvements in tooth form and in chain design and manufacture, the chain drive of to-day is vastly superior to that with which they were acquainted in the days when a chain was regarded simply as a means of connecting a jack shaft and a rear wheel, without any attention being paid to the conditions under which it would have to run.
Neglecting the lighter types of commercial vehicle —that is to say, those capable of dealing with loads up to 25 owt., which are, in the majority of cases, adaptations of touring-car chassis—the real choice of final drive lies between chains and shaft with worm. The double-reduction axle enjoyed a certain amount of popularity a few years ago, owing, chiefly, to the War Office subvention scheme, but, in my opinion, it is somewhat heavy and calls for very careful design, otherwise the difficulties of repair and maintenance must be increased.
A study of the claims made for chain drive and a critical comparison of the chain versus shaft and worm may, therefore, ,be instructive and, in addition, point the way for those makers who are at present wavering in their allegiance to the worm.
Points in Favour of Chain Transmission.
The aim of a commercial vehicle manufacturer is, or should be, to supply a vehicle which will give to the purchaser a maximum of trouble-free service at a minimum cost ; the manner in which the fitting of a chain final drive will help towards the fulfilment of this ideal may be siinamarized under the headings given below:— Initial Cost.—The elimination of universal and sliding joints ; the permissible lightening a the differential gear and the drive shafta, due to the greater speed at which they rotate, combined with the better enviromfient possible ; and, last, but certainly not least, the immeasurable simplification of. the axle itself, leaves a balance heavily in favour of the chain drive.
Service.—The ehain-driven lorry is a "go-any where" vehicle, as opposed.to the shaft-driven live-. axle type' which is a product of good roads ; it is the ideal vehicle for builders, railway or public works contractors and for all transport which is required to operate off the beaten track. The greater axle clearance obtained with the chain-driven lorry is also an advantage when " Colonial " conditions are encountered.
The Ease With Which Gear Ratio May be Varied.
Chain drive permits of the gear ratio being varied by the user to suit the conditions; for instance, it can be lowered when a trailer is to be used, with an appreciable effect upon fuel consumption. A lowerload platform can be obtained with the chain-driven lorry, since there is no gear housing in the middle of the axle; this, in many instances, means a saving in time and money.
Chains are a positive means of power transmission; at the same time, a chain drive possesses a certain amotuit of elasticity, which has the effect of protecting other parts of the chassis from shock, and E40 keeps down the repair bills. Owing to the greater accessibility, transmission repairs can be executed more quickly in the case of a chain-driven lorry than with a live axle and shaft drive ; the interruptions to service are, therefore, not so great when the former type of transmission is employed. Running Costs.—All other conditions being equal, the vehicle with the most efficient transmission system will be the most economical in fuel consumption; it is in this direction that the chain drive has a definite advantage.
Laboratory Efficiency as Compared With Results on the Road.
The question of efficiency is always a delicate matter to touch upon • makers of worm and bevel reduction raring make very high claims for their products, but, upon investigation, it will be found i that it s the laboratory efficiency, with the gear rigidly mounted and all the conditions favourable, that is quoted. There is, however, a considerable difference between the laboratory efficiency of gearing and the efficiency when mounted in the axle of a fully loaded three or four-ton lorry, subjected to the distortion inevitable when traversing bad road surface. On the other hand, the bevel reduction to the jack shaft of the chain-driven vehicle does operate under conditions akin to those in the laboratory, and the chain drive itself is, inherently, able to deal with the practical conditions without any lowering of efficiency. Even in laboratory tests of worm gearing, certain undesirable characteristics are evident, and it is well known that the losses in coasting are greatest with this form of transmission. In addition to the greater efficiency obtained with chain drive, there is the saving_ in weight compared with the shaft-drive live-axle construction to be considered; this also will be reflected in the ton-miles per gallon, since the ratio of the total load to the useful load is not so great. The question of rear tyre life is, of course, bound up with that of the weight imposed upon the wheels. Here, again, the chain drive shows to advantage, since not only is the total weight on the rear axle less, due to the lighter vehicle and the better distribution, but the unsprung weight—a most potent • cause of tyre wear—is considerably reduced.