The Final Drive for Commercial Vehicles.
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Chain-driven versus Gear-driven Chassis.
In motor vehicles, whatever the source of power, or whatever system of transmission is employed, the final drive is by one of two means, viz., chain or gear. The application of the chain drive may be either with one chain to a live back axle or by two side chains from a countershaft to a fixed back axle. Figs. i and 2 show these two methods. The gear drive offers more scope for originality in its application and may be by bevels, worm, or spur gear, as outlined in later illustrations. Each method has its advocates, who, if approached on the subject, would undoubtedly assert that his or their own particular method had advantages which no other possessed and would indignantly deny that it had any disadvantages of its own. There are examples of each of the methods of transmission to be found successfully running in London and in the provinces, and a consideration of the various types and their respective merits will not be out of place at the present time, when the exhibition of commercial vehicles at Olympia claims the attention of all interested.
For the purpose of dealing more efficiently with the various types, they have been divided into two classes, and each class has been further sub-divided into sub-classes as follow :— Class 1. (a) Central-chain drive to live back axle.
(b) Side-chain drive to fixed back axle.
Class 2. (a) Bevel-gear drive to live back axle.
(b) Straight-cut or double-helical spur gear to live back axle.
(c) Worm-gear drive to live back axle.
(d) Spur-gear drive to internal gear wheels bolted to back wheels running on fixed axle.
Chain Gearing.
The manufacturers of chains have done much in recent years to provide motor engineers with really well-designed, well-made chains in a great variety of sizes and designs, the material of which they are made being fine examples of the metallurgists' skill.
Whilst the chain manufacturer has done all in his power to provide the motor engineer with a simple and efficient means of transmitting the power of the engine to the back wheels, it cannot be said that the motor engineer has done his part in designing commercial-vehicle chassis in such a way as to use chains ta the 1,zst advantage. Few have taken any means to lubricate them effectively or to protect them from the grit or mud of which our roads appear to be made. To enclose these chains in oil-tight cases is, undoubtedly, the best means of doing this, but such a refinement is very rarely seen on a commercial-vehicle chassis. When one considers the very great difficulties which are to be overcome, in order to fit chain cases, it is, perhaps, excusable if they are omitted ; still, some means of protecting the chains from the road grit should be provided. As the lubrication of a chain is always a matter of difficulty, great care should be exercised in the choice of a chain, and the speed at which it is run should never exceed the point at which centrifugal force overcomes the tendency of the lubricant to cling to the chain. According to makers' statements, it is not advisable to run a roller chain at a greater speed than Soo feet per Minute. This-fact would appear to limit the use of roller chains to comparatively slow-speed transmission ; there is, however, the silent type of chain, which can be run at a maximum speed of t,25o feet per minute, and even this speed might be exceeded with this type of chain, if it were not for the fact that, at a speed of 1,300 or 1,400 feet per minute, the lubricant would he thrown off by the centrifugal force.
Possibly, one of the greatest advantages obtained from the use of side chain drive (as shown in Fig. 2) lies in the fact that there is less weight below the springs than in any other form of final drive. The only parts which are below the springs are : the fixed back axle ; the chain rings (bolted to the road wheels); the road wheels themselves ; the roadwheel brakes, and part of the weight of the chains, the differential-gear and chain sprockets being carried in a countershaft casing securely bolted to the main frame. This must certainly relieve the tires of a large amount of the pounding due to bad roads, and should be a great consideration in the life and upkeep of the tires. Whether this is actually the case, or whether the more jerky form of drive which the chain offers, as compared with gear, causes this form of drive to be more destructive to the tires, would form an interesting and instructive subject for the consideration of the Society of Motor Omnibus Engineers. Many engineers object to chains because the teeth of the sprockets cannot be cut theoretically correct in pitch. This is accounted for by the fact that, as soon as the chain is put to work, it commences to stretch ; therefore, in order to remedy the effects of this growing pitch of the chains, the teeth of the sprockets must be cut to a mean between a new chain and a badly worn one. The discrepancy between the pitch of the chain and sprockets means that the whole lead must come on one tooth, and, as the space between two adjacent teeth is not wholly occupied by the roller, there is a certain amount of backlash. This accounts for a large amount of the noise which is always noticeable with chaindriven vehicles. The roller which for the moment is taking the load, bearing hard up against one side of a tooth, is instantly relieved of its work when the vehicle over-runs the engine, and the chain, now taking up the drive in an opposite direction, takes up the backlash between the roller and tooth, the result being a collision between the two which is productive of great noise and is destructive to the chain.
The life of a chain is dependent, to a very great extent, on these shocks of reversal, but such shocks may be kept down to a minimum if a chain is carefully selected, having
as little weight as possible consistent with the load it will be called upon to sustain. Large bearing and wearing surfaces are of much more importance than mere tensile strength, although, of course, this must not be lost sight of. By keeping the weight ot a chain down to a minimum, the noise due to the rapid collisions between roller and tooth is very much reduced, and another important factor in the noise question is the size of the driving sprocket. The difference between the sizes of the driver and follower should be as small as possible. Driving sprockets with as small a number of teeth as 8 or 9 are fitted but their use is not advisable, as the joints are thereby compelled to move through a much greater angle than would be the case if a sprocket with, say, 20 teeth had been fitted. The latter number of teeth should, in the writer's opinion, form the minimum, as, the angle of motion between the links being smaller, the wear and noise is much less. A silent drive is never obtained by using a small sprocket ; therefore, for town use on omnibus chassis or any other form of commercial vehicle where silence of running is desired, the sprockets should be as large as possible within the limits of the speed of the chain.
The silent type of chain which has iately been so highly developed is free, to a very great extent, of many of the objectionable features of the roller or block type. Its principal advantage is that it automatically adjusts itself to the teeth and distributes the load over all the teeth which are in mesh at one time. Within reasonable limits, the stretch of the chain does not affect this quality. A manufacturer is sometimes called upon to quote for, or to supply, a vehicle for special work where the roads are either very hilly or very fiat. The standard gear ratio of his standard chassis is not a suitable one, it requires gearing up or gearing down, in order that it may show off to the best advantage and give out the highest efficiency. It is in a case of this kind that the chain-driven chassis has a great advantage over its gear-driven rival. The necessary alterations can be made at small cost to the manufacturer and in a very little time. Taking Fig. t as an example, it would only mean the substitution of a sprocket having more or less teeth, for the standard size which is fitted to the chassis, and a few links taken out or a few added to the chain to complete the alteration. In the case of a vehicle employing the side chain drive, it would mean two sprockets and two chains altered in length. Comparing the cost of this alteration with the cost of cutting new spur, bevel, or worm gears, and the much longer time required to fit them, it will he found that the advantage is decidedly on the side of the chain drive. Of course, the alteration is not always such an easy one, but, if the designer gives the matter his consideration at the time the vehicle is in his hands, there is no reason why provision should not be made for subsequent alterations. With a chain drive, fixed centres are not necessary, as in the case of all gear drives, and, if the driving sprocket becomes slightly out of alignment with its follower, due to twisting of the frame occasioned by careless loading or bad roads, the results are not nearly so destructive to the shafts and other parts as would be the case with a gear drive. The distance between the shafts often being four or more times greater than would be the case with gears of the same ratio and pitch, it follows that a quarter of an inch out of line with chains would only be equal to t-16 inch or less, in the case of gears. It is, of course, desirable that every known means should be employed to prevent the shafts being out of alignment.
Bevel-gear Drive to Live Back Axle.
The bevel-gear drive, to a live back axle, as shown in Fig. 3, whilst it may be satisfactory on comparatively light touring cars, cannot claim to be equally satisfactory on commercial vehicles. The greater axle loads and lower speeds which prevail in the latter type of chassis entail greatly increased ratios between pinion and wheel and, therefore, greatly increased tooth loads, two factors which tell against bevel gearing. The teeth must be much stronger and heavier to stand up to the extra work which they are called upon to transmit. Bevel gears are costly to produce, as they must be planed or shaped on a special machine which generates the correct tooth formation. This process is much slower than milling but, in order to have gears which will mesh correctly and run well, the teeth must be planed and not milled. Well-cast bevels are often much better than milled ones. Bevel gear is, at best, a noisy means of transmitting power from one 'shaft to another, unless the teeth are of fine pitch and fairly narrow. As the tooth loads often demand great strength, the pitch must be coarse or, if fine in pitch, the teeth must be very wide. Fine-pitch teeth of great width are expensive to produce and, if the wheels are to be subsequently hardened, they are much more prone to warp than narrow teeth, whilst, on the other hand, narrow teeth of coarse pitch are very objectionable and very noisy.
Where the tooth loads are light and peripheral speeds low, they may sometimes be used with advantage, as they offer a very simple means of transmitting motion from one shaft to another at an angle to it. Such a gear may easily be enclosed and, therefore, well lubricated. Under such conditions, it is, possibly, better than chain transmission.