The Final Drive for Commercial Motors.
Page 4
Page 5
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Chain-driven versus Gear-driven Chassis.
(Continued from page 4.)
Spur-gear Drive to Live Back Axle.
Coming now to the system of transmission outlined in Fig. 4, we have a reliable and well-tried final drive for heavy Steam or petrol wagons, etc., where the axle loads are great but the speed does not exceed, say, to miles an hour, The universal shaft transmits the power of the engine from the change-speed countershaft to a floating countershaft, on which is mounted a straight-cut or, preferably, double-helical gear pinion, which meshes with a wheel or ring bolted to the differential-gear spider on the live back axle. The distance between the centres is maintained by triangular brackets pivoted on the axle, and prevented from rotating by anchor rods suitably arranged and hinged to the main frame. These bra±ets may, with advantage, take the form of a gear case, and the gear would then run in oil and be protected from road grit, and this design would prolong the life of the gears and ensure silent running. It will be seen that the countershaft, always at the correct meshing distance from the axle, is free to rise and fall with the axle as the springs are deflected, this motion being allowed for by the use of the universal or propeller shaft. Many wagons with this type of final drive have been in use for transport purposes by the British army for some years, over very rough ground where no roads exist, and have given no trouble whatever. A large number of them were constantly used during the South African war and earned very high praise from the Commander-in-Chief. Wherever double-helical gears are used, the pinion should be free to move axially, in order that it may adjust itself to the teeth of its mating gear. They are very quiet in their action and the teeth are enormously strong for a given width of wheel and pitch, and there is no end thrust, if fitted as above stated, but, if the pinion is fixed on the oountershaft or otherwise has its motion restricted, it is possible to get end thrust, due to the wheel and pinion teeth not being quite in alignment, and the result is a noisy and grinding gear.
For heavy, slow-speed vehicles, this system commends itself as one of the best, both for efficiency and smoothness of running.
Worm-drive to Live Back Axle.
Undoubtedly the most silent and smooth running drive for a live back axle, or, for that matter, any other case where power is to be transmitted from one shaft to another at right angles to it, is the worm gear.
It has been objected to by many engineers, but this objection has been, chiefly, the result of early experience with badly-designed and badly-cut worms and wheels made of unsuitable materials. The impression gained ground that the modulus of efficiency was very low, on account of excessive friction.
Whilst this might have been true of worm gears io years ago, it cannot truthfully be said that such is the case at the present time. Worm gearing can be, and is, designed and made, by those who thoroughly understand the principles involved, which will work with a minimum of friction.
Where a large reduction of gear is required, this form of transmission is particularly favourable. It has been successfully applied to touring cars, vans, omnibuses, and heavy oil tractors. In the latter case, the load was particularly heavy and although, when at first put to work, much heat was generated, this trouble very soon remedied itself. The heat was caused through the worm and wheel not having a very high degree of finish but, as soon as the gear became somewhat polished, the gear ran remarkably smoothly and the efficiency was very high. The vehicle was designed to haul a load of 7 tons over very bad roads, and the official test of the tractor consisted of hauling 14 tons over bad roads for a long period, and finally drawing 7 tons through, rather than over, very soft ground. To those who advocate a live axle for commercial vehicles, careful consideration of the worm drive will repay the labour and outlay by its many virtues. Of course, high efficiency can only be obtained with this form of transmission when the gear is well designed and most carefully cut and fitted.
Spur-gear Drive to Fixed Back Axle.
Fig. 5 shows a method of transmission to the road wheels which will, at first sight, be recognised as of the type adopted on one of the best known omnibus chassis, but closer inspection of the diagram will show that there is an important difference in relation to the perch bars. A reference to the diagram will show that the two perch bars or radius rods meet at the forward ends and terminate in a ball end. This ball fits in a ball socket fitted to the frame in a central position. The advantage of this over the older method of keeping the perch bars parallel is that the countershaft and spur gear are relieved of all bending or twisting strains through any lateral movement of the back axle, due
to swaying of the vehicle when taking a curve, or from any other cause. As the countershaft is mounted on the perch bars, this swaying motion must be reckoned with, or noise and inefficient transmission will be the result. If parallel perch bars are employed, some means of preventing any lateral motion of the road wheels, relative to the countershaft, should be adopted.
Parallel perch bars were tried on some of the earliest steam wagons and were abandoned for the above reason. One has only to hear a vehicle with parallel perches, rounding a curve, to be convinced of the truth of this. Attempts have been made to stiffen up the perches, but this does not very much improve matters; the tendency to twist is there and, unless it can be prevented, independently of the perch bars, it would be much better to use bars in the triangular form.
This form of gear should be highly efficient, provided that the pitch and shape of the teeth are carefully considered, and equally carefully cut in the case of the pinion, and cast in the case of the internal toothed ring. A ring with cut teeth would, of course, be much more efficient, but it would bring up the cost of production and renewals considerably. The gears should be enclosed in an oil-tight case, or, at least, protected from stones which may be thrown up by the tires. Whether the use of this method of driving has any advantages over the side chain drive, other than that it can be made much more silent if properly designed, lubricated much more efficiently, and is more easily enclosed, is a question on which many engineers have grave doubts. It has been remarkably successful and has been extensively used, but whether it will continue so, in its present form, is a question which the more exacting police standard of noise will determine. Although in this system, the back axle is relieved of all torsional strains, there is present the same objection which is present with the live axle type, namely, the axle, countershaft, differential gear, spur pinions, internal gear rings, countershaft and back wheel brakes, the road wheels, and part of the weight of the perch bars, are all below the springs and subjected to all road shocks. The result must be increased vibration and noise, unless the maintenance is perfectly carried out. Apart from the question of tire upkeep, the writer is of opinion that the ideal drive for commercial vehicles, running at speeds of to miles an hour or over, is the one which gives the minimum of weight below the springs, as all parts unsprung must have comparatively short lives, on account of the vibration setting up crystallisation, and, also, that such parts are productive of more noise than any other parts. It is the object of every motor engineer to produce a vehicle which will be at once, reliable, silent, economical, and simple in construction. All these questions must depend on the general efficiency of the transmission gear, its cost of manufacture, erection, and frequency of renewals. A careful consideration of the relative merits of the various methods of final drive set out above, with the object of arriving at the best possible design, is highly desirable. Summing up the various advantages of the final drives dealt with in this article, we have :— Chains.
1. Adaptability to long or short centres.
2. Fixed centres not necessary.
3. Simplicity. 4. High efficiency, if well lubricated and kept clean.
5. Offers a very simple means of altering the gear ratio at a minimum of cost and in a minimum of time. The silent type of chain offers, also, the following additional advantages.
i. It is noiseless, as compared with roller or block chains.
2. There is no sliding friction between the chain and the teeth of the wheels.
3. It is smoother in action than roller or block chains and, consequently, much more durable.
With the use of side chains of either type, a further advantage is that there is less weight below the springs than in any of the other methods shown above.
The principal objections to chain drive are : the noise, increasing as the chain and sprockets wear out; and the very great difficulty in properly lubricating them.
Bevel Gear.
I. Offers a simple means of transmitting motion from one shaft to another at an angle to it. 2. It is more generally understood than worm gear. 3. It may more readily be enclosed and made to run in oil.
4. Has a fairly high efficiency, when well cut, hardened, and fitted in an oil-tight case, The principal objections to bevel gear are :— 1. It is very noisy and inefficient, unless very well fitted and lubricated,
2. It is costly to produce.
3. Tooth speed must be kept low, or noise is the result.
4. Teeth must be planed, necessitating special machines.
Worm Gearing.
r. Perfectly smooth and silent in action.
2. Is highly efficient, if properly designed. 3. Is easy to enclose in an oil bath to ensure proper lubrication.
4. Greater range of gear reduction can be obtained.
Its disadvantages are, chiefly, that it is not so well understood as bevel gear, and that, like bevels, special machines are required for its proper manufacture.
Spur Gearing.
1. Is most generally understood.
2. May be cut either by milling cutters or in the shaper. 3. Is cheap to produce.
4. Is very efficient.
5. Is very silent, if the teeth are properly proportioned to the load, and tooth speed is not excessive.
6. If made of the double-helical type it may be run at high speeds.
7. There is no end thrust.
Although this list does not include all the evidence for and against the various methods of final drive,: it is fairly representative of the motor engineer1/2 ,experience.