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British Makers Must Start Afresh
• says A. W. Haigh, A.M.I.Mech.E., THAT the gross weight of a vehicle is the controlling factor in its performance, assuming constant engine output, is a well-known and accepted fact. Yet there are many operators who consistently overload their machines and expect them to function with maximum efficiency and be as free from component failures as though they carried no more than the maximum as laid down by the maker.
I remember a complaint being received in the service department of a well-known manufacturer that a 30-cwt. vehicle was immovable halfway up a local hill. A breakdown van was sent out, and, after the machine had been towed in, it was found to be loaded with approximately 10 tons of lime. Such gross misuse of a machine does not, of course, take place nowadays, but overloading, sometimes through ignorance, does occur, as was shown recently in a letter to a wel4nown trade journal in which the writer assumed that his 5-ton truck was designed to carry loads up to 8 tons.
Margin for Overloading
Designers realize that this state of affairs exists and, much against their will, allow for a proportion of overloading in their calculations, not by assuming a greater load to be carried. but by employing a larger factor of safety when estimating detail dimensions required for the guaranteed payload. There is, naturally, a limit to the allowance which can be made or the classification of vehicles by payload-rating would be futile.
In many instances the limit is fixed by the licensing weight, the maximum load which can be efficiently and safely carried on a machine weighing, say, under 3 tons, being guaranteed by the manufacturer. When, however, the engine size and gear ratios are 'calculated, a fixed gross weight must be assumed, so that a satisfactory result can be reached.
Deciding on Engine Size in the first place, the designer of a modern goods vehicle must bear in mind that the machine must be suitable for export. Countries abroad, especially in the colonies, possess many roads which are little better than cart tracks, and most of the life of the vehicle will, in all probability, be spent in operating under conditions which are simulated in this country only in quarries and woodland. To deal with such conditions, it is essential that the engine size be adequate. But what can be considered to be adequate?
Assuming 100 per cent. efficiency in the transmission system, it is possible to propel any vehicle up any
B2 hill, using any size of engine. For instance, with the correct gear ratio, a machine weighing 20 tons could be driven up a gradient of I in 4 by a 1 h.p. engine, but no one, even assuming that a reasonable speed could be obtained in top gear, would employ such an absurdly lowpowered unit.
In the early days of the commercial vehicle, it was customary to provide a big, cumbersome machine, powered by a heavy and somewhat inefficient engine, for comparatively light loads. Then, when the weight tax was first introduced, we suffered a period of ultra-lightness in which manufacturers were compelled to skin every item to the bone to bring their vehicles within the desired licensing weight. It was during this period that overloading became increasingly apparent.
Because of the need for aluminium in aircraft construction during the war, taxation weights were increased, and still remain at the higher figure. We have, therefore, entered a new era in which a vehicle that previously had to be kept down to 50 cwt. unladen is now permitted to weigh 3 tons, hence it can be made more robust.
The increase in size of components permits a greater payload to be carried, for the additional 10 cwt. allowed on the unladen weight of former " under 50 cwt." machines is more than enough for the increase of strength required to carry the former load with perfect reliability. Thus, to the delight of sales managers, it has been found possible to guarantee a payload of 6 tons,on a machine in the 3-ton-unladen class.
The choice of an engine for a vehicle is, of course, to a certain extent a matter of opinion. There are, however, some formalities to be
followed in determining the minimum size of power unit which will give adequate and economical performance under all predictable conditions, no matter what the size of the machine might be. As a starting ,point, let us assume an engine-torque output of 20 lb.-ft. per ton, which, for 9 tons gross weight, is a total Output of 180 lb.-ft.
The yardstick by which the performance of a commercial vehicle is measured is the gradient which it will climb. It has been found that if a machine will negotiate a tangential gradient of 1 in 4 (sine gradient most commonly used, is 1 in 4.15), and if it be correctly geared, it will give a satisfactory top-gear perform ance, and will not be defeated by the conditions existing, even in quarry and timber work. Furthermore, the engine will be sufficiently large to attract overseas buyers, but not too large to be uneconomical in fuel consumption.
Determining Overall Ratios Our aim, then, is to make our vehicle climb a sine gradient of 1 in 4.15, which it will do with an overall gear ratio of 52 to 1, assuming 80 per cent. transmission efficiency. This 52 to 1 is the product of the bottom ratio in the gearbox and the axle reduction, so there remains to be determined the actual values of these ratios. There is, of course, an infinite number of combinations which, when multiplied, give a total reduction of 52 to 1, but few of them would be of practical value.
As an example of two unsuitable values, let us take 13 to 1 gearbox ratio and 4 to 1 axle ratio. It is a fact that, with certain reservations which are dictated by operating conditions, the ratios in a four-speed box should be in geometrical pro gression. Slight deviations from the _ true figures required in theory must be• made because of the difficulty of using gears with the requisite number of teeth to give the ideal ratios, but for my purpose of discussing 'he practicability of employing a 13 to 1 bottom-gear ratio, the theoretical figures can be used. These ratios ar-; 1 to 1, 2.35 to 1, 5.53 to 1 and 13 to I.
Now let us assume that we wish to change down from top to third gear with the engine turning at 1,200 r.p.m. The mainshaft, and consequently the third-speed mainshaft gear, will be turning at 1,200 r.p.m., so, in order that the two third-speed gears shall be synchronized (assuming that the mainshaft maintains its 1,200 r.p.m.) the engine must be speeded up to 2,820 r.p.m.
Close Ratio Boxes
To increase the engine revolutions with accuracy by such an amount in the short time available is wellnigh impossible. In fact, it has been found by experiment that the greatest increase which can be reasonably employed for efficient gear changing is 2 to 1, but it is advisable to provide gear ratios with a common factor of less than this amount, as the closer the ratios are made, the easier it is to change gear.
Furthermore, when gear ratios are being decided, it is customary to fix the ratio of the speed next to top (third in the case of a four-speed box), closer to 1 to 1 than to the adjacent lower speed_ That is, the common factor of the gears is normally 2 to 1. The difference between top and third is made about 1.8 to 1 in order to provide-a gear in which slow road speeds are possible in traffic without the need for dropping engine revolutions too low, and to prevent roughness in the engine when top is engaged at low speeds Is a "Traffic Top" Essential ?
There are two opposed schools of thought on the subject of the "traffic top" for commercial vehicles. But if all advantages are to be adopted, the slightly better engine performance allowed by the close-third gear must be sufficient argument for its use on any machine.
So, if we are to provide a box with easy-changing characteristics and include a "traffic top," we must have a common factor between the ratios of a figure preferably below 2, say, 1.95, and a third-gear ratio of 1.8 to 1. The four speeds will, therefore, be, 1 to 1, 1.8 to 1, 3.51 to 1 and 6.84 to 1. This bottom-gear ratio automatically fixes the rearaxle ratio at 7.6 to 1, a figure which remains to be proved either satis factory or otherwise. .
We know that the vehicle will have an adequate performance in bottom gear, but we are, as yet, uncertain of its capabilities in top gear. A machine in the licensing class under consideration is permitted by law to travel at 30 m.p.h., but to be able to maintain this speed and be manceuvrable on the road, its maximum possible speed must be in the region of 45-50 m.p.h. To attain such a speed with a 7.6 to I rear-axle ratio the engine must revolve at approximately 3,270-3,650 r.p.m., or 3,450 r.p.m. for 47.5 m.p.h., which, for a commercial power unit, is rather high.
Commercial vehicle engines, in order that they may have long life and give adequate and trouble-free service, are designed as low-revving units, which deliver their maximum torque within the 1,000-1,400 r.p.m. range, and peak at around 3,500 r.p.m. If, therefore, we intend to treat the engine in a reasonable manner, the axle ratio must be chosen so that the peak revolutions shall not be exceeded at the desired maximum road speed. A road speed of 45-50 m.p.h. at 3,000-3,300 engine Limn. demands an axle ratio of 7.2 to 1, which gives 47.5 m.p.h. at about 3,260 r.p.m.
The Crown Wheel and Pinion
A further point which must be considered when determining the axle ratio, is the design of the crown Wheel and pinion necessary to give the desired reduction. First, assume that the 7.6 to I ratio is to be made up of two alternative sets of gears, namely, a pinion with seven teeth meshing with a gear of four to six teeth (ratio 7.667 to 1), and a pinion having five teeth operating with a 38-toothed gear (ratio 76 to 1). .
In the first set the crown wheel must be approximately 14.7 ins, in diameter to be strong enough to transmit the engine torque through bottom gear. In the case of a representative spiral-bevel pinion, the root angle of the teeth is such that the radius to the root gashes is 0.81 in., which allows a shank of maximum diameter 1.62 ins., if the gear cutter is not to remove metal from it, and thus weaken it.
Similarly, the 5/38 set requires a crown wheel of 13.75-in,. diameter, which permits a smooth pinion shank of only 1.5-in diameter. As the minimum shank diameter permissible for the transmission of the torque under rigid conditions and for the accommodation of a suitable bearing is 1.75 ins., it will be appreciated that the pinions for the 7.6 ratios must be of inferior design because of -the gashing of their shanks.
Let us conclude then, for the two reasons already given, that it has been decided to use a 7.2 to 1 rear axle. We have already fixed the gear ratios to the best advantage and, unless we are prepared to accept other inferior ratios, we cannot alter them materially. We must, therefore, in order to maintain the desired performance, increase the engine size to provide a torque of 190 lb.-ft.
Engine 'Torque and Axle Ratio This increase in engine torque, combined with the 7.2 axle ratio, does not require any appreciable increase in crown-wheel diameter, but such are the root angles (due to the ratio) that a shank of l ins. diameter, completely free from gashing, is permissible.
Thus, by using an engine giving 190 lb.-ft. torque, a gearbox with a bottom ratio of 6.84 to 1 and a 7.2 to 1 axle reduction, we have a vehicle which is (1) higher powered than was previously considered necessiry; (2) is not too highly powered that it will be uneconomical; and (3) will give adequate performance under most exacting conditions, both at home and abroad.
It is not suggested, of course, that the figures arrived at are the only ones possible, for it is evident that many combinations of engine torque and gear-axle ratios would be suitable. It is true, however that no combination which could give firstclass performance , would have a smaller engine than that quoted, and it is also true that any combination arrived at must be obtained from scratch, that is, no attempt to patch up existing vehicles with smaller engines by altering gear and axle ratios can be truly satisfactory.
A Fresh Start Needed We must, if we are to compete successfully in world markets against the Americans, realize that vehicles which were used in this country, under the old licensing regulations, are out of date, and, no amount of dodging can give them the desired modern characteristics to deal with the loads imposed on them. It is imperative, therefore, that the commercial vehicle industry in this country should start afresh.
Old ideas, and with them the temptation to make do, must be scrapped and new vehicles, which will operate as efficiently as those of our competitors but at a lower cost, produced on the lines suggested The project may be initially expensi‘e, but it will pay dividends in the long run.