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The Variable Transmission Compromise

7th August 1936, Page 36
7th August 1936
Page 36
Page 37
Page 36, 7th August 1936 — The Variable Transmission Compromise
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Which of the following most accurately describes the problem?

Is it the Last Word?

ONE of the chief stumbling blocks in the development of an ideal transmission device affording an infinitely variable ratio is the high degree of satisfaction given by the conventional step-up gearbox, employing toothed pinions, which, in a sense, may appropriately be described as a compromise.

Whilst it is indisputable that the best mechanism would be one that, under all conditions, provided the ideal ratio between power unit and road n-heels, there is no reason why the existing general practice should not be made more fully to meet present-day requirements. Increasing the number of ratios available is the obvious ,way of doing this, but to change gear requires an effort, therefore an easychange device seems to be an essential feature of the multi-speed gearbox.

It appears reasonable, disregarding individual engine characteristics, to provide ratios in a number which is roughly in inverse proportion to the power-to-weight ratio. For example, if there be, say, 1.5 b.h.p. per cwt., three ratios might be regarded as enough, whilst if only 0.25 b.h.p. be available per cwt. of gross weight, then at least five ratios should be provided.

Even when the power-to-weight factor be high, however, it is undesirable to reduce too low the gear number, for, flexible as modern engines are, there are maximum torque and maximum-economy crankshaft speeds. Moreover, if under easy driving conditions the engine be running comparatively slowly, its life is prolonged and the driver becomes less fatigued. A slight down gradient, or a level road with a following wind, to traverse which only a light effort on the part of the power unit is required to maintain the cruising speed, is a case for a high top gear, and one that is freely encountered.

It might be argued that one set of ratios is needed for driving under fully

laden conditions and another for running with no load. Such an arrangement is given by manufacturers who equip their chassis with .a four-speed box and an auxiliary two-speed gear. On the other hand, it is quite practicable to allow the intermediate gears to serve for both circumstances, provided that there be a low first gear for the heaviest work and a high top gear for light running.

There -are alternative arrangements for the auxiliary box. One gives direct drive and a reduction, and the other direct and a geared-up top ratio, commonly called an over-gear.

A point in favour of the latter is this. For prolonged use when pulling hard, a direct drive is desirable. Both conditions--that of protracted period and that of heavy load—rarely obtain simultaneously in circumstances when the over-gear could advantageously be used. It is mainly employed either for a considerable duration of time running moderately light, or at nigh torque for a matter of only a few minutes. The geared-up " top " is a convenience and economy for the intermediate stage between driving and coasting.

With regard to the actual question of changing gear, it is obvious that the loss of time occasioned by the operation, in either direction, represents a reduction of average speed, and an unnecessary expenditure of fuel. Therefore it is uneconomic.

An Effect of Time Loss.

When ascending a hill, a delay in shifting into a lower gear may mean that, by the time the ratio has been engaged, the road speed has fallen to a figure for which the selected gear is now too high, and the driver finds himself faced with the choice of continuing the climb with a labouring engine, or dropping yet another step.

Similarly, on an easing gradient, a delay in changing up may mean that the vehicle loses the velocity just gained, so that the higher ratio cannot, after all, be employed.

This difficulty can be combated by minimizing the need for changing gear, by ensuring that the operation can be performed quickly, by making the step up or down as low as possible, or by combining two or more of the foregoing.

The only way of adopting the first method is to install a big engine which exerts a high torque over a wide range of revolutions, and to keep down the total weight. From the point of view, of simplicity, it is the best, but it has obvious drawbacks.

The second scheme, especially in the case of a wide-ratio gearbox, necessitates the incorporation of some mechanical means for quickly synchronizing the wheels about to be engaged and for generally simplifying the driver's task, and certainly does not get to the root of the matter.

Many Close Ratios Desirable. The third plan ties the designer's hands neither in respect of engine capacity and type nor in respect of pay-load. Close ratios alone and a well-designed gearbox and selecting mechanism enable full advantage to be taken of whatever' engine characteristics exist, and the changes to be effected with the minimum time loss. If a gearbox of this type be used in conjunction with a mechanism such as that for the second scheme, then maximum efficiency results.

There is another and important reason for applying an easy-change device to the close-ratio multi-speed box. The average driver is often disinclined to change gear when to do so does not appear absolutely necessary. Moreover, if it happens that there are two levers, instead of one, to manipulate, he will be all the more hesitant to use, to their full value, the many ratios at his disposal. Thus, to compensate for the extra work he is required to do, the operation needs to be made as easy as possible. Where four speeds are used the average inter-gear step (that is the ratio of one gear to the next above it) is about 1.7 for second to third and third' to fourth, the step between first and second probably amounting to about 2. A Maybach seven-speed box that we understand will shortly be introduced has maximum inter-gear steps of 1.6 and minimum steps of only 1.43.

In illustration of the foregoing remarks, the accompanying graphs refer to the question under discussion as it affects two main types. The one is based upon the performance of a Fordson 3-tonner with eight-cylindered petrol engine, and the other has, as its basis, characteristics of a Scammell 13 tonner equipped with a Gardner six cylindered oil engine. It must be understood that in neither case are the graphs strictly accurate, the curves and figures being rnerely approximations of the actual performances of these two models.

For each type there is a set of curves showing the road speeds at all engine speeds in all gears, with the torque curve and the b.h.p. curve superimposed. In addition, we have repro duced from our road-test reports of these two vehicles, their respective acceleration curves and above the curve for each -of the gears used (in each case omitting first gear as being required only for special conditions) we have drawn, against scales approximately graduated according to the varying rate of change of the velocity, the engine torque curve.

These graphs show, in striking manner, the importance of determining and using the correct gears for conditions of weight and characteristics of power unit.

The engine of the 3-tonner is capable

of a very high speed and maintains its torque over a wide range of r.p.m. Accordingly, the steps, even between the lower gears, may be wide, Furthermore, because of the relative lightness of the vehicle, it would not matter greatly if the power curve dropped at either end of the corresponding acceleration curve.

In the case of the 13-tonner the power unit is of a much lower-speed type, and although its torque is higher and -better maintained than that. of the petrol unit, the need for avoiding a dropping curve, particularly at the lowspeed end, is obviously greater. The manner in which, by the use of a sixspeed gearbox, the vehicle is accelerated up to 30 m.p.h. by a torque that never drops more than about 25 lb. ft. below maximum is a fine demonstration of the advantages of six speeds.

A Case for an Over-gear.

Were it desired to run this machine empty, the crankshaft speed at 30 m.p.h. is shown to be just over 1,400 r.p.m. This is only 300 r.p.m. below the probable governed Maximum, and as power output is, in such circumstances, not a main consideration, a seventh geared-up ratio, giving, say, 900 r.p.m. woukl he an advantage.

Assuming the case for the multispeed box to be proved, one is faced with the problems attached to its design and manufacture, its additional bulk and weight and so forth. Although these might appear formidable, that they are far from insuperable is evidenced by the excellent examples that are standard products of several makers, and that are little larger and not greatly heavier than those affording only four ratios.

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