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Hydraulics May Replace Gearing

25th January 1946
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Page 21, 25th January 1946 — Hydraulics May Replace Gearing
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THE torque characteristics of the ordinary petrol or oil engine, when such a unit is applied to road transport purposes and in many other cases, require conversion to meet varying conditions. In connection with vehicles its operation has, hitherto, been almost exclusively carried out by the hand-operated, so-called, change-speed gearbox, which, alongside the throttle control, adjusts the torque in stages. For the past 50 years or so this combination has performed with-reasonable satisfaction the duties demanded of it.

Various proposals have been put forward from time to time to improve or replace the present comparatively simple gearbox, which has, of necessity, been developed to a high state of mechanical efficiency. Only a comparatively small proportion of these propositions has come into commercial operation. Infinitely variable gears, belts and other friction devices, chains, automatic gear changing and other arrangements have been tried out and, in many cases, found wanting.

Some Early and Modern Devices Many years ago electric transmission was developed. The Tilling Stevens, the Stevens, and the Owen Magnetic were examples of this system. During more recent years hydraulic transmission has received attention both in this country and in America. In its simplest form—that of the fluid coupling—it replaces only the clutch, and in that sense is not truly a torque converter. Perhaps the best known of this type here is the Daimler Fluid Flywheel. Then there are the Fottinger in Germany, and the Sinclair used here and in America, but none of these takes the place of the gearbox. , A further development was the Leyland torque converter, using the Lysholm Smith system, which is also employed in the United States Another modern development is the Brockhouse Turbo Transmitter.

On some types a hand change for the direct gear cuts out the hydraulic transmission, for without such a provision, and working purely through the hydraulic device, there would be a slip of approximately 5 per cent. It is also necessary to provide a second reduction by gears in heavy vehicles where greater torque is required. The hydrokinetic type, whilst approaching the smoothness of the steam engin e, presents the disadvantage that, at low speeds, there is a certain amount of hydraulic drag, also the efficiency drops as engine revolutions rise in proportion to vehicle speed. The slight drag means that the `.` neutral " position is never quite neutral. Several patents have been taken out recently avowedly to obviate this slight disadvantage, and certain American applications have included a conventional clutch in the transmission, although this appears to be overstressing the benefit derived.

Hydrostatic v. Hydrokinetic Systems There is. however, another system of hydraulic transmission of power, the hydrostatic, which operates by the displacement of fluid under pressure. It is thus distinct from the hydrokinetic, which depends largely upon speed being imparted to the fluid, this being returned in doing work.

Many -years ago a hydrostatic transmission was designed by Lentz, of Berlin, and applied to a few road vehicles and locomotives. One of them was under test about 1910 by what was then the General Motor Cab Co. Ltd., of London. The system comprised two hydraulic engine-driven pumps, one double the capacity of the other, which fed oil to two hydraulic motors, each driving one of the axle half shafts, thereby eliminating the differential gear; thus the clutch, gearbox, gear final drive and differential were replaced by these. The pumps were of the vane type, both of the same diameter, but one double the width of the other. The first speed was afforded by using only the narrow pump, the second by the wide, and top by the two working together. The ratio of the total pump capacity to that of the two motors was 4 to 1, thus providing something approaching the equivalent of the normal axle ratio. A control valve in the oil circuit diverted the fluid to the forward or reverse direction of the axle motors.

At that time difficulties were experienced with frothing and beating, also with alterations in the specific gravity of the oil owing to atmospheric temperatures. The materials employed and the means for obviating leakage were also not such as are now available, and with modern methods and means this system, to-day, might prove more successful.

Until this war, hydraulics were almost totally undeveloped in the way that, for instance, electricity has been, but aeroplane designers saw the possibilities of hydraulic transmission, and for the first time, design, materials and, above all, the accuracy of workmanship placed it in a position to do things alongside, or in conjunction with, electricity that would otherwise have been impossible, or at least unsuitable.

Incidentally, there have been considerable developments iti connection with hydraulic springing for aircraft, which may soon be applied to motor vehicles.

Hydraulic Equipment Gaining Ground To-day some thinking people in the engineering world, particularly in the motor industry, are viewing with favour the further application of hydraulics. Some modern tractors are equipped with hydraulic power-driven pumps to ease the labour of the ploughman. Hydraulic tipping gears have, of course, been in use for many years with great success. It is clear also, from information from American sources, that soon there will be hydraulic engine-driven pumps on vehicles of all types to perform such operations as wheel jacking, the handling of spare wheels, the control of doors, windows, sliding roofs and drop heads, as well as for driving windscreen wipers. Further develop ments with the transmission of engine power to the road wheels by such means are well under way. The American Ford Company is said to have on the road a car with a hydraulic transmission of the Lentz type.

It should be remembered that there are 17 gears in the transmission system of an average goods vehicle. On other than direct drive, 12 of these are concerned with the transmission of the power. It is said that the Ford method replaces the whole 17.

In one recent British patent, provision is made for two hydraulic pumps with an epicyclic gear between them. The pumps are of the multi-piston type operated by swash plates. One plate has an adjustable throw, affording a neutral position, or, by a further movement, a negative one, thus giving a reverse. Both pumps can be operated as motors, and according to which is the motor, acting through the epicyclic gear, the changes of ratio are provided.

Problems of Cost and Efficiency The question of the cost of developing such hydraulic systems need not deter technical advancements, for it always reacts to the influence of mass production. Their efficient operation can be arrived at and expressed only in terms of miles per gallon of the fuel employed, or, perhaps, in ton-miles per gallon. Broad comparisons on the basis of mechanical versus hydraulic systems are inclined to be inaccurate and misleading.

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Organisations: US Federal Reserve
Locations: Berlin, London

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