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Automatic Non-preseiective Gearbox

8th September 1939
Page 38
Page 39
Page 38, 8th September 1939 — Automatic Non-preseiective Gearbox
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MANY au tomobile engineers are convinced that the time will come when the internal-combustionengined vehicle can be controlled in a similar way to the steam or electric machine; that is, by means of only one lever. Drivers now have to be content with a threeor four-speed gearbox of the " crash " type or with one or pther of the synchromesh or preselective gearboxes, which reduce the gear-changing operation to the movement of a small lever before the actual change has to be effected.

It is obvious that such a gearbox does not fulfil all demands, and the idea of a fully automatic gearbox has for long been in the heads of designers. Up to the present only a few such designs have been made public and none has reached a stage of development suitable for incorporation in coLmercial chassis.

Any Number of Ratios.

Now technical details have been released of a new automatic gearbox which is operated by centrifugal clutches ingeniously combined with free-wheel devices. In spite of the fact that the model described below has only three speeds, the fundamental principle allows for as many ratios as may he required for lorries, buses and special vehicles.

The gears themselves are arranged in practically the same way as is customary in usual standard and synchromesh gearboxes, a mainshaft running 'free in the driving shaft and a layshaft with three pairs of gears being positioned below it. This gives two indirect drives and one direct drive. The engaging of the different speeds is A 46

effected by three centrifugal clutches in combination with two free-wheel clutches. A third free wheel serves as an automatic sprag or " hill holder." The gear-change lever is, in this gearbox, only of subordinate importance and serves merely for operating the reverse gear and to check the functioning of the free wheel in the second gear. In top gear no free-wheel action is provided. When the lever, which can be mounted on the instrument board, is brought into its neutral position, the engine can be speeded up without causing movement of the vehicle

How First Speed Works.

If, however, the gear lever be in the starting position and the engine speed be increased in excess of idling speed, the first centrifugal clutch, acting similarly to an internal brake, becomes engaged and the first pair of gears (from left to right) drives through the layshaft the third pair. Thus, the first speed comes into action.

If the speed be further increased by acceleration of the engine, the second centrifugal clutch comes into operation. Here the centrifugal segments cause an axial movement of the pressure balls against conical faces. The second clutch body is connected by a hollow shaft, with the second pinion on the driving shaft. The lastnamed meshes with a smaller layshaft wheel, so the layshaft is speeded up and the inner member of the first layshaft free wheel runs faster than the outer member.

Direct Drive in Top Gear.

On further acceleration of the engine the spring pressure on the third clutch, which is of like design to the second clutch, is overcome and the transmission shaft is directly driven. In this case the second free-wheel clutch, incorporated in the right-hand mainshaft pinion, permits the picking up of the higher speed. It may be remarked that in all higher gears the centrifugal clutches of the lower gears remain in engagement, thus in the third or top gear all clutches are in engagement.

A special characteristic of this gearbox is the adjustment of the centrifugal clutches in such a way that they disengage at a lower speed than that at which they become engaged. Accordingly any in-and-out movement at specific speeds is avoided, especially )etween the second and top ratios. Phis overlap can be reduced at will )57 a corresponding throttle movement, :ither by accelerating or decelerating vhen required.

In any case, the operation of gear. hanging is effected with this nechanism much more quickly than vith ordinary gearboxes. Therefore, he vehicle accelerates more rapidly ; or instance, a speed of 37.5 m.p,h, can ie attained in 12 seconds, whereas with n ordinary gearbox' only 31 m.p.h. an be reached in this time.

The automatic sprag or " hill 'older " consists of a third free-wheel lutch arranged between the driving ;ear for the second speed and the main haft. Backwards motion of the .ehicle is resisted by the short hollow

shaft between the second and third free-wheel clutches.

It is stated that this gearbox, which is named the Kreis, is out of its first trials and that gearboxes have proved absolutely satisfactory and without wear over very long distances. The design is based on wide experience in this-field, which appears to have been successfully used in order to avoid the faults of former designs.

The writer recollects, in this connection, the experiences with the NAG. automatic clutch, introduced about ten years ago. Then it was difficult to obtain a satisfactory adjustment of the springs for the centrifugal weights, and as only one clutch was provided, it was always necessary to return to nearly the same idle speed to effect a gear change without using the clutch pedal. In the hands of experienced drivers the. device was successful, but it was not quite so satisfactory in the hands of inexperienced drivers.

It may be helpful to summarize the features of the Kreis gearbox. The operation of the gear lever can be dispensed with for usual acceleration and deceleration. It serves only for auxiliary purposes such as locking the free wheel and selection of reverse Aid neutral. The next gear always conies into action at the right time, namely, when the torque is at its maximum. In a similar way, when the vehicle reduces its speed a lower gear comes into operation ; however, in this case, the speed range of the particular gear is extended tatvards lower speed.

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