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The Hobbs Transmission tried out on the road

18th September 1936
Page 54
Page 54, 18th September 1936 — The Hobbs Transmission tried out on the road
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TRANSMISSION system's are clearly I the topic of the year, both in the private-car and in the commercialvehicle fields of activity. There are several schools of thought on the subject. One maintains that a pure torque converter is required, another that the present-day gearbox merely requires to be rendered foolproof, whilst a third— and doubtless the most correct in the practical sense—requires a gearbox which affords a variation in torque, but is controllable in some simple way.

The Hobbs gearbox conforms to the requirements of the last-mentioned class and we can say from experience— having recently tried on the road a vehicle equipped with this transmission—that the balance between automatic action and controllability allows the simplest practical driving methods to be adopted. That is to say, only two pedals are provided, an accelerator and a brake.

A description of the Hobbs gear has already appeared in The Commercial Motor so that there is now no need fully to describe the mechanism. Suffice it to say that the device comprises a planetary gear, with a reversible roller clutch and a resilient member which is set in torsional oscillation. The planetary gearing comprises a sunwheel and a number of pillions to which are attached eccentrically placed weights. The casing forming the planet carrier is attached to the crankshaft of the engine usually through a resilient medium to avoid any vibrational " beat " effect, whilst the stationary member of the roller clutch is attached to the gear housing and the torsion shaft to the part to be driven.

Rotation of the weighted planet pinions about the sunwheel produces a varying torque upon the teeth of the sunwheel and, in certain positions, there is a tendency to drive it in the same direction as the planet carrier: Naturally, there is a .negative torque produced when the weights have' passed through 180 degrees, but the force is prevented from taking effect by the roller clutch.

B40 The torsion shaft plays an essential part in the functioning of the gear. It responds to impulses from the weights at certain frequencies and the most important consideration in the design is the natural frequency of vibration of the shaft and the parts of the gear attached at the gear end. Except at relatively low speeds of the propeller shaft, there is a constant impulse frequencY which enables the gear ratios at any speed to be predicted, and it follows that suitable ratios may be provided for any particular application.

Behaviour on the Road.

Turning now to a consideration of the manner in which the gear handles on the road.' We might commence by saying that there is nothing of a finicky nature connected with the mechanistn itself, or its controls. The method of operation to get away from rest merely entails releasing the hand brake and depressing the accelerator when a smooth start always ensues. By maintaining the throttle in the fully open position a gradual increase in overall ratio occurs, and later a quick step-up in ratio will automatically take place when torque and resistance conditions are appropriate.

Still maintaining a wide open throttle a further step-up will take place again when the torque and resistance conditions are appropriate for the control of the operating shaft to take effect. Similarly to change down, as it were, On a hill, the throttle is" merely maintained in a wide open position and as exerted by the weights to equal the torque developed by the engine, the drive becomes direct, the planets and the sunwheel then rotating as a unit..

The conditions already' outlined are maintained even when the driver has a controlling influence upon the gearbox, for he can regulate the available and required torque by manipulation of the accelerator. Thus, a quick step-up can be attained merely by releasing the accelerator, allowing the engine speed to die down below that of the propeller shaft speed when he can accelerate again on a higher effective ratio.

, Conversely, a quick step-down can be obtained by merely , generating a sudden increase in engine torque, such as is 'occasioned by opening the throttle quickly—an action bringing in an harmonic of the oscillating shaft which, of course, controls the speed range over which the rotating masses on the planets are effective.

Our test run showed conclusively that the claims made for the Hobbs gearbox are justified and the simplicity of control is proved by the manner in which we were able to take the vehicle over and drive it in a crowded thoroughfare without practice, or tuition. Within five minutes we were able to judge the speeds at which he , steps-up and down during hillclimbing could be effected advantageously and there is little doubt that the variable range of torque permitted contributes to the ." average" speed maintained.

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