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An Automatic
Page 85
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Variable-ratio Transmission
New Invention Makes Use of Rollerclutchcontrolled Centrifugal Impulses from Planetary Weights
FOR more than a year we have been in close touch with Mr. H. F. Hobbs, of Adelaide, Australia, the inveiator of an ingenious variable-ratio transmission gear, the action of which is, except so far as reversing', entirely automatic. Mr. Hobbs has now been in this country for a -considerable time, and his company, the Hobbs Gearless Drive, Ltd., has offices at Parliament Mansions, Orchard Street, London, S.W.1. We have been able to watch every stage of the development, which is of some promise, and now publish an exclusive account of the device.
Between two discs, separated by an annular distance piece, are located three gearwheels, the discs being fixed to the rear end of the engine crankshaft, whilst the largest of the gears (which is the driven member) is mounted centrally on ball bearings on the disc remote from the engine. The two smaller, or planetary, gears, which are formed integrally with out-of-balance weights, are carried in ball bearings in both the discs.
If the central gear be held stationary while the discs are rotated by the engine, obviously the planet gears must, in rolling around the central gear, rotate together with their eccentric weights. These weights are so arranged that both masses fly outwards and inwards in unison.
When the weights are moving inward, the centrifugal force acting upon them tends to drive the central gearwheel, whilst when the weights are moving outward, the pressure upon the central gearwheel is in the opposite direction. In order to take advantage of the impulses in only one direction, a roller-clutch mechanism is incorporated between the central gearwheel and the final stage of transmission. It will be seen that the engine is able to rotate when, by means of the brakes, the vehicle is held stationary and the engine torque developed is small. In the opposite condition of things, when the vehicle is travelling at considerable speed, the weights take up their position at the outermost point of their path, and, as the resistance of the vehicle to motion is not great enough to overcome the strong centrifugal force holding them in that position, the discs and the central gearwheel rotate as one, giving direct drive.
In the intermediate positions, the ratio depends upon the relation between the power and the speed of the engine on the one hand, and the resistance of the vehicle to motion on the other hand. Thus, as initial resistance to rolling, on the part of the road wheels, is gradually overcome, the engine speed remains roughly constant and the planetary wheels roll more and more slowly whilst making their orbit, the ratio meanwhile approaching and finally reaching direct drive.
The roller-clutch is in operatim only during these stages of intermediate ratio and has but moderate resultant torque to transmit. The roller clutch is capable of being reversed by means of a simple hand control.
As the drive is conveyed, during the intermediate ratio positions, in a series of impulses, a flexible transmission shaft has to be incorporated from the driven clutch member to the final transmission stage. This allows the road wheels of the vehicle to rotate uniformly, whilst the central gearwheel is revolving alternately slowly and quickly in response to accelerating and decelerating loads imposed by. the Weighted planetary gears.
. At speed, the impulses are transmitted in rapid succession, so that, to all intents and purposes, the drive is continuous ; the roller clutch may then actually be thrown out of use. It is at the low road speeds that the flexible shaft is important, for torque is built-up in the partially twisted shaft.
We have driven a light vehicle • equipped with the Hobbs gear and it appears that a torque converter has been obtained, for the results are surprising. After approaching a hill at speed in direct drive, the road speed falls until, at a critical moment, the weights are caused to leave their outward position, and to rotate on their axes at a velocity corresponding to the indirect ratio suiting the engine power and speed and the resistance of the vehicle. The engine speed remains practically constant as the gradient increases and the road-speed falls, thus giving maximum engine efficiency. If the engine throttle be nearly closed the vehicle will come to rest, the sprag effect of the free wheel preventing it from travelling backward down the hill.
A box about 12 ins, high, placed before one of the driving wheels is climbed with steadiness at even the slowest rate, dependent on the throttle opening. There is, of course, no gear lever or clutch pedal.
The device is unostentatious and pleasant to handle, the absence of all gearbox noise being rather curious. No power impulses can be detected.
When accelerating it is possible to remain in indirect drive beyond the critical road speed, for the balance of forces is such that to obtain direct drive calls for a momentary release of the accelerator pedal. •
There is certainly much interest in this device, as has been apparent ever, since, in the spring of 1931, it was first introduced to us. Further developments will be reported to our readers.