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Recent TRANSMISSION Progress
Page 94
Page 95
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A Review of Achievement with an Outline of Possible Future Developments
TO mention the word "transmission" when in the company ef automobile designers is to invite argument, for all appear to believe that
we are on the eve of great developments in the means employed for the transmission of power from the engine to the road wheels, and it is difficult to find two technical men with exactly the same ideas.
When power-driven road vehicles first came into being the simple power control of an electric vehicle, which virtually necessitates the movement of one lever, immediately became the ideal to be aimed at. For 30 years or more this ideal has prevailed, but, despite the unremitting toil of hundreds of clever engineers, the attainment of the objective has yet to be recorded.
The reason seems to be that no suitable and adequately efficient means for transforming energy developed at relatively high revolution speeds into the equivalent power for low revolution speeds is possible with the appliances which at present we have at our disposal, unless a mechanical connection (by gearing, etc.) be used. High efficiencies can be attained with fixed ratios, but if a mechanism such as this be enipinyed the ideal of single-lever control is side-stepped.
The first 25 years of progress appear to have afforded few tangible results, for the general design of transmission lines have remained similar throughout that period. Certainly a cone clutch, a three-speed gearbox, and a long (and often " whippy ") propeller shaft gave place to a plate clutch, a four-speed gearbox and a generally stiffened propeller shaft, whilst rear-axle design has become really sound in construction, although the general layout of the various components has not been made vastly different.
To-day the position appears to he changing basically, for there are new transmission systems in existence, some of which have been tried and proved to be sound, whilst others are merely in
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the experimental stage, and it is this that gives rise to the thought that big developments are in the offing. Certain of the outstanding new designs warrant close study. The Daimler system, which incorporates a hydraulic clutch combined with a pre-selective gearbox, naturally comes first to mind, as it has been standardized for two Cr three years in various chassis produced by this famous Coventry concern, and from personal experience we can testify to its effectiveness and ease of operation. The hydraulic clutch, familiar now as the Fluid Flywheel, is so arranged that, to all intents and purposes, the power transmission at really low rates of revolution is negligible, but the torque capacity at first rapidly increases with the speed, and then runs more or less parallel to it • claims are made that a maximum efficiency in the neighbourhood of 97 per cent, is attained.
Everyone interested in transport vehicles must be aware of the method of operation of an epicyclic gearbox so that there is no necessity here to explain at length the construction and operation. Suffice it to say that the combination of Fluid Flywheel and selfchanging box provides a smooth power take-up and makes driving a simple matter, whilst the silent operation of epicyclie gearing is an additional desirable feature.
Before leaving this consideration of hydraulic transmissiain, mention should • be made of the numerous attempts to obtain conversion of torque. This ap pears to be fairly easy when the ratio of revolution speeds between 'driving and driven components is fairly close, but when there is a great disparity effi ciencies drop to such a level as to render many systems hardly worth while.
Naturally, a turbine action is necessary, a solid fluid (i.e., not steam or air) being utilized for the transference of pres sures, and as the relative speeds of the driving and driven elements are widely dissimilar at times, it is necessary to have swivelling vanes in order to obtain a suitable path for the fluid at all speeds. Time, only, will tell whether greater efficiency can be achieved. If it can, then the ideal of single-lever (or pedal) control also will materialize.
We now come to the consideration of mechanisms of more orthodox construction. Clutches, nowadays, are quite different in action from what they were even a few years ago, for shafts have been stiffened up considerably and, more
recently, efforts have been made ta obtain an insulation from possible building-up of vibrations between the power-unit and the transmission line. To this end rubber discs of the Hardy flexible-joint type have been interposed between the friction surfaces and the splined driving shaft which enters the gearbox. Another development concerns the inclusion of a flexible disc built up of steel spiders, which, by virtue of their construction, give a propressive clutch engagement regulated by the contacting area of the friction surfaces. Actually two spiders are used, the bosses being separated by a distance piece, whilst the outer circumferences are constrained together, giving a double conical form to the whole assembly. The spiders are flexible enough to allow the clutch-spring pressure to -flatten the arms, and as this deformation takes place during engagement, the contacting area is progressively increased,
The most notable advance with regard to gearbox construction
possibly is represented by the inclusion of helical gears for the third-speed pinious a a four
Speed box. In the private-ear field gearboxes of the Synchro mesh type are new common, the
action of this mechanism being simple, but extraordinarily effective in giving a foolproof change. Actually a small high-duty cone clutch is built into one of the indirect gear pinions on the main shaft and is brought into operation during gear changing. After the clutch pedal has been depressed it Is necessary for the driver only to push the lever steadily through neutral into the required position. When it has moved through about three-quarters of its total travel a slight resistance is felt, but by applying continuous pressure the lever will go "home" after a very slight pause. During this pause the friction cones have 'been engaged and have brought the mating parts to synchronous speeds, when they engage without the slightest, clash or shock.
A free wheel behind the gearbox not only permits coasting, but also helps gear changing, for it is possible (due to the isolation of the gearbox from the propeller shaft, when the latter is overrunning the engine) to push the gear lever into any forward gear without even using the clutch. The explanation of this, of course, is that the inertia of the rotating parts of the gearbox when running at engine idling speeds is practically A development of the ordinary automatic free wheel is that of a controlled free wheel or clutch situated behind the gearbox on the transmission line. This system has been incorporated on Cross ley vehicles and gives a foolproof change, but the driver must first depress the clutch pedal—an action which entirely isolates the gearbox shafts from the engine (through the ordinary friction clutch) and the propeller shaft (by the controlled free wheel or after 'clutch). To change gear, therefore, it is merely necessary to pause a few moments after disengaging the friction surfaces and then to push the lever through.
A forecast of future developments can be visualized by studying the drawing of the Lagonda-Maybach preselective gearbox. The four forward speeds which are normally employed are preselected by two small levers located above the steering wheel; they come into action
automatically when the accelerator is momentarily released. Additionally there is a gear lever on the box itself to bring into action a pair of auxiliary reduction gears, the main object of which is to provide an emergency bottom gear. This is a necessarily brief outline of the construction of the gear ; it would take several pages adequately to describe the action in detail.
Finally, we come to a consideration of another development which is receiving attention in various parts of the world—an automatic clutch control. In one of these systems the centrifugal clutch depends for its action upon engine speed, but another type employs a servomotor to disengage the friction surfaces, power being supplied by the partial vacuum in the engine intake pipe when the throttle is closed, or nearly so. It may be anticipated that with this arrangement the number of pedals will be reduced, for if the clutch be operated automatically there is little necessity for a manual control to be provided as well.
With all these schemes in the offing it would be a bold prophet who could forecast the lines of transmission development during the next decade. Even the 'immediate future is a little obscure. New ideas succeed each other rapidly, and advances are made almost daily.