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Britain Pioneere( utomatic Gearboxes

Credit for the Successful Development of Fully Automatic Transmissions Lies in Great Britain: Epicyclic Gear Trains, First Used in 19th Century, Still Form Their Basis

HENEVER the subject of automatic transVVis under consideration, most people's thoughts immediately turn to visions of large American cars with high fuel-consumption characteristics. Few people realize that the epicyclic gearbox, which is, after all, the heart of most automatic boxes, was used as long ago as 1899 on a Lanchester car, and that the Daimler Motor Co., Ltd., broke much fresh ground in 1932 with their decision to standardize on the Wilson pre-selector gearbox.

The credit for the successful evolution of such gearboxes undoubtedly lies in Great Britain, which makes it all the more strange that fully automatic transmissions so far have not met with the general approval of passenger and goods-vehicle operators. In the privatecar field, however, public demand is widespread.

First in importance among the advantages of automatic gearboxes is that they relieve the driver of the need for finesse when changing gear. Even drivers who take a pride in their gear changing will not deny that after a few hours on the road it is pleasant not to have to bother with a clutch pedal and gear lever, By reducing the driver's physical exertions by anything up to 50 per cent., he can concentrate entirely on safety. The second important advantage is tied up with the design of the particular gearbox and the chassis to which it is mated. Provided that this design and mating are correct, an automatic gearbox will yield the best possible performance from an engine, with the minimum of wear and tear on both the engine and transmission. This in turn can lead to high-speed transport with economy, particularly under conditions which demand extensive use of the gearbox.

Tests have shown that most types of automatic transmission will create a greatly improved fuel-consumption rate in heavy traffic and high-density stage-carriage working, but under certain conditions a corresponding saving can be obtained on long-distance high-speed running.

There are three basic types of transmission in this field. The earliest could be termed quasi-automatic. They comprise pre-selective gearboxes in which a gearchange pedal and lever are used, but synchronization of engine and road speeds is relatively unimportant. The best-known transmission of this type is the Wilson system, designed by Self-Changing Gears, Ltd., Coventry, and used in several private cars and in AEC., Daimler, Guy and Leyland passenger chassis.

The Wilson gearbox is usually employed with a hydraulic coupling, as this gives a smooth drive take-up and allows the drive to be discontinued automatically when changing gear. The box itself consists of trains of epicyclic gears, with a multi-plate clutch for direct drive. In its original form the brake bands were applied by a powerful spring compressed by movement of the gear-change pedal.

Thus, although no art is required to drive a vehicle with this gearbox, some physical force is still necessary. This can be abolished by the use of an air-operated cylinder to replace the bus-bar main spring, the gear pedal merely operating an air valve.

It was a logical step, therefore, that greater use should be made of air pressure to reduce driving effort still further, hence the evolution in 1953 of direct-acting airoperated epicyclic gearboxes, the first British semiautomatic transmissions. With these systems the third pedal is abolished and gear changing is effected solely by the movement of a short lever placed in any convenient position.

The two better-known semi-automatic transmissions made in this country are the A.C.V. Monocontrol and the Leyland Pneumo-Cyclic systems. Both use the same basic design of Wilson gearbox and hydraulic coupling, but instead of one main air cylinder acting through a bus-bar, with selective strut-engagement of the brake bands, each band has a separate power cylinder.

These cylinders are controlled directly by the driver, and movement of the gear lever causes a smooth change from one ratio to the next at a controlled, shock-free rate. In the case of the A.C.V. system the lever actuates electrical contacts which control a series of electropneumatic valves on the gearbox itself, whilst the Leyland layout has the air valves combined with the gear lever.

Another design by Self-Changing Gears, Ltd., is the RV28 oil-operated gearbox. This is basically similar to the A.C.V. and Leyland layouts, but instead of air power, hydraulic fluid operates the brake bands. As with the Monocontrol system, electrical connections between the gear lever and the gearbox valve block are used. The advantage of this system is that the gearbox is self-contained and not dependent upon an air supply from the chassis.

A new form of transmission introduced at last year's Commercial Motor Show by Self-Changing Gears, Ltd., and installed in the Leyland Buffalo heavy-duty overseas six-wheeler, is the RV30 eight-speed air-operated gearbox. This is similar in design to the conventional electro-pneumatic direct-acting epicyclic gearbox, but a two-speed auxiliary section is incorporated behind the main box.

Although these semi-automatic systems do much to ease the driver's lot, there is no difference in performance between vehicles with such transmissions and those with ordinary pre-selector boxes. Relative slip at the flywheel still increases fuel consumption.

There is, however, a solution in the guise of the automatic centrifugal friction clutch, of which there are several in existence for use on heavy vehicles. One, the Twiflex, was tested by The Commercial Motor on a Daimler double-decker last year. The test revealed that there was no deterioration in. passenger comfort because of the absence of a hydraulic cushion between the engine and gearbox, whilst fuel consumption was improved. It is known that A.C.V. and Leyland are developing similar clutches for heavy vehicles.

A particular advantage of a friction clutch is that when the engine is idling and a gear is engaged, there is no coupling drag (with consequent fuel wastage). This is of great importance in buses, because it is the habit of drivers to wait at stops with first or second gear engaged in readiness for an instant get-away. A recent compromise, designed to give cushioned drive, limited slip at low speeds and full drive at normal road speeds has been developed by Self-Changing Gears, Ltd. It is known as the Fluid-Friction clutch and consists of a small hydraulic coupling combined

Gearboxes

with a centrifugal clutch. Imitial slip during the take-up of drive is thereby permitted, but no power losses are present at higher engine speeds.

All but one of tha semi-automatic gear systems so far considered have been suitable in the main for buses, coaches and heavy goods vehicles only, principally because of the use made of compressed air. Lighter goods vehicles cannot be expected to have air compressors, but there are several " two-pedal " control systems which give most of the advantages of the air-actuated layouts.

These transmissions have been developed primarily to appease the public clamour for cars with clutchless gear changes and the emphasis has been on cheapness and adaptability. They are particularly suited to 5,ehicles engaged on local delivery work.

Two-pedal Systems The two systems most in use in Great Britain so far are the Manumatic transmission, made by Automotive Products Co., Ltd., and the Newtondrive design, developed by Newton and Bennett, Ltd., and employed

by the Standard Motor Co., Ltd., under the name Standrive.

Both these designs incorporate a centrifugal clutch, which can be mechanically released by the driver for gear changing. This is done by an electrically controlled vacuum-servo motor, the switch for which is incorporated in the gear-lever knob. A clutch servo and a control unit are other principal components. Thus the original gearbox is retained and use of either system in an existing design creates no complication.

The Manumatic system differs from the Newtondrive layout in that a speed-synchronization switch is incorporated in the clutch assembly to give the correct ratio between engine and propeller-shaft speeds during a change. As a test on a Standard van recently proved, however, the Newton and Bennett system works satisfactorily without this synchronizer. The Newton clutch itself has only one set of springs, which act in both pressure and pull-off roles.

The Newtondrive is offered by Standard for an over all price increase of £19 17s. 6d. The Manumatic control raises the price of, say, an Austin A55 van by £41 13s. 4d. (These figures include British purchase tax.) Desirable as semi-automatic transmissions may be, the designer of any vehicle fitted with them has still to contend with the human element, in that any driver can make or mar the success of the design by his use of the gears. The only answer to that is a fully automatic transmission, whereby the engagement of any gear ratio is controlled solely by engine torque and road speed. One of the first types of fully automatic gearbox to go into quantity production was the General Motors Hydra-Matic transmission, first used on Cadillac cars in 1941. Even so, the epicyclic gear train remains at the bottom of it all, albeit disguised with torque converters, modulators and a thousand-and-one other hydraulic devices.

It is probably the complexity of these hydraulic-cumepicyclic transmissions that has done more than anything else to prejudice the British operator against them, so it was a relief to see, in September, 1956, the Wilson semi-automatic epicyclic gearbox brought out as a fully automatic design. Here, then, was a transmission that everybody knew from experience could be relied upon and which could give everything required of an automatic gearbox without complicated mechanism.

This original Wilson automatic gear, which utilized the V.S. (voltage-sensitive) control system, was designed by Self-Changing Gears, Ltd. It was exhibited at Earls Court on a Leyland Titan double-deck bus now in operation by Glasgow Corporation. The Commercial Motor published a test report on March 8.

Antomono-control Transmission

At the beginning of this year the Wilson V.S. design was joined on the market by the A.C.V. Automonocontrol transmission. This was developed in conjunction with C.A.V., Ltd., and, as its name implies, it is a logical sequel to the Monocontrol semi-automatic system.

Both work on similar principles. Current from a generator driven from the output shaft of the gearbox is fed into a control box and " compared" with another current flow, the strength of which is proportional to the throttle opening and to the engine torque output. The voltage difference in the control box decides which relay and brake-band cylinder are to operate. Thus, changes are made entirely proportionate to engine torque and road speed.

A fluid coupling is, of course, retained in both designs, but there is little reason why a centrifugal clutch or even the Fluid-Friction coupling should not be employed with either to reduce transmission losses to a minimum. An important fuel-saving feature of the A.C.V. design is that neutral is automatically engaged at about 2 m.p.h. when braking, so that it,is impossible for the engine to idle against flywheel drag when the vehicle is stationary.

From the gear-change pedal and selector lever of the 1932 Daimlers it has taken 25 years to adapt the same basic design to give fully automatic gear control. It has been a slow process, but a thorough one tuned to the reactions, favourable or otherwise, of the buying public.

Not unnaturally, emphasis has so far been placed on the Wilson epicyclic gearbox and its stages of refinement, this being the principal British transmission which falls into the categories under consideration. This is not by any means to suggest that its makers have to face no competition in Britain. Indeed, within 10 miles of the Self-Changing Gears factory, substantial developments are taking place on another automatic gearbox— the Hobbs.

The Hobbs transmission is also an entirely British invention, developed principally for use in private cars but with a larger version suitable for heavy vehicles and, indeed, already fitted to 11 single-deckers operated by the Birmingham and Midland Motor Omnibus Co., Ltd. An important feature of the Hobbs transmission is that it does not rely on hydraulic couplings, and the drive is always positive and idling drag is negligible.

Two Friction Clutches

In principle, the transmission consists of a clutch unit incorporating two hydraulically actuated disc-type friction clutches; a gear unit with one epicyclic gear train controlled by three disc brake units; a hydraulic unit, which is engine-driven and supplies power forMe clutches, gear brakes and hydraulic valves; and a rear pump unit which supplies pressure for the governor valve and actuates the clutches for use when starting the vehicle by towing.

Although at first sight this transmission appears complicated, its principle is simple, its working parts are relatively few and little running maintenance should be required in service. Furthermore, its efficiency is high and the device is not bulky, so that it forms a suitable fully automatic replacement for existing types of transmission.

First, second and third gears are obtained with the front clutch engaged and the gear brakes applied to different portions of the epicyclic train, whilst for top gear, the gear brakes are released and both clutches are locked. When in reverse, the rear clutch is engaged and the epicyclic cage is held stationary. The speeds at which changes are made depend on the throttle opening, through governor jets, and "kick-down" engagement of lower ratios is possible. A manual overriding control is provided for use when descending steep hills and so forth.

There are two other forms of automatic transmission made in Great Britain, both of which are based on American designs. One, the Rolls-Royce heavy-duty automatic gearbox, has already been used in Dennis fire-engines and the Harrington Contender coach. These are instances in which high speed, good acceleration and effortless driving are essential.

Rolls-Royce Automatic Gearbox

The Rolls-Royce box consists of a fluid coupling and two epicyclic gear trains. Two members of either gear unit can be locked together to give direct drive, whilst brake bands can lock one member of each gear to give a reduction gearing. A hydraulic system controls the actuation of these two gear trains. This hydraulic section consists of a forward pump, driven by the torus cover of the fluid coupling, and a rear pump driven by the gearbox output shaft. The front pump delivers oil whenever the engine is running, whereas the rear pump delivery oil only when the vehicle is moving. Gear changes are determined by the outputs of these pumps.

The other fully automatic gearbox to be manufactured in this country is the Borg-Warner, now in full production at the Letchworth factory of Borg-Warner, Ltd.

Basically, this transmission consists of a hydraulic torque converter and two epicyclic gear trains in series. Although only three forward speeds are given, the drive is taken through the torque converter in the low and intermediate ratios. The mechanical reductions effected by the gear trains are 2.308 and 1.435 to 1, whilst the torque converter has an input-output ratio ranging from slightly over 2 to 1 down to 1 to 1. There is, therefore, a smooth gradation from a maximum reduction of 4.616 to 1 on low gear to a minimum reduction of 1.435 to 1 on intermediate. For top gear a single-plate clutch couples the engine output shaft directly to the gearbox output shaft, thus by-passing the torque converter and the gear trains.

Torque converters

Another form of automatic transmission is the torque converter. Although used in the Borg-Warner gearbox as an integral part of the transmission train, a torque converter can be employed separately and is particularly useful for heavy equipment, excavators, railcars and shunting locomotives. By means of a torque converter the engine can run at or near its governed speed under all load conditions, whilst the torque converter will multiply the engine torque output according to the needs of the transmission.

Rolls-Royce, Ltd., manufacture a three-stage torque converter, made under licence from the Twin Disc Clutch Co., Ltd., of America, and matched to suit RollsRoyce oil engines. Self-Changing Gears, Ltd., also make a torque converter. Devices of this kind were employed in British buses, mainly before the war, but it would appear that their use in road vehicles is limited unless incorporated with some form of epicyclic transmission.

A number of Leyland buses incorporating the Swedish Lysholm-Smith torque converter was made pre-war: they had a direct-drive clutch for normal running, the converter being used for all other working. A constantmesh gearbox was used for forward and reverse gears.