The automatic gearbox

November 22, 2011 at 6:21 pm
filed under Automatics
Tagged , , , , , , , , , , , , , , , , , , , ,

An automatic transmission is more than a device that relieves the driver of the effort of making manual gear-changes. Certainly it makes the clutch and gearlever redundant, but it has other advantages in that theoretically all gearchanges can be made at precisely the correct moment in terms of the engine’s power characteristics, the unit’s speed and load, and the vehicle’s road speed. In this way, maximum performance or fuel economy can be ensured. This represents an ideal situation, but it is one towards which automatic transmission engineers have been working from the earliest days of motoring. A sliding-gear arrangement nor dog clutches are necessary for gear changing which is effected by band brakes or multi-plate clutches; torque transmission can be continuous during gear changing, the drive not having to be interrupted as with other forms of gearing; and tooth loading is distributed over several gears instead of only one pair, thus minimizing noise and wear.

After Dr. Fred, the next major user of the epicyclic system was Henry Ford who adopted it for the unique two-speed and reverse transmission of the Model T, introduced in 1908 and in continuous production until the 1920s.

The two principal elements of most of today’s automatic transmissions—the hydraulic torque converter and the epicyclic gearbox with its control systems—were not, surprisingly, developed together but evolved entirely separately, and were not brought together for many years. However, they were but two of several approaches to automatic transmission in the motor car’s history.

The epicyclic gearbox

The epicyclic gearbox must be considered first since it featured on motor vehicles from quite an early stage. Though first developed during the industrial revo-lution, there is no record of the epicyclic gearing principle having been applied to car transmissions by anyone before “Dr Fred” Lanchester. He started to in-vestigate the possibilities of such gearing in 1899 and used it on his first production car in 1902.

Lanchester firmly believed in tackling his engineering problems from first principles, so why did he employ epicyclics ? The answer is that the system offers numerous advantages over the conventional layshaft type. Since the various gears are in constant mesh, neither for the layshaft gearbox, there were various other digressions into epicyclics. In 1912, Perret evolved an epi-cyclic-bcvel system with electro-magnetic operation of the gear changing band brakes; the mechanical side of his invention was the ancestor of that employed by Automotive Products in their ingenious four-speed automatic offered by BMC Leyland (now BL) on Mini and 1100/1300 models from 1966.

Electromagnetic actuation of the band brakes and clutches was specified also, but with parallel axis (that is to say non-bevel) cpicyclic gearing, in a transmission devised in 1917 by Edward Reeve of Wolselcy Motors, but the system never reached production. A similar arrangement was also proposed two years afterwards in France by Jean Cotal, another prominent engineer in the history of the gearbox.

Wilson makes his mark

Though various manufacturers experimented with the cpicyclic gearbox, there was one man who, more than any other, established it firmly on the map of automotive de-velopment. He was Walter G. Wilson (later Major Wilson) who, born in 1874, was involved shortly after the turn of the century with the Wilson-Pilcher and Armstrong Whitworth cars and subsequently with the Hollford truck and the WWI tank.

Wilson’s thoughts must have turned to epicyclics at quite an early stage, since gearing of this kind featured on the Wilson-Pilcher of around 1904. He joined the Beardmore company in 1919 on leaving the army, and there he de-veloped the “compound” epicyclic system as a means of making the gear ratio spread and spacing appropriate to a car transmission. Compounding, in this context, consists of using more than one gear train and coupling different elements of adjacent ones—for example, planets to annulus or planets to sun gear. This step forward led Wilson to develop the gearbox that bears his name.

Epicyclic gearing or planetary gearing as used...

Image via Wikipedia

The first commercially successful transmission of power from one rotating shaft to another by means of a hydro-kinetic device (one in which the fluid moves bodily in performing its function) was made by Dr. Hermann Fottinger in 1905. Dr. Fottinger was working at that time for the Vulcan company in Hamburg, and he soon progressed from the original paddle concept to that of vaned half-toroids (hollow rings of semi-circular section) within which the fluid circulates in corkscrew fashion.

For many years fluid couplings of this type were used only for industrial duties, in particular where smooth take-up under load was required of an electric motor, which was provided by the coupling’s characteristic of progressively reducing slip as the speed of the output member increased.

Four-speed unit with reverse, obtained from four epicyclic sets, and with mechanical actuation of the band brakes on the annulus gears.

Wilson set the seal on his epicyclic work in 1928 by evolving his “pre-selector” mechanism whereby the next gear required could be selected by the driver (by moving a lever in a quadrant below the steering wheel ) but would not engage until the driver operated a pedal similar to the clutch pedal on conventional cars. The Wilson system was adopted that year by Armstrong Siddeley , and it survives to this day in bus transmission.

In the Armstrong Siddeley application of the Wilson gearbox (and in other later ones, notably by Riley and ERA) there was no separate provision for taking up the drive from rest in either first or reverse gear. The duty was performed by the relevant brake bands which consequently had rather a short life, apart from not giving the smoothest of of take-ups. Since the best-known solution to this problem —and that with the greatest influence on subsequent progress—resulted from the blending of mechanical and hydraulic transmission, this is a good point to consider the use of hydraulics in automatic transmissions.

Then, in 1926, Harold Sinclair, a British engineer, had the idea of utilizing such a coupling in the transmission of a bus to give the passengers (particularly standing ones) a less jerky journey than would a conventional friction clutch. He came to an agreement with Vulcan and formed the Vulcan-Sinclair company to pursue this line of development in the UK.

Sinclair’s work on buses came to the notice of the Daimler company which made vehicles of this type as well as cars. Percy Martin, a senior transmission engineer at Daimler, had been very attracted by the Wilson gearbox but wanted something smoother and more durable for luxury cars than the brake-band take-up mentioned above. Sinclair’s Fottinger-typc coupling was the obvious answer to Martin’s worries, so in 1930 the Daimler Double Six appeared with the first combination of the preselector box and what the car company called the “fluid fly-wheel”. The system gave a remarkable (for those days) blend of smoothness and ease of driving, and its use soon extended down the Daimler range and into models made by the Lanchester and BSA subsidiaries.

At this stage too, there appeared an alternative to the fluid coupling as a means of getting a Wilson-equipped car under way without relying solely on the brake bands.

Talbot’s famous Swiss Chief Engineer, Georges Roesch, was another engineering purist who liked the pre-selector gearbox but not its unassisted take-up, so he produced what was called the “traffic clutch”. This was a centrifugal friction clutch which automatically freed itself at low engine speeds; first or reverse gear could therefore be engaged by the lever and pedal when the car was at rest but no drive was transmitted until opening the throttle caused the clutch to speed-up and so re-engage.

Jean Cotal had continued his work on electro-magnetic-ally controlled epicyclic gear systems for cars during the 1920s and early 1930s. He too was impressed by the smoothing-out capability of the fluid coupling, so he incorporated one into his design in 1935 later adopted by Delage and Delahaye. His control arrangement was not of the pre-selector type, though, gearchanging occurring immediately on movement of a diminutive lever in a gate, with or without use of the “clutch” pedal. Another unusual feature of the Cotal box was that it incorporated a separate gear train for reverse, in which all four forward ratios were also available!

The Wilson and Cotal gearboxes were, of course, not automatic, gear changing having to be initiated by the driver. However, the seeds of automaticity sown by Riescler were nutured in France in 1923 by Gaston Fleis-chel who devised a system in which speed and load sensors and actuators were used to operate the clutch and to move the gear lever of an orthodox layshaft gearbox—presumably with synchromesh.

Despite Ricseler’s pioneering, the marriage of the hydraulic coupling or converter and the epicyclic gearbox, with automatic control of gear changing, was really solemnized by General Motors’ work in the USA during the late 1930s. The first manifestation of GM’s efforts was a “hot- shift” (power-sustained gear changing) system introduced by Oldsmobilc in 1937 and subsequently taken up by Buick the following year. Then came the multiple-birth— the first Hydra-Matic automatic transmission, fitted to 1940 Oldsmobiles as an option. It had a fluid coupling, not a torque converter, and provided four forward ratios, the governing and hydraulic actuating arrangement for gear changing setting the pattern for many years.

Towards the automatic

The two-element fluid coupling inherently has an output torque that is lower than the input torque—by an amount that varies with the degree of slip. Some means of auto-matically multiplying the output torque (by an amount depending on the demand) has. Obvious advantages for vehicle transmissions, though, and work in this direction began in the 1920s using a third element—the “reactor”.

A British pioneer in this field was Allan Coates who in 1924 proposed a design with the reactor mounted on a freewheel to enable the unit to operate either as a torque converter (a torque-multiplying device) or a fluid coupling. One of Dr. Fottinger’s successors at Vulcan in Germany, Hermann Rieseler, also spent a lot of time developing torque converters. In 1927 he designed a very advanced vehicle transmission with a converter (which could be locked to eliminate slip, and hence loss of efficiency, in cruising conditions) and two-speed epicyclic gearing controlled by a speed-sensitive governor. Nothing came of this project at at the time but it is noteworthy that, 22 years later, Packard in the USA came up with an almost identical design.

While considerable progress has been made since those days in what has become the “conventional” automatic transmission, it has been evolutionary rather than revolu-tionary. Most manufacturers went over from fluid couplings to converters, although Mercedes were one of the last to give way, preferring the trade-off of an extra gear ratio for the higher efficiency.

There was a period after WW2 when the American converters became almost frightcningly complex, some even having five elements and other sophistications. The goals were laudable enough—good response to the throttle and a high torque multiplication ratio—but although they were usually achieved the overall efficiency suffered. As the consequently high fuel consumption has become increasingly out of tune with modern conservation considerations, the trend has reversed towards less complexity, higher efficiency and the wider adoption of Ricsclcr’s lock-up idea for the converter. 6 An automatic transmission was offered on the Renault 16 in Variations on an automatic theme 1970 a breakthrough in the popular-price car market With the ever rising cost of fuel and demands from motor- ists for greater reliability and smaller cars, the traditional mechanical/hydraulic technique of controlling gear changing in automatic transmissions is giving way increasingly to electronic methods. These were initially viewed with some misgivings by the vehicle firms, owing to reliability problems in other areas, but in 1972 Renault took the plunge in the popular-price car category with the Renault 16, and others are following suit.

Two interesting post-war ideas are attributable to Howard Hobbs, a brilliant English engineer of limited financial resources. The first was the Hobbs Mecha-Matic transmission originally announced in 1947 and at one stage very nearly adopted by Ford for their mid-range models. It was a four-speed epicyclic unit which, in the interest of high mechanical efficiency, had neither a fluid coupling nor a torque converter.

The layout embodied two main friction clutches and three others for the epicyclic trains which had no annulus gears; there was no rotation, and hence no whine, when the gearbox was in neutral. Because of the Mecha-Matic’s considerable technical merit—albeit with quite a degree of complexity—it now seems a pity that Ford had to decide against it (in favour of a conventional Borg-Warner unit), but its adoption would have necessitated setting up a complete new production facility.

Howard Hobb’s second notable invention—which seems destined for greater success—was his VKD (variable kinetic drive) unit, first seen in 1967.

This, in effect, is a torque converter which, due to an additional clement having variable-angle vanes, provides a high enough torque mul- tiplication to eliminate the gearbox in car applications; even for a heavy truck it should not need more than a two or three-speed auxiliary gearbox, giving a significant saving in weight, bulk and cost over the normal truck transmission with up to 13 ratios.

Development of the VKD is at an advanced stage and several vehicle makers are very interested though it still has to be adopted. It should be pointed out, however, that the idea of variable-angle blades in a converter goes back to 1932, when it was conceived by the Brazilian engineer Dimitri de Lavaud, and was employed by Buick in the Dynaflow transmission of 1958.

Two basically gcarless transmission systems which lend themselves to stepless automatic control are worthy of consideration since, although old in concept, today they have considerable potential which is attributable to advances in design, materials and manufacture. The first is the so-called “friction” drive based on two discs with their axes at right angles; the face of one disc is contacted by the edge of the other so rotation of either turns its fellow. To vary the gear ratio it is necessary merely to alter the contact radius at which the second disc operates.

Drives of this type had an industrial origin, the first known vehicle usage being on the US Cartercar in 1906. GWK featured such a scheme also, in 1911, and it worked quite well on some pre-WWI GN light cars. No real advance was made on the archetypal scheme until the mid-19205 when Frank Hayes, an American vehicle engineer, evolved a more compact system with superior torque-•t transmitting properties. In the Hayes arrangement there were two driving members of part-toroidal form and a central correspondingly shaped driven member. Between driving and driven members were rollers whose axes did not orbit but had variable inclination; the assembly ran in oil to ensure adequate lubrication.

Any change in the roller inclination altered the ratio be-tween the roller-contact radii on the driving and driven members, thus changing the input/output speed ratio and, inversely, the torque ratio. This ingenious system, with automatic control of the roller inclination, was adopted for a while in the early 1930s by Austin in Britain. It performed well when new but wear eventually caused some sticking of the rollers, leading to a tendency not to vary the ratio when required.

After a spell in which its future was uncertain, Hayes’ principle was taken up in 1956 by Perbury Engineering, a small British firm set up by Forbes Perry and David Stutchbury. By 1960 the two had developed the trans-mission, now re-named Perbury , far enough to attract funding from the National Research Development Corporation, and they were assigned the latest patents. The snags of the Hayes box were overcome by mechanical improvements and better “traction fluids” and control methods, Commendably high efficiency—over the whole speed/load range—was added to the inherent compactness and stepless ratio changing which enabled the engine to operate always in its most efficient regime.

For more than 20 years, Perry has striven to convince the car makers and industry in general of the merits of this transmission, until recently with very limited success. Now, though, it seems that the Perbury could be on the verge of lift-off, since BL have been involving themselves with it to such an extent that a production version seems now to be a strong possibility. The second type of CVT (continuously variable transmission) is basically on the V-belt kind, the ratio being changed by contracting or expanding one or both pulleys. Relatively primitive systems of this sort, with manual control, were used successfully on motorcycles—notably by Rudge Whitworth and Zenith— before WWI, but little was done on the car side because of the limited torque-transmitting capacity of the V-belt.

In 1958, though, Hubb van Doorne, of DAF in Holland, startled the car world with his new light car incorporating an automatic belt transmission christened Variomatic. Hubb van Doorne, who died in 1979, used two V-belts instead of one to increase the capacity; he” also utilized both speed and load signals to vary the pulley diameters, whereas earlier automatic control systems had only been speed-sensitive and hence often in the wrong ratio.

The Variomatic was a clever design and it worked reasonably well, both in racing and rallying as well as on the road, the ease of driving being especially appreciated by the inexperienced. On the debit side, its overall efficiency was rather low, giving the small-engined DAF cars a poor performance, and it had operational imperfections which made it less popular. The Dutch company was not discouraged, though, and has subsequently developed a more advanced system, the Transmatic, which has steel block-link belts for longer life and higher efficiency.

This third-generation belt drive has given such impressive results that it looks likely to form the basis of a forthcoming Borg-Warner automatic transmission for small cars.

In such cars (indeed, in all), the conventional torque converter automatic transmission is hardly ideal owing to its considerable power loss and fuel consumption penalty. Howard Hobbs and Hubb van Doorne have not been the only ones to eschew it for its inefficiency, two others quali-fying for a mention here. One was the Smiths Easidrive which had a short production life in the 1960s as an option on some Rootes Group cars. It featured a largely orthodox layshaft gearbox—with gear selection by solenoids—and a pair of Smith-Eaton magnetic-particle clutches.

The use of twin clutches (one of which could be disen-gaged while the other was engaged) gave the gearbox a “hot-shift” characteristic, and the system was mechani-cally as efficient as a normal manual gearbox. On paper, Easidrive looked workable at the time but it foundered on high cost and an unacceptably poor gear change quality, being considerably inferior to epicyclic systems.

Towards the future

Finally, twin clutches and layshaft gearing appear too in the latest and very promising entrant in automatic gearbox development. This invention sprang from the fertile brain of Harry Webster, Technical Director of Automotive Products, whose epicyclic-bevel arrangement has already been mentioned. Webster, apart from seeking high efficiency, did not want his company to become involved in making gear sets, and he knew that car manufacturers would not take kindly to having to install new plant for this purpose, as would be necessary with an epicyclic system. Webster as would be necessary with an epicyclic system. Webster realized that if his ideas were to be accepted production costs would have to be kept to reasonable levels.

He solved his various problems in two ways—a friction clutch at each end of the gearbox and a revision of the conventional gear cluster whereby, instead of having the ratios in sequence, first and third were paired, as were second and fourth. In this way, the next-required ratio (in the “idle” half of the gearbox) could be pre-engaged and the gear-change effected by simultaneously engaging one clutch and freeing the other, as in an epicyclic assembly.

Scans from a three-volume textbook of 1928, on...

Image via Wikipedia

By doing this Webster achieved a clever and simple adaption of established ingredients. Assuming a good, modern system it should function well, and is potentially appreciably cheaper than the orthodox automatic. The AP transmission (which lends itself to a number of variations giving up to six forward ratios in the search for fuel economy) is still very much a development project. How’s ever, it has attracted great interest in the US as well as D Europe, and it would be surprising if the production stage was delayed any longer than is necessary to nudge auto-matic transmissions into a new era.

no comments

RSS / trackback

Comments are closed.