The use of trains of epicyclic gears is a popular form of change speed gear, starting around 1895 with the three speed new Mercedes Benz Comfortable and the Roots and Venables two speed Petrocar as well as the Rolls Royce Phantom. In a simple three element epicyclic gear train there are three main elements, a central gear called the sun, an outer gear with internal teeth known as the annulus, and one or more planet gears between the sun and the annulus. The planets are held on a planet carrier. The sun, planet carrier and annulus all have a common axis and, by holding one and using the other two as driving elements, changes can be rung in the ratios produced and the directions of rotation. If the planet carrier is held, the sun gear will drive the annulus at a slow speed in the reverse direction. If the annulus is held, the sun will drive the planet carrier in the same direction at an even slower speed. If the planet carrier and the annulus are clutched together, then the whole gear train will rotate at the same speed and in the same direction as the sun gear. The epicyclic gear has the advantage that a change of ratio is not dependent on engaging gears but on braking or clutching. This can be frictional and therefore smooth. Earlier trains provided either reverse or two speeds, often in conjunction with a belt drive. Later gears used epicyclic trains to provide all the required ratios. Usually, as in the new Bentley of 1912, one train was used for bottom, two trains acting together for second, a direct drive clutch for top and an additional train for reverse. One of the most significant cars in the entire history of motoring, Henry Ford’s Model T, of which over 15 million were built, used a slightly different form of epicyclic gearing. In the Model T Ford there were no annulus gears but three planet gears, which were fixed together and free to rotate on a spindle fixed to the back of the flywheel. The planet gears were of different sizes and meshed with sun gears, also of different sizes and on tubular shafts. The front sun gear was connected to the output, or propeller shaft. At the end of its own tubular shaft it carried one side of a direct drive friction clutch. The other side of the clutch was driven by the crankshaft extension, so that, when the clutch was engaged, there was direct drive from the engine to the rear wheels. The other two sun gears were coupled to two separate brake drums having external contracting brakes. By selectively braking the drums, low, forward and reverse ratios were produced. The Model T had direct and low forward speeds and reverse, all of which were engaged by three pedals. The left pedal, when depressed, engaged the low speed brake band. At mid travel, it freed both that band and the direct drive clutch and, when fully released, it engaged the clutch to provide direct drive. The centre pedal engaged the reverse brake band and the right pedal operated the vehicle brake, which was a transmission brake on the outside of the clutch. To save holding the left pedal in neutral, the clutch was also connected to the handbrake lever and was disengaged at mid travel. When working with Beardmores in 1919, Walter Wilson devised the compound epicyclic gear train used later in the Wilson preselector gearbox, fitted to the 1930 Rolls Royce Ghost and later classic cars. This gear system had four sun planet-annulus trains, four brakes and a direct drive clutch to give four forward and one reverse ratio. The Wilson compound train was the first of the “accelerating trains”, in which the epicyclic gear sets were used in series to obtain different ratios. It is known as an accelerating train because the first annulus is driven faster for each upward ratio shift. A preselection mechanism, devised in 1928, allowed a ratio to be selected by a lever mounted on the steering column and engaged some time later, when the driver wanted it, by depression of a pedal. The lever turned a camshaft in the gearbox and the camshaft selectively operated a row of struts used to engage each of the brake bands which locked the gear trains. When the pedal was depressed, the appropriate strut end moved into a grooved bar and any other strut moved out. Releasing the pedal caused the grooved bar to rock and engage the appropriate brake (and gear train) while at the same time allowing the remaining brakes to be free. In 1912, George Perret introduced an epicyclic gearbox in which the planet carrier had spindles at right angles to the transmission axis. All the gears were bevel gears, similar to the gearing in the Automotive Products’ automatic transmission used in new Porsche cars today. Perret also employed electro magnetic brakes for obtaining the various ratios, as did George Pollard of the Menco Elma Syndicate from 1916. Pollard tilted the planet spindles to obtain a better choice of gear ratios. A parallel spindle location for spur planets, again with electro-magnetic clutches and brakes, was used by Edward Reeve of Bentley Motors in 1917. Two years later, Jean Cotal employed a similar system with the addition of a pin (turned with a screwdriver) which could lock the gear in the lowest ratio if the electric current failed. An addition to this gearbox was a power take off shaft which could be used for a dynamo or starter motor. By 1935 the Cotal gearbox was being driven through a hydraulic coupling. A means of sensing, from the throttle pedal, the precise stage when the coupling was neither driving nor on the overrun was used to inhibit the gear change except in that condition and to provide a smooth and clutchless transition. A facet of the Cotal gearbox was a separate reverse train, with its own control, enabling all four ratios to be obtained in both forward and reverse. This gave the 1935 Delahaye interesting possibilities.
<< Do You Need to Fix the Garage Door or Just Move Something
