Tea Break: Manual transmission
The history of manual gearboxes.
Although the compact internal combustion engine heralded the birth of the modern automobile, it introduced significant engineering drawbacks. Unlike the cumbersome steam engine, it couldn’t develop full torque from rest, making it impossible to connect the engine crankshaft to the wheels directly. This meant that not only was a clutch required, a transmission was also needed to vary the relationship between wheel and engine speeds, so that the driver could operate within the relatively narrow torque band.
Gaining teeth
Toothed gearing had been used for thousands of years in applications, such as windmills and in factories of the Industrial Revolution. It provided a far more satisfactory solution than earlier attempts at belt drives, despite requiring a separate clutch.
While James Watt (1736-1819) designed a steam-powered car fitted with a sliding shaft multi-speed transmission, it was never built. The breakthrough for the manual gearbox was to occur almost 100 years later, when business partners in a woodwork machinery shop, Emile Levassor and René Panhard, were inspired by the gearing of wood-turning lathes to adopt sliding gears on a shaft for motor car use. The earliest Panhardlevassor transmissions had their gears exposed but the three forward ratios were enclosed within their own casing in later developments. The layout was shared by other pioneers, including Louis Renault.
These very early transmissions used straight-cut teeth on their gears, which tended to be noisy and cause vibration. Incidentally, only reverse gears on certain modern car gearboxes tend to have straight-cut teeth, which explains their characteristic audible whine when transmitting drive. A solution was to use helical angled gears instead, despite
being less efficient mechanically. This is one reason why gear oils smell so pungent, because of the extra additives required for the extreme pressure (EP) lubricant to protect the meshing surfaces adequately. As it is virtually impossible to mesh helical gears together on the move, a pair can be made to run continually (constant mesh) although one gear is not fixed to its shaft but can be locked onto it by means of a dog clutch. The first known merging of these technologies appeared circa-1906 in the Riley V-twin, one of the former British brands that is owned now by BMW.
Avoiding the grind
Even Levassor described the action of changing gear on the move with his straight-cut gears as “brutal”, which led to De Dion Bouton’s rudimentary method of slowing down the faster spinning gear, so that it did not clash against its slowermoving opposite number. The solution was to bolt a rubber ring to one gear that slowed down the other gear as they were forced together. Unsurprisingly, the gearbox was not very durable and could not tolerate the faster, ‘racier’ shifts demanded by impatient drivers.
The arrival of dog clutches saw Bernard Thompson, a British engineer, incorporate a baulking system that did not allow the mechanism to engage unless the two components rotated at the same speed. He also employed mating conical surfaces to provide the friction for synchronisation, all of which was contained within one dog clutch for the constant mesh application. General Motors (GM) purchased the inventor’s patents, naming it ‘Synchromesh’, and fitted transmissions thus equipped first to its North American luxury brand, Cadillac. For lesser models not equipped with any kind of synchronisation, the driver was required to ‘double-declutch’
while moving, which involves matching the input shaft (engine) speed with that of the desired ratio, using the throttle in neutral gear, before making the selection.
While it was possible to buy cars in Britain with bottom gear lacking synchromesh until the early 1970s, the first ‘all-synchro’ transmission appeared on the 1933 Alvis Speed Twenty. Various alternatives to cone-type synchromesh exist and can be manufacturer dependent, many of which have been developed through motorsports with its desire for quicker gear-changes. Notable among these is Porsche’s split-ring synchromesh that appeared in the late 1940s and included a baulking feature, unlike GM’S production gearboxes of the period. This meant the synchromesh could be ‘beaten’, when making a fast gear-change, and it was the US Borg-warner Corporation that developed the cone synchromesh to include baulk rings. This arrangement remains in production to this day.
Latest developments
The manual gearbox has scarcely changed in the past 50 years, apart from packaging improvements, such as transverse installations, most notably in BMC’S Mini that saw the transmission share the engine oil for lubrication purposes. One of the main refinements has been to improve selection and quieten reverse gear, by adopting both helical gears and synchromesh, or using one of the forward ratio’s cones to stop the input shaft from spinning when reverse is selected.
Due to considerable weight and efficiency advantages over normal automatic gearboxes, automated manual transmissions became more common from the beginning of this century. Single-clutch transmissions are simply manual gearboxes with the shift and clutch operation controlled robotically, while twin-clutch automated gearboxes consist of two separate gear sets linked together by internal clutches. However, both types still have the same operating principles of a conventional manual gearbox and it is likely that developments in these areas will continue to improve, especially while the internal combustion engine remains viable.