Gearbox Designs

Types Of Gearboxes

Epicyclic gearboxes

Epicyclic differential gearing, used for calendrical computation, has been identified in the Greek Antikythera mechanism dating to around 87 BC.
Richard of Wallingford, an English abbot of St Albans monastery is credited for reinventing epicyclic gearing for an astronomical clock in the 14th century.
In 1588, Italian military engineer Agostino Ramelli invented the bookwheel, a vertically-revolving bookstand containing epicyclic gearing with two levels of planetary gears to maintain proper orientation of the books.

Wilson gearbox 

The Wilson pre-selector gearbox is the best known design and is the archetype generally meant when the term "pre-selector gearbox" is used without further qualification.

Major W. G. Wilson (1874–1957) was rewarded as one of the major co-inventors of the tank after World War I. He had mainly been involved with the development of transmissions for tanks, particularly the problem of their steering gearbox. He had become an advocate for the benefits of the epicyclic gearbox, which allowed large torques to be transmitted whilst still being controllable through a small input force. In 1917, Wilson designed the Mark V tank, which incorporated his epicyclic steering gear. This was the first of the heavy tanks that could be driven by a single driver, without requiring him to signal orders inside to others working the secondary gear levers.

The Wilson gearbox was produced with a variety of clutches. The best-known is the fluid flywheel, used for touring cars such as the Daimler and the Armstrong-Siddeley. Sports cars used a Newton centrifugal clutch. This was a multiple plate dry clutch, similar to racing manual clutches of the time, but with the pressure plate centrifugally actuated to engage at around 600rpm. Pure racing cars, such as the ERA, avoided a clutch altogether and relied on the progressive engagement of the gearbox's band brake on lowest gear when starting.

The Wilson gearbox relied on a number of epicyclic gears, coupled in an ingenious manner that was also Wilson's invention. A separate epicyclic was required for each intermediate gear, with a cone clutch for the straight-through top gear and a further epicyclic for reverse.Four gears were provided, at a time when the usual gearbox had only three. This was owing to the sporting, or indeed racing, market for the Wilson gearbox, so that the ratios could be more closely spaced. Although this same arrangement of epicyclics would become the precursor for the post-war automatic transmission, the automatic transmission's use of a torque converter, together with the broad power band and excess power of US V8 engines, meant that wider-spaced, thus fewer, ratios were acceptable. Unlike the "crash" gearboxes of the first half of the 20th century, the gearwheels in a preselector box are permanently in mesh in an epicyclic layout.

Multi-clutch gearboxes

A multi-clutch gearbox avoids the difficulties of shifting gear by avoiding the need to shift at all. It operates as a number of separate gearboxes, each one controlled by a separate clutch, interlocked to avoid multiple selection. Selecting a gear is a matter of selecting the appropriate clutch. An advantage of this type is that it's simple to arrange remote operation, as there is no gear shift linkage as such, merely duplication of a clutch linkage.

Single ratio per clutch

This type of gearbox appeared in the March 1917 Oldbury gearbox trials between 8 different World War One tanks. Each ratio has its own shaft, and its own clutch. Provided the clutches are interlocked so that only one may be engaged at a time, the system is simple.

In the early 1980s this transmission was developed for urban buses in the UK as the Maxwell, with a four-speed gearbox. UK buses are mostly double-deckers with rear-mounted transverse engines. Their use also involves much stop-start driving, thus heavy clutch wear for conventional gearboxes. The advantage of this arrangement was its use of four clutches, all easily serviced from outside the engine assembly, without needing to remove the engine. The clutches were multi-plate oil-supplied wet clutches. The first gearboxes gained a reputation for unreliability and in 1985 the original developers, Brockhouse, licensed them to Avon Maxwell Transmissions. A developed version, with an enlarged oil pump, was available as a retrofit for most UK buses. An unusual feature of this gearbox was the ability to engage first and second gear simultaneously. This acted as a 50 hp hydraulic retarder.

Tiger tanks

The Tiger tank of World War Two used a form of pre-selective gearbox, offering 8 speeds. Clutches were used in combinations, allowing many more ratios than actuators. The shift mechanism was hydraulic, to reduce driver effort. There were three hydraulic cylinders, each with two positions and controlling dog clutches for gear trains arranged on four shafts. The cylinders were controlled by a rotary valve on a simple quadrant gear lever and activated by pushing the lever sideways into its gate. The combination of the three cylinders, effectively a 3-bit binary code, permitted 8 different ratios, although in reverse these were limited to just the lowest 4. When the captured Tiger 131 was studied by the British in 1943, the report on the gearbox was carried out by Armstrong-Siddeley motors, as experts in such a field.

Dual clutch

The idea of rapid shifting by clutch alone has also been developed as the dual clutch transmission, in recent high-performance cars. This combines the simplicity of a shifting gearbox with the rapid shifts of a clutch-based design. There are effectively two separate gearboxes, each offering alternate ratios from the overall set, and the two clutches select which gearbox is in effect. Changes within the gearbox are done automatically, to predict which ratio the driver will require next. Provided that the next ratio has been selected correctly (i.e. the computer guessed correctly as to an up-shift vs. a down-shift) the shift itself is merely a rapid movement of the clutch. Unexpected shifts may confuse the system though and it must first select the correct ratio before engaging the clutch, giving a far slower shift.