Preselector gearbox
Compared with the contemporary (non-synchromesh) manual transmissions, preselector gearboxes were easier for drivers to operate smoothly, since they did not require techniques such as double de-clutching.
[citation needed] A design advantage of many preselector gearboxes is that the friction components are brakes,[4] rather than clutches.
This meant that non-rotating brake bands could be used for the parts which are subject to wear, which results in a simpler design than a rotating component such as a typical clutch.
The wearing components could also be mounted on the outside of the mechanism (rather than buried within it), providing easier access for maintenance and adjustment.
One of the company directors, Walter Gordon Wilson, 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.
The best-known is the fluid flywheel, used for touring cars such as the Daimler (Armstrong Siddeley used a centrifugal clutch).
[17] 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.
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.
Changing gear with the Wilson box relied on the control of the brake bands that held each epicyclic's annulus in fixed position.
This small hand-operated lever could not provide enough power to engage and lock the brake band, thus the need for the foot pedal.
The actual movement of the brake bands was controlled by the 'toggle' mechanism, a distinctive feature of the Wilson design.
If the cam (for each gear) held the linkage in place, rather than allowing it to swing out of the way, the busbar finger would then press, via the operating strut, onto the toggles controlling the brake bands themselves.
These toggles provided the additional leverage necessary to hold the brake band in place, under the force of the coil spring, until the pedal was next pressed.
The action of engaging and disengaging each band was sufficient to advance the ratchet nuts and so adjust for wear by taking up any slack.
[21] A downside to this approach was that a loss of electrical power meant that the engine could no longer drive the wheels.
There was no mechanical servo action, so no need for the Wilson's cam or toggle arrangements and the change gear pedal.
[22] It was broadly similar to the Wilson, but used direct hydraulic actuation of the brake bands (selected via a lever on the steering column) therefore avoiding the need for a change-gear pedal.
[29][30][31] The 1942–1945 German Tiger I armoured tank used a form of pre-selective gearbox that was built by Maybach and offered 8 ratios.
There were three hydraulic cylinders, each with two positions and controlling dog clutches for gear trains arranged on four shafts.
[32] When a captured Tiger I tank was studied by the British in 1943, the report on the gearbox was carried out by Armstrong Siddeley motors.
The first gearboxes gained a reputation for unreliability and in 1985 the original developers, Brockhouse, licensed them to Avon Maxwell Transmissions.
The preselector buses were also exported to various countries – the AEC Regent III with the Wilson type gearboxes were used in Western Australia for 25 years as part of the public transport fleet from 1950 until 1976.
