Borg-Warner Overdrive
The Borg-Warner overdrive unit was in production up to the early 1960's, usually fitted to cars with engines of about three litres capacity. Production ceased when Borg-Warner concentrated on developing
automatic transmissions. The gear train layout of the Borg-Warner design was similar to the
Laycock overdrive - the annulus and the planet carrier act as the output and input members respectively and between them was fitted a roller clutch.
However, this overdrive did not have a cone clutch. Instead, a blocker ring and pawl were used to engage the overdrive. Once engaged, the pawl locked the sun gear and so allowed the drive to pass through the planet carrier and the annulus. The pawl was directly operated by the solenoid so no hydraulic mechanisms were involved. Normal drive was obtained by sliding the sun gear so that the teeth on its perimeter engaged with the planet carrier and locked up the epicyclic train. This movement was achieved by a selector fork, cable-operated by a lever in the car.
The electrical control circuit consisted of a centrifugally-operated switch, a relay, a kick-down switch and the overdrive solenoid. When the control lever inside the car was in the overdrive position, overdrive would automatically be selected when the car's speed exceeded about 48 km/h (30mph). At this speed the centrifugal switch closed and overdrive would engage if the driver momentarily lifted their foot from the accelerator pedal. At speeds below 48 kmh (30mph) the transmission would revert to direct drive. However, because the sun gear was not locked to the planet carrier, the transmission would "freewheel" on the over-run clutch.
If engine braking or reverse gear was required the control lever had to be moved to the direct drive position. Another feature of this overdrive was that if direct drive was required for acceleration or overtaking whilst the overdrive was in operation, the accelerator pedal could be pushed right down in order to operate a kick-down switch in the solenoid circuit and give direct drive. When the kick-down was used, the ignition was momentarily cut in order to take the load off the locking pawl and to allow it to disengage.
