Hydra-Matic automatic transmission

One of the most well-known forms of construction of an automatic transmission is the Hydra-Matic; great numbers of which were manufactured in the U.S.A. in the post-war years. It comprises two planetary gear sets for the four forward gears and one set for reverse. Transmission of force is effected by a fluid coupling.

Gear-changing is effected automatically, depending on the speed of the car (speed of rotation of the driven shaft of the transmission), engine load, and accelerator position (through a governor). The transmission of force is effected in the mechanical part of the system by means of planetary gear sets and a fluid coupling.

In first and second gear the total power is transmitted through the fluid coupling, which is driven at a higher speed in second gear than in first because the planetary gear set is then locked, and the slip losses in the coupling become less.

On the other hand, the greater slip that occurs in first gear ensures great smoothness in take-up of the clutch.

In third and fourth gear, however, the power is transmitted in two ways. At the point when the power coming from the engine via the planetary gear set 1 is transmitted partly (through the fluid coupling) to the sun wheel of the planetary gear set 2 and partly (through the hollow shaft and the locked clutch to the annulus of the planetary gear set 2. In the last-mentioned gear set the two flows of transmitted power converge upon the planetary gear. The fluid coupling is by-passed. The slip then affects only that portion of the power which is transmitted by the coupling. In third gear the drive of the fluid coupling is effected through the operative planetary gear set 1, as in first gear, with reduced speed of rotation.

In fourth gear, on the other hand, the planetary gear set 1 is locked by the clutch. As a result of this the fluid coupling receives a higher operating speed in relation to third gear, so that the slip is reduced. In the parking position the clutches are locked. The clutch locks the annulus and therefore the sun wheel. The clutch locks the annulus, so that the planetary gear between the sun wheel and the annulus is likewise immobilised. The drive to the rear axle is thus locked.

The gear-changing operations are controlled by a hydraulic regulating device. The centrifugal governor is driven by the driving shaft of the transmission. It comprises a centrifugal piston which is eccentrically and movably mounted in a housing. Controlled by the centrifugal force, whose magnitude depends upon the road speed of the vehicle, the centrifugal piston feeds a regulated supply of hydraulic pressure oil to the control valve. This oil pressure, regulated by the centrifugal governor, is now applied to one side of a piston, while a spring thrusts against the other side. This spring pressure is augmented by the oil pressure which is developed in the load regulator by the action of the accelerator pedal.

If the oil pressure controlled by the load centrifugal governor exceeds the pressure controlled by the load regulator, the piston in the control valve clears the way to the relevant planetary gear set. At the same time the brake is released and the clutch is locked, as corresponds to the gear-changing operations described above. On the other hand, if the oil pressure controlled by the load regulator is greater than the pressure controlled by the centrifugal governor, the whole operation is reversed. The transmission system is thus controlled only by the accelerator (load regulator) in relation to the road speed (centrifugal governor).

When there is no pressure in the pipe from the control valve to the relevant planetary gear set, then the spring action locks the brake and keeps the clutch released. A hydraulic regulator of this kind is provided for each planetary gear set.