Quick Quote



The first automatic gearbox (transmission) was invented by Alfred Horner Munro, of Regina, Saskatchewan, Canada in 1921. On November 15th 1923, he received GB patent GB215669 for a Pneumatic Power Transmission Gear.

Being a steam engineer, Munro designed his device to use compressed air rather than hydraulic fluid, so it lacked power and was not commercially produced.

The first Commercially produced automatic gearbox using hydraulic fluid was General Motors’ Hydra-Matic Transmission. Since the mid twenties, GM president Alfred P Sloan had been pushing research chief Charles Kettering to develop a workable automatic transmission.

The impetus was personal as well as commercial; Sloane was by his own admission a poor driver and struggled with the conventional gearbox.

The project was handed over to a team of engineers lead by Earl Thompson, who had developed the first synchromesh gearbox back in the Twenties (when you change gears the action of a synchromesh transmission matches the speeds of the gear you are leaving to the speed of the gear you are selecting, making the transition smooth and quiet. So it “synchronises” the “meshing” of the gears.)

Thompson and engineers Ralph Beck and Walter Herndon started work on their own automatic transmission in early 1932. Three more staffers, William Carnegie, Maurice Rosenburger and Oliver K Kelly, joined the group to handle experimental testing.

The Hydra-Matic used a fluid coupling to transfer power in place of a conventional friction clutch, and three planet gearsets to provide four forward speeds and reverse. In the absence of a conventional clutch a parking pawl (a pin that engaged in a notched wheel on a shaft) was included to lock the drive wheels when the engine was off.

The Hydra-Matic made its debut in 1939 as an option on 1940 Oldsmobiles, and featured on Cadillac and Pontiac’s option list the following year. Soon afterwards, civilian car manufacture ceased for the war effort. The Hydro-Matic, however, continued as part of war-time production in Cadillac-powered tanks (one engine and transmission per track) and some GMC 6×6 military trucks.

This testament to the Hydra-Matic’s durability was successfully exploited by GM’s advertising department after the war, who could claim the system’s “battle-tested” durability. The system’s reputation also grew within the industry and was incorporated in vehicles produced by Hudson, Nash, Kaiser, Willys and even rivals Ford. A real compliment to the Hydra-Matic design came when Rolls-Royce licensed it, keeping a version in production through to 1967.

From 1964 onwards, the Hydro-Matic was phased out in favour of the turbo Hydra-Matic, a lighter and more efficient system. The automatic transmission division at GM is still called Hydra-Matic today.


(written by Alan Warwick, London N12)