Hydra-Matic

Hydramatic (also known as Hydra-Matic) was an automatic transmission developed by both General Motors' Cadillac and Oldsmobile divisions. Introduced in 1939 for the 1940 model year vehicles, the Hydramatic was the first fully automatic mass-produced transmission developed for passenger automobile use.

During the 1930s, automakers sought to reduce or eliminate the need to shift gears. At the time, synchronized gear shifting was still a novelty (and confined to higher gears in most cases), and shifting a manual gearbox required more effort than most drivers cared to exert. The exception here was Cadillac's break-through synchromesh fully synchronized manual transmission, designed by Cadillac engineer Earl A. Thompson and introduced in the fall of 1928.

Cadillac, under Thompson, began working on a 'shiftless' transmission in 1932, and a new department within Cadillac Engineering was created, headed by Thompson and including engineers Ernest Seaholm, Ed Cole, Owen Nacker, and Oliver Kelley. During 1934, the Cadillac transmission group had developed a step-ratio gearbox that would shift automatically under full torque. This same group of engineers was then moved into GM Central Research, building pilot transmission units during 1935-36 which were then handed to Oldsmobile for testing.

The Automatic Safety Transmission (AST) was a tangent outgrowth of this work. The AST was a semi-automatic transmission using planetary gears and a conventional friction clutch, requiring the driver to use the clutch to shift into or out of gear, but not between the two forward gears. Oldsmobile offered the AST from 1937-1939, while Buick offered it only in 1938.

The HydraMatic was designed to combine hydraulic operation of a planetary gearbox (allowing much shifting to be automated) with a fluid coupling instead of a friction clutch, eliminating the need for de-clutching. The transmission would have four forward speeds (3.82:1, 2.63:1, 1.45:1, and 1.00:1) plus reverse, with all acceleration provided by gearing; its fluid coupling did not multiply the engine output as a torque converter does. (In this way, it was less sophisticated than the 1924 Vulcan (Vulcan-Werke Hamburg und Stettin ) prototype, which had a torque converter.) It incorporated a parking pawl which was engaged when the shift selector was placed in reverse with the engine off. There was no separate Park position as found with modern transmissions.

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