Hydra-Matic WE have been producing the Hydra-matic trans- mission since 1939. During the years since that time, much has been said about its operation. I thought I would not attempt to describe the operation of the Hydra-matic, but would rather tell you briefly of its history, of some of the fea- tures that have made for its success, and of the improvements that have just appeared or are about to appear in the transmission. The original design of the transmission that eventually turned out to be the Hydra-matic started in 1932. The first attempt was to build a single-unit automatic underdrive to attach to the back end of a synchromesh transmission. It soon became apparent to us that the best way to do the automatic transmission job was to start with a complete redesign of the clutch and the transmis- sion. Following along these lines, General Motors introduced a semiautomatic transmission in 1937. This was a four-speed power-shifted device, but it retained the well-known friction clutch and clutch pedal. Further development produced the fluid flywheel combined with the fully automatic four-speed transmission, which was put into production in the 1940 Oldsmobile and named the Hydra-matic. In 1941, Cadillac adopted the transmission as optional equipment. Next month, we will deliver to a car manufac- 128turer our three-millionth production Hydra-matic. Three million automatic transmissions are a lot. Installed in cars, the cars would fill a three-lane highway bumper to bumper from Boston to Los Angeles. The weight of these finished transmis- sions exceeds 375,000 tons. In addition to the above, we have built 85,000 military-type Hydra- matics for installation in tanks, armored cars, and amphibious vehicles. IN this paper, the author tells briefly of the history of the Hydra-matic, of some of the features that have made for its success, and of the improvements that have just appeared or are about to appear in the transmission. The latter include a new type of reverse and a new variable-capacity vane type of pump. W. B. HERN DON (M '48) has been chief engineer of the Detroit Transmission Division, General Motors Corp. since 1949. He received.his B.S. degree in mechanical en- gineering from the State College of Washington in 1928, and his M.S. degree from the University of Michigan in 1930. SAE Quarterly TransactionsDownloaded from SAE International by University of Leeds, Monday, August 13, 2018Transmission Walter B. Herndon, Detroit Transmission Division, GMC This paper was presented at the SAE Summer Meeting, French Lick, Ind., June 4, 1951 We are now building 13 different models of Hydra-matics, 11 for passenger car and bus use and two for military use. In addition to our General Motors divisions, Cadillac, Oldsmobile, Pontiac, GM Truck and Coach, and GM of Canada, we are now supplying Hydra-matic transmissions to Lincoln, Hudson, Nash, and Kaiser-Frazer. There have been and are many other requests for our Hydra-matic, both from this country and from Europe. We are continually working with new applications. We are now producing some 3400 units a day and our division is now experiencing a large-scale expansion program. Since the time that the Hydra-matic first went into production, our engineering department has been continually striving to build a better and more easily produced automatic transmission at lower cost. We have built many experimental transmissions - transmissions that were similar to many of the types that are on the market today. However, we have not yet found any other type of automatic transmission that combines the advantages in pleasibility, efficiency, economy, performance, and adaptability to high-volume pro- duction of the Hydra-matic. The fundamental design of the first Hydra-matic is still the basic design today and the features of that design are still definite advantages. Let's look at a few.First, we have split torque. In the Hydra-matic, after the car is in third gear, engine torque is transmitted to