Hydramatic - Design

Design

The Hydramatic used a two-element fluid coupling (not a torque converter, which has at least three elements, the pump, turbine and stator) and three planetary gearsets, providing four forward speeds plus reverse. Standard ratios for the original Hydra-Matic were 3.82:1, 2.63:1, 1.45:1 and 1.00:1 in automotive applications, and 4.08:1, 2.63:1, 1.55:1 and 1.00:1 in light truck and other commercial applications. The Jetaway Hydramatic used 3.96:1, 2.55:1, 1.55:1, and 1.00:1.

The Hydramatic was fitted with two pumps to pressurize its hydraulic control system and provide lubrication of internal components. The front pump was a variable displacement vane unit driven from the fluid coupling housing, which meant oil pressure would be available immediately upon starting the engine. A relatively constant pressure was maintained by moving a slide inside the pump, which had the effect of changing the pump's displacement and therefore the volume of oil being delivered.

The rear pump was an unregulated spur gear type driven from the transmission output shaft, which meant it was capable of pressurizing the transmission if the vehicle was in motion. This feature made it possible to push-start a vehicle with a dead battery if the vehicle could be accelerated to at least 15–20 mph (24–32 km/h). At higher speeds, the rear pump provided all the oil volume that was needed to operate the transmission and the front pump's slide was nearly centered, causing that pump to produce little output.

In first gear, power flow was through the forward planetary gear assembly (either 1.45:1 or 1.55:1 reduction, depending on the model), then the fluid coupling, followed by the rear gear assembly (2.63:1 reduction) and through the reverse gear assembly (normally locked) to the output shaft. That is, the input torus of the fluid coupling ran at a lower speed than the engine, due to the reduction of the forward gear assembly. This produced an exceptionally smooth startup because of the relatively large amount of slippage initially produced in the fluid coupling. This slippage quickly diminished as engine RPM increased.

When the transmission upshifted to second gear, the forward gear assembly locked and the input torus now ran at engine speed. This had the desirable effect of "tightening" the coupling and reducing slippage, but unfortunately also produced a somewhat abrupt shift. It wasn't at all uncommon for the vehicle to lurch forward during the 1-2 shift, especially when the throttle was wide open.

Upon shifting to third, the forward gear assembly went back into reduction and the rear gear assembly locked. Due to the manner in which the rear gear assembly was arranged, the coupling went from handling 100 percent of the engine torque to about 40 percent, with the balance being handled solely by the gear train. This greatly reduced slippage, which fact was audible by the substantial reduction that occurred in engine RPM when the shift occurred.

The shift from third to fourth gear locked the forward gear assembly, producing 1.00:1 transmission. The fluid coupling now only handled about 25 percent of the engine torque, reducing slippage to a negligible amount. The result was a remarkably efficient level of power transfer at highway speeds, something that torque converter equipped automatics could not achieve without the benefit of a converter clutch.

Many Hydramatics did not execute the 2-3 shift very well, as the shift involved the simultaneous operation of two bands and two clutches. Accurate coordination of these components was difficult to achieve, even in new transmissions. As the transmission's seals and other elastomers aged, the hydraulic control characteristics changed and the 2-3 shift would either cause a momentary flare (sudden increase in engine speed) or tie-up (a short period where the transmission is in two gears simultaneously), the latter often contributing to failure of the front band. Much of the difficulty in staging a "clean" 2-3 or 3-2 shift the any cast iron Hydramatic was the changing elasticity of the governing springs in the valve bodies. Even ambient temperature would affect this variable, so that a Hydramatic that would shift perfectly on a summers day would usually exhibit 2-3 "flare" when cold. Only the most knowledgeable and experience Hydramatic technicians, using specifications for spring length and tension from the factory, could even approach getting the "2-3" and "3-2" shifts to be even near perfect. Another long-standing driver complaint would be "flair" when trying to get a "3-2" downshift when going around a corner, which usually resulted in a neck snapping jolt upon band application.

From 1939-1950, the reverse anchor was used to lock the reverse unit ring gear from turning by engaging external teeth machined into that ring gear. From 1951 on, a cone clutch did the same thing when oil pressure was up, and a spring-loaded parking pawl was allowed to lock the same ring gear in the absence of oil pressure. This worked better as the anchor would not grind on the external teeth if that ring gear were turning (that is, unless the engine stalled as reverse was engaged). Reverse was obtained by applying torque from the front unit (band on, in reduction) through the fluid coupling to the rear unit sun gear. The planet carrier of this gearset was splined to the planet carrier of the reverse unit. The rear unit ring gear hub had a small gear machined on its end which served as the reverse unit sun gear. Because the rear unit band was not applied for reverse, the rear unit and reverse unit compounded causing the combined planet carriers to rotate opposite to the input torque and at a further reduced speed. The output shaft was machined onto the rear unit and reverse unit planet carriers.

Shutting off the engine caused the transmission oil pressure to rapidly dissipate. If the selector lever was in reverse or moved to reverse after the engine stopped, two mechanical parts combined to provide a parking brake. The reverse unit ring gear was held stationary by the reverse anchor. The drive shaft could still turn causing the reverse unit sun gear and attached rear unit ring gear to rotate at a very high speed, were it not for the fact that the rear unit ring gear band was now applied by a heavy spring. Usually, bands are applied by a servo and released by spring pressure, but in this case, the band was held off by the servo and applied by spring pressure (actually, when the engine was running, the band was applied by a combination of spring pressure assisted by oil pressure). With the engine off, this brake band acting on the rear unit ring gear had a tremendous mechanical advantage. Since the rear unit ring gear with its attached reverse unit sun gear and the reverse unit ring gear were both locked to the transmission case, the planet carriers and driveshaft could not turn. As such, it provided an effective driveshaft mounted parking brake to be used alone or supplementing the hand brake.

The first-generation Hydramatic (not the Jetaway version that succeeded it in 1956) did not have a separate park position as found in modern automatic transmissions. The driver had to shut off the engine and then place the transmission in reverse in order to lock the driveline to prevent the car from moving. Also, the original Hydramatic required periodic band adjustments as a routine maintenance item that later versions did not. Early 1940 model Oldsmobiles with Hydra-Matic Drive could be started with the transmission selector lever in any position. The car would then start to move, unless the transmission lever had been left in N, neutral.

The all cast-iron Hydramatic was the heaviest automatic transmission ever produced for automobiles. The heaviest of them all was the Truck Hydra-Matic version offered by GM Truck and Coach Division in its line of light- and medium-duty trucks and conventional buses, as well as with its transverse mounted gas L6 engined transit buses produced until 1963. That particular version weighed in at an incredible 655 pounds, when equipped with the angle drive for the transit bus application, while the ¾ ton and up pickup truck model (HM270) still tipped the scale at a solid 435 pounds. When coupled to GMC's heavy V6 powerplant of 1960-1962, the powertrain weight was not too much lighter than the weight of the entire body of a ¾ ton P-2500 model pickup truck. Even its successor, the Controlled Coupling Hydramatic was reviled by shop mechanics having to remove or reinstall such a unit, as they, too, were quite heavy when compared to other contemporary units. In the end, the true Hydramatic was rendered obsolete because of its cost, both in raw materials used as well as the machining needed. The successor, Turbo Hydramatic, was a much simpler, lighter and cheaper, if less efficient, transmission.