GM Roto Hydramatic transmission

From Wikipedia, the free encyclopedia
  (Redirected from Roto Hydramatic)
Jump to: navigation, search

Roto Hydramatic (sometimes spelled Roto Hydra-Matic or Roto-Hydramatic) was an automatic transmission built by General Motors and used on some Oldsmobile and Pontiac and Holden models from 1961-1965. It was based on the earlier, four-speed Hydramatic, but was more compact, providing only three forward speeds plus a small 8" fluid coupling with a stator in place of the Hydramatic's fluid coupling. Oldsmobile, one of the users of this transmission, called the torque converter's stator the "Accel-A-Rotor." The lightweight, aluminum-cased transmission was sometimes nicknamed the "Slim Jim."

There were two models of the Roto Hydramatic: the lightweight Model 5, which weighed 145 lb (66 kg) and had ratios of 3.03, 1.58, and 1.00, and the larger Model 10, which weighed 154 lb (70 kg) and had ratios of 2.93, 1.56, and 1.00. The stator provided a maximum torque multiplication of 1.2:1.

In 1961-62, the "Model 5" was used on the Opel Kapitan, Vauxhall Velox/Cresta and EK Holden. The Roto Hydramatic was cheaper and smoother than the previous Hydramatic, but its slower, softer shifts sacrificed performance for refinement. Owners discovered that it was also less durable than Hydramatic, and was prone to various mechanical problems.

The Roto Hydramatic was phased out after the 1964 model year in favor of the two-speed Super Turbine 300 and three-speed Turbo-Hydramatic. As with previous Hydramatic transmissions, auto safety experts criticized GM for the Hydramatic design which had a shift quadrant sequence of Park-Neutral-Drive-Second-Low-Reverse (P-N-D-S-L-R) due to the placement of reverse adjacent to a forward gear as opposed to the more common P-R-N-D-S-L sequence found in most other automatic transmissions at the time that placed "reverse" between "park" and "neutral", which was also incorporated in the new Turbo Hydramatic design introduced on Buicks and Cadillacs in 1964, and then other GM divisions in 1965.

The Roto Hydramatic was used in all full-sized Oldsmobile models including the Dynamic 88, Super 88, Ninety-Eight and Starfire from 1961-1964 as well as the compact Oldsmobile F-85 from 1961-1963. Pontiac used the Roto Hydramatic from 1961-1964 on its shorter-wheelbase full-sized cars including the Catalina, Ventura and Grand Prix, but continued with the older four-speed Super Hydramatic design in the longer-wheelbase Star Chief and Bonneville models.

In the familiar Oldsmobile and Pontiac shift quadrant, P-R-N-D-S-L or P-N-D-S-L-R, the "S" quadrant was for "Super" and not second. In the earlier four-speed Hydra-Matic, the "S" or "Super" quadrant was actually third gear, allowing 1-2-3 shifts. This nomenclature was also used by Mercedes-Benz with the introduction of the four-speed type-3 automatic transmission. As with the Hydra-Matic, the "L" position was actually 1st and 2nd gears, holding the transmissions from shifting above 2nd gear.

The Roto-Hydramatic did not have a conventional fluid coupling or torque converter but had an internal fluid coupling with a torque multiplier. There was no real bell-housing; instead, the Roto-Hydramatic had a kind of flange connecting the transmission to the engine. The power flow from the engine crankshaft to the transmission input shaft was connected via a flywheel containing a set of springs to absorb torsional vibrations at the flex plate.

The shifting sequence was very different and much rougher than the Hydra-Matics that preceded it, or the Super Turbines and Turbo Hydramatics that followed it. The shift from 1st to 2nd was long and drawn-out, ending with a bang. The reason for this was that three things were happening. Not only was the transmission shifting to a higher range, but it was also simultaneously emptying the fluid coupling and making a mechanical lock-up. The wide gap in the reduction ratio between first and second gears combined with the direct mechanical connection to the engine to create a shift that could be firm enough to lug the engine. Because of the mechanical lock-up, there was sometimes a shudder felt as the transmission shifted into third gear by refilling the fluid coupling. Direct drive (1:1 ratio) is achieved by splitting engine power between the fluid coupling (used for first gear) and the front clutch (used for second gear), which are both applied to provide third gear, effectively locking the planetary gearsets and forcing them to spin at engine speed. The design is unique in that it provides third gear by applying first and second gears at the same time.

By historical context of comparison, the '50 Packard Ultra-Matic which was a torque converter applied two speed lock up design with high and low range for four forward speeds and the '50 Studebaker/Detroit Gear featured a lock up torque converter with four forward speeds. The lock-up feature of Packard's Ultramatic and Studebaker's DG 250 is commonplace in virtually all modern 4+speed, overdrive automatic transmissions.

The smaller model 5 Roto Hydramatic uses the same principles of operation as the model 10. It is 9 lb (4.1 kg) lighter and much shorter in length — a necessary design parameter considering its placement in compact cars. The model 5 is not related to the "Dual Path Dynaflow" transmission used in the Buick Special and Skylark models of 1961-1963. Buick's Dual Path was an air-cooled 2-speed unit with a planetary gearset inside the flywheel-mounted torque converter. The first Pontiac Tempests used a modified version of the Powerglide transaxle shared with the Chevrolet Corvair. But unlike the Corvairs, the TempesTorque was connected via a flexible drive shaft to an unconventional slant-4 (half of a 389 V-8) or an aluminum 215 c.i. V-8, which would be later sold to the Rover Group and be renamed the 3500 V-8. The smaller 61-05 as fitted to the Vauxhall Cresta was dubbed the "smoothamatic from Luton" in Cars Illustrated in Feb 1961 being quite taken with the overall refinement with the test car, also noting that full throttle starts could be made on slushy roads, perhaps giving away the rather tall 1st gear and minimal torque conversion. They also stated that the gearshifts were so smooth as to be practically imperceptible. The magazine The Motor, Dec 1960, also praised the high standard of smoothness with very lively performance. The performance was claimed to be almost as fast as with synchromesh gears and the 2-3 shift could be so gradual in gentle driving that you don't always realise it has happened. Certainly, the examples I drove were equal to this and it seemed disappointing when the 61-05 model was discontinued in favour of the Powerglide in 1965.

The Roto Hydramatic was a 3-speed transmission, and both the 5 and 10 models worked in a similar way. The mechanical part was based on the old turboglide layout with the original converter changed to a more normal item,but was still unique,as it only operated in converter mode in 1st gear. 2nd gear was totally mechanical with the converter drained of oil as a multi plate clutch took the place of the 2nd turbine in the Turboglide. Third gear filled the converter while the rear planetary set was released, the 2nd gear clutch still in operation.

A diagram is helpful in understanding the above, of the Roto Hydramatic, and a Turboglide section shows the similarity.

Notes[edit]