RHM meaning in General ?

Roto Hydramatic (sometimes spelled Roto Hydra-Matic or Roto-Hydramatic) was an automatic transmission built by General Motors and used on some Oldsmobile, 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 inside of the fluid coupling (a type of torque converter, only integral with the transmission rather than being an independent unit). Oldsmobile, one of the users of this transmission, called the fluid couplings stator the "Accel-A-Rotor." The lightweight, aluminum-cased transmission was sometimes nicknamed the "Slim Jim." HydraMatic Division calls the Roto a four range, three gear HydraMatic. It counts the stator multiplication at 3.50:1 as a first gear, and when road speed and the two coupling halves speed match, it counts the same gear with fluid now passing straight through the stator as 2nd gear at 2.93 to one. Second gear (or third range) has a ratio of 1.56 and because the fluid coupling is drained for this gear ratio making the front clutch apply makes this a rare automatic that is in FULL mechanical lock-up (coupling drained) in second gear. Fourth range the coupling fills releasing the front clutch makes a ratio of 1 to 1. This transmission, like single and dual range, and dual coupling hydramatics also have the feature of split torque in the transmission whereby in fourth or high gear only 40-to 50% depending on transmission, 40% in Roto's case, but because of the design the coupling is only required to carry 40% of the engine torque. The rest is (60%) is in full mechanical connection making these hydramatics the most efficient automatic until the wide spread use of the lock-up torque converter.

Roto's downfall was the 2-3 range or 1-2 gear change because it is not only a huge ratio jump from 2-3 range or 1-2 shift from 2.93 to 1.56, but also there is no fluid slippage in the coupling because the coupling drains (4 tenths of a second) to engage or apply the front clutch and so the trans goes to full mechanical connection. The other downfall of Roto HydraMatic was the extreme oil pressures in the small 8" fluid coupling which caused transmission leaks. All HydraMatic transmissions suffer some shift quality with today's ATF fluid. The original Type "A" fluid that these transmissions take is available only at O'Reilly Auto Parts.

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 3.50, 2.93, 1.56, and 1.00.