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Rolls-Royce built “Hydra-matic” Automatic Transmission
Page 1 of 4 The four speed automatic transmission fitted to the Silver Cloud and other Rolls-Royce and Bentley models owed its origins to General Motors in the United States. The Hydramatic transmission was the world’s first mass produced fully automatic gearbox. G.M. tested their first prototypes in 1938. After further refinement the transmission was finally launched late in 1939 with some fanfare and became known as “Hydramatic drive”. Initially it was fitted as an option to Oldsmobile’s 1940 models costing around £12. Within a year of the launch of “Hydramatic drive” Cadillac offered it as an option on their cars. Pontiac not wanting to be out done by the competition also started fitting the Hydramatic shortly afterward. 200,000 Hydramatic transmissions were built before the 2nd world war stopped production. The tooling was put away until hostilities ceased. By 1946 Hydramatics were once again in production.
Rolls-Royce knew they must offer an automatic transmission as quickly as possible. The design department at Crewe were instructed to evaluate the very few automatic transmissions available on the world market in the late 1940’s. All the transmissions were American in origin and varied greatly in quality. Having gained an insight into the complexities of producing such a transmission, Rolls-Royce came to the speedy conclusion they could not afford to design an automatic transmission of their own. The General Motors Hydramatic transmission had proved to be the very best fully automatic transmission available, and it was amazingly cheap. A deal was struck with G.M. giving Rolls-Royce a license to build the Hydramatic transmission with modifications to suit the rationalised range of post war chassis then in production. 1952 saw the very first Hydramatic transmissions fitted to Rolls-Royce cars. The first 100 transmissions were bought in direct from G.M. These transmissions were fitted with makers plates which read; ‘G. M. Detroit Transmission Division – Built for Rolls-Royce Ltd’. Once at Crewe they were modified to Rolls-Royce specification with the addition of a modified tail housing, and output shaft. These housed and powered the ride control pump and fiction servo for the brakes. Before long, Rolls-Royce had tooled up to manufacture the transmission themselves. The necessary machine tools were made and or purchased and a clean room was built in the factory at Crewe to facilitate assembly. In basic terms the transmission consists of a fluid coupling which transmits the engine torque through 3 epicyclic gear trains to the propeller shaft. The fluid coupling is driven by the Torus cover which is bolted to the engine flywheel. The fluid coupling does not transmit all the drive from the engine at low engine speeds. The coupling becomes more efficient as the engine speed rises. This allows drive to be taken up progressively and smoothly as engine power increases, effectively doing the job of the clutch in a manual transmission. Two of the epicyclic gear trains in the transmission provide all four forward ratios. The remaining rear epicyclic gear train is responsible for reverse. Both of the forward drive epicyclic gear trains each provide two ratios, direct drive and reduced ratio drive. When working together the combined ratios of both forward epicyclic trains dictate the ratio of drive to the road wheels. For example;
The gear changes are controlled by a valve block which directs high pressure fluid from the main fluid pump to the clutches and brake band operating servos. The automatic changes are also controlled by hydraulic pressure which is provided by a second pump which is driven by the output shaft. A governor assembly works together with the pump to provide a signalling pressure directly proportionate to the road speed of the car. This signalling pressure is used in the valve block to operate the shift valves which determine which gear ratio is appropriate for a given load and road speed. Engine output power is sensed by a valve connected to the engine throttle linkage. This valve alters the signalling pressure in the valve block, causing the gear changes to delay or advance according to engine output. On top of these automatic controls manual controls are also provided to enable the driver to override the system. It is surprisingly difficult to explain how the transmission works, if a more “in depth” understanding is needed an owner should acquire a workshop manual which will provide all the information one could wish for. During the period Rolls-Royce built and fitted the Hydramatic transmission to their own cars very few changes were introduced. The earlier 6 cylinder transmissions run at a lower pressure to the later V8 compatible transmissions. The earlier transmissions also have a lower gear ratio used to drive the friction servo for the brakes. The last Silver Dawn / Bentley R type cars were fitted with Silver Cloud transmissions without ride control pumps. The friction servo drive gear ratio was raised with the advent of the Silver Cloud range. The main fluid pressure was raised with the advent of the V8 engine mainly to help ensure the clutches and brake bands were able to cope with the additional engine torque. An extra clutch plate was also added to the forward clutches to ensure the added engine power could be handled with confidence.
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With the launch of the Bentley MK VI of 1946 behind them, Rolls-Royce ever watchful of other motor manufacturers innovations, very soon became interested in producing a fully automatic transmission of their own. The need to keep pace with the American motor industry had never been greater. The British Government’s ‘Export or die’ campaign after the 2nd world war meant Rolls-Royce could not just supply their traditional home market customers even if sufficient numbers could still be found with the resources to afford the companies products. Only companies who exported their products were allocated steel, therefore if British companies wanted to stay in business they had to produce products that were appealing to export markets as well as the home market. Rolls-Royce needed far greater appeal in the American market. That meant matching the equipment levels provided by U.S. manufacturers which were so enjoyed by American car buyers. Automatic transmission was to become a huge success in the U.S. American buyers loved it, and to prove it automatics soon began to out sell every other type of transmission. 
The engineering department at Crewe tried hard to improve on the G.M. product. Rolls-Royce’s engineers at first scoffed at what they perceived as poor finish on some components. They decided to tighten up the tolerances and improve the finish of all the components. Unfortunately when assembled, these improved parts refused to perform satisfactorily. Rolls-Royce’s engineers went back to G.M. for advice. G.M. told them they had gone through the same problems during their development work. They had found certain components needed a rougher finish to make them work. In the end Rolls-Royce were forced to exactly replicate nearly all of the G.M. parts in order to get their version of the transmission to work properly. G.M. had of course got their transmission right at the outset. The Hydramatic transmission was in fact such a fundamentally sound and successful design, it would take over 10 years for anyone to better it.
Each forward drive epicyclic gear train is controlled by a hydraulically operated clutch and a hydraulically operated brake band. If the clutch is engaged the brake band is released and vice versa. When the brake band is applied the unit is in reduced ratio mode, when the clutch is engaged the brake band is off and the unit is in direct drive mode. 
It is small wonder the Hydramatic transmission was so well liked. Its robust construction and its smooth silent operation made it an immediate success. Rolls-Royce supplied versions of the Hydramatic transmission to the British Motor Corporation who offered it as an optional extra on their Vanden-Plas Princess range of large saloons and limousines. B.M.C. in turn supplied complete power trains to other manufacturers, Jensen being their best customer, fitting the hydramatic to their 541 model. If automatic transmission was specified by any customer requiring one of B.M.C.’s top range power trains, they would have been supplied with a Hydramatic transmission built at Crewe. Rolls-Royce also supplied Hydramatic transmissions to Armstrong Siddeley who offered it as an option on their Star Sapphire cars. 
