The hydraulic torque converter originated as a hydraulic speed transformer to provide speed reducing gears for marine steam turbines. This was patented in 1905 by Dr Hermann Föttinger and was known as the Föttinger Transformer. The development of helical reduction gearing for steam turbines by Britain's Charles Parsons made the hydraulic speed transformer redundant and other applications for the device were investigated. Föttinger designed a transmission for a 1600 hp locomotive in 1926. This transmission employed two torque converters and a fluid coupling operating in sequence. Although the locomotive was never constructed, the design principle was later used in transmissions manufactured by Voith.

During the late 1920's other engineers such as Sweden's A J R Lysholm and Britain's Coates interested themselves in the development of the hydraulic torque converter. Lysholm developed the multi-stage torque converter, while Coates designed a torque converter with pivoted guide vanes that automatically adapted themselves to the direction of flow of the transmission fluid. Alf Lysholm (1893-1973), was Chief Engineer of Swedish engineering company, Ljungstroms Angturbin AB, and these early torque converters were based on many of his developments. Lysholm made several inventions including, a turbine blade with a thick, rounded inlet edge. This was intended for use in steam turbines, but found even wider application in hydraulic torque converters. This new turbine blade was used in hydraulic transmissions for buses, railcars and locomotives and notably in military vehicles manufactured during World War II. The Lysholm-Smith type torque converter found great acceptance in the USA where it was the subject of further development. These transmissions were generically known as the "twin disc" type.

In 1933, a twin-engined, bogie rail car with hydraulic transmissions began operating for the London Midland and Scottish Railway (LMS) subsidiary in Northern Ireland in 1933. This vehicle used underfloor Leyland engines coupled to Lysholm-Smith hydraulic torque converters. LMS also built an experimental three-car articulated rail car train in 1938, using the same engines and transmissions. It is probable that the engines and transmissions used in these vehicles were the same as those used in NSW Rail Motor No.38 and the later 400 Class.


The fluid coupling uses a transmission fluid as the medium to transmit power from the engine to the drive train. The coupling consists of two sets of facing rotating vanes or blades encased in an sealed housing containing the transmission fluid. One set of vanes, called the pump or impeller, is connected to the engine by the input shaft, while the other, called the turbine, is connected to the drive train by the output shaft. There is no physical connection between the impeller and the turbine. Rotation of the impeller imparts torque (or energy in the form of motion) to transmission fluid which in turn causes the turbine to rotate. In a fluid coupling the torque delivered to the turbine is equal to the torque absorbed by the pump. Efficiencies with this form of coupling can reach in excess of 90%. Excess energy is generated in the form of heat in the transmission fluid and the fluid is passed through a heat exchanger to provide cooling and maintain the viscosity of the fluid.


Like the fluid coupling, the torque converter uses a transmission fluid as the medium to transmit power from the engine to the drive train. The energy in the moving fluid in turn causes a turbine that is attached to the vehicle's drive train to rotate thus providing the power to move. The principle difference to the fluid coupling is that in addition to the pump (impeller) and turbine, the basic torque converter has a fixed set of vanes known as the reactor that causes the reaction torque (that is fluid returning from the turbine) to be returned back to the turbine. This torque is in addition to the input torque and amplifies the power being imparted to the turbine. In later models, a freewheeling set of vanes or blades, called a stator, is located between the impeller and the turbine, and redirects the moving fluid back onto the turbine in place of the reactor. The faster the engine turns the impeller, the more torque is applied to the fluid resulting in more power being output to the turbine and then to the drive train. The torque converter provides an infinite range of ratios between the input and the output speeds of the transmission. Once the rotational speed of the turbine approaches that of the impeller, the effectiveness of the torque converter diminishes.

Section showing a simple torque converter


Hydraulic transmissions, such as those used in rail car applications, generally consist of a torque converter to get the vehicle moving and then either a mechanical direct drive mechanism or fluid coupling for higher speed operations. Hydraulic transmissions for high speed applications may have more than one fluid coupling.

Direct drive is where the torque converter part of the transmission is bypassed and the engine drives the output shaft directly. This procedure is termed "lock up" and is achieved by two sets of clutch plates that engage either converter mode or direct drive mode. Hence the term "twin disc". In early transmissions, lock up was a manual process that was performed by the driver, while in later transmissions, lock up is automatically actuated by a Woodward governor that is controlled from the road speed of the vehicle. Most rail car transmissions of this type have three positions, neutral, converter and direct drive. The neutral position disconnects the transmission from the engine and enables the vehicle to remain stationery without causing the torque converter to rotate and generate heat in the transmission fluid. When the transmission is engaged with the vehicle stationery, this is called the "stall" state. The neutral position also enables the vehicle direction to be reversed without impact on the transmission. The torque converters used in the 100 Class (Allison Model TCLA 965) and the 600 Class (Allison Model TCLA 655) did not have a neutral position and required the final drive to moved to the neutral position if they were left standing in the "stall" position for longer than 5 minutes. The engines then had to be shut down to engage the final drive for the direction of travel before proceeding. Hydraulic transmissions also feature a freewheel arrangement that disconnects the transmission from the engine in direct drive when the vehicle's road speed exceeds the speed of the engine. This feature prevents damage to the engine and transmission by over speeding. Transmissions of this type were produced in the USA by Twin Disc Incorporated and the Allison Transmission Division of General Motors. Twin Disc Incorporated designs have been built under licence by the Niigata Converter Company (Japan) and by Rolls-Royce (Great Britain).

The Voith transmissions used in NSW applications featured the torque converter and fluid coupling arrangement. Both the Voith T113r, T211r and T311r models used in NSW had one torque converter and one fluid coupling. The advantage of the fluid coupling over direct drive mechanism is the much reduced rates of wear on both the transmission itself and the rest of the drive train. Voith transmissions incorporate an integrated reverse gear train. The latest Hunter rail cars use the Voith T312r which features a torque converter and two fluid couplings for improved high speed operation.

Reversing using hydraulic transmissions is achieved in one of three ways - either:

  1. by using a reversing final drive
  2. by using a reversing gearbox associated with the transmission
  3. by having an integrated reversing gear train inside the transmission itself.

The early hydraulic transmissions (the Voith-Sinclair, Leyland Lysholm-Smith, Twin Disc DFF 10024, Allison TCLA 655 and TCLA 965 and Torcon) all used a reversing final drive. The Twin Disc DFFR 10034, Rolls-Royce DFR 11500 and Niigata DBRG2115 and DAFRG2001 (all being basic Twin Disc designs) used a reversing gearbox, while the Allison RC3 and Voith transmissions used an integrated reversing gear train.