A cross-drive steering transmission is a transmission, used in

tracked vehicle Continuous track or tracked treads are a system of vehicle propulsion used in tracked vehicles, running on a continuous band of treads or track plates driven by two or more wheels. The large surface area of the tracks distributes the we ...

s to allow precise and energy-efficient steering. It consists of the following main parts: * two identical single-stage

planetary gearing An epicyclic gear train (also known as a planetary gearset) is a gear reduction assembly consisting of two gears mounted so that the center of one gear (the "planet") revolves around the center of the other (the "sun"). A carrier connects the ...

s, * a differential, * a

hydraulic pump A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy ( hydrostatic energy i.e. flow, pressure). Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. They gen ...

connected to the engine (similar to an oil pump of cars), * a

hydraulic motor A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder as a linear actuator. Most broadly, ...

A steering transmission combines the two functions needed for a tracked vehicle transmission: a

transmission Transmission or transmit may refer to: Science and technology * Power transmission ** Electric power transmission ** Transmission (mechanical device), technology that allows controlled application of power *** Automatic transmission *** Manual tra ...

to couple the relatively constant engine speed to varying road speeds and a steering gearbox to drive the two output shafts (and so the tracks) at different speeds, thus steering the vehicle. Cross-drive transmissions developed after WWII, at a time when

tanks A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and battlefield mobility provided by tracks and a powerful engine; t ...

were growing larger and heavier. A growing problem with earlier gearboxes had been the amount of heat dissipated in the brakes used for steering. As tanks grew heavier, a more efficient system was required not just for efficiency (i.e. more available power from the same engine) but also to reduce the wasted power having to be disposed of through heat and wear in the steering brakes. Cross-drive transmissions were developed from the earlier controlled-differential steering gearboxes. Their innovation was to cross-couple the two sides, so that excess power from the slow side of the gearbox could be supplied to the faster side. This reduced the power to be dissipated by braking and increased the power available to drive the fast track. Implementing this was difficult for most transmissions, as it required a varying gear ratio between the sides. Most gearboxes simply offered a single ratio, through fixed gears and clutches, which restricted steering to a single radius. The particular innovation of the cross-drive, over earlier attempts, was to use a

torque converter A torque converter is a device, usually implemented as a type of fluid coupling, that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the ...

in the cross shaft. This allowed a continuously variable ratio between the two sides and so more precise steering. Torque converters are also easier to make for the high torques required in tank design, saving weight and cost. Cross-drive transmissions also improved the drivability of the manual transmissions then in use. Manual transmissions were heavy and tiring to drive, requiring skill and strength from the driver, and also frequent adjustments as the clutch plates wore. The

pre-selector gearbox A preselector gearbox is a type of manual transmission mostly used on passenger cars and racing cars in the 1930s, in buses from 1940–1960 and in armoured vehicles from the 1930s to the 1970s. The defining characteristic of a preselector gearb ...

had been used to simplify the driving task, but at the cost of mechanical complexity. The cross-drive transmissions offered a mechanically actuated self-changing gearbox, where the ratio selection could be either manual, automatic, or by pre-selection. The control input for steering also became lighter, allowing the use of a single hand-controlled joystick or "wobble stick", as used for the CD-850-1 transmission in the T44 Cargo Tractor of 1950. The construction of these transmissions also integrated the transmission and steering gearbox functions into a single casing. This gave reduced overall weight and volume and reduced servicing times. A related development at this time was the "

power pack Power Pack is a superhero team consisting of four young siblings appearing in American comic books published by Marvel Comics. Created by writer Louise Simonson and artist June Brigman, they first appeared in their own series in 1984, which las ...

" concept, where engine and transmission could be removed as a single unit. As size and complexity increased, fitter time became short, but mechanical handling through

mobile cranes A mobile crane is a cable-controlled Crane (machine), crane mounted on Continuous track, crawlers or rubber-tired carriers or a hydraulic-powered Crane (machine), crane with a telescoping boom mounted on truck-type carriers or as self-propelled ...

became available in the field. It was now quicker and simpler to exchange one large unit than to dismantle and re-assemble more connections to remove a smaller component.

X1100

A later design of cross-drive transmission, the Allison X1100, was used in the 1970s experimental US

MBT-70 The MBT-70 (German: or ) was an American–West German cancelled joint project to develop a new main battle tank during the 1960s. The MBT-70 was developed by the United States and West Germany in the context of the Cold War, intended to counte ...

and XM1 tanks, then later adopted in the

M1 Abrams The M1 Abrams () is a third-generation American main battle tank designed by Chrysler Defense (now General Dynamics Land Systems) and named for General Creighton Abrams. Conceived for modern armored ground warfare, it is one of the heavies ...

. This adopts a different principle for the steering cross-coupling: instead of a hydro-dynamic torque converter, it uses a hydrostatic combination of a

and a

. The X1100 was designed as a modular system, allowing its easy adaption to vehicles with different power plants, ranging from diesels to gas turbines. The central module is matched to the engine driving it, the outer steering modules to the weight and speed of the vehicle.

Notes

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References

Automotive transmission technologies Automotive steering technologies