M47 tank at the Harel Memorial, close-up 2

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

tracked vehicle Continuous track is 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 weight of the vehicle ...

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) consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates the planet and sun ge ...

s, * a differential, * a

hydraulic pump Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy (hydrostatic energy Fluid statics or hydrostati ...

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 broadl ...

powered by the hydraulic pump, * hydraulic control valves. A steering transmission combines the two functions needed for a tracked vehicle transmission: a

transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...

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 good battlefield mobility provided by tracks and a powerful en ...

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 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 torque converter connects the pow ...

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 gearbox ...

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 l ...

" 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 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: ''KPz 70 or KpfPz 70'') was an American–West German 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 ...

and XM1 tanks, then later adopted in the

M1 Abrams The M1 Abrams is a List of main battle tanks by generation, third-generation American main battle tank designed by Chrysler Defense (now General Dynamics Land Systems) and named for General Creighton Abrams. Conceived for modern Armoured warfa ...

. 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.

References

{{Reflist, refs= {{Cite book , work=Technical Manual , title=Principles of Automotive Vehicles , id=TM-9-8000 , publisher=US Department of the Army , date=29 October 1985 , chapter=24: Cross-Drive Transmission , url=http://automotiveenginemechanics.tpub.com/TM-9-8000/TM-9-80000502.htm {{Cite journal , date=1 February 1977 , issue=770339 , title=The New X1100 Automatic Transmissions for the XM1 Tank , journal=SAE Technical Papers , doi=10.4271/770339 , url=http://papers.sae.org/770339/ , first1=J. W. , last1=Schmidt , first2=G. L. , last2=Hadley , series=SAE Technical Paper Series , volume=1 , quote=Abstract: The X1100 is a fully automatic shifting transmission which has been designed and developed for vehicles in the 49 to 60 ton class, operating at speeds of 40 to 50 mph. A modular design is utilized to provide application flexibility for diesel or turbine engines of 1300 to 1500 GHP, as well as adaption to the current M60 vehicle. This automatic transmission features a hydrostatic steer system with pivot steer, a four speed range pack, integral power brakes and a high speed reverse. The torque converter can be locked up in all gear ranges to provide optimum transmission performance. {{Cite book , work=Technical Manual , title=Principles of Automotive Vehicles , id=TM-9-8000 , publisher=US Department of the Army , date=29 October 1985 , chapter=25: X1100 Series Cross-Drive Transmission , url=http://automotiveenginemechanics.tpub.com/TM-9-8000/TM-9-80000523.htm Automotive transmission technologies Automotive steering technologies