Differential steering

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Transmission (foreground) and engine (background) of a Centurion tank

Differential steering is the means of steering a land vehicle by applying more or less drive torque to one side of the vehicle than the other.[1] Differential steering is the primary means of steering tracked vehicles, such as tanks and bulldozers, is also used in certain wheeled vehicles commonly known as skid-steer, and even implemented in some automobiles, where it is called torque vectoring, to augment steering by changing wheel direction relative to the vehicle. Differential steering is distinct from torque steer, which is usually considered a negative side effect of drive-train design choices.

History[edit]

Hornsby tractor, 1909

A British agricultural company, Hornsby in Grantham, developed a continuous track, which was patented in 1905.[2] The Hornsby tractors featured a track-steer clutch arrangement.

Mechanisms[edit]

There are several mechanisms that have been developed to vary the torque applied to different sides of a vehicle. These include clutch-brake steering, braked-differential steering, controlled-differential steering, geared steering, Maybach double-differential steering, double-differential steering, triple-differential steering,[1] hydraulic,[3] and electric.

Clutch-brake[edit]

In clutch-brake differential steering, power is disconnected to one side or the other with a clutch, and the unpowered side may also have a brake applied to tighten the turn.[1] This method is simple to implement but inefficient and only suitable for light vehicles.[1][4]

A Gehl skid-steer loader

Braked-differential[edit]

In brake-differential steering, power is applied to both sides through a differential and a brake is applied to one side or the other. The slowing of one side causes the other side to speed up, because of the differential, and so the vehicle maintains a constant speed. A subsequent disadvantage is that changes in rolling resistance or traction from one side to the other automatically causes the vehicle to steer unless counteracted by the driver.[1]

Controlled-differential[edit]

In controlled-differential steering, pinions within the differential are locked causing one side to rotate faster than the other. An advantage is that no power is lost to braking. A disadvantage is that only one turning radius is possible. This method was developed by the Cleveland Tractor Company in 1921 and called the Cletrac Regenerative Steering System.[1]

Geared[edit]

In geared differential steering, two complete gearboxes are used to provide power to each side, and one distinct turning radius can be derived from each gear ratio. The main disadvantage to this system is that it doubles the size and weight of the total transmission and therefore it has only been implemented experimentally.[1]

Maybach double-differential[edit]

Differential steering mechanism, either double-differential minus the clutches, or triple-differential minus the brakes

In the Maybach double-differential, power is transmitted to each side through a single, main transmission and then through an epicycle gear on each side. A different drive speed is caused on one side or the other by feeding torque from a separate transmission into one epicycle gear or the other. Each gear ratio of this second transmission generates a distinct turning radius. This system was implemented on German Panther tanks during World War II.[1]

Double-differential[edit]

In double-differential steering, power from a second transmission is fed an epicycle gear in the final drive of one side or the other, as in the Maybach double-differential system, but constant drive speed is achieved by adding an idler to apply the opposite torque to the epicycle gear on the other side. This system was developed in 1928 by Major Wilson.[1]

Triple-differential[edit]

Triple-differential steering is similar to double-differential steering except that brakes are used instead of clutches.

Hydraulic[edit]

Hydraulic differential steering consists of a hydraulic drive system with one hydraulic pump and two hydraulic motors, one for each side.[3] This system is often employed on skid-steer loaders and zero-turn mowers.

Electric[edit]

A three-wheeled differentially steered robot

Electric differential steering consists of two electric motors, one for each side, that are driven at different speeds, depending on steering needs. It is often implemented in wheeled robots.

Hand[edit]

Most traditional wheelchairs are manuvered by differential steering when propelled by the occupant.

Turning radius[edit]

Depending on implementation, friction between drive mechanism and ground, and available power, a vehicle with differential steering may have a zero turning radius or a curb-to-curb turning circle equal to the length of the vehicle by driving each side at the same speed but in opposite directions. This is also called a neutral turn.[5] Vehicles on which only one drive wheel on each side is rigidly aligned and all others are free to caster, such as wheelchairs and wheeled robots, require the least power to turn. Vehicles with long continuous tracks on each side which must slide on the ground in order to turn at all require more power.

Examples[edit]

The German Solo 750 (WP:DE)
A Toro Z Master Commercial Zero-Turn mower

See also[edit]

References[edit]

  1. ^ a b c d e f g h i Phillip Edwards (September 1988). "Differentials, the Theory and Practice". Constructor Quarterly. Retrieved 2017-11-13.
  2. ^ British Patent No. 16,345 (1904)
  3. ^ a b Karim Nice. "How Caterpillar Skid Steer Loaders & Multi Terrain Loaders Work". How Stuff Works. Retrieved 2017-11-23.
  4. ^ Stuart J. McGuigan; Peter J. Moss (November 1998). "A Review of Transmission Systems for Tracked Military Vehicles". Journal of Battlefield Technology. Retrieved 2017-11-23.
  5. ^ Michael Green, James D. Brown (2008). Tiger Tanks at War. MBI Publishing Company. p. 46.