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A typical continuously tracked vehicle
Closeup of a small bulldozer's tracks

Continuous tracks are large (modular) tracks used on crawler-type tanks, construction equipment and certain other off-road vehicles. Unlike the Kégresse tracks which use a flexible belt, most continuous tracks are made of a number of rigid units that are joined to each other. The tracks help the vehicle to distribute its weight more evenly over a larger surface area than wheels can. Tracks do this because as the tracked vehicle moves forward the segments are laid out flat on the ground at the front and are picked up again at the back. The segments in between the front and the back end carry load too as they are supported by rollers. This keeps it from sinking in areas where wheeled vehicles of the same weight would sink. The seventy-ton M1 Abrams tank has an average ground pressure of just over 15 psi (100 kPa). Since tire air pressure is approximately equal to average ground pressure, a typical car will have an average ground pressure of 28 psi (190 kPa) to 33 psi (230 kPa).

Contents

History

Lombard Steam Log Hauler (Designed, patented 1901)
David Roberts tracked steam tractor
Hornsby tractor

A crude continuous track was designed in the 1770s by Richard Lovell Edgeworth. Polish mathematician and inventor Józef Maria Hoene-Wroński conceived of the idea in the 1830s.[1] The British polymath Sir George Cayley patented a continuous track, which he called a "universal railway" (The Mechanics' Magazine, 28 January 1826). In 1837, a Russian inventor Dmitry Zagryazhsky designed a "carriage with mobile tracks" which he patented the same year, but due to a lack of funds he was unable to build a working prototype, and his patent was voided in 1839. Steam powered tractors using a form of continuous track were reported in use with the Western Alliance during the Crimean War in the 1850s. An "endless railway wheel" had been patented by the British engineer James Boydell 1846.[2]

An effective continuous track was invented and implemented by Alvin Lombard for the Lombard Steam Log Hauler. He was granted a patent in 1901. He built the first steam-powered log hauler at the Waterville Iron Works in Waterville, Maine, the same year. In all, 83 Lombard steam log haulers are known to have been built up to 1917, when production switched entirely to internal combustion engine powered machines, ending with a Fairbanks diesel powered unit in 1934. Undoubtedly, Alvin Lombard was the first commercial manufacturer of the tractor crawler. At least one of Lombard's steam-powered machines apparently remains in working order.[3] A gasoline powered Lombard hauler is on display at the Maine State Museum in Augusta.

In addition, there may have been up to twice as many Phoenix Centipeed versions of the steam log hauler built under license from Lombard, with vertical instead of horizontal cylinders. In 1903, the founder of Holt Manufacturing, Benjamin Holt, paid Lombard $60,000 for the right to produce vehicles under his patent. There seems to have been an agreement made after Lombard moved to California, but some discrepancy exists as to how this matter was resolved when previous track patents were studied. Popularly, everyone claimed to have been inspired by the dog treadmill used on farms to power the butter churn, etc. to "invent" the crawler on their own, and the more recent the history, the earlier this supposed date of invention seems to get.

At about the same time a British agricultural company, Hornsby in Grantham, developed a continuous track which was patented in 1905.[4] The design differed from modern tracks in that it flexed in only one direction with the effect that the links locked together to form a solid rail on which the road wheels ran. Hornsby's tracked vehicles were given trials as artillery tractors by the British Army on several occasions between 1905 and 1910, but not adopted. The patent was purchased by Holt. The Hornsby tractors featured the track-steer clutch arrangement, which is the basis of the modern crawler operation, and some say an observing British soldier quipped that it crawled like a caterpillar. The word was shrewdly trademarked and defended by Holt.

American James B. Hill, working in Bowling Green, Wood County, Ohio, patented what he termed "apron traction"[5] on September 24, 1907.

Following a merger and name change, The Holt Manufacturing Company became the Caterpillar Tractor Company in 1925. Caterpillar brand continuous tracks have since revolutionized construction vehicles and land warfare. Track systems have been developed and improved during their use on fighting vehicles. During World War I Holt tractors were used to tow heavy artillery by the British and Austro-Hungarian armies, and stimulated the development of tanks in several countries. The first tanks to go into action, built by Great Britain, were designed from scratch and inspired by but not directly based on the Holt, but the slightly later French and German tanks were built on modified Holt running gear.

Perhaps the oldest implementation of something like tracks is to be found in theories of prehistoric erection of large stone monuments, when megaliths may have been slid atop rounded wooden logs. The logs were grooved near their ends to be held in alignment and rotation by belts out past the edge of the megalith and lubricated by some means, probably organic. The logs are carried from the back of the procession to the front in an endless chain, like continuous track. Attempts by experimental archaeologists to reconstruct these methods have met with varying success. The system is a precursor to development of the axle, which keeps a rotating cylinder fixed relative to its cargo.

A concept vehicle called the Hyanide proposes a continuous track drive motorcycle. It involves a steerable continuous track to enable the vehicle to corner.[6]

Engineering


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Function

Diagram of tracked suspension.(1=rear drive wheel (rear wheel drive), 2=track, 3=return rollers, 4=front drive wheel (front wheel drive), 5=road wheels, 6=idler)

Modern tracks are built from modular chain links which compose together a closed chain. These chain links are often broad and made of manganese alloy steel for high strength, hardness, and abrasion resistance.[7] The links are jointed by a hinge. This allows the track to be flexible and wrap around the set of wheels to make the endless loop.

The vehicle's weight is transferred to the bottom length of track by a number of road wheels, or sets of wheels called bogies. Road wheels are typically mounted on some form of suspension to cushion the ride over rough ground. Suspension design is a major area of development; the very early designs were often completely unsprung. Later-developed road wheel suspension offered only a few inches of travel using springs, whereas modern hydro-pneumatic systems allow several feet of travel and include shock absorbers. Torsion-bar suspension has become the most common type of military vehicle suspension.

Tracks are moved by a toothed drive wheel, or drive sprocket, driven by the motor and engaging with holes in the track links or with pegs on them to drive the track. The drive wheel is typically mounted well above the contact area on the ground, allowing it to be fixed in position. Placing a suspension on the driving wheel is possible, but is mechanically more complicated. A non-powered wheel, an idler, is placed at the opposite end of the track, primarily to angle the front (or rear) of the track to allow it to climb over obstacles, and also to tension (take up the slack of) the track properly - loose track could be easily thrown (slipped) off the wheels. To prevent throwing, the inner surface of the track links usually have vertical guide horns engaging grooves in or gaps between the doubled road and idler/sprocket wheels. Some track arrangements use return rollers to keep the top of the track running straight between the drive sprocket and idler. Others, called slack track, allow the track to droop and run along the tops of large road wheels. This was a feature of the Christie suspension, leading to occasional misidentification of other slack track-equipped vehicles. Many WW II German military vehicles, including all half-track and all later tank designs (after the Panzer IV), had slack-track systems, usually driven by a front-located drive sprocket, running along the tops of the often overlapping, and sometimes interleaved large diameter doubled road wheels, as on the Tiger I and Panther, in their suspension systems. The choice of overlapping/interleaved road wheels allowed the use of slightly more torsion bar suspension members, allowing any German tracked military vehicle with such a setup to have a noticeably smoother ride over challenging terrain, but at the expense of mud and ice collecting between the overlapping areas of the road wheels, and freezing solid in cold weather conditions, often immobilizing the vehicle so equipped.

Advantages

A Russian tracked vehicle designed to operate on snow

Tracked vehicles have better mobility than pneumatic tires over rough terrain. They smooth out the bumps and glide over small obstacles. Riding in a fast tracked vehicle feels like riding in a boat over heavy swells. Tracks are tougher than tires since they cannot be punctured or torn. Tracks are much less likely to get stuck in soft ground, mud, or snow since they distribute the weight of the vehicle over a larger contact area, decreasing its ground pressure. In addition the larger contact area, coupled with the cleats on the track shoes, allows vastly superior traction that results in a much better ability to push or pull large loads where wheeled vehicles would dig in. Bulldozers, which are most often tracked, use this attribute to rescue other vehicles, (such as wheel loaders), which have become stuck in, or sunk into, the ground. Tracks can also give higher maneuverability, as a tracked vehicle can turn in its own radius by driving the tracks in opposite directions. In addition, should a track be broken, assuming the correct tools are available, it can be repaired without the need for special facilities; something which is crucial in a combat situation.

Disadvantages

Tracked vehicles may be put on semitrailers or railway cars for long-distance hauling.
Small tracks on a roadworks machine, note the rubber pads to reduce wear on the carriageway.

The disadvantages of tracks are lower top speed, much greater mechanical complexity, and the damage that their all-steel versions cause to what passes beneath them: they can severely damage hard terrain like asphalt pavement, but deal less damage to lawns and farm fields than wheeled analogs. A compromise between the all-steel and all-rubber tracks for military vehicles to ensure their smoother, faster, quieter and non-damaging movement on paved surfaces at a slight reduction in cross-country traction has been found in attaching rubber pads to individual track links. Prolonged use places enormous strain on the drive transmission and the mechanics of the tracks, which must be overhauled or replaced regularly. It is common to see tracked vehicles such as bulldozers or tanks transported long distances by a wheeled carrier such as a tank transporter or train, though technological advances have made this practice less common among tracked military vehicles than it once was. Additionally, the loss of a single segment in a track immobilises the entire vehicle, which can be a disadvantage in situations where high reliability is important. Tracks can also ride off their guide wheels, idlers or sprockets, which can cause them to jam in an overly tight position or to come completely off of the guide system (this is called a 'thrown' track). Jammed tracks may become so tight that the track may need to be broken before a repair is possible, which requires either explosives or special tools. Multi-wheeled vehicles, for example, 8 X 8 military vehicles, may often continue driving even after the loss of one or more nonsequential wheels, depending upon the base wheel pattern and drivetrain.

Recently many manufacturers have used rubber tracks instead of steel, especially for agricultural use. Rather than a track made of linked steel plates, a reinforced rubber belt with chevron treads is used. In comparison to steel tracks, rubber tracks are lighter, make less noise, create less maximal ground pressure and don't damage paved roads. The disadvantage is that they are not as solid as steel tracks. Previous belt-like systems, such as used for half-tracks in World War II, were not as strong, and during military actions were easily damaged. The first rubber track was invented and constructed by Adolphe Kégresse was patented in 1913; rubber tracks are often called Kégresse tracks.

"Live" and "Dead" track

Tracks may be broadly categorized as "live" or "dead" track. "Dead" track is a simple design in which each track plate is connected to the rest with hinge-type pins. These dead tracks will lie flat if placed on the ground; the drive sprocket pulls the track around the wheels with no assistance from the track itself. "Live" track is slightly more complex, with each link connected to the next with a bushing that causes the track to bend slightly inward. A length of live track left on the ground will curl upward slightly at each end. Although the drive sprocket must still pull the track around the wheels, the track itself tends to bend inward, slightly assisting the sprocket and conforming to the wheels somewhat.

See also

References

External links


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