Caterpillar track

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Image:AmericanTank.jpg
U.S. M60 Patton tank. The rear drive sprocket and return rollers holding up the track are clearly seen.

Caterpillar tracks are large (modular) tracks used on tanks, construction equipment and certain other off-road vehicles. Unlike the Kegresse tracks which use a flexible belt, most caterpillar 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, keeping it from sinking in areas where wheeled vehicles of the same weight would sink. For instance, the ground pressure of a car is equal to the pressure of the air in the tires, perhaps 30 psi (207 kPa), whereas the seventy-tonne M1 Abrams tank has a ground pressure of just over 15 psi (103 kPa).

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[edit] History

A crude caterpillar track was designed in 1770 by Richard Edgeworth. The British polymath Sir George Cayley patented a caterpillar 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 that same year. However, due to a lack of funds he was unable to build a working prototype. As a result his patent was voided in 1839. Steam powered tractors using a form of caterpillar track were reported in use with the Western Alliance during the Crimean War in the 1850s.

An effective caterpillar 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, eighty-three 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 Lombard was the first commercial manufacturer of the tractor crawler. At least one of Lombard's steam-powered machines apparently remains in working order. Also, 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 travelled out 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 tread mill" once 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 date of "invention" seems to get.

At about the same time a British agricultural company Hornsby based in Grantham, in the United Kingdom developed a caterpillar track which was patented in 1905. 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 used as artillery tractors by the British Army from 1906. Their patent was also 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". What is known is that caterpillar became a generic term for this type of machine. The word was shrewdly trademarked and defended by Holt.

Following a merger and name change, The Holt Manufacturing Company became the Caterpillar Tractor Company in 1925. Caterpillar tracks have since revolutionized construction vehicles and land warfare. Track systems have been developed and improved during the years. The first tanks to be fielded were developed from Holt tractors which were already in use towing artillery over the difficult terrain of the Western Front of the First World War.

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 caterpillar 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.

The Israeli Defence Forces have developed an improved suspension system, called Mazkum מ?-ק?·"ם (or ?-?--לים for short), which enables greater mobility than regular tracks. The Mazkum is installed on the Israeli Merkava tank which helps improve mobility and speed, some of the Israeli patents were sold to Caterpillar Tractor.

[edit] Engineering

Image:T-55 skos RB.jpg
Soviet T-55 tank with "slack track" and rear drive sprocket.
Image:Armored vehicle on trailer p1040729.jpg
Tracked vehicles may be put on semitrailers or railway cars for long-distance hauling
Image:Caterpillar track segments.jpg
The tread of a construction machine's tracks.

Modern tracks are built from modular chain links which compose together a closed chain. These chain links are often broad and made of alloy steel. 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 unsprung at all, 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.

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 track links usually have the vertical guide "tongues", engaging the grooves or gaps between the doubled road and idler/sprocket wheels. Some track arrangements, usually called "live track", 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.

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. 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 as well give higher maneuverability, as a tracked vehicle can turn on spot.

The disadvantages of tracks are lower top speed 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 (first used on German semitracks, then on the US post-war tanks). 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 semitrailer 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 disadavantage in situations where high reliability is important.

Caterpillar also manufacture a rubber track system for agricultural use, such as the Cat™ Challenger Tractor, MT700 and MT800 series. Instead of a track made of linked steel plates, it uses a reinforced rubber belt with chevron treads for improved traction and reduced soil compaction. Having a rubber belt also means that the vehicle can relocate itself on public roads without damaging the road surface. Previous belt-like systems, such as used for half-tracks in WW2, were unable to deliver the tractive effort, and have been limited to lighter vehicles.

[edit] See also

[edit] References

de:Kettenlaufwerk

he:מזקו"ם ja:無限軌道 no:Beltedrift pl:Napęd gąsienicowy ru:Гусеница (движитель) fi:Telaketju sv:Bandfordon

Caterpillar track

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