Caterpillar track

Tracks on a modern lifeboat carrier trailer by Clayton Engineering

Continuous tracks are large (modular) tracks used on the so-called caterpillar tanks, construction plant 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. From equilibrium, the ground pressure of a car must be equivalent to the pressure of the air in the tires. For most cars this is approximately 30 psi (207 kPa), whereas the seventy-ton M1 Abrams tank only exerts a ground pressure of just over 15 psi (103 kPa).

History

A Working scale Model of the Hornsby track tractor on display at the GDSF in 2008

A crude continuous track was designed in 1770 by Richard Lovell Edgeworth, but the first conception of it was made by Polish inventor Józef Maria Hoene-Wroński. 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 in 1846.

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. 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. 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. 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.

A Quadtrac track unit. the whole unit oscillates slightly for uneven ground

James B. Hill, working in Bowling Green, Wood County, Ohio, patented what he termed "apron traction" on September 24, 1907. Following a merger with the Best Company 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. 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 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.

The Israeli Defence Forces have developed an improved suspension system, called Mazkum מזקו"ם , 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.

Engineering

The Front ider and suspension system on a Caterpillar Challenger tractor

The Track suspension system on a Russian DT75 crawler

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. 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 track links usually have the vertical guide horns engaging the grooves 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.

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.

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 wheeled loaders) which have become stuck in or sunk into the ground. Tracks can also give a higher maneuverability, as a tracked vehicle can turn in its own radius by driving the tracks in opposite directions.

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 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 disadvantage in situations where high reliability is important. Multi-wheeled vehicles, for example, 8 X 8 military vehicles, may often continue driving even after the loss of one or two wheels.

Recently many manufactures use rubber tracks instead of the steel ones, especially for agricultural use. Instead of 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, do 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 so strong, and during military actions were easily damaged. The first rubber track was invented and constructed by Adolphe Kégresse and are often called Kégresse track, and was patented in 1913.

See also

• Excavator
• Snowmobile
• Snowcat

References

Base on a wikipedia article, to build on and cross reference Tractor wiki articles.

External links

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Caterpillar track