Caterpillar track
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Caterpillar tracks are large (modular) tracks used on tanks, construction equipment and certain other off-road vehicles. Unlike the rubber-made Kegresse tracks used on snowmobiles, the Caterpillar tracks are made or metal or some rigid material. 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 70 tonne M1 Abrams has a ground pressure of just over 15 PSI (103 kPa).
History
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A crude caterpillar track was designed in 1770 by Richard Edgeworth, and steam powered tractors using a form of caterpillar track were reported in use during the Crimean War.
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 were built. In 1903, the founder of the Holt Manufacturing, Benjamin Holt, paid Lombard $60,000 so they could produce vehicles under his patent.
At about the same time a British agricultural company Hornsby based in Grantham, UK developed and patented a caterpillar track in 1905. Holt also purchased their patent.
Following a merger and name change, The Holt Manufacturing Company went on to become the Caterpillar Tractor Company in 1925. Caterpillar tracks have since revolutionized construction vehicles and land warfare. The tracks system 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.
Perhaps the oldest concept of something like tracks is to be found in theories of pre-historic erection of large stone monuments, when megaliths may have been slid along atop rounded wooden cylinders. While most of the workers pushed or pulled the rock along the timber track, a task of a smaller group was to take each wooden log that the rock had already passed over and put it in front. This would have been a more efficient method to transport heavy rocks great distances than simply pulling it along the ground, though attempts by experimental archaeologists to reconstruct these methods have met with variable success. The system is a pre-cursor to development of the axle which keep a rotating cylinder from moving.
George Cayley in the early 1800's invented a pre-cursor to modern Caterpillar tractor which he called the 'Universal Railway'.
The Israeli Defence Forces have developed an improved suspension system, called Mazkum מזקו"ם (or זחלים 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 Company.
Engineering
Modern tracks are built from modular chainlinks which compose together a closed chain. These chainlinks are often broad and made of strong metal. Between every two pieces of the chain there is a joint enabling the chains to change the angle between them. This allows the track to be flexible and maintain its elliptical shape.
The vehicle weight is suspended from a number of wheels known as bogies or road wheels. These wheels are typically mounted on some form of suspension to even out the ride over rough ground. Suspension design is a major area of research; early designs offered only a few inches of travel using springs, whereas modern hydropneumatic systems allow several feet of travel and include shock absorbers.
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Tracks are moved by a driving wheel connected to the motor. The driving-wheel has teeth around its perimeter which engage with holes in the links and drag it while it rotates. The driving-wheel is typically mounted well above the contact area on the ground, allowing it to be fixed in position. Although it is possible, placing a suspension on the driving wheel tends to be difficult mechanically. A non-powered wheel, the idler, is placed at the same "height" above the ground at the opposite end of the track, primarily to angle the front (or rear) of the track to allow it to climb over obstacles.
The "top" of the track is not as much of a concern as the bottom, although it does have to be held so it doesn't fall off to the side of the wheels. On tanks two systems have developed. One, which is a single element of the whole track suspension system called Christie suspension uses oversized road wheels and the track simply lies on top of the wheels. The shape of the track as a whole is somewhat banana-like as the track droops onto the wheels after running over the driving wheel and idler. The other system, sometimes called a Vickers suspension arrangement, uses smaller return rollers to hold the track straight from the idler to the driving wheel, leading to a sideways D shape.
The advantages of tracks result in increased mobility through rough terrain. First, tracks are much more resistant to shrapnel (nails, broken glass etc) and sharp fluctuations in the ground (holes, small pits and ditches) than wheels, thus enabling the vehicle to drive over small obstacles that would stop a wheeled vehicle. Secondly, tracks distribute the weight of the vehicle over a larger area, thus decreasing its ground pressure. When working on sandy or muddy ground, this can prevent the vehicle from sinking. Bulldozers, which are most often tracked, uses this attribute to rescue other vehicles (such as wheel loaders) which have become stuck in or sunk into the ground.
The disadvantages of tracks are decreased speed and the damage that they cause to what passes beneath them - rocks, shrubbery and asphalt roads. In addition, prolonged use places enormous strain on the drive transmission and the mechanics of the tracks, which must then be overhauled or replaced on a regular basis. It is common to see tracked vehicles such as bulldozers or tanks transported long distances by a wheeled carrier such as a semi-trailer or train, though technological advances have made this practice less common among tracked military vehicles than it once was.