- Continuous track
Continuous tracks or caterpillar tracks are a system of vehicle propulsion in which modular metal plates linked into a continuous band are driven by two or more wheels. The large surface area of the tracks distributes the weight of the vehicle better than steel or rubber tires on an equivalent vehicle, enabling a continuous tracked vehicle to traverse soft ground with less likelihood of becoming stuck due to sinking. The prominent treads of the metal plates are both hard-wearing and damage resistant, especially in comparison to rubber tires. The aggressive treads of the tracks provide good traction in soft surfaces but can damage paved surfaces. Special tracks that incorporate rubber pads can be installed for use on paved surfaces to prevent the damage that can be caused by all metal tracks.
Continuous tracks can be traced back as far as 1770 and today are commonly used on a variety of vehicles including bulldozers, excavators and tanks, but can be found on any vehicle used in an application that can benefit from the added traction, low ground pressure and durability inherent in continuous track propulsion systems.
Perhaps the oldest implementation of something resembling continuous 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. The system is a precursor to development of the axle, which keeps a rotating cylinder fixed relative to its cargo.
In modern times, continuous track propulsion systems can be traced back to a crude continuous track system designed in the 1770s by Richard Lovell Edgeworth. Polish mathematician and inventor Józef Maria Hoene-Wroński conceived of the idea in the 1830s. 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.
In 1877, a Russian, Fyodor Blinov, created a tracked vehicle called "wagon moved on endless rails" (caterpillars). It lacked self-propulsion—it was pulled by horses, instead. Blinov got a patent for his "wagon" the next year. Later, in 1881-1888 he created a steam-powered caterpillar-tractor. This self-propelled crawler was successfully tested and showed at a farmer's exhibition in 1896.
A little known American inventor, Henery T. Stith, developed a continuous track prototype, which was, in multiple forms, patented in 1873, 1880, and 1900. The last, was for the application of the track to a proto-type off road bicycle built for his son. The 1900 proto-type is retained by his surviving family.
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.
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 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.
Caterpillar Tractor Company began in 1925 from a forced reorganization of the Holt Manufacturing Company; an early successful manufacturer of crawler tractors. 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.
Construction and Operation
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.
Track construction and assembly is dictated by application. Military vehicles use a track shoe that is integral to the structure of the chain in order to reduce track weight. Reduced weight allows the vehicle to move faster and decreases overall vehicle weight to ease transportation. In contrast, Agricultural and Construction vehicles opt for a track with shoes that attach to the chain with bolts and do not form part of the chain's structure, this allows track shoes to break without compromising the ability of the vehicle to move and decrease productivity but increases the overall weight of the track and vehicle. Extra weight is an advantage when optimizing for traction and power over speed and mobility.
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 in military vehicles 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. Construction vehicles have smaller road wheels that are designed primarily to prevent track derailment and they are normally contained in a single bogie that includes the idler wheel and sometimes the sprocket.
Transfer of power to the track is accomplished by a 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. In military vehicles, the drive wheel is typically mounted well above the contact area on the ground, allowing it to be fixed in position. In agricultural crawlers it is normally incorporated as part of the bogie. Placing suspension on the sprocket is possible, but is mechanically more complicated. A non-powered wheel, an idler, is placed at the opposite end of the track, primarily to tension the track - 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. In military vehicles with a rear sprocket, the idler wheel is placed higher than the road wheels to allow it to climb over obstacles. 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.
Tracked vehicles have better mobility than pneumatic tyres over rough terrain. They smooth out the bumps, glide over small obstacles and they are capable of crossing trenches or breaks in the terrain. Riding in a fast tracked vehicle feels like riding in a boat over heavy swells. Tracks are tougher than tyres 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, or grousers, 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 some tracked vehicle can turn in place with no forward or backward movement 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.
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).
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 are perceived/misconceived to severely damage hard terrain like asphalt pavement, but in actuality, often have significantly lower ground pressures than equivalent or lighter wheeled vehicles, but often cause damage to less firm terrains such as lawns, gravel roads, and farm fields, as the sharp edges of the track easily routs the turf. Vehicle laws and local ordinaces often require rubberised tracks or track pads due to this ideal. However, a compromise between all-steel and all-rubber tracks exists. Attaching rubber pads to individual track links ensures continuous tracked vehicles can travel more smoothly, quickly, and quietly on paved surfaces. While these pads slightly reduce a vehicle's cross-country traction, they keep it from (in theory) damaging any pavement.
Additionally, the loss of a single segment in a track immobilizes 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 non-sequential 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.
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.
"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.
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