Continuous track or tracked treads are a system of vehicle propulsion used in tracked vehicles, running on a continuous band of treads or track plates driven by two or more wheels. The large surface area of the tracks distributes the weight of the vehicle better than steel or rubber tyres on an equivalent vehicle, enabling continuous tracked vehicles to traverse soft ground with less likelihood of becoming stuck due to sinking.
Modern continuous tracks can be made with soft belts of synthetic rubber, reinforced with steel wires, in the case of lighter agricultural machinery. The more common classical type is a solid chain track made of steel plates (with or without rubber pads), also called caterpillar tread or tank tread, which is preferred for robust and heavy construction vehicles and military vehicles.
The prominent treads of the metal plates are both hard-wearing and damage resistant, especially in comparison to rubber tyres. The aggressive treads of the tracks provide good traction in soft surfaces but can damage paved surfaces, so some metal tracks can have rubber pads installed for use on paved surfaces. Other than soft rubber belts, most chain tracks apply a stiff mechanism to distribute the load equally over the entire space between the wheels for minimal deformation, so that even the heaviest vehicles can move easily, just like a train on its straight tracks.
The stiff mechanism was first given a physical form by Hornsby & Sons in 1904 and then made popular by Caterpillar Tractor Company, with tanks emerging during World War I. Today, they are commonly used on a variety of vehicles, including snowmobiles, tractors, bulldozers, excavators and tanks. The idea of continuous tracks can be traced back as far as the 1830s, however.
History
The British polymath Sir George Cayley patented a continuous track, which he called a "universal railway" in 1825. Polish mathematician and inventor Józef Maria Hoene-Wroński designed caterpillar vehicles in the 1830s to compete with the railways. In 1837, Russian army captain Dmitry Andreevich Zagryazhsky (1807 – after 1860) designed a "carriage with mobile tracks" which he patented the same year, but due to a lack of funds and interest from manufacturers he was unable to build a working prototype, and his patent was voided in 1839.
Heathcote's Steam Plough
Patented in 1832 by John Heathcoat (also Heathcote) M.P. for Tiverton, the Heathcote steam plough was demonstrated in 1837 and press coverage fortunately provided a wood cut of this unusual tracked vehicle. The continuous tracks were made of sections of wood bolted to continuous iron bands which were driven by the 'drums' at each end. A strong chassis provided the bearings for the drums, and carried the steam engine, fuel and winch. The chassis was supported on "numerous small wheels or rollers" which ran upon the lower iron bands, which "thus form a perfectly portable and smooth road for the platform". The drums were in diameter, apart. The tracks were each wide with a gap in-between giving an overall width of . The twin-cylinder steam engine could be used either to drive the plough winch or to drive the vehicle along, at a speed of up to . Although the machine weighed 30 tons complete with 6 tons of fuel, its ground pressure was only , considerably less than a man. The successful demonstration was carried out on Red Moss at Bolton-le-Moors on 20 April 1837. The steam plough was lost when it sank into a swamp by accident and was then abandoned as the inventor did not have the funds to continue development.
Dreadnaught wheel by Boydell (1846)
Although not a continuous track in the form encountered today, a dreadnaught wheel or "endless railway wheel" was patented by the British Engineer James Boydell in 1846. In Boydell's design, a series of flat feet are attached to the periphery of the wheel, spreading the weight. A number of horse-drawn wagons, carts and gun carriages were successfully deployed in the Crimean War, waged between October 1853 and February 1856, the Royal Arsenal at Woolwich manufacturing dreadnaught wheels. A letter of recommendation was signed by Sir William Codrington, the General commanding the troops at Sebastopol.
Boydell patented improvements to his wheel in 1854 (No. 431) – the year his dreadnaught wheel was first applied to a steam engine – and 1858 (No. 356), the latter an impracticable palliative measure involving the lifting one or other of the driving wheels to facilitate turning.