Steam plough

A traction engine is a self-propelled steam engine used to move heavy loads on roads, plough ground or to provide power at a chosen location. The name derives from the Latin tractus, meaning 'drawn', since the prime function of any traction engine is to draw a load behind it. They are sometimes called road locomotives to distinguish them from railway locomotives – that is, steam engines that run on rails.

Traction engines tend to be large, robust and powerful, but heavy, slow, and difficult to manoeuvre. Nevertheless, they revolutionized agriculture and road haulage at a time when the only alternative prime mover was the draught horse.

They became popular in industrialised countries from around 1850, when the first self-propelled portable steam engines for agricultural use were developed. Production continued well into the early part of the 20th century, when competition from internal combustion engine–powered tractors saw them fall out of favour, although some continued in commercial use in the United Kingdom well into the 1950s and later. All types of traction engines have now been superseded, in commercial use. However, several thousand examples have been preserved worldwide, many in working order. Steam fairs are held throughout the year in the United Kingdom, and in other countries, where visitors can experience working traction engines at close hand.

Traction engines were cumbersome and ill-suited to crossing soft or heavy ground, so their agricultural use was usually either "on the belt" – powering farm machinery by means of a continuous leather belt driven by the flywheel – or in pairs, dragging an implement on a cable from one side of a field to another. However, where soil conditions permitted, direct hauling of implements ("off the drawbar") was preferred – in America, this led to the divergent development of the steam tractor.

Limits of technical knowledge and manufacturing technology meant that practicable road vehicles, powered by steam, did not start to appear until the early years of the 19th century.

The traction engine, in the form recognisable today, developed partly from an experiment in 1859 when Thomas Aveling modified a Clayton & Shuttleworth portable engine, which had to be hauled from job to job by horses, into a self-propelled one. The alteration was made by fitting a long driving chain between the crankshaft and the rear axle. Other influences were existing vehicles which were the first to be referred to as traction engines such as the Boydell engines manufactured by various companies and those developed for road haulage by Bray. The first half of the 1860s was a period of great experimentation but by the end of the decade the standard form of the traction engine had evolved and would change little over the next sixty years.

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Wikipedia