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) steam locomotives – that is, steam engines that run on rails.
Traction engines tend to be large, robust and powerful, but heavy, slow, and poorly maneuverable. 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 UK 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 All types of traction engines have now been superseded, in commercial use.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 UK, 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 "in 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 the U.S., this lead to the divergent development of the steam tractor.
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.
Development of traction engines, and indeed all forms of steam road transport, was hampered in the UK by a series of government acts that tried to balance various competing interests, those of other types of vehicles as well as those maintaining the roads. Until the quality of roads improved there was little demand for faster vehicles and engines were geared accordingly to cope with their use on both roads and farm tracks.
Right through to the first decades of the twentieth century, manufacturers continued to seek a solution to realise the economic benefits of direct-pull ploughing, and, particularly in North America, this led to the American development of the steam tractor. British companies such as Mann's and Garrett developed potentially-viable direct ploughing engines, however market conditions were against them, and they failed to gain widespead popularity. These market conditions arose in the wake of the First World War when there was a glut of surplus equipment available as a result of British Government policy. Large numbers of Fowler ploughing engines had been constructed in order to increase the land under tillage during the war, and many new light Fordson F tractors had been imported from 1917 onwards.
The last new UK-built traction engines were constructed in the 1930s, although many continued in commercial use for many years, while there remained experienced enginemen available to drive them.
From the 1950s, the 'preservation movement' started to build up as enthusiasts realised that these lumbering beasts were in danger of dying out. Many of the remaining engines were snapped-up by enthusiasts, and restored to working order. Traction engine rallies began, initially as races between engine owners and their charges, later developing into the significant tourist attractions that takes place in many locations each year. It has been estimated that over two thousand traction engines have been preserved.
Although the first traction engines employed a chain drive, it is more typical for large gears to be used to transfer the drive from the crankshaft to the rear axle.
The machines typically have two large powered wheels at the back and two smaller wheels for steering at the front. However, some traction engines used a four-wheel-drive variation, and some experimented with an early form of caterpillar track.
A simple animation showing the steam cycle of a traction engine, the operation of the valve gear and the reversing mechanism, may be found here: .
The most common form in the countryside. They were used for hauling and as a stationary power source. Even when farmers did not own such a machine they would rely upon it from time to time. Many farms would use draught horses throughout the year, but during the harvest, threshing contractors would travel from farm to farm hauling the threshing machine which would be set up in the field and powered from the engine – a good example of the moveable stationary engine.
Favourable soil conditions meant that U.S. traction engines usually pulled their ploughs behind them, thereby eliminating the complexities of providing a cable drum and extra gearing, hence simplifying maintenance. American traction engines were manufactured in a variety of sizes, with the 6 horsepower Russell being the smallest commercially made, and the large engines made by Russell, Case, and Reeves being the largest.
A distinct form of traction engine, characterised by the provision of a large diameter winding drum driven by separate gearing from the steam engine. Onto the drum a long length of wire rope was wound, which was used to haul an implement, such as a plough, across a field.
The winding drum was either mounted horizontally (below the boiler), vertically (to one side), or even concentrically, so that it encircled the boiler. The majority were under-slung (horizontal), however, and necessitated the use of an extra-long boiler to allow enough space for the drum to fit between the front and back wheels. These designs were the largest and longest traction engines to be built.
Mostly the ploughing engines worked in pairs, one on each side of the field, with the rope from each machine fastened to the implement to be hauled. The two drivers communicated by signals using the engine whistles.
A variety of implements were constructed for use with ploughing engines. The most common were the balance plough and the cultivator - ploughing and cultivating being the most physically demanding jobs to do on an arable farm. Other implements could include a mole drainer, used to create an underground drainage channel or pipe, or a dredger bucket for dredging rivers or moats.
The engines were frequently provided with a 'spud tray' on the front axle, to store the 'spuds' which would be fitted to the wheels when travelling across claggy ground.
The man credited with the invention of the ploughing engine, in the mid-nineteenth century, was John Fowler, an English agricultural engineer and inventor.
Ploughing engines were rare in the U.S.; ploughs were usually hauled directly by an agricultural engine or steam tractor.
They were popular in the timber trade in the UK, although variations were also designed for general light road haulage and showman's use.
Designed for haulage of heavy loads on public highways, it was not uncommon for two or even three to be coupled together to allow heavier loads to be handled.
The characteristic features of these engines are very large rear driving wheels fitted with solid rubber tyres; three-speed gearing (most traction engine types have only two gears); rear suspension; and belly tanks to provide a greater range between the stops needed to replenish water. All these features are to improve the ride and performance of the engine, which used to be used for journeys of hundreds of miles. Most road locomotives are fitted with a winch drum on the back axle. This can be used by removing the driving pins that secure the rear wheels, allowing the drive train to power the winch drum instead of the wheels.
A number of road locomotives are fitted with a crane boom on the front. The boom pivot is mounted on the front axle assembly, and a small winch is mounted on an extension to the smokebox in front of the chimney; the cable passing over a sheave at the top of the boom arm. The winch is powered by bevel gears on a shaft driven directly from the engine, with some form of clutch providing raise/lower control. These road locomotives can be used to load a trailer as well as to haul it to a new location. They are often referred to as 'crane engines'.
A particularly distinctive form of road locomotive was the Showman's engine. These were operated by travelling showmen both to tow fairground equipment and to power it when set up; either directly or by running a generator. These could be highly decorated and formed part of the spectacle of the fair. Some were fitted with a small crane that could be used when assembling the ride.
Some traction engines were designed to be convertible: the same basic machine could be fitted with either standard ('treaded' or tyred) road wheels, or else smooth rolls – the changeover between the two being achieved in less than half a day.
A number of other steam-powered vehicles share design features with the traction engine, usually because the same technology was re-used in a new application.
A portable engine is a type of self-contained steam engine and boiler combination that may be moved from site to site. Although bearing a strong family resemblance, in both appearance and (stationary) operation, the portable engine is not classed as a traction engine as it is not self-propelled. However, it is included in this list because the traction engine is a direct descendant.
These were the earliest steam lorries and came in two basic forms. The earlier over-type designs resembled traction engines by having a cab built around a horizontal boiler with a round smokebox and chimney (eg Foden). And they resembled lorries in having a load-carrying body and being built around a chassis (so they cannot really be called traction engines).
The more modern under-type designs have the engine under the chassis (although the boiler - usually a vertical type boiler - remains in the cab), and generally resemble lorries rather than traction engines.
The earliest examples of either type had steel or wooden wheels, later followed by solid rubber tyres. Various developments fully enclosed cabs and pneumatic tyres were later tried by companies such as the Sentinel Waggon Works in a bid to compete with internal combustion engine-powered lorries. Some wagons built to run on solid tyres were later converted to pneumatic tyres.
John I. Thornycroft & Company was an established marine engineering company that successfully spawned the Steam Carriage and Wagon Company for the production of steam-powered road vehicles. They supplied steam lorries to the British Army, commercial steam wagons and vans, steam cars (for a few years), and buses – London's first powered bus was a Thornycroft double-decker steam bus.
Manufacturers who specialised in the construction of steam lorries include:
Steam lorries were the best road lorries of their day. The Sentinel S4 was easily capable of attaining 60 mph at a time when petrol lorries would be lucky to achieve 35 mph, the Sentinel DG's could reach 40 mph (considerably more than their listed speed) and the Foden lorries were limited only by speed limits placed on them – the makers even offered alternative gearing capable of twice the speed limit although they were still not as fast as the Sentinels.
The lorry was based on an 'unusual' prototype, a long wheelbase undertype, with a small vertical boiler mounted to one side of the cab, and no windscreen.
Throughout the 1920s and 1930s successive governments placed tighter restrictions on road steam haulage, including speed, smoke and vapour limits, and a 'wetted tax', where the tax due was proportional to the size of the wetted area of the boiler; this made steam engines less competitive against domestically-produced internal combustion engined units (although imports were subject to taxes of up to 33%).
As a result of the Salter Report on road funding, an 'axle weight tax' was introduced in 1933 in order to charge commercial motor vehicles more for the costs of maintaining the road system, and to do away with the perception that the free use of roads was subsidising the competitors of rail freight. The tax was payable by all road hauliers in proportion to the axle load; it was particularly damaging to steam propulsion, which was heavier than its petrol equivalent.
Initially, imported oil was taxed much more than British-produced coal, but in 1934 Oliver Stanley, the Minister for Transport, reduced taxes on fuel oils while raising the Road Fund charge on road locomotives to £100 pounds a year, provoking protests by engine manufacturers, hauliers, showmen and the coal industry. This was at a time of high unemployment in the mining industry, when the steam haulage business represented a market of 950,000 tons of coal annually. The tax was devastating to the businesses of heavy hauliers and showmen, and precipitated the scrapping of many engines.
Model traction engines, powered by steam, are manufactured by several companies, notably Mamod and Wilesco. Larger scale model engines are popular subjects for model engineers to construct, either as a supplied kit of parts, or machined from raw materials.