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A ballast tractor is a heavy haulage road vehicle designed to pull or push heavy or exceptionally large loads. Unlike the tractor unit from an articulated trailer, the ballast tractor is designed or adapted to pull or push loads from a drawbar. By contrast an articulated tractor unit pulls from a "fifth wheel", and only pushes when reversing its load.
The name ballast is derived from the nautical term (see sailing ballast) describing heavy material added to a vessel to improve stability. For a ballast tractor, ballast is added over the driving wheels to increase the available tractive effort. The additional weight increases the friction between the tyres and the road surface. Without such ballast, the tractor would be unable to overcome the inertia and friction of rolling of a heavy trailed load, and its wheels would rotate without generating forward motion (termed wheelspin). With a semi-trailer, the weight of the trailer presses down through the fifth wheel and adds ballast. In the case of a ballast tractor, the load is supported separately and its weight provides no ballast: the drawbar only transmits a horizontal force to the load.
High inertia is encountered when starting to move a heavy load. To overcome this, ballast tractors tend to have high power engines and engines that provide lots of torque, especially at low speeds. Ballast tractors are often fitted with heavy duty hub reduction axles, or high reductive gear boxes to increase torque at the wheel, therefore heavy duty ballast tractors tend to have low maximum speeds.
A strong chassis is required, to support the extra weight of the ballast and the pulling forces imposed by the drawbar. A reinforced chassis allows multiple tractors to be coupled together to increase the total power and traction available. Heavy-duty versions of commercial tractor units may be fitted with a ballast box and suitable drawgear, alternatively, a ballast tractor may be purpose-built for the role. The ballast is located above the driving axle, and tractors often have multiple driving axles to reduce the axle load, with the ballast distributed to avoid overloading a particular axle.
A ballast tractor that is built as a ballast tractor tends to be the heaviest class of on-highway trucks. In some cases, the weight of the chassis alone of these trucks (the kerb weight) can be even greater than their axle configuration legal Gross Vehicle Weight (GVW) permits, so require special permission to use the roads.
The main use of ballast tractors is to move heavy and abnormal loads using an independent drawbar trailer, modular wheel-trailer units, or a dolly trailer. An advantage of using ballast tractors is that one can be used to push-steer a trailer around a corner. A girder trailer, for example, is double-articulated so the front tractor can pull the load around a corner whilst the rear tractor can push the rear end (at an angle) around the same corner. Therefore a push/pull combination can make an abnormal load more manoeuvrable than a rigid solution pulled by several tractors. Use of a following tractor can increase the control and brake force available when descending a hill.
Another specialized use for ballast tractors is in showman vehicles. The ballast tractor used in this case may be a typical tractor unit fitted with a ballast box used to tow one or more specialized trailers that contain fairground rides which may not fit onto a standard fifth wheel connecting trailer. The overall weight of showman's vehicles tends to be much less than those used for heavy haulage to comply with vehicle weight legislation, particularly axle loads. Sometimes, one or more generator sets for powering the rides could act as the ballast; a more economical solution than having dead-weight as ballast on a tractor and providing a separate vehicle for the generators. Modern generators tend to be smaller and can fit between the cab and the fifth wheel of a conventional tractor unit, reducing the need for fairground use of ballast tractors. A vehicle used for towing fairground rides may also be called a "showman's diesel road locomotive", reflecting back to the steam-powered showman's road locomotives that originally provided this function.
A ballast tractor is somewhat limited in its use in modern day road freight because tractor-trailer combinations are more flexible and practical to move normal heavy loads. A heavy duty tractor unit equipped with a temporary ballast box may also be used as a normal fifth wheel -based tractor-unit, but its kerb weight is often high thus reducing the payload of the entire vehicle, compared to a lighter-weight unit.
Few well-known manufactures produce purpose-built ballast tractors. Most high-volume manufacturers offer heavy duty chassis versions of certain tractor units which enable a ballast box to be fitted. In Europe, manufacturers tend to send some of their products to another company (owned by the parent) to be converted into a special heavy duty version.
Built-to-order machines tend to be the heaviest and most powerful machines designed for one company for a specific purpose. Nicolas Tractomas (France) for example, currently builds 8x8 and 10x10 ultra-heavy-duty tractors for Rotran in South Africa.
- Czech Republic
- DAF (GINAF)
- Russia (or former Soviet Union)
- Atkinson Vehicles Ltd
- Foden Trucks
- Rotinoff Motors Ltd, Slough – 35 heavy haulage tractors were built by the company between 1952 and 1959, of which 11 are known to survive.
- Scammell Lorries – Many early examples, now used and seen as showman's vehicles, originally served with the army.
Atkinson ballast tractor
- Diamond T - Ex-army tank transports were popular post-war with heavy hauliers in the UK.
- Oshkosh Corporation
Most heavy haulage and heavy lift engineering firms employ heavy duty tractor-unit models that can accommodate a ballast box. Countries where modernisation is taking place, such as regions of the Middle East and South Africa, operate larger number of ballast tractors due to the greater frequency of heavy loads (such as power station components).
Apart from being used by fairgrounds as a direct replacement for the steam-powered showman's engine, specialist moving and haulage companies use ballast tractors. These include:
- Abnormal Load Engineering - specialist in moving transformers and turbines for power stations, from GEC at Stafford.
With the increasing versatility of Self Propelled Modular Transporter (SPMT) and modular low loaders, pure ballast tractors are not as common, with most being a heavy duty tractor unit with a removable "Ballast" box fixed in place of the fifth wheel unit. Ultra heavy loads of 200 ton plus generally are only moved short distances and SPMT's are more maneuverable. Items commonly moved include: oil rig modules, bridge sections, buildings, sections of ships, industrial machinery.
- "Thimbleby & Shorland - COMMERCIAL VEHICLES SPARES and ACCESSORIES MEMORABILIA, MODELS, LITERATURE PICTURES and PRINTS AUCTION SATURDAY 25th MARCH 2006" (pdf). 2006-03-25. p. 16. Archived from the original on 2007-09-26. Retrieved 2007-09-14. "This very smart tri axle [Scammell Junior Constructor] ballast tractor was one of six supplied new to Pickfords in 1958."
- High, Wide & Heavy by David Lee, Published by Round Oak, ISBN 1-871565-47-2
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