Description
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.
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