Ballast tractor

A ballast tractor is a specially weighted tractor unit of a heavy hauler combination. It is designed to utilize a drawbar to pull or push heavy or exceptionally large trailored loads. When feasible, lowboy-style semi-trailers are used to minimize a load's center of gravity. Typical drivetrains are 6×4 and 6×6.

Typical ballast tractor loads include oil rig modules, bridge sections, buildings, ship sections, and industrial machinery such as generators and turbines.

Only a handful of manufacturers produce dedicated ballast tractors. Extra-heavy-duty chassis versions of mass-production tractor units are fitted with drawbar hitches and a separate ballast box as an alternative.

Increasingly, remote-controlled, self-propelled modular transporters (SPMT) are being employed in traditional ballast tractor/trailer roles.

The ballast tractor's name derives from the nautical term "sailing ballast" describing heavy material added to a vessel to improve stability. For a ballast tractor, ballast is added over the driving wheels to maximize traction. The additional weight increases the friction between the tyres and the road surface, allowing the tractor to overcome the inertia and friction of moving a heavy trailed load. Without it, there would be unproductive wheelspin.

With a semi-trailer, ballast is added in the form of the weight of the attached trailer pressing down upon the tractor's fifth wheel. Since the load is separate from a ballast tractor, it provides no ballast: the drawbar only transmits a horizontal force.

High inertia is encountered when starting to move a heavy load. To overcome this, ballast tractors tend to have high powered, low geared engines that provide substantial torque, especially at low speeds. Additionally, ballast tractors are often fitted with heavy duty hub reduction axles, a combination resulting in exceptionally low maximum speeds.

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Wikipedia