Tractive force

As used in mechanical engineering, the term tractive force can either refer to the total traction a vehicle exerts on a surface, or the amount of the total traction that is parallel to the direction of motion.

In railway engineering, the term tractive effort is often used synonymously with tractive force to describe the pulling or pushing capability of a locomotive. In automotive engineering, the terms are distinctive: tractive effort is generally higher than tractive force by the amount of rolling resistance present, and both terms are higher than the amount of drawbar pull by the total resistance present (including air resistance and grade). The published tractive force value for any vehicle may be theoretical—that is, calculated from known or implied mechanical properties—or obtained via testing under controlled conditions. The discussion herein covers the term's usage in mechanical applications in which the final stage of the power transmission system is one or more wheels in frictional contact with a roadway or railroad track.

The term tractive effort is often qualified as starting tractive effort, continuous tractive effort and maximum tractive effort. These terms apply to different operating conditions, but are related by common mechanical factors: input torque to the driving wheels, the wheel diameter, coefficient of friction (μ) between the driving wheels and supporting surface, and the weight applied to the driving wheels (m). The product of μ and m is the factor of adhesion, which determines the maximum torque that can be applied before the onset of wheelspin or wheelslip.

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