Tractive effort

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 railroad track.

The product of μ and mg is the factor of adhesion, which determines the maximum torque that can be applied before the onset of wheelspin or wheelslip.

[1] Vehicles having a hydrodynamic coupling, hydrodynamic torque multiplier or electric motor as part of the power transmission system may also have a maximum continuous tractive effort rating, which is the highest tractive force that can be produced for a short period of time without causing component harm.

The period of time for which the maximum continuous tractive effort may be safely generated is usually limited by thermal considerations.

In order to start a train and accelerate it to a given speed, the locomotive(s) must develop sufficient tractive force to overcome the train's resistance, which is a combination of axle bearing friction, the friction of the wheels on the rails (which is substantially greater on curved track than on tangent track), and the force of gravity if on a grade.

Once in motion, the train will develop additional drag as it accelerates due to aerodynamic forces, which increase with the square of the speed.

Drag may also be produced at speed due to truck (bogie) hunting, which will increase the rolling friction between wheels and rails.

In Britain main-line railways generally used a constant of 0.85 but builders of industrial locomotives often used a lower figure, typically 0.75.

The constant c also depends on the cylinder dimensions and the time at which the steam inlet valves are open; if the steam inlet valves are closed immediately after obtaining full cylinder pressure the piston force can be expected to have dropped to less than half the initial force.

Possibly the highest tractive effort ever claimed was for the Virginian Railway's 2-8-8-8-4 triplex locomotive, which in simple expansion mode had a calculated starting T.E.

Of more successful steam locomotives, those with the highest rated starting tractive effort were the Virginian Railway AE-class 2-10-10-2s, at 176,000 lbf (783 kN) in simple-expansion mode (or 162,200 lb if calculated by the usual formula).

The relationship between power and tractive effort was expressed by Hay (1978) as where Freight locomotives are designed to produce higher maximum tractive effort than passenger units of equivalent power, necessitated by the much higher weight that is typical of a freight train.

In modern locomotives, the gearing between the traction motors and axles is selected to suit the type of service in which the unit will be operated.