Maximum tractive effort
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The maximum tractive effort is the highest force that a vehicle can exert. The term tractive effort is usually used in connection with trains and rail locomotives. Like all forces, tractive effort is measured in newtons.
For a locomotive to accelerate from a stationary position, it must apply a force to overcome the static friction of the train, and to accelerate it against its inertia. To do this, a particularly high tractive effort is required, usually the maximum tractive effort of the engine is applied. This means that the engine works to produce the highest possible force that it can exert onto the wheels to cause movement or motion.
Few engines can maintain work at the maximum tractive effort for very long, but neither is it usually necessary for an engine to do this.
Once the train is running at a constant velocity the train no longer needs to overcome its inertia to remain at the same velocity, and hence must only provide power to compensate for frictional forces; the tractive effort can hence be reduced to at or below the continuous tractive effort.
A table to illustrate the speed the maximum tractive effort, continuous tractive effort and the speed at which the tractive effort should be reduced on a selection of trains operating in the United Kingdom:
| Class | Type | Top speed | Maximum tractive effort |
Speed to reduce tractive effort |
Continuous tractive effort |
Maximum power at rail |
Mass | |
|---|---|---|---|---|---|---|---|---|
| mph | km/h | |||||||
| Class 08 | Shunter | 15 | 156 kN | 8.8 mph | 49 kN | 194 kW | 49.6 - 50.4 t | |
| Class 33 | Passenger | 85 | 200 kN | 17.5 mph | 116 kN | 906 kW | 77.7 t | |
| Class 56 | Light freight | 80 | 275 kN | 16.8 mph | 240 kN | 1790 kW | 125.2 t | |
| Class 58 | Light freight | 80 | 275 kN | 17.4 mph | 240 kN | 1780 kW | 130 t | |
| Class 59 | Heavy freight | 60 or 75 | 506 kN | 14.3 mph | 291 kN | 1889 kW | 121 t | |
| Class 60 | Heavy freight | 60 | 500 kN | 17.4 mph | 336 kN | 1800 kW | 129-131 t | |
| Class 66 | Heavy freight | 75 | 409 kN | 15.9 mph | 260 kN | 1850 kW | 126 t | |
| Class 67 | Light freight | 125 | 200 | 141 kN | ?? mph | 90 kN | 1860 kW | 90 t |
The power at rail of a train follows the equation power (kW) = force (kN) x speed (m/s)
In general, it is more common for heavy freight trains (such as Class 59, Class 60 and Class 66 locomotives) to have a high maximum tractive effort due to the mass which they haul. Light freight trains (such as Class 56, Class 58 and Class 67 locomotives) and passenger trains (such as Class 33 and Class 43 / Intercity High Speed Train locomotives) usually have much lower maximum tractive efforts.
