Tractive Effort is a measure of the force that a locomotive can apply to the train it is pulling. For steam locomotives, Tractive Effort was usually the limitting factor in the size of the train it could pull. Therefore, Tractive Effort was more important than HorsePower for steam locomotives.
Tractive Effort is a quantity determined almost entirely from the locomotive's geometry. It is at a maximum when the locomotive is motionless and is therefore sometimes called Starting Tractive Effort. As the locomotive gains speed, Tractive Effort decreases.
The equation for Tractive Effort is
c P (d)^2 s TE = ----------- D
TE = tractive effort in lbs|
c = a constant determined by the mean effective pressure and friction (usually 85%)
P = boiler pressure
d = piston diameter
s = piston stroke
D = driver diameter
The application below will allow you to play around with the geometry of a steam locomotive and see how it affects the Tractive Effort.
For each revolution of a two-cylinder steam locomotive drive wheel, torque is at a maximum four times per revolution. Because of this, slippage can be initiated at any of those four maximums. To keep a steam locomotive from being too "slippery", it is important to have sufficient weight on the driving wheels. For any steam locomotive, the ratio of the weight on drivers divided by the Tractive Effort is called the Factor of Adhesion. It has been found that a Factor of Adhesion of around 4 is a good balance of pulling force and weight on driver. If the Factor of Adhesion is too low (3.5, for example), the locomotive will be "slippery" when forced to work hard.