Locomotive Engine Running and Management

by Angus Sinclair

PROPORTION OF ADHESION TO TRACTION.

Tractive force is the power which the pistons of the engine are capable of exerting through the driving-wheels, to move the engine and train. The efficiency of the engine’s traction is dependent upon the adhesion of the wheels to the rails; for, where the adhesion is insufficient, the pistons will slip the wheels, and no useful effect will result. To prevent the wheels of ordinary American engines from slipping on dry rails, the weight resting on the drivers must be about five times the power exerted by the pistons to slip the wheels. To prevent slipping on wet, unwashed rails, more than double the above weight would be needed. In practice, locomotives are not provided with weight enough to prevent the wheels from slipping on a greasy rail: the sand-boxes provide the means of obtaining adhesion where the rails are in bad order. A common practice is to place upon the drivers weight equal to about six times the power exerted to slip the wheels, which leaves a small margin for wet rails. Many locomotives have power sufficient to slip the wheels on dry rails; but such engines generally have boilers too small for the cylinders, or the distribution of weight on the drivers is badly effected.