CALCULATING POWER OF LOCOMOTIVES.
The capacity of engines is generally expressed in horse-power, which is a measurable quantity; but, for several reasons, that method of indicating power has not been usually applied to the locomotive. When practical railroad men hear the size of cylinders, the diameter of driving-wheels, and the boiler dimensions of a locomotive, mentioned, they understand what kind of service the engine is adapted for, and about the weight of train it can haul. As it has been found necessary for designing and other purposes, to estimate, with some degree of accuracy, the work a locomotive is capable of doing, it has become usual to reckon the power of a locomotive by the tractive force it can exert upon the rails.
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.
ESTIMATING TRACTIVE POWER.
The easiest way of calculating the tractive power of a locomotive is by use of the following simple formula, first propounded by Pambour:—
d2Lp
T = —————.
D
d = the diameter of the cylinder in inches.
L = the length of the stroke in inches.
D = the diameter of driving-wheels in inches.