Alberta Railway Museum

Tractive Force (effort) is the power which the pistons of the engine are capable of exerting through the drive-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 this, the weight resting on the drivers must be about five times the power exerted by the pistons.

An easy calculation...................

d2 LpT= D

Where:T=Tractive force to the rails (%TE)d=diameter of the cylinder in inchesL=length of piston stroke in inchesD=diameter of the driving whels in inchesp=effective pressure on the piston in psi

Horse-power is the measure of the rate at which work is performed, and is equal to 33,000 pounds lifted one foot in one minute, or one pound lifted 33,000 feet in one minute, or one pound lifted 550 feet in one second; therefore, one horse-power equals 550 foot-pounds per second.

Calculation is:P x L x A x N

H.P= 33,000

P=effective pressure on the piston in psi

L=length of piston stroke in feet.A=area of the piston in square inches.N=number of strokes (four times the number of revolutions) per minute.

Or, by cancellation and substituting the diameter of the driving wheels, the formula can be rearranged as:C2 x S x P x (M.P.H.)H.P= D x 375 (personaly, I never liked thisformula!!)

C=diameter of cylinder in inches.

P=mean effective pressure at given speed.

S=length of stroke in inches.M. P. H.=miles per hour.D=diameter of driving wheels in inches.

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