REVISED 06/01/03

The following calculations sizes the drive system for a locomotive.

A) ______ lbs Operating weight of engine

B) ______ lbs Total weight of cars

C) ______ lbs Total weight of passengers

D) ______ % % of steepest grade

E) ______ ft Radius of sharpest curves on grade

F) ______ lbs Total train weight = A + B + C

G) ______ lbs Rolling resistance = F x .006

H) ______ lbs Curve resistance if E = 35 ft to 45 ft H = F x .008

if E = 45 ft to 60 ft H = F x .006

if E = 60 ft to 90 ft H = F x .005

if E greater than 90 ft H = F x .003

I) ______ lbs Grade resistance = D x F x .01

J) ______ lbs Total train resistance = G + H + I (tractive effort required)

we ignore wind resistance and accretion

K) ______ Coefficient of friction – wheel on rail, range 0.1 to 0.3

we assume 0.25 for dry rail, 0.1 for wet rail

L) ______ lbs Weight on powered drivers

CHECK L x K must be greater than J

M) ______ in. Drive wheel diameter

N) ______ in Drive wheel circumference = M x 3.1416

P) ______ MPH Desired travel speed

Q) ______ in/min Travel speed in inches per minute = P x 1056

R) ______ RPM Wheel rpm = Q / N

S) ______ lb-in Total axle torque required = (M / 2) x J

T) ______ HP Total hp required at wheels = ( S x R) / 63025


From the load calculations we can determine the torque and rpm of the drive axles. One must decide on the number of motors to be used on the locomotive. This depends on the wheel arrangement, and the type of drive train the builder wishes to use.


A) ______ lb-in Torque required per load calculations “S”

B) ______ RPM Axle speed per load calculation “R”

C) ______ Number of drive motors you wish to use

D) ______ : 1 Expected reduction between motor and axle

E) ______ RPM rpm of motor = B x D

F) ______ lb-in Motor torque = (A / D) / C

G) ______ Volts system voltage selected

We now go to the motor curves and find a motor that will deliver the required torque (F) at the required RPM (E) at the selected motor voltage. You will probably have to adjust the reduction or other parameters to find a motor to fit. See illustration 1 for a typical performance curve.

Will it fit the space that is available?