Moreover, the average tractive power per locomotive and the average capacity per freight car advanced greatly in this period, although specific figures cannot be given.
The magnitude of F when p and e are put each equal to unity, is usually called the tractive force of the locomotive per pound of mean effective pressure in the cylinders.
A side wind causes excessive flange friction on the leeward side of the train, and increases the tractive resistances therefore very considerably, even though its velocity be relatively moderate.
In the first case all the driving is done on one or at most two axles, sufficient tractive force being obtained by coupling these axles when necessary to others carrying heavy loads.
If p is the mean pressure at any speed the total tractive force which the engine is exerting is given by equation (25) above.
The fundamental condition governing the design of all tractive machinery is that the wheels belonging to the axles to which torque is applied shall roll along the rails without slipping, and exert a tractive force on the train.
To obtain the tractive force the weight on the coupled wheels must be about five times this amount - that is..
Thus if the maximum horse-power which a locomotive can develop is woo, the tractive resistance R, at 60 m.
When the road leads the train up an incline, however, the tractive force must be increased, so that the need for coupled wheels soon arises if the road is at all a heavy one.
The engine can only exert this large tractive force so long as the mean pressure is maintained at 149 lb per square inch.