The weight upon any one pair of wheels is as their distance from the centre of gravity; by changing their position we change the applied weight.
The flange base[[10]] must increase as the engine becomes heavier, when applied to fast trains, as more leverage is necessary to keep it on the rails. Heavy freight engines with four or five pairs of wheels, and no truck, wear the rails and strain themselves very much. We should make the wheels of such very small and near together, in order to contract the flange base.
[10]. Wheel base,—Horizontal length between centres of extreme wheels. Flange base,—Horizontal length between centres of extreme fixed flanged wheels.
DISTRIBUTION OF WEIGHT.
346. Suppose the whole load upon the wheels is 60,000 lbs. If the centre of gravity is half-way between the wheels (there being two pairs), each will support 30,000 lbs. If the centre of gravity is twice as near to one axle as to the other, the furthest one will support 20,000 lbs., and the nearest one 60,000–20,000, or 40,000 lbs.
Suppose the engine has six points of support, or three points in the side elevation, (the ordinary four driving wheels and a truck engine). Let the centre of gravity be one foot behind the middle axle and the distances between the wheel centres eight feet.
The weight upon the middle axle being H, that upon the hind axle is H
7, because that axle is seven times more distant from the centre of gravity than the middle one, and for the same reason the weight upon the front axle is H
9.
Now H + H
7 + H
9 = 60,000 lbs.
Whence H = 47,976 lbs.
Also, H
7 = 6,853 lbs.