291. The object of the switch is to adjust a single line of rails to two or more pairs, so that any two lines may be made continuous. The form in general use consists of two rails, as at a b, a b, fig. 144, moving upon a and a as centres. Here the tangent point of the turnout curve is at c. The data given for the switch are the length of switch rail and the motion at the toe (c) (which determine the direction of the starting tangent) and the radius of curvature of the turnout curve. The required elements are, the angle of frog at b and the distance from a to the point of the frog.
Fig. 144.
The following formula and table are by Josiah Hunt, Esq., (at present chief engineer of the Hannibal and St. Joseph Railroad, Mo.). The formula was first published in Appleton’s Mechanics’ Magazine, vol. 1, p. 575.
D = 2(g – s) × cot. S × cot. F
cot. S + cot. F.
Where S = angle of switch.
F = angle of frog.
s = the movement.
g = the gauge.
Example.—How far from the toe of the switch is the point of the frog, the gauge being 4′ 8½″, the rail twenty feet long, and moving five inches; the frog being six feet long, six inches wide across the head, and three inches at the mouth?