Fig. 59. Automatic Coupler.
Rate of Retardation in Braking. The rate of retardation of cars in braking is usually 1 to 2 miles per hour per second. In other words a car going at a speed of 40 miles an hour will usually be stopped in 40 to 20 seconds.
The plotted results of some braking tests ([Fig. 58]) show a higher rate of acceleration. These tests were made on an interurban car weighing about 63,000 pounds, equipped with straight air brakes. Of the six curves shown, that giving the highest rate of retardation is No. 4. This shows a stop from a speed of 38 miles per hour in 9½ seconds or a rate of retardation of about 4 miles per hour per second. All of the curves shown are for emergency stops. They show about the highest rate of retardation that could be made with the equipment.
Drawbars and Couplers. For small surface cars a crude drawbar is usually provided consisting simply of a straight iron bar pivoted under the car and provided with a cast-iron pocket near the end. A coupling pin passing through the pocket of one coupler and through a hole in the end of the bar of the other, holds the two cars together.
The requirements of a coupler for heavier cars such as those used on interurban and elevated roads are more exacting. The ends of the bars are usually pivoted under the car about five feet back from the bumper. A spring cushion intervenes between the pivot point and the drawbar head. The illustrations, Figs. [59] and [60], show the action of the Van Dorn Automatic coupler, which is the one used by all the elevated lines in the United States. The link is placed in one of the drawbar heads and the pin in the other. As the cars come together the wedge-shaped end of the link forces its way between the pin and a spring. When the faces of the drawbar heads meet, the spring forces the link to engage the pin. The mechanism is designed especially to prevent lost motion between coupler heads because, unlike steam railroad drawbars, electric car drawbars must swivel to round curves and a great amount of play at the point of coupling with swiveling drawbars would allow the couplers to bend under a pushing strain.
Fig. 60.
CAR CONSTRUCTION.
Car Bodies. In cities there are three general types in common use; namely, box cars, suited for winter use only; open cars, suited for summer use only; and semi-convertible cars, which can be adapted to either summer or winter use. The open and box cars are the older types. The semi-convertible car is usually provided with a center aisle, and cross seats on each side of this aisle. The windows are large, so that they can be lowered or raised in summer to make something approaching the character of an open car. The car bottom, which forms the basis for the entire car structure, is constructed with longitudinal sills either of steel or of wood combined with steel. One form of construction employs as the main supports two steel channel bars extending the full length of the car. Steel I-beams are sometimes used. Where wood is used in combination with steel for longitudinal sills, the steel is usually in the form of flat steel plates between the timbers. Most cars seat about one passenger per foot of length over all.