As the pull rods from each side of the truck are attached to the truck quadrant, the stresses in the brake rods are double this, or 2,076 pounds.
Fig. 51. Diagram of Brake Shoe Pressures and Strains.
The position of the brake cylinder under the car restricts the length of the “live” and “dead” cylinder levers to 16 inches. To obtain 2,076 pounds pull on one end of the levers with the previously computed 2,695 pounds on the other, the proportions must be 20764771 = x16, since 2076 + 2695 = 4771. Then x = 7 inches, the distance from the brake piston to the pivotal point.
Since 2,695 pounds pressure is exerted and 36,000 pounds results the proportion of the whole system of levers is 36,000 to 2,695 or 13.3 to 1. In other words the travel of the piston in the cylinder will be 13.3 times that of the shoes if there were no lost motion to be taken up. The piston travel should be from 4 to 5½ inches. This gives about ⅜-inch travel of the brake shoes. Increased travel of the brake shoes necessary to set them as they wear away causes increased travel of the piston of the air cylinder. Not only is more air used at each application of the brakes but the brakes are slower in acting. It is therefore necessary to adjust the brakes frequently. This is done in the system shown in the diagram by the use of a turnbuckle in the connecting rod between the live and dead levers of the truck.
When two motors are on one truck and none on the other, allowance must be made in the levers for the increased weight of the motor truck and the inertia of the armature. The leverage on the motor truck must be greater than on the other.
Air Brakes. Air brakes used on electric railway cars are usually of what is called the straight air brake type in distinction to the Westinghouse automatic air brake. A straight air brake is one in which the air is stored in a reservoir; and, when the brakes are to be applied, air from this reservoir is turned directly into the brake cylinder, in which works a piston operating the brake levers. Air admitted behind the piston forces it out with a pressure which applies the brakes. When the air is let out of the brake cylinder, a spiral spring forces the piston back to its original position and the brakes are released. The motorman’s valve by which he applies the brakes, therefore, provides, first, for turning air from the storage reservoir to the brake cylinder to apply the brakes, and, second, for closing the opening to the storage reservoir and opening an exhaust passage from the brake cylinder so that the air can escape from the brake cylinder to release the brakes.
Straight air brakes of this kind would not be suited to the operation of long trains, because, if the air-brake hose connection between cars should be broken, the brakes would be useless; but for trains of one or two cars, such as are common in electric railway practice, the simplicity of the straight air brake outweighs its disadvantages and this is the type of brake usually employed. (See [Fig. 52].)
The Westinghouse and other forms of automatic air brake are used on electric railways where cars are operated in long trains; but it is out of the province of this paper to describe these brake systems fully, as they are rather complicated. It may be said in general, however, that the Westinghouse automatic air brake is so arranged that, should the hose connection between cars be broken, should the train pull in two, or should anything happen to reduce the pressure which is maintained in the train pipe that runs the length of the train, the brakes would immediately be applied on the entire train.
