Every-day Science: Volume 7. The Conquest of Time and Space

by Henry Smith Williams; Edward Huntington Williams

Other inventors had tried to produce a practical system of brakes, such as those using steam as a working force, or systems of hand-wound springs; but Mr. Westinghouse utilized compressed air, and from the first his brakes proved effective.

His first air brake, operated successfully in 1869, was the "straight air brake" type—one that has now been replaced almost universally by the automatic. In this brake system there was an air reservoir on the locomotive, and steam was used for making the compression. From this reservoir a line of gas pipe ran through the cab of the engine beneath the tender and under each car, the space between the cars being bridged by rubber tubes and easily-adjusted couplings. This line of pipe, called the train pipe, was connected near the centre of each car with a cylinder which contained a piston with a stem which acted upon the brake shoes by means of a series of levers and connecting rods.

In the cab, placed conveniently for the engineer, was a valve by means of which he could cause the compressed air to flow into the train pipe and thus act upon the brake cylinders of the cars. This could be done gradually for making a slow stop, or with full force as the case required, and the brakes could be released by turning the valve to a point which opened a vent and allowed the air to escape.

The effect of this invention was revolutionary. Stopping the train was no longer dependent upon manual labor applied intermittently at different points, but was placed entirely in the hands of the engineer who applied the required power almost simultaneously at all points along his line of cars. Thus the brakeman was relieved of one of his perilous tasks, which on freight trains took a heavy toll in loss of lives.

This relatively simple, and usually effective, system had two grave defects. The first of these lay in the fact that if there was a leak—even a very small one—anywhere along the line of the train pipe or the brake cylinders, the brakes would not work, the compressed air being exhausted into the atmosphere instead of acting on the brake cylinders. The common accident of having his train "break in two" rendered the engineer powerless to stop the cars, and disastrous "runaways" sometimes resulted. The second defect, which became more and more apparent as the length of trains was increased, was the impossibility of applying the air to the brakes of the rear cars as quickly as to those near the engine, since the compressed air could not travel the length of the train pipe instantaneously, on account of the frictional resistance.

These defects were quickly recognized by Mr. Westinghouse, and in 1876, seven years after he applied his first invention, he produced his automatic air brake which overcame them effectually. In this brake the train pipe and the air reservoir were retained as in the straight air brake system, but in addition each car was equipped with a storage reservoir of sufficient size to supply the brake cylinder. In place of the older arrangement in which the train pipe simply retained air at atmospheric pressure when not in use, the new system kept the air in the train pipe under a considerable pressure at all times when the brake was not in use. And, reversing the conditions of the straight air brake, the engineer in order to apply the brakes let out the air in the train pipe instead of forcing air into it, a "triple valve" on each car performing the work of operating the brake cylinder automatically.

The advantage of this system over the older one is obvious. Whereas the detachment of a portion of the train, or a leak in any part of the air brake system heretofore had left the engineer helpless, exactly the reverse condition was produced in the new system. Any leakage of air, either from a break or a defect, caused every brake on the entire train to be applied to the wheels and brought the train to a stop. Moreover, with the new system it was now possible to equip each car with a valve which would lessen the pressure of air in the train pipe so that the train could be brought to a stop by the trainmen in the rear or intermediate coaches as readily as by the engineer.

This system worked perfectly on passenger trains; but on long freight trains the resistance to the passage of the escaping air through the train tube was so great that if an emergency required the full force of the brake to be applied suddenly, the brakes of the rear cars did not come into use until several seconds after those of the forward cars. The result was that the momentum of the rear cars caused them to strike the forward cars with great violence. But Mr. Westinghouse overcame this defect by an ingenious use of the triple valve mechanism of each car, whereby the application of the emergency brake by the engineer caused the air in the train pipe on each car to be discharged simultaneously into the brake cylinder. In this manner the discharge of air not only allowed the brakes to act, but assisted them in doing so. This was only the case, however, when the emergency application of the brake was made, this system of venting on each car into the brake cylinder not being brought into play when ordinary stops were made. Thus the engineer in this quick-action automatic air brake has really two brakes at his command, one for making ordinary stops, the other for emergencies.

In 1891 a so-called high-speed air brake was perfected, this brake being really a modified quick-action automatic brake. This modification consists of the addition of an automatic pressure-reducing valve connected with each brake cylinder. In the high-speed air brake as applied when the train is running rapidly, the highest possible pressure is applied at once to the wheels, but this pressure is lessened by the automatic pressure-reducing valves as the speed diminishes. This method of applying the brakes is the most effective way of getting the full benefit of their stopping power. This high-speed brake, therefore, represents the highest perfection in train-stopping devices.

We have referred here specifically to the air brake as used on steam railroads. In another chapter the subject has been touched upon in connection with electric railroads. In such brakes the compression of the air is accomplished by electricity instead of steam, but the general principles involved are the same as those just described.