Brake Tests

225 force or the amount of shock between cars and the indications of the slack action recorders is not direct nor can it be expressed in any simple terms as would be the case if the resistance to relative motion between cars could be considered the equivalent of a dynamometer element for which the amount of movement between cars would be directly proportional to the force acting. Braking POWER, Serial Action and Shocks—Low Speed Stops. 424. There is but little on record concerning the likelihood of shocks due to a high emergency braking power quickly applied at low speeds, although the general impression is that emergency applications, even at sixty miles per hour or over, are likely to be rough or even dan- gerous to passengers. This is a wrong impression. It is well known to all who have observed the action of brakes at high speeds (and it was the invariable experience during these tests) that the higher the speed the less noticeable is the application of the brakes. 425. The use of the UC pneumatic and electro-pneumatic equip- ments at the same speeds, percentages of braking power and otherwise similar circumstances, afforded an opportunity to demonstrate the most important fact in this connection, namely, that the amount of braking power, by itself, has but little to do with the shock experienced but that the rate of transmission of serial brake application in relation to the rate of build up of brake cylinder pressure on each car is the controlling factor. With the electro-pneumatic brake, in which serial quick action is entirely eliminated, there was no shock at any speed or percentages of braking power except the slight shock on the first few cars due to the running out of slack which was to be expected on account of the relatively low braking power on the locomotive. On the other hand with the pneumatic equipment, having an appreciable time interval between the application of the successive cars in the train, shocks were experienced. 426. The most marked evidence of the effect of the time element in the serial action of the brakes and resultant shocks, occurred during the emergency stops made from very low speeds. Test Nos. 648, 649 and 650 were made with the UC pneumatic equipment at ten and at twenty miles per hour. The resulting shocks, especially in the last third of the train, were extremely severe being in effect a collision between the forward end of the train, which was almost stopped, and the rear end upon which the brake application was having but little, if any, effect at the time the rear end run-in occurred. The fact that this test was made with the UC pneumatic equipment, which was relatively slow in transmitting serial quick action (Par. 194 and 199), undoubtedly caused more severe shocks than will be experienced with the considerably smaller time element of the universal valve as subse- quently modified (Par. 242).