TRIPLE VALVES 219 



valve. The safety valve, being set at a pressure of 62 lb., 

 will prevent the brake-cylinder pressure from rising above 

 this amount during a service application. 



Full-Service Position. When a service reduction is made 

 with a short train, brake-pipe pressure will reduce faster than 

 when the train is long, resulting in a greater difference between 

 brake-pipe pressure and auxiliary-reservoir pressure being 

 formed. This will cause the triple piston to compress the 

 graduating spring slightly and move the slide valve and gradu- 

 ating valve a little beyond quick-service position until port o 

 ceases to register with port y. The triple is then in full-service 

 position. When the slide valve is in this position, ports z 

 and r register fully, the quick-service port y is blanked by the 

 slide valve, and no brake-pipe air can pass to the brake cyl- 

 inder from chamber Y. The local reduction of brake-pipe 

 pressure at each triple valve is thus prevented, for the reason 

 that it is not necessary; the reduction is as quick as desirable. 

 The brakes apply promptly, because the service port z is then 

 fully opened. 



Service Lap Position. The lap position assumed by the 

 triple valve from quick-service position differs from the posi- 

 tion it assumes from full-service position, owing to the fact 

 that the slide valve is not moved when the piston moves 

 the graduating valve to lap the service ports. The triple 

 valve is held in service position as long as the brake-pipe 

 reduction continues. When the brake-pipe reduction ceases 

 auxiliary-reservoir air continues to flow into the brake cylinder 

 until auxiliary-reservoir pressure is reduced below brake-pipe 

 pressure sufficiently to cause the triple piston to be moved 

 toward release position and the shoulder of the piston stem 

 to come in contact with the slide valve. The difference in 

 pressure necessary to move the piston and graduating valve 

 is not sufficient to overcome the additional friction encountered 

 in moving the slide valve, so that further movement of the 

 piston is stopped by the slide valve. 



When the piston starts to lap position from quick-service 

 position, the parts come to rest in quick-service lap position. 

 In this position the graduating valve 7 closes port z and its 

 cavity v is moved from over port /, so that no more air can 



