Зображення сторінки

comes into contact with the spring head, 60, at which point the piston's movement ceases, with the triple valve in the service-application position, as shown; the first movement occurs when the brake-pipe pressure has been reduced by 2 to 4 pounds, when the piston responds by moving far enough for the shoulder on the end of the stem to strike against the left end of slide valve 31, and if the brake-pipe pressure reduces slowly-as is always the case when a service application is made from a long train line—there will be a slight pause of the triple-valve piston at this point, due to the frictional resistance of the slide valve from the air pressure upon it, but the feed-groove, u, will have been closed, thereby trapping the auxiliary-reservoir air against any back flow, and graduating valve 28 will have been drawn from over port z in the slide valve; when the brake-pipe reduction has amounted to from 5 to 7 pounds, the resistance of slide valve 31 will have been overcome and the triple piston will complete its service travel, carrying the slide valve with it and bringing port z into register with port h in the slide-valve seat; the auxiliary-reservoir air in chamber x now flows through port z to port h, and enters the tube L, through the auxiliary reservoir, that carries it to the brake cylinder, where, acting upon the brake piston, the air pressure accomplishes the application of the brake.

Automatic Graduation

The automatic graduation of the pressure supplied to the brake cylinder is the paramount feature of the triple valve. It may be asked at this point—why, if graduating port z is fully opened to port h, does not the brake apply full-on, by the auxiliary-reservoir air continuing to feed to the brake cylinder until their pressures are equal? And the answer to this question explains the “secret” of the triple valve-gives the reason for its automatic graduation of braking power: As the result of a 10-pound reduction the brake-pipe pressure stands at 60 pounds; the auxiliary-reservoir air (“you can't have your penny and spend it, too), in supplying the brake cylinder, drops in pressure until it becomes just a trifle less than the brake-pipe pressure, and when that occurs the superior pressure acting upon the triple-valve piston is on the rightthe brake-pipe side-of it, and although the difference in pressures may only be a matter of ounces there is but little frictional resistance to piston 26's leftward movement until it slides the short distance necessary to bring the other shoulder of the piston's stem against slide valve 31, and then the frictional resistance of the slide valve is encountered and the triple piston is again stopped; the slight, leftward movement of piston 26 closed the graduating valve, 28, cutting off further supply to the brake cylinder and arresting the fall of auxiliary-reservoir pressure when it has become just

enough weaker than that of the brake pipe to permit of the back lash of the piston, but not weak enough to permit movement of the slide valve; the triple valve has then assumed the position of service lap, as shown in Fig. 4 C.

It should be stated that the graduating spring that offers an effective resistance to the movement of the triple-valve piston beyond the service-application position is not absolutely necessary in order that the piston shall stop at that point; a triple valve may work very well without the graduating spring; it may be removed, and if the brake-pipe pressure is not reduced more rapidly than the rate provided for in the equalizing-discharge port of the engineer's brake-valve, and if the triple valve is not sticking and sluggish in movement as the result of dirt and lack of lubrication, the service movement will be accomplished just as well and the triple-valve piston will stop in exactly the same position as though the resistance of the graduating spring had been interposed; the reason is that when the triple valve has reached the service-stop position the auxiliaryreservoir pressure that moved it thus far begins to reduce through the graduating port to the brake cylinder, and this fall of the motive force will be as rapid as, or more so than, the reduction of the brake-pipe pressure, with the result that the air pressures on both sides of the triple-valve piston are nearly equalized, and the

Emergency Application

frictional resistance of the slide valve overcomes any slight balance of application force, and the whole mechanism is halted until the back lash to service lap occurs.

Fig. 4 D-EMERGENCY-APPLICATION POSITION: Port h in the slide-valve seat is of greater area than appears, as, instead of being a circular hole, it extends transversely across the valve seat to nearly the width of the slide valve, and only its narrowest diameter appears; but at service application the pressure can not flow through it any faster than the smaller, round, graduating port, z, can supply, which is a rate of flow desirable for service action of the brake; when it is desired to stop quickly, however, the full capacity of the large port in the seat is demanded to supply the reservoir air to the brake cylinder rapidly enough-say in “spotting” the engine on the turntable, at a water column, or in case of emergency on the road—and when such an occasion arises, the engineer's brake-valve being thrown to the emergency-application position, the sudden and heavy reduction of brake-pipe pressure induces the movement of the triple-valve piston to the right in the same manner as explained in connection with Fig. 4 B, except that the stroke is quicker, and instead of the piston being halted by the graduating spring the latter is compressed and the piston completes its full travel, assuming the emergency application position as represented by Fig. 4 D, in which it is seen that the slide

valve, 31, is drawn completely off of port h in the seat, exposing the full opening of the latter for the more rapid passage of the auxiliary-reservoir pressure to the brake cylinder.

Referring again to service application: In order to apply the brake with full-service force it is only necessary to reduce the brake-pipe pressure to the same figure at which the auxiliary reservoir and brake cylinder will equalize; with an initial pressure of 70 pounds in the former they will not always equalize at the same figure, and this is due to the variation in volumes of the pressure spaces in the brake cylinders of a train, this pressure space being greater or less as the brake piston may have a longer or shorter travel; the length of piston travel is proportionate to the amount of slack in the brake rigging—the longer the piston travel the greater the space that must be filled in the cylinder; and with increased expansion of the air there is decreased pressure; the piston should never travel much more than one-half of its full stroke, and if properly adjusted, the auxiliary reservoir and brake cylinder should equalize at 50 pounds per square inch, and to secure this equalization calls for a 20-pound brake-pipe reduction (from 70 pounds to 50 pounds); in regular train service, the equalization will give brakecylinder pressures anywhere between 45 and 55 pounds, due to minimum and maximum piston travels, and

« НазадПродовжити »