« НазадПродовжити »
chamber L, and passage e, d, d. Regulating valve 12 is held open by the force of regulating spring 17 against the center of the diaphragm 14, upon which the regulating valve bottoms.
When the air in the feed-valve pipe reaches the pressure at which the feed valve is adjusted-say 70 pounds -its power upon the diaphragm in chamber L overcomes the pressure of regulating spring 17, which is then further compressed (refer now to Fig. 43, CLOSED POSITION), and this permits the small spring 13 to drive the regulating valve 12 to the right until it seats, closing port K and thus cutting off communication between chamber G and the feed-valve pipe; the feed of main-reservoir air from chamber B by piston 8 continues, and the pressure in chamber G quickly becomes equal to that of chamber B; the air pressures on both sides of piston 8 now balancing each other, piston spring 6 forces the piston and supply valve 9 to the right, closes port c and stops the flow of air to the feedvalve pipe.
When the feed-valve-pipe pressure begins to lessen, the regulating spring, 17, again the stronger, unseats valve 12, and the pressure of chamber G equalizes with that of the feed-valve pipe; spring 6 is of very light tension, and the drop of pressure in chamber G results in piston 8 again being forced to the left by mainreservoir pressure, supply valve 9 is opened and the
flow of pressure to the feed-valve pipe is resumed; and
In this type of feed valve the duplex adjusting arrangement eliminates the necessity of the two feed valves formerly provided for high- and low-pressure service. The spring box 18 has two rings encircling it, which are split through the lugs marked 19 and 20 in the diagram, and which may be secured in any position by the screw 21. The pin forming part of adjusting handle 22 limits the movement of the handle to the distance between stops 19 and 20. When adjusting the valve, stop 19 is located so that the compression of regulating spring 17 will give the desired high brakepipe pressure, and stop 20 so that the spring compression is enough less to give the low brake-pipe pressure. After which, by simply turning handle 22 until its pin strikes either one of these stops, the regulation of the feed valve is changed from one brake-pipe pressure to the other.
To change to other minimum and maximum pressureadjustment positions of the stops, slacken screws 21, which allows stops 19 and 20 to turn around spring box 18. Then turn the adjusting wheel handle 22 (compressing the regulating spring to increase the pressure; slackening the spring tension to decrease it); adjust for the lower pressure first, and turn until the valve closes at the minimum brake-pipe pressure desired,
when stop 20 should be moved to contact with the handle pin, and securely fastened in that position by tightening its set-screw. Then turn wheel handle 22 clockwise until the higher adjustment is obtained, bring stop 19 in contact with the handle pin, and tighten set-screw 21. The stops are generally placed to give 110 pounds high, and 70 pounds low, brake-pipe pressures, the former being the pressure commonly accepted for HighSpeed Braking, and the latter is the long-established pressure for all other conditions of air-brake service.
When replacing the feed-valve on its bracket after removal, the gasket, shown in Fig. 41, must always be in place between the valve and bracket, to insure a tight joint.
The FEED-VALVE PIPE: As has been stated, besides carrying the feed-valve pressure to the automatic brakevalve, this pipe has but one other connection-the branch pipe leading to the regulating-spring chamber of the excess-pressure top of the pump governor. When the automatic brake-valve is in release position the brake pipe does not receive feed-valve pressure, and to guard against the possibility of a slight leak through the feed valve overcharging the feed-valve pipe at this timewhen it is important that the supply to the governor top shall be no greater than the regulation brake-pipe pressure-the warning port of the brake valve is supplied from feed-valve pressure; this gives relief, and