When the pressure in the train pipe is diminished by the brakes being applied, the diaphragm is restored to the position it occupies in the figure by the action of the spring 16. Then valve 13 is seated by the spring 12, and the steam pressure accumulates on the upper end of valve 10, forcing it down and letting the steam from a into b and thence into the steam cylinder, starting it into action, which continues until the pressure in the train pipe exceeds 70 lbs. per square inch.

The use of this governor not only prevents the carrying of an excessive air pressure in the train pipe, which may result in entirely preventing the wheels from revolving and causing a flat place to wear on the wheel tire, but it also causes the accumulation of a surplus of air pressure in the main reservoir, while the brakes are applied, which insures the release of the brakes without delay. It also obviates the unnecessary working of the pump when the desired air pressure has been obtained.

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Fig. 3346.

A sectional view of the steam and air cylinders is shown in [Fig. 3346], the construction being as follows:

Steam is distributed to the steam cylinder by means of a piston valve, composed of three pistons, marked 16, 14, and 20 respectively, the steam entering between pistons 16 and 14, and, in the positions in which the parts occupy in the figure, steam can pass through the bushing 18 and beneath the steam piston 7, propelling it upwards until the bottom of the hole in its piston rod strikes the end of rod 12, and raises it and valve 13. The chamber 23, in which valve 13 works, receives steam through a suitable port from the steam space between valves 14 and 16; and the steam from chamber 23, it is that (in the positions the parts occupy in the figure), acting on the area of the large valve piston 20, holds the valve down against the pressure on the bottom face of piston 14 of the valve. As soon, however, as the piston rod 7 strikes and raises rod 12 and valve 13, the steam is exhausted from the top of piston 20 of the valve, and the steam beneath piston 14 of the valve raises the valve, opening the lower port in the sleeve 18 for the exhaust, and piston 14 admits steam to the upper side of the steam piston 7. The construction of the air cylinder differs somewhat from that shown in the freight locomotive, [Fig. 3326], this air pump corresponding with that shown on the engine and tender, [Fig. 3342]. A detail list of the parts may be given as follows:

No.
2.Steam cylinder head (with reversing cylinder, piston, and valve bushes).
3.Steam cylinder (with main valve and bushes).23.Reversing valve bush.
4.Centre piece.24.Reversing valve cap.
5.Air cylinder (with lower discharge valve).25.Piston rod nut.
6.Air cylinder head.26.Piston packing gland.
7.Steam piston and rod.27.Piston packing nuts.
8.Air piston.28.Packing glands.
9.Piston rings.29.Right Chamber cap.
10.Steam piston plate.30.Left chamber cap.
11.Steam piston bolt.31.Air valve bushing.
12.Reversing rod.32.Air valve.
13.Reversing valve.33.Air valve.
14.Piston valve.34.Air valve.
15.Piston valve rings.35.Delivery union.
16.Piston valve rings.36.Exhaust steam outlet.
17.Upper valve bushing.40.Steam cylinder gasket.
18.Lower valve bushing.42.Top air-pump gasket.
19.Reversing piston casing.43.Bottom air-pump gasket.
20.Reversing piston.44.Waste water pipe.
21.Piston rings.46.Exhaust union stud.
22.Reversing cylinder cap.49.Air exhaust union stud.