42. Q. What is the duty of the air compressor?

A. To furnish the compressed air used in the operation of the brakes, and all other air operated appliances on both locomotive and cars.

43. Q. Explain the operation of the steam end of the compressor.

A. When steam is turned on at the boiler it flows through the steam pipe and governor, entering the compressor at the steam enlet, then through the steam passage "a" to the reversing valve chamber "C" also to the main valve chamber "A" between the differential pistons 77 and 79. The area of the piston at the right being greater than the one at the left, the main valve is moved to the right, (See Fig. 2) admitting steam to port "b" which leads to the lower end of the steam cylinder; steam is now free to flow under the main piston, forcing it upward. When the piston has almost completed its upward stroke, the reversing plate 69 on top of the piston 65 engages a shoulder on the reversing rod 71, moving the rod and reversing valve 72 upward (See Fig. 3). The upward movement of the reversing valve closes the ports "f" and "h" and opens port "g"; thus permitting steam to enter the chamber at the right of the large piston 77, balancing the pressure on this piston, and the pressure acting on the right side of the small piston 79-the chamber at the left being open to the exhaust-will force the main valve to the left.

Fig. 2: Diagrammatic View, Up Stroke.

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When the main valve moves to the left, steam is admitted through port "c" to the upper end of the cylinder on top of the piston 65, forcing it downward. At the same time the lower end of the cylinder is connected through exhaust cavity "b" of the main valve to the exhaust port "d", allowing the steam below the piston to escape to the atmosphere.

44. Q. When the piston has about completed its downward stroke, what takes place?

A. The reversing plate 69 engages the button "k" on the end of the reversing rod 71 pulling the rod and the reversing valve down. This movement of the reversing valve closes port "g" and the cavity in the face of the valve connects ports "f" and "h", which allows the steam in chamber "D" at the right of the large differential piston to escape to the exhaust, thus allowing the main valve to move to the right, exhausting the steam from the top end of the cylinder, and at the same time admitting steam to the lower end, causing an upward stroke of the piston.

45. Q. Explain the operation of the air end of the compressor.

A. The movement of the steam piston 65 is imparted to the air piston 66 by means of the piston rod. When the air piston moves up, a partial vacuum is formed below it, and air from the atmosphere will enter through passage "F" thence through passage "n" to the under side of receiving valve 86b (see Fig. 2), lifting this valve from its seat, and will fill the cylinder with air at about atmosphere pressure. 

 

 

Fig. 3: Diagrammatic View, Down Stroke.

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In the meantime the air above the piston, being compressed, will hold the upper receiving valve 86a to its seat, and when the pressure is slightly greater than that in the main reservoir, this pressure acting under the upper discharge valve 86c, will lift this valve from its seat and now the air will be free to flow through passage "G" to the main reservoir connection. On the down stroke the action is similar, air is taken in through the upper receiving valve 86a, while the air below the piston is being compressed and forced past the lower discharge valve 86d, to the main reservoir. (See Fig. 3.)

46. Q. What lift should the air valves have?

A. All valves should have a lift of three thirty-second of an inch.

47. Q. At what speed should the compressor be run to obtain the best results?

A. At 100 to 120 single strokes per minute.

48. Q. What kind of oil should be used in the air end of the compressor and on the swab?

A. Valve oil.

49. Q. How often should the air end of the compressor be oiled?

A. No fixed rule can be given as so much depends on the condition of the compressor, as well as the amount of work required; but in any case it should be used sparingly.

Cross - Compound Compressor

50. Q. What do Figures 4 and 5 represent?

A. These are diagramatic views of a cross-compound compressor.

51. Q. Why is this called a cross-compound compressor?

Fig. 4: Diagram of 8½" Cross-Compound Compressor.
High Pressure Steam (Low Pressure Air) Piston on Its Upward Stroke

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A. Because both steam and air are compounded, that is, the steam is used the second time before it is exhausted to the atmosphere, while the air is com[Pg 179]pressed the second time before it is delivered to the main reservoir.

52. Q. How many cylinders have the cross-compound compressor?

A. Four; two steam cylinders and two air cylinders.

53. Q. What is the diameter of the different cylinders?

A. The high pressure steam cylinder is 8½ inches; the low pressure steam cylinder 14½ inches; the low pressure air cylinder 14½ inches; high pressure air cylinder 9 inches.

54. Q. Explain the valve gear of this compressor.

A. The valve gear is the same as that of the 9½ or 11 inch compressor, only that a piston valve is used to distribute the steam instead of a slide valve.

55. Q. Where does the steam come from that is used in the high pressure steam cylinder?

A. Direct from the boiler.

56. Q. Where does the steam come from that is used in the low pressure steam cylinder?

A. The steam after doing work in the high pressure steam cylinder is exhausted into the low pressure steam cylinder, where it becomes the working pressure of this cylinder.

57. Q. Explain the operation of this compressor.

A. When steam is first turned on, it enters the compressor at the steam inlet (see Fig. 4) and flows through passage "a" into the reversing valve chamber "C" and on to chambers "b" and "y" against the inner faces of the differential pistons, causing the main valve to move to the right. In this position of the main valve, port "g" is open to chamber "b", thus admitting live steam to the lower end of the high pressure steam cylinder, causing an upward movement of the piston 7. When the piston 7 has nearly completed its up stroke, the reversing plate 18, which is attached to the top of this piston, comes in contact with a shoulder on the reversing rod 21, forcing it upward, carrying with it the reversing valve 22, the movement of which closes port "m", at the same time opens port "n", filling chamber "D" with live steam from chamber "C" and passage "a". This balances the pressure on the two sides of the large piston of the differential pistons, and the pressure acting against the inner side of the small piston causes the main valve to move to the left (see Fig. 5). The main valve moving to the left closes port "g" to the live steam and at the same time connects this port with port "f" leading to the lower end of the low pressure steam cylinder, causing an up stroke of the low pressure steam piston 8. In the meantime port "c", which leads to the upper end of the high pressure steam cylinder, is open to chamber "y", allowing live steam to flow down on top of the high pressure steam piston 7, forcing it downward. As the high pressure steam piston about completes its downward stroke, the reversing plate 18 engages the button on the lower end of the reversing rod 21, pulling the rod and reversing valve 22 down, closing port "n" and at the same time connecting port "m" and "l" through the exhaust cavity "q", thus allowing the steam in chamber "D" to escape to the exhaust. The pressure being removed from the outer face of the large differential piston, the main valve will again move to the right, opening port "g", admitting live steam beneath the piston 7, and at the same time connecting the upper end of the high pressure steam cylinder through port "c", chamber "h" and port "d" to the upper end of the low pressure steam cylinder, causing a downward movement of the low pressure steam piston; the steam below this piston will now be free to escape to the exhaust through port "f", chamber "i" and port "e". Thus it will be seen that the steam used in the high pressure steam cylinder is live steam from the boiler, while the steam used in the low pressure steam cylinder is the exhaust steam from the high pressure steam cylinder.

58. Q. Explain the operation of the air end of the compressor.

A. As the low pressure air piston 9 moves up, a partial vacuum is created beneath it and air from the atmosphere enters the air inlet and passage "r" past the lower receiving valve 38 and fills the lower end of the cylinder with air at about atmospheric pressure (see Fig. 4). In the meantime the air above the piston being compressed will hold the upper receiving valve 37 to its seat, thus preventing a back-flow of air to the atmosphere; at the same time the upper intermediate discharge valves 39 are forced from their seats, allowing the air from the low pressure air cylinder to flow through passage "u" to the high pressure air cylinder, the piston of which is now moving downward. The air beneath the high pressure air piston 10 being compressed will hold the lower intermediate discharge valves 40 to their seats, thus preventing the air in the high pressure air cylinder flowing back to the low pressure air cylinder. When the pressure in the high pressure air cylinder becomes slightly greater than the main reservoir pressure, the[Pg 182] final discharge valve 42 will be forced from its seat and the air beneath the piston allowed to flow to the main reservoir through passage "w". On the opposite strokes of these pistons air is compressed in a similar manner, but the opposite air valves are used.

Fig. 5: Diagram of 8½" Cross-Compound Compressor.
The High Pressure Steam (Low Pressure Air) Piston on Its Downward Stroke

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59. Q. How many valves are there in the air end of the compressor?

A. Ten; two upper and two lower receiving valves; two upper and two lower intermediate discharge valves; one upper and one lower final discharge valves.

60. Q. Are the air valves all the same size?

A. No; the receiving and final discharge valves are the same size and of the size used in the 11-inch compressor, while the intermediate valves are the same as used in the 9½-inch compressor. The receiving and final discharge valves are two inches in diameter, while the intermediate valves are one and one-half inches.

61. Q. What lift is given the different air valves?

A. All valves have 3/32-inch lift.

 

 

 

 

 

Deffects of the Compressor

62. Q. What are some of the common causes for the compressor stopping?

A. Lack of lubrication; bent, worn or broken reversing rod; loose or worn reversing plate; nuts on air end of piston rod coming off; defective compressor governor; and, in addition with the cross-compound compressor, final discharge valve broken or stuck open, or packing rings in main valve pistons breaking and catching in the steam ports.

63. Q. What will cause the piston to make an uneven stroke?

A. This may be caused by a broken or stuck open air valve, or air valves not having proper lift. Where the piston short strokes, it is generally caused by over-lubrication of the steam end.

64. Q. What are some of the common causes for the compressor running hot?

A. The overheating of the compressor may be due to any one of the following causes: Running at high speed; working against high pressure; packing rings in air piston badly worn; air cylinder worn; defective air valves; air passages or air discharge pipe partially stopped up; leaky piston rod packing; lack of lubrication.

65. Q. What will cause the compressor to run slow?

A. This may be caused by leaky air piston packing rings; final discharge valves leaking, or air passages partially stopped up. A defective governor may also cause the compressor to run slow.

66. Q. What will cause the compressor to run very fast and heat, and not compress any air?

A. This may be caused by the strainer becoming clogged with ice or dirt, preventing air entering the cylinder.

67. Q. If, when steam is first turned on, the piston makes a stroke up and stops, where would you look for the trouble?

A. The shoulder on the reversing rod may be worn; the opening in the reversing plate too large to engage the shoulder on the reversing rod; loose reversing plate studs preventing the piston traveling far enough to reverse the compressor, or the main valve stuck in its position at the right.

68. Q. If the piston makes a stroke up and a stroke down and stops, where is the trouble?

A. This may be caused by a loose reversing plate, or the button on the lower end of the reversing rod worn or broken off, or the nuts off the piston rod in the air end, or the main valve stuck in its position at the left.

69. Q. What will cause the piston to make a quick up stroke?

A. This may be caused by a broken or stuck open upper receiving or lower discharge valve.

70. Q. What will cause the piston to make a quick down stroke?

A. Lower receiving or upper discharge valve broken or stuck open.

71. Q. If a receiving valve breaks or sticks open, how may it be located?

A. The air will flow back to the atmosphere as the piston moves toward the defective valve and may be detected by holding the hand over the strainer.

72. Q. If a receiving valve in a cross-compound compressor breaks, what may be done?

A. Remove the broken valve, blocking the opening made by its removal, and as there are two upper and two lower receiving valves the compressor will now take air through the other valve.

73. Q. If an intermediate discharge valve breaks or sticks open, how may it be located?

A. No air will be taken in to that end of the compressor as the piston moves from the defective valve, and may be located by holding the hand over the strainer.

74. Q. If an intermediate discharge valve breaks, what may be done?

A. Remove the broken valve, blocking the opening made by its removal, and as there are two upper and two lower intermediate discharge valves the air will now pass from the low pressure cylinder to the high pressure cylinder through the other valve.

75. Q. If a final discharge valve breaks, what effect will it have on the compressor?

A. Will cause the compressor to stop when the main reservoir pressure is in excess of forty pounds.

76. Q. How would you test for a defective final discharge valve?

A. To test for this defect, bleed the main reservoir pressure below forty pounds, and if the compressor starts it indicates a defective discharge valve.

77. Q. If a final discharge valve breaks, what may be done?

A. As the receiving valves and final discharge valves are the same size, the defective valve may be replaced by one of the receiving valves, blocking the opening made by the removal of the receiving valve.

78. Q. Where piston rod packing is blowing bad, what may be done to stop it?

A. This generally indicates lack of lubrication, and by cleaning and oiling the swab the trouble may be overcome. However, there are times when leakage by the packing is so great that the oil is blown off the swab as fast as it is applied, therefore is of no value in lubricating the parts. Where this condition exists, a little hard grease wrapped up in an old flag and tied around the piston rod will ensure its being lubricated.

79. Q. If the compressor stops, how can you tell if the governor is responsible for the trouble?

A. By opening the drain cock in the steam passage between the governor and the compressor; if steam flows freely, the trouble is in the compressor; if not, it is in the governor.

80. Q. How may a compressor often be started when it stops?

A. By closing the steam throttle for a few seconds, then opening it quickly; if this does not start it, try tapping the main valve chamber. This will usually overcome the trouble where the compressor stops on account of lack of lubrication.

81. Q. What will cause a compressor to short-stroke or dance?

A. Too much oil in the steam end; bent reversing rod; or low steam pressure, as when the governor has almost shut off the steam.

ENGINEER'S BRAKE VALVE

82. Q. Name the different positions of the G-6 and H-6 brake valves.

A. Release, running, lap, service, and emergency position, with the G-6; release, running, holding, lap, service, and emergency positions, with the H-6.

83. Q. What is the purpose of release position?

A. To provide a large and direct opening from the main reservoir to the brake pipe, for the free flow of air, when charging and recharging the brakes.

84. Q. What pressure will be had in the brake pipe if the brake valve be left in release position?

A. Main reservoir pressure.

85. Q. Can the locomotive brake be released by the automatic brake valve in release position, when using the H-6 valve?

A. No; as the port in the automatic brake valve to which the distributing valve release pipe is attached is blanked in this position of the valve.

86. Q. What is the purpose of running position, and when should it be used?

A. This is the proper position for the brake valve when the brakes are charged and not in use, also when it is desired to release the locomotive brake with this valve. In this position the brake pipe pressure is maintained at a predetermined amount by the feed valve, as all air that now enters the brake pipe must pass through the feed valve.

87. Q. What is the purpose of holding position?

A. To hold the locomotive brake applied while recharging the brakes. The charging of the brake pipe and equalizing reservoir is the same in holding as in running position.

88. Q. What is the purpose of lap position?

A. To hold both the locomotive and train brakes applied after an automatic application.

89. Q. What is the purpose of service position?

A. This position of the brake valve enables the engineer to make a gradual reduction of brake pipe pressure, thus causing a service application of the brakes.

90. Q. What is the purpose of emergency position?

A. In this position of the brake valve, the brake pipe is connected directly with the atmosphere through[Pg 189] the large ports in the valve, causing a sudden reduction of brake pipe pressure, this in turn causing the distributing valve on the engine and all operating triple valves on cars in the train to move to emergency position, thus insuring a quick and full application of the brake.

91. Q. How should the brake valve be handled when making an emergency application of the brake?

A. The valve should be placed in full emergency position and left there until the train stops, even though the danger may have disappeared.

Defects of the Brake Valve

92. Q. What will cause a constant blow at the brake pipe exhaust port, and what may be done to overcome it?

A. This indicates that the brake pipe exhaust valve is being held off its seat, due no doubt to dirt; tapping the side of the valve will sometimes stop the blow; if not, close the brake pipe cut-out cock and make a heavy service reduction; next, place the brake valve handle in release position. This will cause a strong blow at the exhaust port, which will invariably remove the trouble.

93. Q. If the pipe connecting the brake valve with the equalizing reservoir breaks, can both locomotive and train brakes be operated with the automatic brake valve?

A. Yes; by placing a blind gasket in the pipe connection at the brake valve and plugging the brake pipe exhaust port. To apply the brake, move the handle carefully toward emergency position, making a gradual reduction of brake pipe pressure through the direct exhaust ports of the brake valve; when the desired reduction is made, the handle should be moved gradually back to lap position.

94. Q. What would be the effect if the handle were moved to lap quickly?

A. Would cause the release of the brakes on the head end of the train.

95. Q. What will cause air to blow at the brake pipe exhaust port when the handle is moved to lap position?

A. This is caused by a leak from the equalizing reservoir or its connections, which reduces the pressure in chamber "D" above the equalizing piston, allowing brake pipe pressure under the piston to force it up, unseating the brake pipe exhaust valve, permitting brake pipe air to flow to the atmosphere.

96. Q. What is the purpose of the equalizing reservoir?

A. The purpose of the equalizing reservoir is to furnish a larger volume of air above the equalizing piston than is found in chamber "D", thus to enable the engineer to make a graduated reduction of the pressure above the equalizing piston.

97. Q. What defect will cause the brake pipe and main reservoir pressure to equalize when the handle is in running position?

A. This may be caused by leakage past the rotary valve, defective body gasket, or leakage by the feed valve or its case gasket. To determine which part is at fault, close the cut-out cock under the brake valve and move the handle to service position, exhausting all air from chamber "D" and the brake pipe; return the handle to lap position. Leakage of air past the rotary valve is generally into the brake pipe port which allows the air to come in under the equalizing piston, thus forcing it upward, unseating the brake pipe exhaust valve, allowing this air to escape to the atmosphere at the brake pipe exhaust port. Leakage past the body gasket allows air to enter chamber "D", above the equalizing piston, holding it in its lower position, keeping the brake pipe exhaust port closed, thereby preventing the escape of this air to the atmosphere. Since the capacity of the equalizing reservoir and chamber "D" is small, such a leak will cause the black hand to quickly move up to the position of the red hand. To determine if the leakage be in the feed valve or its gasket, recharge the brake pipe to some pressure below the adjustment of the feed valve, then place the handle in lap position. If the black hand on the air gauge remains stationary, it is fair to assume that the trouble is in the feed valve or its gasket, as in this position of the brake valve the feed valve is cut out.

98. Q. With the engine alone, the brake pipe pressure will equalize with that in the main reservoir, while when coupled to a train the pressure will remain at that for which the feed valve is adjusted; where is the trouble?

A. This is caused by light leakage of main reservoir air into the brake pipe, and may come past the rotary valve, body gasket, or feed valve, and with the lone engine is sufficient to raise the brake pipe pressure to that in the main reservoir; while, when coupled to a train, the brake pipe leakage of which is greater than this amount, this leakage will not be noticed.