80 AIR COMPRESSORS 



operation the pressure in the main reservoir increases until 

 it reaches 90 Ib. When the pressure below the diaphragm 

 slightly exceeds the force exerted by the regulating spring, the 

 diaphragm is raised, carrying the diaphragm valve with 

 it. The air below the diaphragm passes by the unseated dia- 

 phragm and into the chamber on top of the piston, forcing it 

 down and thus seating the steam valve. As long as main- 

 reservoir pressure remains at 90 Ib., the diaphragm valve 

 will be held from its seat and the pressure in the chamber 

 above the piston will hold the steam valve to its seat. If the 

 main -reservoir pressure falls below 90 Ib., the thrust of the 

 spring tending to force down the diaphragm will overcome 

 that of the air pressure tending to force it up; consequently, 

 the diaphragm will move downwards and seat the diaphragm 

 valve. This shuts off the air supply from the chamber above 

 the piston, and the air confined therein by the diaphragm valve 

 closing will escape to the atmosphere through the vent port c. 

 The pressure now being removed from above the piston, the 

 piston spring, aided by the steam under the steam valve, forces 

 the piston upwards, unseating steam valve, and allowing steam 

 to pass through the governor to the pump. The piston is made 

 enough larger than the steam valve to enable a moderate 

 air pressure to hold the steam valve to its seat against the 

 combined upward force of the steam pressure under the valve 

 and the push of the piston spring. 



Regulating the Governor. To increase main-reservoir pres- 

 sure, remove check-nut 17 and turn regulating nut 18 to the 

 right, increasing the tension of the regulating spring 19 until 

 the desired pressure is obtained; then replace check-nut 17. 

 To decrease main-reservoir pressure, turn the regulating nut 18 

 to the left, decreasing the tension on the regulating spring 19 

 until the pressure is decreased to the desired amount. 



Testing the Governor. The pump governor should be 

 tested to see whether standard pressure is obtained when it 

 stops the pump, also to see whether it will start the pump 

 promptly when a light reduction of not more than 2 Ib. is 

 made in the pressure that operates the governor. If the 

 pump stops either before or after standard pressure is obtained, 

 adjust the governor by means of the adjusting screw, until it 



