Air hoses and fittings

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Relay Emergency Valve

Construction Mechanic Basic Volume 2

Servicing air brakes

AIR HOSES AND FITTINGS
Air hoses and fittings (fig. 7-55) provide a means of making a flexible air connection between points on a vehicle which change their position in relation to each other or between two vehicles. All air brake assemblies used to connect the air brake systems from one vehicle to another are equipped with detachable fittings and spring guards.

When installing a hose assembly where both ends are permanently connected, use the air hose connector assembly at each end as the union to permit tightening the hose connectors in place. Loosen the nut on one of the connector assemblies and then turn the hose in the loose connector to avoid kinking the hose.

To prevent dirt and moisture from entering unused air lines, use dummy couplings (fig. 7-56). The two types of dummy couplings are as follows:

Bracket-type couplings are mounted to the vehicle for storage of unused hose.

Chain-type couplings are attached to the vehicle by a chain and placed in couplings mounted on the vehicle.

SYSTEM SWITCHES AND INDICATORS
The switches and indicators in an air brake system are designed as safety devices. The two most common safety devices found in an air brake system are the low-pressure warning indicator and the stoplight switch.

Low-Pressure Warning Indicator
The low-pressure warning indicator (fig. 7-57) is an electro-pneumatic switch connected with a warning buzzer and, in some designs, a warning light or both. It remains in the OPEN position when air pressure is above approximately 60 psi. When pressure drops below 60 psi, the spring forces the diaphragm down and closes the contacts, which operate the warning

Figure 7-56.- Dummy couplings.

Figure 7-57.- Low-pressure warning indicator.

device. Normal operating pressure is 60 psi, plus or minus 6 pounds.

Stoplight Switches
Stoplight switches (fig. 7-58) in an air brake system are electro-pneumatic devices, which operate in conjunction with the treadle valve to close the

Figure 7-55.- Air hose and fittings.

Figure 7-58.- Stoplight switch.

stoplight circuit when the brakes are applied. When air pressure from the treadle valve enters the cavity on the one side of the diaphragm, the diaphragm changes position. This action overcomes the force of the spring and moves the contact plunger until the contacts close. This closes the stoplight electrical circuit causing the brake lights to come on. The switch is designed to close as soon as 5 psi is delivered to it. This means that the stoplight circuit closes immediately on brake application.