BENDIX BW1971 Manual Do Utilizador

2

Powered by the vehicle engine, the 

builds the air pressure for the air brake system. The air 

compressor is typically cooled by the engine coolant 

system and lubricated by the engine oil supply.
The compressor's unloader mechanism and 

governor 

(along with a synchro valve for the Bendix

®

 DuraFlo 596

™ 

air compressor) control the brake system air pressure 

between a preset maximum and minimum pressure 

level by monitoring the pressure in the service (or 

“supply”) reservoir.  When the air pressure becomes 

greater than that of the preset “cut-out”, the governor 

controls the unloader mechanism of the compressor to 

stop the compressor from building air and also causes 

the air dryer to purge.  As the service reservoir air 

pressure drops to the “cut-in” setting of the governor, 

the governor returns the compressor back to building 

air and the air dryer to air drying mode.  
As the atmospheric air is compressed, all the water 

vapor originally in the air is carried along into the air 

system, as well as a small amount of the lubricating 

oil as vapor.
The

 duty cycle is the ratio of time the compressor 

spends building air to the total engine running time.  Air 

compressors are designed to build air (run “loaded”) up 

to 25% of the time. Higher duty cycles cause conditions 

that affect air brake charging system performance which 

may require additional maintenance. Factors that add 

to the duty cycle are: air suspension, additional air 

accessories, use of an undersized compressor, frequent 

stops,  excessive  leakage  from  fittings,  connections, 

lines, chambers or valves, etc.
The

 discharge line allows the air, water-vapor and 

oil-vapor mixture to cool between the compressor and 

air dryer.  The typical size of a vehicle's discharge 

line, (see column 2 of Table A on page 3) assumes 

a compressor with a normal (≤ 25%) duty cycle and 

operating in a temperate climate.  See Bendix and/

or other air dryer manufacturer guidelines as needed. 

When the 

temperature of the compressed air that 

enters the air dryer is within the normal range, the air 

dryer can remove most of the charging system oil.  If 

the temperature of the compressed air is above the 

normal range, oil — as oil-vapor — is able to pass 

through the air dryer and into the air system.  Larger 

diameter discharge lines and/or longer discharge line 

lengths can help reduce the temperature. 
The discharge line must maintain a 

constant slope 

down from the compressor to the air dryer inlet fitting to 

avoid low points where ice may form and block the flow.  

If, instead, ice blockages occur at the air dryer inlet, 

insulation may be added here, or if the inlet fitting is a 

typical 90 degree fitting, it may be changed to a straight 

or 45 degree fitting.  For more information on how to 

help prevent discharge line freeze-ups, see Bendix 

Bulletins TCH-008-021 and TCH-008-022 (available 

on www.bendix.com).  Shorter discharge line lengths 

or insulation may be required in cold climates.
The

 air dryer contains a filter that collects oil droplets, 

and a desiccant bed that removes almost all of the 

remaining water vapor.  The compressed air is then 

passed to — and stored in — the air brake service 

(supply) reservoir.  The oil droplets and the water 

collected are automatically purged when the governor 

reaches its “cut-out” setting. 
For vehicles with accessories that are sensitive to 

small amounts of oil, we recommended installation of 

a Bendix

®

 PuraGuard

®

 oil coalescing filter, designed to 

further minimize the amount of oil present.

™

supplies the 

compressed air for the braking system as well as other air 

accessories for the vehicle. The system usually consists 

of an air compressor, governor, discharge line, air dryer, 

and service reservoir.