RE: [ihc] RE: Shifty Mandera, air brakes

["Mac McMuffin" <mac@xxxxxxxxxxxx>]

## >> >I'm curious how the big-rig air brake compressors know when to cycle
## >> on/off.
## >>
## >> Good question. Electric clutch? Normally on? Constant duty
## >> with a bypass?
## >> Dave N? Greg?
## >>
## >> Jim

for all the air brake systems i'm familiar with, pressure in the air system
is regulated by an in-line pressure-activated mechanical governor and
constant-duty compressor.  when the system pressure falls below 95psi, the
governor cuts in and the compressor feeds more pressure to the system.  the
governor cuts back out again when overall pressure hits 110psi.  the
pressure values can vary a bit up or down from one truck or manufacturer to
another, but 95 to 110 is the standard.  on my new-to-me B182 the compressor
is driven by a belt from the engine to a direct-drive pulley on the front,
no clutch or other cut-out device, and i believe the B184 is the same way.
the governor is a stand-alone device a few feet up the line on one of the
two air lines coming out of the compressor, the other line going from the
compressor to a one-way check valve on the air tank.

from what i can determine, there is a mechanical air-controlled valve on the
top of the compressor.  the governor senses low pressure on the system-side
of itself and either pressurizes or depressurizes the line (i don't remember
which) from it to the valve on top of the compressor, opening the line to
the air tank one-way check valve.  when pressure on the system-side of the
governor peaks, it reverses the line to the valve on the compressor,
shutting off the feed to the air tank and venting the compressor to the open
air.  nothing in the system is electric other than the low pressure buzzer
in the cab.

standard nickel/grain of salt as per normal

--Mac

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Krystof "Mac" MacBryghde
TyrGothi

http://master.triad.ath.cx/
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