Bendix Commercial Vehicle Systems R-12P PILOT RELAY VALVE 4/11 - Manual (Page 3)

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FIGURE 4 - NORMAL (RAPID) APPLICATION
LOAD PISTON
INLET/EXHAUST
VALVE
CONTROL
PORT
FIGURE 5 - BALANCE
FIGURE 3 - CHARGING
DELIVERY
PORT
SUPPLY PORT
NORMAL (RAPID) APPLICATION
Figure 4 shows a Bendix ® R-12P ™ valve receiving a
“normal” service brake application. “Normal” means the
control signal is applied rapidly enough to build up pressure
on the relay piston and “open” the inlet valve, causing the
R-12P
™ valve to react like a relay valve. The relay piston
opens the inlet, allowing supply air to fl ow out the delivery
port to the next valve in the system.
Note that these events occur quickly, due to the “preload”
on the relay piston. This preload, provided by supply air
pressure acting on the load piston, is a feature unique to
the R-12P
™ valve.
BALANCE
Air pressure being delivered by the open inlet valve is
also effective underneath the R-12P
™ valve relay piston.
When air pressure beneath the piston approaches control
pressure on top of the piston, the piston and inlet valve lift
together and close the inlet valve. The exhaust remains
closed, and the valve achieves “force balance.” That is,
the piston and inlet/exhaust valve are in their neutral
state. This is the position commonly known as “balanced”
when referring to conventional relay valves. However,
conventional relay valves cannot achieve true air pressure
balance because of their inherent differentials. The R-12P
™
valve achieves this balance because of its equalizing
orifi ce. The orifi ce allows control pressure and delivery
pressure to be “common” and equalize. In this position,
shown in Figure 5, sustained service brake applications
can be held, with no “residual” differential.
RELAY PISTON
LOAD PISTON
INLET/EXHAUST
VALVE
CONTROL
PORT
SUPPLY PORT
RELAY PISTON
LOAD PISTON
INLET/EXHAUST
VALVE
CONTROL PORT
DELIVERY
PORT
SUPPLY PORT
RELAY
PISTON
EQUALIZING
ORIFICE
PROVIDES AIR
PRESSURE
BALANCE