Bendix Commercial Vehicle Systems LQ-5 BOBTAIL RATIO VALVE 4/04 - Manual (Page 2)

2
TRACTOR-TRAILER MODE
Brake Application—LQ-5 ™ Valve Limiting
During tractor-trailer operation, the trailer supply valve (red
octagonal button) is depressed and delivers air pressure
simultaneously to the trailer and to the LQ-5
™ valve control
port. Air pressure enters the LQ-5 ™ valve control port and
displaces the blend back piston, against the resistance of
its spring, until the piston contacts the cover.
Control pressure holds the blend back piston against the
cover and the “initial delivery” spring forces the proportioning
piston to its seat in the body, which opens the inlet. The
“initial delivery” pressure is designed to ensure that the brake
linings contact the brake drums before proportioning occurs.
The exhaust valve is sealed due to its spring. Brake valve air
pressure enters the supply port, moves through the center
of the pistons, around the inlet/exhaust valve, and out to the
front axle actuators.
Air pressure being delivered to the brake actuators is also
present beneath the proportioning piston. Because the area
on the delivery side of the proportioning piston is larger than
that on the supply side, the delivery side needs less air
pressure to apply the same force.
Therefore after the initial delivery pressure, the LQ-5 ™ valve
delivers less air pressure to the front axle than it receives
from the brake valve.
The LQ-5 ™ valve also allows full service pressure to pass
through it when the trailer supply valve (red octagonal button)
is pulled out—when the tractor is running in bobtail mode.
Note: LQ-5 ™ valve control pressure, which comes from the
trailer supply line, will vary. As a result, the LQ-5 ™ valve
delivery and blend back ranges will also vary, because the
blend back piston works against control pressure. Figure 3
shows the LQ-5
™ valve delivery range at two control
pressures: 90 psi and 120 psi.
FIGURE 2 - LQ-5 ™ BOBTAIL RATIO VALVE INSTALLATION
FIGURE 3 - LQ-5 ™ BOBTAIL RATIO VALVE PERFORMANCE
CURVE
CONTROL PRESSURE
120 PSI
NO CONTROL
PRESSURE
0
BRAKE VALVE DELIVERY
PRESSURE
CONTROL
PRESSURE
90 PSI
FRONT
AXLE
BRAKE
CHAMBER
PRESSURE
INITIAL
DELIVERY
PRESSURE
PP-7 ™
TRAILER
SUPPLY
VALVE
SUPPLY
LQ-5 ™
VALVE
QUICK RELEASE
VALVE
BRAKE
CHAMBER
BRAKE CHAMBER
FRONT AXLE
SERVICE RESERVOIR
GAUGE
LP-3 ™ LOW
PRESSURE
INDICATOR
BRAKE
VALVE
DELIVERY