Bendix LQ-5 BOBTAIL RATIO VALVE User Manual
®
Bendix® LQ-5™ Bobtail Ratio Valve
3/8 P.T.
SUPPLY PORT
1/8 P.T.
CONTROL PORT
SD-03-953
1.5"
.340”
MOUNTING
HOLES (2)
STAMPED LQ-5
REPLACEMENT
PC. NO.
3/8 P.T.
DELIVERY PORT
FIGURE 1 - LQ-5™ BOBTAIL RATIO VALVE
DESCRIPTION
The LQ-5™ bobtail ratio valve is used on the front (steering)
axle of tractor air brake systems to reduce brake application pressure during normal tractor-trailer operation. It is
designed for tractor systems only and replaces the existing
front axle limiting valve. It is not to be used on straight trucks
or towing trucks. Also, a quick release valve must be installed between the LQ-5™ valve delivery and the brake
chambers.
IMPORT ANT: Do not replace the BP-1™ front brake proportioning valve with the LQ-5™ valve. The LQ-5™ valve
will not function properly in the BP-1™ valve system.
Caution: The LQ-5™ valve can replace the LQ-4™ ratio valve
on tractors. The LQ-5™ valve control port must be connected
to the trailer supply line. When the tractor is disconnected
from the trailer, bobtail tractor braking performance is improved
because full brake valve pressure will be applied to the
steering axle brakes.
Although it is not mandatory, the LQ-5™ valve is used in
conjunction with the BP-R1™ bobtail brake proportioning relay
valve, only when a customer specifies a front axle ratio valve.
The BP-R1™ valve and LQ-5™ valve combine to reduce
stopping distance and improve tractor control in bobtail mode.
(For more information on the BP-R1™ valve, see SD-03-1067.)
OPERATION
GENERAL
After an initial delivery , the LQ-5™ valve limits front axle brake
pressure during normal tractor-trailer operation. The LQ-5
valve has two spring-loaded pistons: a blend back piston
and a proportioning piston. The “stepped” proportioning piston
determines actual air pressure reduction and the blend back
piston allows the valve’s delivery pressure to “blend back”
and reach a 1:1 ratio during high pressure brake applications.
™
1
BRAKE
CHAMBER
SUPPLY
™
PP-7
TRAILER
SUPPLY
VALVE
BRAKE
VALVE
QUICK RELEASE
VALVE
™
LQ-5
VALVE
DELIVERY
BRAKE CHAMBER
FIGURE 2 - LQ-5™ BOBT AIL RA TIO VALVE INSTALLATION
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.
CONTROL PRESSURE
120 PSI
FRONT
AXLE
BRAKE
CHAMBER
PRESSURE
CONTROL
NO CONTROL
PRESSURE
INITIAL
DELIVERY
PRESSURE
FIGURE 3 - LQ-5
CURVE
2
0
BRAKE VALVE DELIVERY
PRESSURE
™
BOBTAIL RATIO VALVE PERFORMANCE
PRESSURE
90 PSI
™
LP-3
LOW
PRESSURE
INDICATOR
GAUGE
FRONT AXLE
SERVICE RESERVOIR
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.
COVER
DUAL
BRAKE
VALVE
BLEND
BACK
SPRING
BODY
TRAILER
SUPPLY
VALVE
FRONT
BRAKE
CHAMBER
FIGURE 4 - LQ-5™ BOBTAIL RATIO V ALVE LIMITING: TRACTOR-TRAILER MODE (SHOWN DURING INITIAL DELIVERY)
INLET/
EXHAUST
VALVE
BLEND
BACK
PISTON
INITIAL
DELIVERY
PISTON
PROPORTIONING
PISTON
BALANCED BRAKE VAL VE APPLICA TION - LQ-5
™
V ALVE HOLDING
The forces acting on the proportioning piston are equal when
air pressure beneath the piston is approximately one-half of
that above. (For example, if a 40 psi brake application is
made, approximately 20 psi would be present beneath the
proportioning piston.) At this point, the proportioning piston
lifts and closes the inlet. The exhaust passage remains
sealed. With both the inlet and exhaust valves closed, the
LQ-5™ valve is balanced—front axle air pressure is neither
increasing nor decreasing.
BRAKE VAL VE RELEASE
Upon brake valve release, air pressure above the blend back
piston returns to the brake valve and exhausts. Air pressure
from the delivery line lifts the proportioning piston and inlet/
exhaust valve away from the exhaust seat in the body . Air
then flows out the open exhaust passage. Air from the
actuators exhausts through the quick release valve. (See
Figure 2.)
If the service brakes are rapidly released, air pressure may
lift the inlet/exhaust valve further against the relatively light
resistance of the valve spring. Air would then also flow back
through the open inlet and out to the brake valve.
BRAKE VALVE APPLICATION
PRESSURE
APPROXIMATELY 1/2 OF
BRAKE VALVE
APPLICATION PRESSURE
FIGURE 5 - LQ-5™ BOBTAIL RA TIO VALVE HOLDING:
TRACTOR-TRAILER MODE
3
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