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Bendix® LQ-5™ Bobtail Ratio Valve
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1.5" |
3/8 P.T. |
.340” |
SUPPLY PORT |
MOUNTING |
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HOLES (2) |
STAMPED LQ-5 REPLACEMENT
PC. NO.
1/8 P.T.
CONTROL PORT
3/8 P.T. 
DELIVERY PORT
FIGURE 1 - LQ-5™ BOBTAIL RATIO VALVE
SD-03-953
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.
IMPORTANT: 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
PP-7™ |
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TRAILER |
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SUPPLY |
BRAKE |
VALVE |
VALVE |
SUPPLY |
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QUICK RELEASE |
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VALVE |
LP-3™ LOW |
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PRESSURE |
LQ-5™ |
INDICATOR |
VALVE |
GAUGE |
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DELIVERY |
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FRONT AXLE |
BRAKE CHAMBER |
SERVICE RESERVOIR |
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FIGURE 2 - LQ-5™ BOBTAIL RATIO 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.
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
120 PSI
FRONT |
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AXLE |
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BRAKE |
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CHAMBER |
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PRESSURE |
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CONTROL |
NO CONTROL |
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PRESSURE |
PRESSURE |
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90 PSI |
INITIAL |
0 |
BRAKE VALVE DELIVERY |
DELIVERY |
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PRESSURE |
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PRESSURE |
FIGURE 3 - LQ-5™ |
BOBTAIL RATIO VALVE PERFORMANCE |
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CURVE |
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2 |
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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
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BLEND |
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BACK |
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SPRING |
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BODY |
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TRAILER |
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SUPPLY |
BLEND |
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VALVE |
BACK |
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PISTON |
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INITIAL |
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DELIVERY |
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PISTON |
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PROPORTIONING |
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FRONT |
PISTON |
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INLET/ |
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BRAKE |
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CHAMBER |
EXHAUST |
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VALVE |
FIGURE 4 - LQ-5™ BOBTAIL RATIO VALVE LIMITING: TRACTOR-TRAILER MODE (SHOWN DURING INITIAL DELIVERY)
BALANCED BRAKE VALVE APPLICATION - LQ-5™ VALVE 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 VALVE 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 RATIO VALVE HOLDING:
TRACTOR-TRAILER MODE
3
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