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SUPPLY
DELIVERY
FIGURE 4 - LQ-3
™
FRONT AXLE RATIO VALVE*
BALANCED BRAKE VALVE APPLICATION AT 40 PSI OR
™
LESS. (*LQ-4
VALVE OPERATION SAME AS LQ-3
Note: The top surface of the inner piston has approximately
one half of the area of the lower surface of the inner piston.
Air pressure acting on the top surface of the inner piston
exerts a force one half as great as the same air pressure
acting on the lower surface. Therefore, to balance the pis-
ton, (both inlet and exhaust closed) the pressure beneath
the inner piston may be 1/2 that above it. An example would
be: 30 psi delivered to the supply port and acting on the top
surface of the inner piston would result in approximately 15
psi at the delivery port and acting on the lower surface.
OPERATION — EXHAUST
When the service brake application is released, the air above
the inner piston of the LQ valve returns to the brake valve
and is exhausted. The air pressure at the delivery port of the
LQ valve acts on the underside of the inner piston and lifts it,
which unseats the exhaust valve. (See Figure 5.) The deliv-
ery volume air exhausts both through the brake valve and
out the exhaust port.
OPERATION — SERVICE BRAKE APPLICATION
40 PSI OR GREATER
When a service brake application of greater than 40 psi is
made, air entering the supply port forces the outer piston
into contact with the inner piston against the resistance of
the outer piston return spring. Both the outer and inner pis-
tons now move as one. Because of the added surface area
of the outer piston, delivery pressure from the LQ valve will
OUTER
PISTON
OUTER
PISTON
RETURN
SPRING
INNER PISTON
INLET & EXHAUST
VALVE RETURN
SPRING
INLET & EXHAUST
VALVE
EXHAUST
™
VALVE.)
SUPPLY
DELIVERY
FIGURE 5 - LQ-3
™
FRONT AXLE RATIO VALVE* EXHAUST.
(*LQ-4
™
VALVE OPERATION SAME AS LQ-3
"catch up" at the rate of 2:1 as supply pressure increases
from 40-60 psi. An example would be: a brake application of
50 psi would result in delivery pressure of 40 psi. For service
applications of 60 psi or greater the LQ valve does not re-
duce delivery pressure. (See Figure 6.)
SUPPLY
DELIVERY
™
FIGURE 6 - LQ-3
FRONT AXLE RATIO VALVE* BRAKE
VALVE APPLICATION AT 60 PSI OR GREATER.
(*LQ-4
™
VALVE OPERATION SAME AS LQ-3
OUTER
PISTON
OUTER
PISTON
RETURN
SPRING
INNER PISTON
INLET & EXHAUST
VALVE RETURN
SPRING
INLET & EXHAUST
VALVE
EXHAUST
™
VALVE.)
OUTER
PISTON
OUTER
PISTON
RETURN
SPRING
INNER PISTON
INLET & EXHAUST
VALVE RETURN
SPRING
INLET & EXHAUST
VALVE
EXHAUST
™
VALVE.)
3
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