Bendix Commercial Vehicle Systems TW-12 User Manual
Bendix® TW-12™ Control Valve
LEVER
CAM
LEVER CAM
PIVOT
MOUNTING
HOLE (2)
1/4" P.T. SUPPLY
PORT
LEVER
YOKE
CAM
FOLLOWER
1/8" P.T.
RETURN
PORT
SUPPLY PORT
CAP NUT
1/4" P.T.
DELIVERY
PORT
LEVER
PIVOT
LEVER
YOKE
CAP
NUT
PISTON O-RING
RETURN PORT
PISTON RETURN
PASSAGE
SD-03-3625
CONTROL
CABLE
CONNECTION
LEVER CAM
PLUNGER
PISTON
PISTON RETURN
SPRING
DELIVERY PORT
TM
FIGURE 1- TW-12
CONTROL VALVE
DESCRIPTION
The TW-12™ control valve is a manually operated, nongraduating, on-off valve. It is extensively used in hydraulic
braking systems to control devices that are responsible for
the application and release of the parking brakes.
Note: The TW-12™ control valve can also be
used in air brake system applications.
The most common configuration of the TW-12™ control valve
uses a levered cam that is manually, remote controlled. A
dash mounted lever or button may be connected to the
aluminum lever cam via Bowden cable control or connected
directly to the transmission selector.
Two .28 inch mounting holes are provided in the die cast
zinc body for panel or frame mounting. Two steel hex cap
nuts at the ends of the cylindrical valve body retain the internal
components and provide attachment for the lever cam. All
connection ports are pipe threads. Lettering embossed in
the valve body identifies two of the three ports in the valve.
Refer to the following chart and figure one.
INLET VALVE
CAP NUT O-RING
SUPPLY PORT
INLET VALVE SPRING
Connection Body Ident. Thread Sizex
Supply SUP 1/4" P.T. xx
Delivery DEL 1/4" P.T. xx
Vent NONE 1/8" P.T. xx
OPERATION
SYSTEM CONNECTIONS
Although the TW-12™ valve can be used in a variety of different
systems and connected in different ways, figure 2 illustrates
a typical hydraulic parking brake system. In this system
the TW-12™ valve controls an HR-1™ hydraulic relay that in
turn applies and releases hydraulic spring brakes.
VALVE OPERATION
Parking Brakes Applied
With hydraulic fluid flowing to the supply port (Figure 1) and
the piston in the upward position, fluid flow into the TW-12
control valve is stopped at the closed inlet valve.
™
1
POWER
STEERING
GEAR
OUT
RETURN
IN
™
TW-12
CONTROL
VALVE
DELIVERY
MASTER
CYLINDER
TO SERVICE
IN
OUT
BRAKES
HYDRAULIC
BRAKE
BOOSTER
IN
HYDRAULIC
SPRING BRAKE
ACTUATOR
SUPPLY
DELIVERY DELIVERY
HYDRAULIC
RESERVOIR
RETURN
POWER
STEERING
PUMP
FIGURE 2 - TW-12™ CONTROL VALVE HYDRAULIC PARK SYSTEM
OUT
The TW-12™ valve delivery port is open to its return port
through a passage in the center of the piston.
SUPPLY
CONTROL
RETURN
HYDRAULIC
RETURN
position to the other three times and observe that the
components controlled by the TW-12™ control valve function
promptly to the ON then OFF condition. Inspect the exterior
Parking Brakes Released
When the TW-12™ control valve plunger is depressed by the
cam action of the lever (Figure 1) the piston contacts the
inlet valve, closing the return passage. Continued piston
movement pushes the inlet valve off its seat in the body,
of the TW-12™ valve. If the TW-12™ valve is used to control
hydraulic brakes, no fluid leakage should be noted. Pay
particular attention to the area around and beneath the lever
cam.
1. When bench testing the TW-12™ control valve, attach a
allowing hydraulic fluid to flow through the valve and out the
delivery port.
SERVICE CHECKS
GENERAL
Read the "General Safety Guidelines" presented in this
manual before beginning any testing or service procedure.
With the TW-12™ control valve installed in the vehicle brake
system, proper valve operation can only be observed by the
reaction of the system and components controlled by the
TW-12™ valve. If malfunction or internal leakage is suspected
or specific operational and leakage testing is desired, the
TW-12™ valve must be removed and bench tested. Bench
testing of the TW-12™ valve is accomplished using air
pressure regardless of its application or the braking media
used in the vehicle brake system.
OPERATING AND LEAKAGE TESTS
Function test the TW-12™ valve while it is installed in the
system by starting the engine and warming the vehicle brake
system for 5 minutes. Move the lever cam from one extreme
2
2. Apply a 120psi air pressure source to the supply port
3. With the TW-12™ control valve in the released position
4. Place the valve in the applied position and again apply a
™
HR-1
RELAY
HYDRAULIC
SPRING BRAKE
ACTUATOR
IN
small reservoir volume to the delivery port with an air
gauge installed.
Note: 6-8 feet of ½” tubing or hose could be
used in lieu of a small reservoir.
and move the lever cam to the applied and released
position three times. Note that delivered air pressure is
the same as supply air pressure in the applied position
and that air is promptly exhausted in the released
position.
and 120psi at the supply port, apply a soap solution to
the return port and around the supply port cap nut.
Leakage should not exceed a 1” bubble in less than 5
seconds (100 SCCM) at the return port. No leakage is
permitted at the cap nut.
soap solution to the return port. Leakage should not
exceed a 1” bubble in less than 5 seconds (100 SCCM)
at the return port.
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