1.21.4. Tow Point Lift System. The tow point lift
system consists of individual switches on the screed
control panels, right auxiliary hydraulic pump, 50/50 flow
divider, flow regulator valve, a second 50/50 flow divider,
solenoid valve, hydraulic cylinders and needle valves.
Power from CB4 is applied to the tow point movement
valve raise and lower solenoids. The tow point
movement switches provide a return path for the
electrical power through ground. The switches are three
position, spring centered switches. The raise and lower
solenoids reposition the tow point movement valve when
energized, redirecting flow to the tow point cylinders.
Hydraulic oil for the tow point lift system is supplied from
the right auxiliary hydraulic pump. The right auxiliary
hydraulic pump contains two pumps, 0.7 and 0.58 cu in.
(11,5 and 9,5 cc) outputs, mounted on a common shaft.
The 0.58 cu in. (9,5 cc) output pump supplies oil to a
50/50 flow divider.
The flow divider splits the hydraulic flow, sending half to
the tow point and screed vibration systems. The flow to
the tow point movement system and vibration system is
sent through a flow regulator valve to control the speed
of the cylinder movement. This adjustable flow is sent
to a second 50/50 flow divider. The flow divider sends
half of the hydraulic oil to each tow point movement
cylinder. Oil flowing from tow point system is sent to the
The output of the flow divider is regulated at 1500 psi (10
343 kPa) by a relief valve. The hydraulic oil is then sent
to the tow point control valves. The tow point control
valve is a three position solenoid valve with a spring
return to the center position. The raise and lower
solenoids are energized through the tow point movement
switches on the screed control panels. The tow point
movement valve routes fluid to the tow point cylinders to
raise and lower the tow point. The fluid exiting the tow
point movement cylinders is combined with the excess
fluid from the 1500 psi (10 343 kPa) relief valves and
sent to the screed vibration system. After exiting the
vibration system the hydraulic oil is sent to the return
manifold and then to the hydraulic reservoir.
A manual shutoff valve and pressure relief valve are at
the lower port of each tow point cylinder. When closed,
the shutoff valves lock the positions of the tow point
cylinder pistons. The pressure relief valves will bypass
oil at pressures greater than 3000 psi (20 685 kPa) to
protect the cylinder. Downward pressures can exceed
3000 psi during paving operations.