1.21.4. Tow Point Lift System

TM 5-3895-373-20 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 vibration system. 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. 1-48