1.21.3. Screed Lift System

TM 5-3895-373-20 1.21.3. Screed Lift System. The screed lift system consists of the screed lift switch on the operator switch panel, right auxiliary hydraulic pump, 50/50 flow divider, solenoid valves, and two hydraulic cylinders. Power for the screed lift circuit is supplied from CBS. From CB5 the power is sent to the screed lift switch on the operator switch panel. The screed lift switch is a three-position center off switch. The switch is momentary contact in the raise position and maintains contact in the lower position. When the switch is placed in the up, LIFT position, power is sent to the screed lift valve solenoid and the power beyond valve solenoid. When the screed lift switch is placed in the down, FLOAT position, power is sent to a bleed valve solenoid to allow the screed to float. Hydraulic oil for the screed 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 stack valve. Stack valve pressure is regulated at 2500 psi (16 472 kPa) by a pressure relief valve internal to the stack valve. This pressure can be read at the test manifold. The relief valve ports excess hydraulic oil to the return manifold and back to the hydraulic reservoir. The oil that is not routed through the relief valve is routed to the power beyond valve and the screed lift valve. In the normal, deenergized position, the power beyond valve routes all hydraulic oil out of the stack valve and to the track tensioning system. The power beyond valve solenoid energizes whenever any hydraulic function on the stack valve is operated. When the power beyond valve solenoid is energized, the valve shifts and does not allow any oil to leave the stack valve by way of the track tensioning system. All hydraulic oil entering the stack valve is therefore directed to the screed lift valve. This temporarily removes hydraulic oil from the track tensioning system. The screed lift valve is normally deenergized, allowing all hydraulic oil to flow to the stack valve supply header. When the screed lift switch on the operator switch panel is placed in the up, LIFT position, the screed lift valve shifts and redirects all hydraulic oil to the screed lift system. The hydraulic oil is sent to the screed lift cylinders causing the screed to raise. When the screed lift switch is placed in the down, FLOAT position, a bleed valve is opened. This allows the hydraulic oil in the screed lift cylinders to return to the return manifold and then to the hydraulic reservoir. The screed travel lock valve installed in the supply line for the screed lift cylinders can be used to prevent the screed from lowering. When the valve is shut, the bypass check valve will seat when hydraulic oil from the cylinders attempts to flow to the return manifold. The screed may not be lowered until the screed travel lock valve is opened. The bypass check valve on the screed travel lock valve allows hydraulic pressure from the stack valve to raise the screed even when the screed travel lock valve is shut. 1-46