773E and 775E Off-Highway Truck Hydraulic System: Control Valve (Hoist)

(1) Hoist proportioning solenoid for SNUB, FLOAT, and LOWER (2) High pressure relief valve (3) Low pressure relief valve (4) Hoist proportioning solenoid for RAISE (5) Rear brake oil cooler relief valve (6) Outlet to the hoist and brake hydraulic tank (7) Outlet to the head end of the hoist cylinders (8) Outlet to the rod end of the hoist cylinders (9) Outlet to the rear brake oil cooling system(10) Inlet for pump oil

(1) Hoist proportioning solenoid for SNUB, FLOAT, and LOWER (2) High pressure relief valve (3) Low pressure relief valve (4) Hoist proportioning solenoid for RAISE (5) Rear brake oil cooler relief valve (6) Outlet to the hoist and brake hydraulic tank (7) Outlet to the head end of the hoist cylinders (8) Outlet to the rod end of the hoist cylinders (9) Outlet to the rear brake oil cooling system (10) Inlet for pump oil (11) Hoist and brake hydraulic tank (12) Brake pump section (13) Hoist pump section (14) Parking brake release hydraulic filter (15) Hoist and brake gear pump (16) Line to the parking and secondary brake valve (17) Check valve (18) Pilot oil line from the parking brake release system (19) Hoist control valve (20) Stem (21) Dump spool (22) Directional spool (23) Actuators (24) Centering springs (25) Hoist cylinders(26) Orifices

Oil flows from the brake pump section (12) of the hoist and brake gear pump. This oil flows through parking brake release hydraulic filter (14). This oil will go through check valve (17). The oil flow is then split. Most of the oil is sent to the parking and secondary brake valve through line (16). The rest of the oil goes to hoist control valve (19) through line (18). This oil is used as pilot oil for hoist proportioning solenoid (1) and hoist proportioning solenoid (4). Pilot oil is always present at both ends of directional spool (22). The hoist proportioning solenoids are used to drain the pilot oil from the ends of directional spool (22), which then allows the spool to move.

Oil flows through hoist pump section (13) of hoist and brake gear pump (15). This oil flows through line (10) to hoist control valve (19). The hoist control valve will send pump oil to one of two locations. When the dump body is being raised or lowered, the hoist cylinders receive the oil. If the hoist is not being used, the oil flows to the oil cooling system for the rear brakes.

Rear brake oil cooler relief valve (5) limits the pressure in line (9) to the rear brake oil cooler. Rear brake oil cooler relief valve (5) will vent oil to hoist and brake hydraulic tank (11), when the pressure in line (9) is too high.

Low pressure relief valve (3) limits the pressure in line (8) to the rod end of hoist cylinders (25) when the dump body is being lowered. Low pressure relief valve (3) will open when the pressure in line (8) becomes too high. When low pressure relief valve (3) opens, dump spool (21) will open and oil will vent to hoist and brake hydraulic tank (11) .

When the dump body is being raised oil is sent to stem (20). Stem (20) will shift. High pressure relief valve (2) will now limit the pressure in line (7) to the head end of hoist cylinders (25). High pressure relief valve (2) will open when the pressure in line (7) becomes too high. When high pressure relief valve (2) opens, dump spool (21) will open and oil will vent to hoist and brake hydraulic tank (11) .

HOLD Position

In the HOLD position, neither of the hoist proportioning solenoids are receiving current. All of the pilot oil is blocked in actuators (23). Centering springs (24) hold directional spool (22) so that line (7) to the head end of hoist cylinders (25) are blocked by directional spool (22) .

The directional spool (22) is in a position that allows all of the pump oil to flow to the rear brake cooling system through line (9). The maximum pressure in line (9) to the rear brake cooling system is controlled by rear brake oil cooler relief valve (5). If the rear brake oil cooler relief valve (5) opens, the oil will vent through line (6) to the hoist and brake hydraulic tank (11) .

RAISE Position

In the RAISE position, hoist proportioning solenoid (4) is receiving a current of 1.9 amperes. Pilot oil in the actuator (23) for hoist proportioning solenoid (4) is allowed to drain to the hoist and brake hydraulic tank (11). Pilot oil in the actuator (23) for the hoist proportioning solenoid (1) will now push directional spool (22) to the left. When directional spool (22) moves, the pump oil that is coming through inlet (10) flows to line (7), and to the head end of hoist cylinders (25). Oil from the rod end of hoist cylinders (25) is allowed to flow to the rear brake oil cooler through line (9). Orifices (26) control the rate of oil that is leaving the rod end of hoist cylinders (25). This will prevent hoist cylinders (25) from moving too fast for the pump to supply oil. This can happen when a load shifts during the dump.

Some of the oil is also sent as signal oil to stem (20). The signal pressure shifts stem (20) so that low pressure relief valve (3) is blocked. This allows all of the pump oil that is flowing through dump spool (21) to flow to high pressure relief valve (2). If the oil pressure opens high pressure relief valve (2), dump spool (21) will shift upward and the pump oil will be vented through line (6) to hoist and brake hydraulic tank (11) .

LOWER Position

In the LOWER position, hoist proportioning solenoid (1) is receiving a current of 1.9 amperes. Pilot oil in the actuator (23) for hoist proportioning solenoid (1) is allowed to drain to the hoist and brake hydraulic tank (11). Pilot oil in the actuator (23) for the hoist proportioning solenoid (4) will now push directional spool (22) to the right. When directional spool (22) moves, the pump oil that is coming through inlet (10) flows through line (8) to the rod end of the hoist cylinders (25) .

Oil in line (7) is vented through line (6) to the hoist and brake hydraulic tank (11) .

Because no signal pressure is sent to stem (20), pump oil can flow to low pressure relief valve (2). If low pressure relief valve (2) opens, dump spool (21) will shift upward and the pump oil will be vented through line (6) to the hoist and brake hydraulic tank (11) .

FLOAT Position

In the FLOAT position, hoist proportioning solenoid (1) is receiving a current of 1.69 amperes. Pilot oil in the actuator (23) for hoist proportioning solenoid (1) is allowed to drain to the hoist and brake hydraulic tank (11). Pilot oil in the actuator (23) for the hoist proportioning solenoid (4) will now push directional spool (22) to the right. Because a smaller amount of pilot oil is allowed to drain from the actuator (23) for hoist proportioning solenoid (1), directional spool (22) does not move all the way to the LOWER position. When directional spool (22) moves, some of the pump oil that is coming through inlet (10) flows to the rod end of the hoist cylinders (25). The remaining pump oil is sent to the rear brake cooling system through line (9) .

SNUB Position

The SNUB position is activated by the Power Train Electronic Control Module (Power Train ECM). The operator has no control over this position. The SNUB position will slow the rate of lowering the dump body as the dump body nears the frame of the truck. The body down or up switch electronic control will signal the Power Train ECM when the dump body is near the frame. The Power Train ECM will then send a signal in order to move the hoist control valve from the FLOAT position to the SNUB position.

In the SNUB position, hoist proportioning solenoid (1) is receiving a current of 0.810 amperes. Pilot oil that is draining from actuator (23) for hoist proportioning solenoid (1) is restricted. Pilot oil in the actuator (23) for the hoist proportioning solenoid (1) will move directional spool (22) to the left. This moves the directional spool (22) out of the FLOAT position and into the SNUB position. When directional spool (22) moves, the oil from the head ends of hoist cylinders (25) is restricted. This lowers the speed of movement of the dump body. Some of the pump oil that is coming through inlet (10) flows to the rod end of the hoist cylinders (25). The remaining pump oil from line (10) is sent to the rear brake cooling system.

Oil in lines (7) to the head end of the hoist cylinders (25) is vented to hoist and brake hydraulic tank (11) .

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773E and 775E Off-Highway Truck Hydraulic System: Control Valve (Hoist) 2013-01-10T18:57:00-08:00 Rating: 4.5 Diposkan Oleh: Unknown