797F Off-Highway Truck: Control Valve (Brake Cooling, Hoist)

(1) Hoist relief valve
(2) Actuator (6 places)
(3) Springs (6 places)
(4) Proportional solenoid valve for brake cooling
(5) Pump check valves
(6) Control valve for hoist and brake cooling
(7) Gear pump for hoist and brake cooling system (front)
(8) Hoist screen for the front cooling brakes
(9) Lines for pump oil supply to the front cooling brakes
(10) Lines for pump oil supply to the rear cooling brakes
(11) Hoist screen for the rear cooling brakes
(12) Gear pump for hoist and brake cooling system (rear)
(13) Pilot oil supply port
(14) Control valve and mounting
(15) Line to the rear brake cooling system
(16) Pilot oil passages
(17) Spools for brake cooling oil
(18) Solenoid for pilot oil
(19) Line to the front brake cooling system
(20) Return oil passage from the rod end
(21) Hoist manifold
(22) Drain port for pilot oil to the hydraulic tank
(23) Line for return oil from the hoist cylinders
(24) Return oil passage from the head end
(25) Proportional solenoid valve for rod end oil that is flowing from the center of hoist control valve (30) to the hydraulic tank
(26) Proportional solenoid valve for rod end oil that is flowing from the pump to the center of hoist control valve (30)
(27) Load check valve
(28) Proportional solenoid valve for head end oil that is flowing from the pump to the center of hoist control valve (30)
(29) Proportional solenoid valve for head end oil that is flowing from the center of hoist control valve (30) to the hydraulic tank
(30) Hoist control valve
(31) Relief valve for the rod end of the hoist cylinders
(32) Relief valve for the head end of the hoist cylinders
(33) Hoist cylinders
(34) Line to the rod end of the hoist cylinders
(35) Pilot oil passages
(36) Spool for rod end oil that is flowing to the hydraulic tank
(37) Spool for pump oil that is flowing to the rod end
(38) Spool for pump oil that is flowing to the head end
(39) Spool for head end oil that is flowing to the hydraulic tank(40) Line to the head end of the hoist cylinders

Oil from the hydraulic tank flows to the gear pump for the hoist and for the brake cooling. The gear pump has four sections. The gear pump has a front section (7) and the gear pump has a rear section (12). The oil from the four sections of the gear pump flows through two hoist screens (8) and (11). Oil from hoist screens (8) and (11) flows through two lines for pump oil supply (9) and (10) into control valve (6). The Chassis Electronic Control Module (Chassis ECM) controls proportioning solenoids (4). Valves (4) control actuators (2). Actuators (2) control the position of spools (17). Spools (17) control the flow of oil from the pump to hoist control valve (30) and to the brake cooling system. If the body is being raised or lowered, the pump oil combines after passing through two pump check valves (5). These two pump check valves protect the pump from reverse oil flow by separating the flow of oil to the front and to the rear brake cooling systems. If the body is not being raised or lowered, the oil from lines (9) flows to the front and rear brake cooling systems through lines (15) and (19) .

Hoist relief valves (1) limit the pressure in the hoist system. Hoist relief valves (1) will vent oil to the front brake and the rear brake cooling systems through lines (15) and (19) when the pressure in the system is too high.

When the body is being raised or lowered, the oil flows from control valve (6) for the hoist and for the brake cooling through hoist manifold (21) into hoist control valve (30). The oil flows through load check valve (27). Load check valve (27) prevents the dump body from dropping slightly when the hoist control is moved from the hold position to the raise position. This will only occur if the dump body is in a partially raised position and the hoist control is moved to the raise position. The Chassis ECM controls proportioning solenoids (25), (26), (28), and (29). These proportional solenoid valves control the pilot oil in actuators (2) on hoist control valve (30). Actuators (2) on hoist control valve (30) control the position of spools (36), (37), (38), and (39). Spools (37) and (38) allow the oil to flow to the hoist cylinders (33). Spools (36) and (39) allow the oil to flow from hoist cylinders (33) to the hydraulic tank. The return oil from the cylinders flows to hoist manifold (21) through line (20) and (24). The return oil flows from hoist manifold (21) to the hydraulic tank through line (23) .

Relief valve (31) for the rod end of the hoist cylinders limits the pressure in lines (34) to the hoist cylinders. Relief valve (31) will vent oil to the hydraulic tank when the pressure in the rod end of hoist cylinders (33) is too high. This could occur when the dump body is being raised and the load shifts suddenly. Relief valve (32) for the head end of the hoist cylinders limits the pressure in lines (40). Relief valve (32) will vent oil to the hydraulic tank when the pressure in the head end of hoist cylinders (33) is too high. This could occur when a load is dumped into a partially raised dump body.

Pilot oil in supply port (13) from control valve and mounting (14) flows into hoist manifold (21). When the body is being raised or lowered, solenoid (18) is activated in order to allow pilot supply oil to flow to all six actuators (2). Proportional solenoid valves (4), (25), (26), (28), and (29) are activated by the Chassis ECM. When the hoist control is moved to the raise, the lower, or the float position, the Chassis ECM sends a specified amount of current to the appropriate proportional solenoid valves. These proportional solenoid valves control the pilot supply oil that moves spools (17), (36), (37), (38), and (39) in order to modulate the flow of oil to the hoist cylinders. Pilot oil that is returning to the hydraulic tank flows through line (22) .

Hold Position

Control valve in the hold position
(2) Actuator
(3) Springs (6 places)
(5) Pump check valves
(6) Control valve for hoist and brake cooling
(14) Control valve and mounting
(18) Solenoid for pilot oil
(21) Hoist manifold
(25) Proportional solenoid valve
(26) Proportional solenoid valve
(28) Proportional solenoid valve
(29) Proportional solenoid valve
(42) Line to the rear brake cooling system(43) Line to the front brake cooling system

When the control valve (6) is in the hold position, pilot oil flows from control valve and mounting (14) to hoist manifold (21). When the engine is started, the Chassis ECM energizes ON/OFF solenoid (18). If the hoist lever is left in the hold position for an extended time, solenoid (18) will be de-energized.

When the hoist control (6) is placed in the hold position, proportional solenoid valves (25), (26), (28) and (29) are not receiving current. The pressurized oil in actuators (2) and the force of springs (3) hold the spools away from the actuators. The lines to the hoist cylinders are now blocked and all of the pump oil will go to the brakes. The pump check valves (5) keep the front brake cooling circuit separate from the rear brake cooling circuit. The position of directional spools (37) and (38) in the control valve block the oil in the head end and in the rod end of the hoist cylinders.

In the hold position, a gauge that is connected to the pressure taps of the hoist system will show the pressure that is present in the brake cooling system. This pressure is the result of the restriction in the brake cooling filters, the brake oil coolers, the brakes, and the hoses.

Raise Position

Control valve in the raise position
(1) Hoist relief valve
(2) Actuator
(3) Springs (6 places)
(4) Proportional solenoid valve
(5) Pump check valves
(6) Control valve for hoist and brake cooling
(14) Control valve and mounting
(16) Pilot oil passage
(17) Spool
(18) Solenoid for pilot oil
(21) Hoist manifold
(22) Drain port for pilot oil to the hydraulic tank
(23) Line for return oil from the hoist cylinders
(25) Proportional solenoid valve
(27) Load check valve
(28) Proportional solenoid valve
(31) Relief valve for the rod end of the hoist cylinders
(32) Relief valve for the head end of the hoist cylinders
(35) Pilot oil passage
(36) Spool
(38) Spool
(42) Passage to the rear brakes
(43) Passage to the front brakes
(44) Passage to the rod ends of the hoist cylinders(48) Passage to the head ends of the hoist cylinders

When the control valve (6) is in the raise position, pilot oil flows from the control valve and mounting (14) to hoist manifold (21). When the engine is started, the Chassis ECM energizes ON/OFF solenoid (18). When solenoid (18) is energized, pressurized pilot oil is directed to both ends of all four of the directional spools.

When the hoist control is placed in the raise position, both proportional solenoid valves (4) are receiving current. Pilot oil in actuators (2) is drained to the hydraulic tank through line (22). Pressurized pilot oil in passage (16) will overcome the force of springs (3). Spools (17) move toward actuators (2). Passages to the brake cooling systems are now blocked. Load check valves (5) unseat. Pump oil flows through hoist manifold (21) to the hoist control valve.

Note: Proportional solenoid valves (4) receive 0.0 to 1.9 amperes from the Chassis ECM, which allows the operator to modulate the oil flow to the hoist cylinders.

When the hoist control is placed in the raise position, the Chassis ECM energizes proportional solenoid valves (25) and (28). Pilot oil in the actuators of the solenoids are drained to the hydraulic tank through line (22). The pressurized oil in passages (35) overcomes the force of springs (3). Spools (36) and (38) move toward actuators (2). Load check valve (27) remains closed until the supply pressure of the pump is higher than the pressure in the hoist cylinders. Load check valve (27) prevents the truck body from dropping before the raise pressure increases. Oil now flows past spool (38) to the head end of the hoist cylinders. Oil from the rod end of the hoist cylinders flows past spool (36) to hoist manifold (21). The oil then flows to the hydraulic tank through line (23) .

Hoist relief valves (1) limit the pressure in the hoist system. Hoist relief valves (1) will vent oil to the front brake and the rear brake cooling systems when the pressure in the system is too high.

Two line relief valves (31) and (32) also protect the hydraulic system from excessive pressure. If the control valve for the hoist and brake cooling is in the hold position and an external force moves the body, the line relief valves will open.

The overcenter control is necessary in order to prevent cavitation in the hoist cylinders. This type of event could result from a sudden shift of load in the truck body. When the body is being raised, at approximately 40 degrees the Chassis ECM de-energizes solenoid (25). The surface area of the head end is larger than the surface area of the rod end, so the hoist cylinders will continue to raise. Oil from the rod end of the hoist cylinders will exceed the setting of relief valve (31) and flow to tank.

Before the hoist cylinders are fully extended to approximately 55 degrees, the Chassis ECM reduces the current to solenoid (28). The reduction in current reduces the flow of oil to the head end of the cylinder. This is called the raise snub control. The speed of the body is reduced in order to prevent the hoist cylinder pistons from making hard contact with the cylinder heads.

Lower Position

Control valve in the lower position
(1) Hoist relief valve
(2) Actuator
(3) Springs (6 places)
(4) Proportional solenoid Valve
(5) Pump check valves
(6) Control valve for hoist and brake cooling
(14) Control valve and mounting
(16) Pilot oil passage
(17) Spool
(18) Solenoid for pilot oil
(21) Hoist manifold
(22) Drain port for pilot oil to the hydraulic tank
(23) Line for return oil from the hoist cylinders
(25) Proportional solenoid valve
(26) Proportional solenoid valve
(27) Load check valve
(28) Proportional solenoid valve
(29) Proportional solenoid valve
(31) Relief valve for the rod end of the hoist cylinders
(32) Relief valve for the head end of the hoist cylinders
(35) Pilot oil passage
(37) Spool
(39) Spool
(42) Passage to the rear brakes
(43) Passage to the front brakes
(44) Passage to the rod ends of the hoist cylinders(48) Passage to the head ends of the hoist cylinders

When control valve (6) is in the lower position, pilot oil flows from control valve and mounting (14) to hoist manifold (21). When the engine is started, the Chassis ECM energizes the ON/OFF solenoid (18). When the solenoid (18) is energized, pressurized pilot oil is directed to both ends of all four of the directional spools.

When the hoist control is placed in the lower position, both proportional solenoid valves (4) for brake cooling are receiving current. Pilot oil in actuators (2) is drained to the hydraulic tank through line (22). Pressurized pilot oil in passage (16) overcomes the force of springs (3). Spools (17) move toward actuators (2). Passages to the brake cooling systems are partially blocked. Pump oil flows past the direction spools. Pump check valves (5) unseat. Pump oil flows through hoist manifold (21) to the hoist control valve.

Note: Proportional solenoid valves (4) receive 0.0 to 1.9 amperes from the Chassis ECM, which allows the operator to modulate the oil flow to the hoist cylinders.

When the hoist control is placed in the lower position, the Chassis ECM energizes proportional solenoid valves (26) and (29). Pilot oil in the actuators of these solenoids are drained to the hydraulic tank through line (22). The pressurized oil in passages (35) overcomes the force of springs (3). Spools (37) and (39) move toward actuators (2). The oil now flows past spool (37) to the rod end of the hoist cylinders. Oil from the head end of the hoist cylinders flows past spool (39) to hoist manifold (21). The oil then flows to the hydraulic tank through line (23) .

In the lower position, the current to proportional solenoid valve (26) is limited. The current that is supplied to the solenoid is limited in order to move the directional spool 8 mm (0.31 inch). Full travel of spool (37) is 16 mm (0.63 inch). Full travel of spool (37) is limited in order to reduce the pressure in the system during the lower function.

Note: Relief valves (31) and (32) will open when the hoist system pressure exceeds the relief valve setting. The cylinder oil will flow to the hydraulic tank.

Note: The hoist lever will have three different functions in the same position. The lower function, the float function and the snub function. The three different functions are determined by the position of the body position sensor. When the body position sensor is at more than 40 degrees, the hoist lever will remain in the lower function. This is called the second stage of the hoist cylinder. When the body position sensor is at less than 30 degrees, the hoist lever will be in the float function. This is called the first stage of the hoist cylinder. When the body position sensor is at less than 15 degrees, the hoist lever will be in the body down snub function.

Float Position

When the body is at less than 30 degrees, the Chassis ECM will reduce the current to valve (29). The oil flow from the head end of the hoist cylinder to tank is reduced. This is called the float position. In this position, the speed of the body that is being lowered will be reduced.

Note: The operator has no control over this situation.

When the hoist control is in the float position, neither of the proportional solenoid valves (4) receive current. The pressurized pilot oil in actuators (2) and the force of springs (3) moves spools (17) away from actuators (2). Pump oil flows past spools (17) and through the supply lines to the brake cooling system.

When the hoist control is in the float position, both proportional solenoid valves (26) and (29) are receiving current. Pilot oil in the actuators of these solenoids are drained to the hydraulic tank through port (22). Pressurized pilot oil in passages (35) works against spring (3) in order to keep the spools shifted upward. Spools (37) and (39) modulate depending on the current that is supplied to the solenoids. When spool (39) shifts, the oil in the head end of the hoist cylinders flows back to the hydraulic tank through line (23). The amount of current that is supplied by the Chassis ECM will determine the amount of oil that is allowed to leave the head end of the hoist cylinder.

SNUB Position

The snub position is activated by the Chassis 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 position sensor for the dump body will signal the Chassis ECM when the dump body is between 15 degrees and 4 degrees from the frame. The Chassis ECM will then send a signal in order to move the hoist control valve to the snub position. The speed of the body is reduced in order to prevent hard contact of the body with the frame.

As the hoist control valve changes from the float position to the snub position, the current to proportional solenoid valve (29) decreases. Less pilot oil is drained from actuator (2). Spool (39) moves slightly away from actuator (2). The flow of oil from the head end of the hoist cylinders past spool (39) is restricted. This lowers the speed of movement of the dump body. After the body is down for 20 seconds, the solenoids will change to the low power float position. The passage from the head end of the hoist cylinder will be slightly open.

MARYGAR