Pilot Operated Relief Valves
The hoist system contains two relief valves which are used to control the maximum pressure of the system. Each of the two relief valves for the hoist system is associated with one of two hoist pump sections. In the RAISE position, both pump sections are used together in order to raise the hoist cylinders. Therefore, the two relief valves are exposed to hoist oil pressure when the hoist is being raised. In the LOWER position, only one pump section supplies oil to the hoist cylinders. Therefore, only one relief valve is exposed to hoist oil pressure when the hoist is being lowered. The pressure setting of both relief valves can be adjusted with shims. For information on setting the proper relief pressures, refer to Testing and Adjusting, "Relief Valve (Hoist) - Test and Adjust".
 | |
| Illustration 1 | g01013565 |
Hoist relief valve for RAISE position only (1) Passage to tank (2) Chamber for signal oil from inlet port (3) Oil port to rear brake cooling (4) Inlet port for pump oil (5) Check valve (6) Valve (7) Passage to hoist control valve for signal oil (8) Unloading valve (9) Orifice (10) Passage to hoist valve for pump oil | |
The relief valve that is shown in Illustration 1 controls the maximum pressure of the hoist supply oil only when the hoist control lever is in the RAISE position. When the hoist control lever is in the HOLD, FLOAT or LOWER position, this particular valve redirects pump oil to the rear brake cooling system. The maximum pressure of the oil that is redirected to the rear brake cooling system is controlled by relief valves in the hydraulic tank.
In the RAISE position, pump oil flows through inlet port (4), check valve (5), and passage (10) to the hoist control valve. Pump oil also enters chamber (2) through orifice (9). The oil in chamber (2) is called signal oil. Signal oil has the same pressure as the pump oil. When the pump oil exceeds the relief pressure setting, the signal pressure in chamber (2) forces valve (6) to open. This allows signal oil in chamber (2) to drain through passage (1) to the hydraulic tank. The pressure of the signal oil in chamber (2) then decreases and unloading valve (8) opens. Oil will then flow from inlet port (4) to the rear brake coolers through port (3) and pump pressure decreases. When the pump pressure has dropped below the relief pressure setting, the unloading valve closes. The unloading valve will open whenever the pump pressure exceeds the relief pressure setting of this valve. For the relief pressure setting for the hoist supply oil, refer to Specifications, "793B Off-Highway Truck Hydraulic System".
Passage (7) in chamber (2) leads to a stem in the hoist valve. When the hoist control lever is in the HOLD, FLOAT or LOWER position, this stem allows signal oil to drain from chamber (2). This decreases the pressure in chamber (2) and unloading valve (8) opens. Oil then flows from inlet port (4) to the rear brake coolers through port (3) instead of to the hoist cylinders. The relief pressure of the oil that flows through port (3) to rear brake cooling is controlled by the relief valves in the hydraulic tank. For the relief pressure setting of the oil that flows to rear brake cooling, refer to Specifications, "793B Off-Highway Truck Hydraulic System".
 | |
| Illustration 2 | g01013564 |
Hoist relief valve for the LOWER position and for the RAISE position (11) Inlet port for pump oil (12) Oil port for rear brake cooling (13) Unloading valve (14) Chamber for signal oil from inlet port (15) Passage to tank (16) Check valve (17) Passage to hoist control valve for pump oil (18) Passage to hoist control valve for signal oil (19) Valve (20) Orifice | |
The relief valve that is shown in Illustration 2 controls the maximum pressure of the hoist supply oil only when the hoist control lever is in the LOWER or RAISE position. When the hoist control lever is in the HOLD or FLOAT position, this particular valve redirects the pump oil to the rear brake cooling system. The maximum pressure of the oil that flows to the rear brake cooling system is controlled by relief valves in the hydraulic tank.
In the LOWER or RAISE position, pump oil flows through inlet port (11), check valve (16), and passage (17) to the hoist control valve. Pump oil also enters chamber (14) through orifice (20). The oil in chamber (14) is called signal oil. This oil is at the same pressure as the pump oil. When the pump oil exceeds the relief pressure setting, the signal pressure in chamber (14) forces valve (19) to open. This allows signal oil in chamber (14) to drain through passage (15) to the hydraulic tank. The pressure in chamber (14) then decreases and unloading valve (13) opens. Oil then flows from inlet port (11) to the rear brake coolers through port (12) and the pressure of the pump oil decreases. When the pump pressure drops below the relief pressure setting, the unloading valve closes. The unloading valve will open whenever the pump pressure exceeds the relief pressure setting of this valve. For the relief pressure setting for hoist supply oil, refer to Specifications, "793B Off-Highway Truck Hydraulic System".
Passage (18) in chamber (14) leads to a stem within the hoist control valve. When the hoist control lever is in the HOLD or FLOAT position, this stem allows signal oil to drain from chamber (14). This decreases the pressure in chamber (14) and unloading valve (13) opens. Oil then flows from inlet port (11) to the rear brake coolers through port (12) instead of to the hoist cylinders. The maximum pressure of the oil that flows through port (12) to rear brake cooling is controlled by the relief valves in the hydraulic tank. For the relief pressure setting of the oil that flows to rear brake cooling, refer to Specifications, "793B Off-Highway Truck Hydraulic System".
Control Valve
 | |
| Illustration 3 | g01013563 |
(21) Orifice for pilot oil (22) Drain port to hydraulic tank (23) Stem (24) Pilot pressure port for RAISE position (25) Drain passage from head end of hoist cylinders (26) Drain passage from rod end of hoist cylinders (27) Pilot pressure port for LOWER position and FLOAT position (28) Supply passage to head end of hoist cylinders (29) Supply passage to rod end of hoist cylinders (30) Drain passage for signal oil (14) from passage (18) (31) Drain passage for signal oil (2) from passage (7) (32) Inlet port for hoist supply oil (33) Passage to counterbalance valve from head end of hoist cylinders (34) Hoist counterbalance valve | |
The following explanations describe the operation of the hoist control valve and the associated relief valves for each of the four positions of the hoist control lever (RAISE, HOLD, LOWER and FLOAT).
RAISE Position
Pilot pressure port (27) for the LOWER position and the FLOAT position is vented to the hydraulic tank. The force of pressurized pilot oil at pilot pressure port (24) for the RAISE position shifts stem (23) 15 mm (0.59 inch) to the left. In this position, stem (23) blocks passage (31) and passage (30) so that signal oil in chamber (2) and chamber (14) cannot drain to the hydraulic tank.
Hoist supply oil enters the relief valves through inlet port (4) and inlet port (11). Some of that oil flows through orifice (9) and orifice (20) into chamber (2) and chamber (14). The oil in chamber (2) and chamber (14) is called signal oil. This oil is at the same pressure as the hoist supply oil. Signal pressure in chamber (2) and chamber (14) closes unloading valve (8) and unloading valve (13). This prevents pump oil from flowing to port (3) and port (12) for rear brake cooling.
Hoist supply oil then opens check valve (5) and check valve (16). The oil flows through passage (10) and passage (17) to inlet port (32) in the hoist control valve. Stem (23) directs this oil through passage (28) to the head end of the hoist cylinders. Oil in the rod end of the cylinders returns to the hydraulic tank through passage (26) .
HOLD Position
Pilot pressure from port (24) and port (27) is vented to the hydraulic tank. Stem (23) is held in the HOLD position by two centering springs. In this position, stem (23) opens passage (31) and passage (30) so that any signal oil in chamber (2) and chamber (14) drains to the hydraulic tank. Pressurized pump oil in inlet port (4) and inlet port (11) causes unloading valve (8) and unloading valve (13) to open. Pump oil then flows from inlet port (4) and inlet port (11) to port (3) and port (12) for rear brake cooling. The position of stem (23) prevents the oil in the rod end and the head end of the hoist cylinders from returning to the hydraulic tank. The hoist cylinders cannot be moved in the HOLD position.
FLOAT Position
The pilot pressure from port (24) for the RAISE position is vented to the hydraulic tank. Pilot pressure at port (27) for the LOWER and the FLOAT position is held at a reduced pressure. At this pressure, pilot oil shifts stem (23) only 12.0 mm (0.47 inch) to the right of the HOLD position. In this position, stem (23) opens passage (31) and passage (30) so that any signal oil in chamber (2) and chamber (14) drains through passage (7) and passage (18), through passage (31) and passage (30) to the hydraulic tank. Pressurized pump oil in inlet port (4) and inlet port (11) causes unloading valve (8) and unloading valve (13) to open. Pump oil then flows from inlet port (4) and inlet port (11) to port (3) and port (12) for rear brake cooling. The weight of the dump body forces oil in the head end of the cylinders to return to the hydraulic tank through passage (25).
When stem (23) is in the FLOAT position, the dump body cannot raise. The dump body remains in contact with the truck frame because pressure at inlet port (4) and inlet port (11) is sensed at the rod end of the hoist cylinders through passage (29). In the FLOAT position, the pressure at the rod end of the hoist cylinders is equal to the pressure in the rear brake cooling circuit. This pressure is controlled by the relief valves for the brake oil cooler in the hydraulic tank.
LOWER Position
The pilot pressure from port (24) for the RAISE position is vented to the hydraulic tank. The force of the pressurized pilot oil at port (27) for the LOWER and the FLOAT position shifts stem (23) 15 mm (0.59 inch) to the right of the HOLD position. In this position, stem (23) blocks passage (30) so that any signal oil in chamber (14) cannot drain to the hydraulic tank. At the same time, stem (23) opens passage (31) so that the signal oil in chamber (2) drains through passage (7), through passage (31) to the hydraulic tank.
Because any signal oil in chamber (2) is open to drain, pressurized pump oil in inlet port (4) causes unloading valve (8) to open. The pump oil then flows to the port (3) for rear brake cooling.
Pump oil enters through inlet port (11). Some of the pump oil that enters inlet port (11) flows through orifice (20) into chamber (14). The oil in chamber (14) is called signal oil. This oil is at the same pressure as the pump oil. Signal pressure in chamber (14) closes unloading valve (13). This prevents the pump oil from flowing to port (12) for rear brake cooling.
Pump oil then opens check valve (16). That oil then flows through passage (17) to inlet port (32) in the hoist control valve. Stem (23) directs this oil through passage (29) to the rod end of the hoist cylinders. Oil in the head end of the cylinders returns to the hydraulic tank through passage (25) .
Counterbalance Valve
 | |
| Illustration 4 | g01015637 |
(34) Hoist Counterbalance Valve | |
Counterbalance valve (34) is located in the hoist control valve. Refer to Specifications, "793B Off-Highway Truck Hydraulic System" for the pressure setting of this valve. This valve is not adjustable.
The counterbalance valve prevents the dump body from running ahead of the hoist pumps when the dump body is raising. This can happen if there is a sudden shift in the load and this action can cause cavitation in the hoist cylinders. The purpose of the counterbalance valve is the prevention of cavitation.
Passage (33) directs a portion of the oil from the head end of the hoist cylinders to the counterbalance valve (34). The counterbalance valve reacts to the pressure of this oil. When the oil pressure in the head end of the hoist cylinders drops below the cracking pressure of the counterbalance valve, the counterbalance valve restricts the flow of the return oil from the rod end of the hoist to the hydraulic tank, in order to prevent cavitation.
When the hoist control lever is in the LOWER position or the FLOAT position, counterbalance valve (34) allows unrestricted oil flow from pump port (11) to the rod end of the hoist cylinders through passage (29) .
Orifice for pilot oil
When the hoist control lever is in the RAISE position, the pilot oil is continuously circulated from port (24) for the RAISE position, through orifice (21), through port (27), back to the pilot valve and to the hydraulic tank. The pressure at pilot port (27) is equal to the hydraulic tank pressure.
When the hoist control lever is in the FLOAT position or the LOWER position, the pilot oil is continuously circulated from port (27) for the FLOAT position and the RAISE position, through orifice (21), through port (24), back to the pilot valve, and to the hydraulic tank. The pressure at pilot port (24) is equal to the hydraulic tank pressure.
The continuous flow of pilot oil to the hoist control valve provides warm pilot oil to the hoist control valve. Orifice (21) is not a component of the actual hoist valve. The orifice is located in an elbow assembly that is connected to the hoist valve.