HOLD Position
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| Illustration 1 | g00910440 |
Control valve in HOLD position (1) Relief valve for the hoist system (2) Inlet for pump oil (3) Load check valve (4) Spring (5) Detent ball (6) Passage to oil cooler for brakes (7) Passage for rod ends of hoist cylinders (8) Passage for head ends of hoist cylinders (9) Valve spool (A) Pump oil (B) Blocked oil | |
The oil from the pump flows into the control valve through inlet (2). When valve spool (9) is in the hold position pump oil flows to the oil cooler. The cooled oil flows to the brakes on the rear wheels, and then to the hydraulic tank.
The HOLD of the valve spool blocks the oil in passage (7), in passage (8), and in the ends of the hoist cylinders. The cylinders cannot be moved. The pump oil is also in the spring chamber of pump relief valve (1).
FLOAT Position
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| Illustration 2 | g00910435 |
Control valve in FLOAT position (1) Relief valve for the hoist system (2) Inlet for pump oil (3) Load check valve (4) Spring (5) Detent ball (6) Passage to oil cooler for brakes (7) Passage for rod ends of hoist cylinders (8) Passage for head ends of hoist cylinders (9) Valve spool (A) Pump oil (C) Restricted pump oil (D) Return oil | |
Valve spool (9) is held in the FLOAT (C) by four detent balls (5) in the detent groove around the end of the valve spool.
The position of valve spool (9) in the FLOAT position (C) permits some of the pump oil flow through grooves in the valve spool. This oil flows through passage (6) to the oil cooler. The cooled oil goes to the brakes on the rear wheels, and then to the hydraulic tank. The grooves in valve spool (9) are restrictions to the flow and there is an increase in the pressure of the pump oil in the control valve.
The pressure of the pump oil opens load check valve (3) enough to permit flow. Oil enters passage (7), and oil flows into the rod ends of the hoist cylinders. The weight of the dump body on the hoist cylinders pushes the oil from the head ends of the cylinders through passage (8) in the control valve. The oil flows from passage (8) through a screen and into the hydraulic tank.
When the second stage of the two-stage cylinder is retracted, the weight of the body forces the first stage of the two-stage cylinder to retract. This completes the lowering of the dump body.
When the body is down and the machine is in normal operation, the hoist lever will be in FLOAT. When the hoist control valve is in the float position pump oil flows to the brake oil cooler. The FLOAT position will hold the body down.
RAISE Position
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| Illustration 3 | g00910436 |
Control valve in RAISE position (1) Relief valve for the hoist system (2) Inlet for pump oil (3) Load check valve (4) Spring (5) Detent ball (6) Passage to oil cooler for brakes (7) Passage for rod ends of hoist cylinders (8) Passage for head ends of hoist cylinders (9) Valve spool (A) Pump oil (D) Return oil | |
The oil from the pump flows into the control valve through inlet (2). When valve spool (9) is held in the RAISE position, passage (6) is not open to pump oil. When the pump oil is restricted to flow through passage (6), there is an increase in the pressure. The pressure of the pump oil opens load check valve (3). The pump oil flows through the open load check valve (3), around valve spool (9), through passage (8), and to the head ends of the hoist cylinders.
The pump oil in the head ends of the cylinders moves the cylinders toward the extended lengths and the dump body lifts. When the pump oil in the head ends of the cylinders moves the cylinders, the oil in the rod ends of the cylinders is pushed through passage (7) around valve spool (9) and through passage (6) to the oil cooler. The cooled oil flows to the brakes on the rear wheels, and then to the hydraulic tank. An orifice plate on the rod end of the hoist cylinder restricts the flow of oil through passage (7). Restricting the return oil prevents cavitation of the cylinders. The hoist cylinders will cavitate when the body is raised at a rate that is greater than the oil that is supplied to the hoist cylinders.
When the hoist control lever is released, spring (4) on the end of valve spool (9) moves the valve spool and the hoist control lever to the HOLD position. When the hoist control valve is in the HOLD position and the body is raised above the frame, valve spool (9) and load check valve (3) prevent the body from lowering. Load check valve (3) will prevent the body from lowering until the pump supply pressure is higher than the pressure in the head end of the hoist cylinders.
LOWER Position
When the valve spool is held in the LOWER position, the pump oil flows through the inlet into the control valve. The LOWER position of the valve spool in the control valve closes the passages to the oil cooler. This causes an increase in the pressure of the pump oil. When the load check valve is open oil is allowed to flow to the rod end of the hoist cylinders. The oil goes around the valve spool and through the passage to the rod ends of the cylinders.
The pump oil in the rod ends of the cylinders and the weight of the body move the cylinders to the retracted positions. The oil in the head ends of the cylinders is pushed from the cylinders through the control valve and through the passage to a screen assembly in the hydraulic tank.
When the hoist control lever is released, the spring on the end of the valve spool moves the valve spool and the hoist control lever to the FLOAT position.
Pilot Operated Relief Valve
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| Illustration 4 | g00908140 |
(1) Shims (2) Pilot valve (3) Chamber (4) Dump valve (5) Orifice (6) Inlet passage from the hoist control valve (7) Spring (8) Shims (9) Spring (10) Outlet passage to the hydraulic tank (11) Screen | |
The pilot operated relief valve is the part of the hoist control valve that limits the pressure in the hydraulic system. The pump oil enters inlet passage (6) from the hoist control valve. This oil flows through screen (11) and orifice (5) into chamber (3). When the pilot operated relief valve is in the closed position, dump valve (4) has the same oil pressure on both sides of the valve. Spring (9) holds dump valve (4) closed.
The pilot operated relief valve is closed when the pressure of the pump oil is less than the pressure setting of the pilot operated relief valve, which is 950 ± 172 kPa (138 ± 25 psi).
When the pressure of the pump oil is more than the setting of the pilot operated relief valve, the pressure of the oil in chamber (3) opens pilot valve (2). The oil in chamber (3) now flows through the open pilot valve (2) and through outlet passage (10) to the hydraulic tank.
The pressure of the oil on the spring side of dump valve (4) is now less than the pressure of the oil in inlet passage (6). The force of the high pressure oil in inlet passage (6) is now more than the force of spring (9) and the pressure of the oil pushes dump valve (4) into chamber (3). The pump oil in passage (6) flows through the open dump valve and through outlet passage (10) to the hydraulic tank.
The pressure setting of the pilot operated relief valve can be changed. When the pressure setting is low add shims (1) behind the pilot valve, or remove shims (8) under the plug. If the pressure setting is high, remove shims (1) from behind the pilot valve or add shims (8) under the plug.