CONTROL G 0389220 - Caterpillar

Illustration 3 g00290877
Main Control Valve (Tilt Stem in HOLD position, Lift Stem in LOWER position and Makeup Valve in FLOAT position)
(10) Inlet passage for implement oil
(11) Tilt stem
(13) Passage
(14) Passage
(15) Outlet passage to hydraulic tank
(16) Lift stem
(17) Chamber for pilot oil
(18) Springs
(19) Makeup valve for rod end of lift cylinders
(20) Vent passage to pilot control valve
(21) Spring
(22) Passage to rod end of lift cylinders
(23) Passage to head end of lift cylinders
(24) Passage
(25) Load check valve for lift circuit
(26) Spring
(27) Chamber LOWER Operation
When the lift control lever or the joystick control is moved to the LOWER position, pilot oil is sent to chamber (27) and lift stem (16) moves to the left. Implement oil is sent from inlet passage (10) to load check valve (25).Load check valve (25) does not open until the oil pressure becomes greater than the force of spring (26) and the rod end pressure of the lift cylinders. When the load check valve opens, oil flows through passage (24) and out of passage (22) to the rod end of the lift cylinders. This causes the bucket to lower.If the lift cylinder rod retracts too fast and the pump cannot provide enough oil to the rod end during this operation, makeup valve (19) moves against spring (21) and against the oil pressure in vent passage (20). This allows the addition of hydraulic tank oil to the implement oil in order to prevent cavitation (vacuum) in the hydraulic system.Oil from the head end of the lift cylinders comes through passage (23), around lift stem (16) and through outlet passage (15) to the hydraulic tank.FLOAT Operation
When the lift control lever or the joystick control is moved to the FLOAT position, pilot oil is sent to chamber (27) and lift stem (16) moves to the left. Oil from the pilot valve (oil manifold) opens vent passage (20) and makeup valve (19) can open.Implement oil holds the float check valve open. The pump oil from passage (24) and the oil in passage (22) now flow through passage (13) and through outlet passage (15) to the hydraulic tank.The lift cylinders can move freely in either direction according to the force on the bucket. As the cylinder rods move, all extra oil flows out of outlet passage (15) to the hydraulic tank and pump oil comes in from inlet passage (10).Load Check Valves
Load check valves (9) and (25) prevent reverse oil flow which can cause cylinder drift or load loss. For example, when tilt stem (11) is in the TILT BACK position, pump oil opens load check valve (9) and the oil flows through passages (6) and (7) to the head end of the tilt cylinder.If the pressure at inlet passage (10) decreases, the pressure of the oil and the force of spring (8) close the load check valve. The oil is blocked in the head end of the tilt cylinder. This prevents any movement of the tilt cylinder so that the bucket cannot dump.Makeup Valves
There are two makeup valves in the main