966G Series II Wheel Loader and 972G Series II Wheel Loader Hydraulic System Implement System - Pilot Hydraulics Caterpillar


Implement System - Pilot Hydraulics
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1.1. Pilot Hydraulic System
2.1. Main Hydraulic System
3.1. Quick Coupler (If Equipped)



Illustration 1g00990525

Schematic for the Implement System

(1) Main control valve

(2) Auxiliary cylinders

(3) Tilt cylinder

(4) Lift cylinder

(5) Manifold

(6) Main relief valve

(7) Line relief valve (auxiliary)

(8) Line relief valve (tilt)

(9) Check valve (float)

(10) Line relief valve (auxiliary)

(11) Line relief valve (tilt)

(12) Check valve (load)

(13) Stem (auxiliary)

(14) Check valve (load)

(15) Stem (tilt)

(16) Check valve (load)

(17) Stem (lift)

(18) Shuttle valve (ball resolver)

(19) Screen (pilot)

(20) Orifice (1.7 mm)

(21) Primary reducing valve

(22) Pilot valve

(23) Shuttle valve

(24) Float valve

(25) Pilot valve (oil manifold)

(26) Secondary reducing valve

(27) Ball valve (hydraulic lockout)

(28) Screen (pilot)

(29) Auxiliary control valve

(30) Tilt control valve

(31) Lift control valve

(32) Implement pump

(33) Pilot/brake pump

(34) Hydraulic tank

(35) Breaker relief valve

Note: Illustration 1 shows a machine with an auxiliary function. Machines without auxiliary functions (logging, side dump bucket, multipurpose bucket, etc) will not have the following component: Auxiliary cylinders (2), line relief valves (7, 10), the stem (auxiliary) (11), the check valve (load) (12), the stem (auxiliary) (13) and the pilot control valve (auxiliary) (29) .

The implement system is composed of the following systems: pilot control system, main hydraulic system and quick coupler (if equipped). Hydraulic tank (34) is common to the systems.

The pilot hydraulic system consists of the following components: pilot control valve (22), the pilot valve (oil manifold) (25), the ball valve (hydraulic lockout) (27), pilot/brake pump (33) and hydraulic tank (34) .

The main hydraulic system consists of the following components: main control valve (1), Implement cylinders (2, 3, 4), main relief valve (6), Implement pump (32) and Hydraulic tank (34) .

Pilot Hydraulic System

Pilot/brake pump (33) draws oil from hydraulic tank (34). The pump provides oil to the pilot system. Also, the pump provides oil to the hydraulic brake system.

The oil that is supplied by the pilot/brake pump to the pilot valve (oil manifold) is filtered by screen group (19). Screen group (19) is installed in the pilot valve (oil manifold) (25). Pilot oil flows through the primary reducing valve (21). Primary reducing valve (21) will reduce the pressure of the pilot oil to approximately 2600 ± 150 kPa (375 ± 22 psi) at a flow of 4.0 ± 0.5 L/min (1 ± 0.1 US gpm). When the pressure adjustment of primary reducing valve (21) is greater than the pressure adjustment of secondary reducing valve (26), the pilot oil from the primary reducing valve (20) will shift shuttle valve (23) to the DOWN position.

The output of shuttle valve (23) flows in two directions. In one direction,the pilot oil flows to the ride control system. In the other direction, the pilot oil flows to the ball valve (hydraulic lockout) (27). If the ball valve (hydraulic lockout) (27) is in the OPEN position, the pilot oil flows to pilot control valve (22).

If the engine is not running and the lift cylinders are in a RAISED position, oil from the head end of lift cylinders (4) will flow back to the pilot valve (oil manifold) (25) through screen (28). The oil flows through secondary reducing valve (26). When the engine is not running, the pressure at primary reducing valve (21) will be less than the pressure in secondary reducing valve (26). Shuttle valve (23) will shift to the UP position and the oil that is coming from the head end of lift cylinders (4) will supply the oil to the pilot system. This will allow the operator to lower the work tool.

Reference: For more information about the pilot valve (oil manifold), refer to the Service Manual module Systems Operation, "Pilot Valve (oil manifold)" for the machine that is being serviced.




Illustration 2g00990695

(22) Pilot valve. (36) Lever (hydraulic lockout). (37) Lift control lever. (38) Tilt control lever.

When lever (36) is in the B position, the pilot oil from shuttle valve (23) will be blocked at the ball valve (hydraulic lockout) (27). If the lever (36) is in the A position, the pilot oil from shuttle valve (23) will be open and the pilot oil will flow through the ball valve (hydraulic lockout) (27) .

The pilot valve (oil manifold) (25) is equipped with a float valve (24). If the lift control lever (37) is placed in the full LOWER position, the float valve will allow the work tool to follow the contour of the terrain.




Illustration 3g00990711

The pilot oil will flow to the inlet of pilot valve (22). The oil flows to the pilot control valves through passage (39). As the operator moves the lever for the control valves, the pilot oil will flow through the moved control valve. The pilot oil flows from ball valve (27) to pilot valve (22). Each output of the pilot control valve with orifice (20) is connected to an end cap on the main control valve. The pilot oil from the activated control valve will flow to the end cap that is requested in main control valve (1) .

Reference: For more information about the pilot control valve, refer to the Service Manual module Systems Operation, "Pilot Valve" for the machine that is being serviced.

Main Hydraulic System

Implement pump (32) draws oil from hydraulic tank (34). Then, the pump provides supply oil to the implement cylinders.

Reference: For more information about the implement pump, refer to the Service Manual module Systems Operation, "Vane Pump (Steering, Implement, Pilot, and Braking" for the machine that is being serviced.

The oil from Implement pump (32) flows to main control valve (1). The supply oil flows to the main relief valve (6). Main relief valve (6) regulates the oil pressure that is flowing into the main control valve. When the oil pressure in the hydraulic system rises above the relief valve setting, main relief valve (6) opens. Oil flows to the hydraulic tank until the pressure in the hydraulic system is reduced to less than the adjusted pressure. The relief valve setting is 20700 ± 340 kPa (3000 ± 50 psi) at approximately 268 ± 4 L/min (71 ± 1 US gpm).

Reference: For more information about the main relief valve, refer to the Service Manual module Systems Operation, "Relief Valve" for the machine that is being serviced.

The 966G Series II Wheel Loader or the 972G Series II Wheel Loader can be equipped with a two-section valve or a three-section main control valve. The three sections control the lift function, the tilt function, and the auxiliary function of the work tool. The two-section valve controls the lift function, and the tilt function of the work tool.

Main control valve (1) has an open center. When the engine is running and the control levers are in the CENTER position, the oil that is flowing into main control valve (1) flows through the main control valve and back to hydraulic tank (34). The return oil will pass through the filter that is assembled within hydraulic tank (34).




Illustration 4g00990720

Main Control Valve

(1) Main control valve. (40) Auxiliary control lever. (41) Tilt control lever. (42) Lift control lever.

When pilot oil is sent to an end cap of main control valve (1) the stem is shifted and the pump supply oil is directed to the appropriate implement cylinder.

Valve stems (13), (15), and (17) control the flow of oil to auxiliary cylinders (2), tilt cylinder (3), and lift cylinder (4). The valve stems are controlled by pilot pressure oil from the pilot control valve.

Tilt stem (15) has three positions: TILT BACK position, HOLD position and DUMP position. When tilt stem (15) moves to the TILT BACK position, pump supply oil flows through the tilt valve (40) to the head end of the tilt cylinder. Oil from the rod end of the tilt cylinder flows through tilt valve (41) back to the hydraulic oil tank.

When the tilt lever is in the HOLD position, all of the oil flow returns to hydraulic tank (34). When tilt stem (15) moves to the DUMP position, high pressure oil flows through the tilt valve (41) to the rod end of the tilt cylinder. Oil from the head end of tilt cylinder (3) will flow to hydraulic tank (34) .

Lift stem (17) has four positions: RAISE position, HOLD position, LOWER position and FLOAT position. In the RAISE position, pump supply oil flows to the head end of the lift cylinders. As lift cylinders (4) are moving the oil that is in the rod end of the lift cylinders (4) will flow through lift control valve (42) back to hydraulic tank (34). When the lift lever is placed in the HOLD position, stem (17) will return to the CENTER position. The pump supply oil returns to hydraulic tank (34). When the operator moves the lift control lever to the LOWER position, stem (17) will shift. Pump supply oil flows to the rod end of lift cylinders (4). Lift cylinder (4) will retract and the oil in the head end of lift cylinder (4) will flow through lift control valve (42) back to hydraulic tank (34) .

When the lift control lever in the cab is placed in the FLOAT position the lift valve spool remains in the LOWER position. Float valve (24) vents the high pressure oil that is in the rod end of lift cylinder (4). The high pressure oil will drain back to hydraulic tank (34) through float valve (24). The bucket will be allowed to follow the contour of the terrain.

The flow of oil through the main control valve changes when the machine has an auxiliary circuit. The oil from implement pump (32) first flows to auxiliary stem (11). The auxiliary stem has three positions: OPEN position, HOLD position and CLOSE position.

In the OPEN position, high pressure oil flows through auxiliary control valve (40) to the rod end of the auxiliary cylinder. In the HOLD position, all of the oil flow returns to the hydraulic oil tank. In the CLOSE position, high pressure oil flows to the head end of the auxiliary cylinder.

The auxiliary circuit, the tilt circuit, and the lift circuit have check valves that are located inside the main control valve. Load check valves (12), (14) and (16) prevent reverse oil flow through main control valve (1). Check valves (12, 14, 16) are built into the control valves. The check valves prevent oil from flowing back to implement pump (32) from a load that is developed in the cylinders.

The tilt function and the auxiliary functions are equipped with line relief valves. If the work tool is met by an outside source, a pressure spike can be induced into the main control valve. If the pressure spike is higher than the setting of the line relief valve, the line relief valve will allow the pressure to dump to hydraulic tank (34) .

Reference: For more information about the operation of the main control valve, refer to the Service Manual module Systems Operation, "Main Control Valve" for the machine that is being serviced.

Reference: For more information about the operation of the relief valves, refer to the Service Manual module Systems Operation, "Relief valve" for the machine that is being serviced.

Quick Coupler (If Equipped)

If a machine is equipped with an optional quick coupler, the operator has the ability to change work tools without leaving the cab. The quick coupler is controlled from a switch in the cab. Illustration 5 shows the location on the panel. In order to activate the pins for the quick coupler, press the bottom of the switch. In order to disengage the pins from the work tool, activate the red button on the switch as the top of the switch is pressed. The red button is a locking feature of the switch that prevents disengaging the quick coupler during operation.




Illustration 5g00990764

Location of the Switch (Quick Coupler)




Illustration 6g01002740

Lines Group (Quick Coupler)

(43) Charge valve (service brakes). (44) Accumulator. (45) Hose between the ball valve and the accumulator. (46) Hose between the diverter valve and the ball valve. (47) Hose between the diverter valve and the hydraulic tank. (48) Hose between solenoid valve and the ball valve on the lift arm (retract pins). (49) Hose between the diverter valve and the ball valve on the lift arm (extend pins). (50) Ball valve. (51) Solenoid valve (quick coupler). (52) Relief valve (quick coupler). (53) Diverter valve (quick coupler). (54) Ball valve. (55) Connector (extend pins). (56) Rear frame. (57) Loader frame. (58) Connector (retract pins).

In the operation of the quick coupler, diverter valve (53) controls the engagement or the disengagement of the coupler pins.

The oil that is used by the quick coupler is provided by the pilot/brake pump. The pressure line for the quick coupler is attached to a tee on brake accumulator (44) for the front brakes. From the tee on brake accumulator (44), the oil flows through hose (45) to ball valve (50). Then, the oil flows from ball valve (50) through hose (46) to diverter valve (53). The return oil flows from diverter valve (53) through hose (47) and back to hydraulic tank (34) .

As the switch for the quick coupler in Illustration 5 is in the DOWN position, solenoid valve (51) is energized in order to retract the coupler pins. The oil flows from diverter valve (53) through hose (48). The oil flows out of hose (48) to ball valve (54) on the lift arm. The hose for the rod end of the cylinder (quick coupler) is connected to connector (58) at ball valve (54).

When the switch is placed in the UPPER position, the oil flows out of diverter valve (53) through hose (49). Hose (49) is connected to the other ball valve on the lift arm. The hose for the head end of the cylinder (quick coupler) is connected to connector (55) at ball valve (54). The oil flows to the head end of the cylinder. The cylinder will extend the coupler pins. The work tool will be attached to the coupler frame.




Illustration 7g01002741

Frame (Quick Coupler)

(59) Cylinder (quick coupler). (60) Locking area of the frame. (61) Pins.

When the boss of the work tool is positioned in locking area (60) of the coupler frame and the operator presses the top of the switch, coupler pins (61) advance. The work tool will be firmly attached to the coupler frame.

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