906 and 908 Wheel Loaders Hydraulic and Steering System Caterpillar


Pilot Hydraulic System

Usage:

906 MER



Illustration 1g01062155

The standard hydraulic system schematic

(1) Charge pump

(2) Piston pump

(3) Accumulator

(4) Shutoff valve for the pilot supply oil

(5) Pilot valve for the auxiliary

(6) Pilot valve for the implements

(7) Bank valve

(A) Supply oil to the fan drive motor

(B) Supply oil to the couplers

(C) Hystat charge oil and pilot supply oil

(D) Supply oil to the bearing for the piston motor

(E) Oil pressure from the load sensing port for steering

(F) Supply oil for the steering cylinder

The pilot hydraulic system receives oil from the charge pump (C). The charge pump is part of the piston pump ( hydrostatic) (2). The pilot oil flows from the charge pump into the hydraulic oil filter and into the speed sensing valve. Refer to Systems Operation, "Piston Pump (Hydrostatic)" for more information on the operation of the charge pump.

Accumulator

Oil flows from the speed sensing valve to the accumulator (3). The accumulator provides limited pilot oil to the pilot valves after the engine is shut down. The accumulator is charged with dry nitrogen.

Shutoff Valve

After the accumulator (3), oil flows to the shutoff valve (4). The shutoff valve is used to disable the pilot supply. Once the shutoff valve is disabled you cannot move the work tool.

The hydraulic shutoff control is located on the right console. Pressing the top of the switch will turn on the hydraulics for the loader. Pressing the bottom of the switch will turn off the hydraulics for the loader. The hydraulics for the loader should be turned off when the machine is roaded.

Pilot Valve for the Implements




Illustration 2g01062178

Location of the pilot valve for the implements

(6) Pilot valve for the implements

(8) FLOAT position

(9) LOWER position

(10) HOLD position

(11) RAISE position

(12) TILT BACK position

(13) DUMP position

If the shutoff valve (4) is engaged, oil flows from the shutoff valve to the pilot valve for the implements (6) and the pilot valve for the auxiliary (5). The pilot valve for the auxiliary (5) controls the functions for the work tool through the bank valve (7). The pilot oil flows from the pilot valve for the implements to the control valves for the work tool functions. The pilot oil shifts the spools inside the control valves. This directs high pressure oil to the work tool in order to control the work tool.




Illustration 3g01062188

(3) Accumulator

(4) Shutoff valve for the pilot supply oil

(5) Pilot valve for the auxiliary

(6) Pilot valve for the implements

(C) Hystat charge oil and pilot supply oil

(G) Line for the pilot oil for the lift function

(H) Line for the pilot oil for the lift function

(J) Line for the pilot oil for the tilt function

(K) Line for the pilot oil for the tilt function

(L) Line for the pilot oil for the auxiliary function

(M) Line for the pilot oil for the auxiliary function

(N) Return line from the pilot valves to the tank

HOLD Position

When the control lever of the pilot valve for the implements (6) is in the HOLD position (10), the pilot supply line (C) is blocked inside the pilot valve for the implements. The other pilot oil lines within pilot valves (5) and (6) are open to the tank. The control lever will return to the HOLD position when you release the lever from any of the positions except for the FLOAT position.

RAISE Position

When you move the control lever of the pilot valve for the implements (6) to the RAISE position (11), the pilot oil flows through supply line (C) and through line (G). The oil in line (G) flows to the bank valve (7). The spool in the lift control valve shifts to the RAISE position and the lift cylinder raises the work tool. The following pilot oil lines for the implements are open to the tank through line (N) : line (H), Line (J) and Line (K). The lever control will return to the HOLD position when you release the lever control. The work tool will remain in the selected position.

LOWER Position

When you move the control lever of the pilot valve for the implements (6) to the LOWER position (9), the pilot oil flows through supply line (C) and through line (H). The oil in line (H) flows to the bank valve. The spool in the lift control valve shifts to the LOWER position and the lift cylinder lowers the work tool. The following pilot oil lines for the implements are open to the tank through line (N) : line (G), Line (J) and Line (K). The lever control will return to the HOLD position when you release the lever control. The work tool will remain in the selected position.

FLOAT Position

When you move the control lever of the pilot valve for the implements (6) to the FLOAT position (8), the pilot oil flows through supply line (C) and through line (H). The pilot valve has a detent in this position. This allows the control to remain in the FLOAT position until the control is pulled out of the detent. The oil in line (H) flows to the bank valve. The spool in the lift control valve shifts to the FLOAT position and the work tool will lower to the ground. The following pilot oil lines for the implements are open to the tank through line (N) : line (G), Line (J) and Line (K). The lever control will return to the HOLD position when you pull the lever control out of the detent position and you release the control lever.

TILT BACK Position

When you move the control lever of the pilot valve for the implements (6) to the TILT BACK position (12), the pilot oil flows through supply line (C) and through line (J). The oil in line (J) flows to the bank valve. The spool in the tilt control valve shifts to the TILT BACK position and the tilt cylinder tilts the work tool backward. The following pilot oil lines for the implements are open to the tank through line (N) : line (G), Line (H) and Line (K). The lever control will return to the HOLD position when you release the lever control. The work tool will remain in the selected position.

DUMP Position

When you move the control lever of the pilot valve for the implements (6) to the DUMP position (13), the pilot oil flows through supply line (C) and through line (K). The oil in line (K) flows to the bank valve. The spool in the tilt control valve shifts to the DUMP position and the tilt cylinder tilts the work tool forward. The following pilot oil lines for the implements are open to the tank through line (N) : line (G), Line (H) and Line (J). The lever control will return to the HOLD position when you release the lever control. The work tool will remain in the selected position.

Pilot Valve for the Auxiliary (if equipped)




Illustration 4g01062255

Location of the pilot valve for the auxiliary

(5) Pilot valve for the auxiliary

(14) ON position for the left hand side auxiliary

(15) HOLD position

(16) ON position for the right hand side auxiliary

(17) DETENT position

The pilot valve for the auxiliary (5) controls the auxiliary function through the bank valve. The auxiliary function is active when you move the lever control of pilot valve (5) .

HOLD Position

When the control lever of the pilot valve for the auxiliary (5) is in the HOLD position (15), pilot supply line (C) is blocked. Pilot oil lines (L) and (M) are open to the tank through line (N). The control lever will return to the HOLD position (15) when you release the lever from the ON position (14) or from the ON position (16) .

ON Position for the Left Hand Side Auxiliary

When you move the control lever of pilot valve (5) to the ON position (14), the pilot oil flows through supply line (C) and through line (M). The oil in line (M) flows to the bank valve. Oil flows from the bank valve to the left hand side auxiliary hydraulic line. The pilot line (L) is open to the tank through line (N). The lever control will return to the HOLD position when you release the lever control.

ON Position for the Right Hand Side Auxiliary

When you move the control lever of pilot valve (5) to the ON position (16), the pilot oil flows through supply line (C) and through line (L). The oil in line (L) flows to the bank valve. Oil flows from the bank valve to the right hand side auxiliary hydraulic line. The pilot line (M) is open to the tank through line (N). The lever control will return to the HOLD position when you release the lever control.

DETENT Position for the Right Hand Side Auxiliary

When you move the control lever of pilot valve (5) to the DETENT position (17), the pilot oil flows through supply line (C) and through line (L). The pilot valve has a detent in this position. This allows the control to remain in the DETENT position until you pull the control out of the detent. The oil in line (L) flows to the bank valve. The bank valve routes a constant supply of oil to the right hand side auxiliary hydraulic line. The pilot line (M) is open to the tank through line (N). The lever control will return to the HOLD position when you pull the lever control out of the detent and you release the control lever.

The DETENT will also be released when the key switch is turned off. The lever control will return to the HOLD position when the key switch is turned off.

Proportional Auxiliary Circuit Control (if equipped)




Illustration 5g01062369

The hydraulic system schematic with attachments

(18) Modulating Valve

(19) Bank Valve

(20) Auxiliary Line

(21) Modulating Valve

(A) Supply oil to the fan drive motor

(B) Supply oil to the couplers

(C) Hystat charge oil and pilot supply oil

(D) Supply oil to the bearing for the piston motor

(E) Oil pressure from the load sensing port for steering

(F) Supply oil for the steering cylinder




Illustration 6g01062476

Proportional auxiliary control

(22) Thumb wheel for the proportional auxiliary control

(23) ON position for the left hand side auxiliary

(24) HOLD position

(25) ON position for the right hand side auxiliary

If the machine is equipped with a proportional control for the auxiliary circuit, the auxiliary circuit is controlled by a thumb wheel (22) on the pilot valve for the implements.

HOLD Position

When the thumb wheel (22) is in the HOLD position (24), pilot oil (C) is blocked at modulating valves (18) and (21). The thumb wheel will return to the HOLD position (24) when you release the thumb wheel from the ON position (23) or from the ON position (25) .

ON Position for the Left Hand Side Auxiliary

When you move the thumb wheel (22) to the ON position (23), the pilot oil flows through supply line (C) and through modulating valve (21). Oil flows from the bank valve (19) to the left hand side auxiliary hydraulic line. The rate of flow that is provided to the left hand side auxiliary line is proportional to the movement of the thumb wheel (22). The thumb wheel will return to the HOLD position when you release the thumb wheel.

ON Position for the Right Hand Side Auxiliary

When you move the thumb wheel (22) to the ON position (25), the pilot oil flows through supply line (C) and through modulating valve (18). Oil flows from the bank valve (19) to the right hand side auxiliary hydraulic line. The rate of flow that is provided to the right hand side auxiliary line is proportional to the movement of the thumb wheel (22). The thumb wheel will return to the HOLD position when you release the thumb wheel.

Continuous Flow




Illustration 7g01062491

(26) Switch for the proportional auxiliary circuit control

In order to provide a continuous flow of oil to the auxiliary couplings, you must press the top of the continuous flow switch (26) and you must hold the top of the continuous flow switch (26) while you move the thumb wheel (22). In order to provide continuous flow to the left hand side auxiliary, move the thumb wheel to position (23). The rate of continuous flow that is provided to the left hand side auxiliary line is proportional to the movement of the thumb wheel (22). In order to provide continuous flow to the right hand side auxiliary, move the thumb wheel to position (25). The rate of continuous flow that is provided to the right hand side auxiliary line is proportional to the movement of the thumb wheel (22). Press the bottom of the proportional switch (26) or move the thumb wheel (22) in order to deactivate the continuous flow.

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