320C and 322C Forest Machines Hydraulic System Pilot Hydraulic System Caterpillar


Pilot Hydraulic System
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1.1. Pilot Oil Circuit
2.1. Power Shift Pressure System
3.1. Pilot Control Valve Circuits
4.1. Pressure Switch Circuits
5.1. Straight Travel Valve Circuit
6.1. Swing Parking Brake
7.1. Boom Priority
8.1. Swing Priority
9.1. Automatic Travel Speed Change Valve



Illustration 1g00947522

(1) Swing parking brake

(2) Displacement change valve (left travel motor)

(3) Displacement change valve (right travel motor)

(4) Pilot line (grapple open)

(5) Solenoid valve (grapple control)

(6) Grapple pressure switch (open/close operation)

(7) Line (pilot oil from swing parking brake solenoid valve)

(8) Travel pilot control valve

(9) Pilot line (grapple close)

(10) Pilot line (pilot oil to travel pilot control valve)

(11) Pilot line (pilot pressure to left travel pressure switch)

(12) Travel pressure switch (left)

(13) Pilot line (pilot pressure to right travel pressure switch)

(14) Travel pressure switch (right)

(15) Pilot line (BOOM LOWER)

(16) Pilot line (boom drift reduction valve)

(17) Boom drift reduction valve

(18) Pilot line (STICK IN)

(19) Pilot line (stick drift reduction valve)

(20) Stick drift reduction valve

(21) Main control valve

(22) Pilot valve (straight travel)

(23) Travel pressure switch (straight travel)

(24) Solenoid valve (straight travel)

(25) Right travel control valve

(26) Boom I control valve

(27) Resolver valve (straight travel)

(28) Variable swing priority valve

(29) Left travel control valve

(30) Straight travel control valve

(31) Pilot line (pilot oil from boom pilot control valve)

(32) Pilot line (BOOM RAISE)

(33) Proportional reducing valve (negative flow control)

(34) Pilot line (pilot oil to pilot control valve for the boom and heel)

(35) Pilot line (pilot oil to the solenoid valve for grapple controls)

(36) Pilot line (pilot oil to pressure reducing valve for swing priority)

(37) Pilot line (pilot oil to the pressure reducing valve for boom priority)

(38) Passage (power shift pressure)

(39) Pilot line (pilot oil to pilot control valve for the stick and swing)

(40) Pilot control valve for boom and heel

(41) Pilot line (pilot oil to straight travel control valve)

(42) Pilot line (pilot pressure to displacement change valves)

(43) Swing parking brake solenoid valve

(44) Valve (hydraulic activation)

(45) Hydraulic activation solenoid valve

(46) Left pump

(47) Pilot pump

(48) Right pump

(49) Pilot control valve for stick and swing

(50) Pilot line (STICK OUT)

(51) Pilot line (STICK IN)

(52) Pilot line (SWING RIGHT)

(53) Pilot line (SWING LEFT)

(54) Pilot line (HEEL CLOSE)

(55) Pilot line (HEEL OPEN)

(56) Pilot line (BOOM LOWER)

(57) Pilot line (BOOM RAISE)

(58) Pilot line (pilot pressure to implement/swing pressure switch)

(59) Implement/swing pressure switch

(60) Pilot line (pilot oil to pilot control valves)

(61) Travel speed solenoid valve

(62) Pilot manifold

(63) Passage

(64) Pressure reducing valve for swing priority

(65) Pressure reducing valve for boom priority

(66) Passage

(67) Passage

(68) Passage

(69) Pilot line (pilot oil flow to pilot oil manifold)

(70) Pilot filter

(71) Pilot relief valve

(72) Passage

(73) Pilot line (pilot oil flow from pilot pump to pilot oil filter)

(74) Passage (power shift pressure)

(75) Proportional reducing valve (power shift pressure)

(76) Pilot line (pilot oil flow to pump regulators)




Illustration 2g00886437

Ports and solenoids at the pilot manifold

(43) Swing parking brake solenoid valve

(44) Valve (hydraulic activation)

(45) Hydraulic activation solenoid valve

(61) Travel speed solenoid valve

Pilot Oil Circuit

The pilot circuit pressure is limited by pilot relief valve (71) .

The oil delivery from pilot pump (47) performs the following main functions.

  • Create pilot oil pressure in order to control the output flows of the main pumps.

  • Provide pilot oil pressure to the pilot control valves for implements, swing and travel in order to perform machine operations.

  • Create pilot oil pressure in order to automatically operate the control devices.

The pilot circuit is classified into the following circuits and each circuit performs one of the above functions.

  • Power shift pressure system

  • Pilot control valve circuit

  • Pressure switch circuits

  • Straight travel valve circuit

  • Swing parking brake

  • Boom priority

  • Swing priority

  • Automatic travel speed change

Power Shift Pressure System




Illustration 3g00886447

(46) Left pump

(47) Pilot pump

(48) Right pump

(75) Proportional reducing valve (PS pressure)

(78) Engine speed sensor (flywheel housing)

(79) Governor actuator

(80) Left pump pressure sensor

(81) Right pump pressure sensor

(82) Feedback sensor

(83) Engine and pump controller

(84) Monitor

(85) Engine speed dial

During machine operation, engine and pump controller (83) receives input signals from the following components:

  • Engine speed sensor (78) that is located on the flywheel housing

  • Left pump pressure sensor (80)

  • Right pump pressure sensor (81)

  • Feedback sensor (82) at governor actuator (79)

  • Monitor in the cab (84)

  • Engine speed dial (85)

The engine and pump controller (83) continually monitors all of the input signals. The input signals are processed by the engine and pump controller and an output signal is sent to proportional reducing valve (75) at the right pump regulator. The proportional reducing valve assists in controlling the output flow of right pump (48) and left pump (46) .

The oil delivery from pilot pump (47) flows through the pilot filter to proportional reducing valve (75) at the right pump regulator. The electrical signal that is sent from engine and pump controller (83) causes proportional reducing valve (75) to regulate the pilot pressure to a reduced pressure. This reduced pressure is called power shift pressure (PS). The proportional reducing valve sends the reduced pilot oil pressure through the right pump regulator and through the left pump regulator. The output flow of right pump (48) and left pump (46) is controlled in accordance with the power shift pressure. The power shift pressure is used to regulate the maximum allowable hydraulic pump output.

The output signal that is sent from the engine and pump controller to the proportional reducing valve will change when the engine and pump controller detects a change in any of the input signals. The power shift pressure that is sent to the regulators at the right pump and the left pump will change in order to regulate the maximum allowable hydraulic pump output. The desired engine speed is maintained.

A decrease in engine speed increases the power shift pressure. An increase in power shift pressure causes a destroke condition of the right pump and the left pump. The maximum allowable hydraulic power output is decreased.

An increase in engine speed decreases the power shift pressure. A decrease in power shift pressure causes an upstroke condition of the right pump and the left pump. The maximum allowable hydraulic power output is increased.

Note: For more information concerning the operation of the engine and pump controller, refer to Systems Operation/Testing and Adjusting, "Engine and Pump Electronic Control System".

Pilot Control Valve Circuits

Oil from pilot pump (47) flows through pilot line (73), pilot filter (70) and pilot line (69) to pilot manifold (62). When the hydraulic activation control lever is shifted to the UNLOCKED position, the engine and pump controller energizes the hydraulic activation solenoid valve (45). The pilot oil then shifts valve (44). The pilot oil now flows through valve (44), pilot line (35), and pilot line (60). The pilot oil now flows to pilot control valve (8), pilot control valve (49), pilot control valve (40), and solenoid valve (5) for implements, swing and travel in order to perform machine operations. When the joysticks and/or travel levers/pedals are moved, the pilot oil flows to main control valve (21) in order to control the machine functions.




Illustration 4g00938804

Pilot lines at the main control valve (top view)

When joystick (40) and/or joystick (49) are operated, the pilot control valves send pilot pump oil through the pilot lines to pilot ports at the main control valve in order to shift the spools in the main control valve. Refer to Illustration 4 and Table 1 for the location of the pilot lines and machine operations.

Table 1
Pilot line     Control Valve     Machine Operation    
(86)     Boom I control valve     BOOM LOWER    
(87)     Heel control valve     HEEL CLOSE    
(88)     Grapple control valve     GRAPPLE OPEN    
(89)     Swing control valve     SWING LEFT    
(90)     Boom II control valve     BOOM RAISE    
(91)     Stick II control valve     STICK IN    
(92)     Right travel control valve     REVERSE RIGHT TRAVEL    
(93)     Left travel control valve     REVERSE LEFT TRAVEL    
(94)     Stick I control valve     STICK IN    

Pilot oil from the pilot control valves flows through pilot lines to the ports on the bottom of the main control valve in order to perform the opposite operation.

The following example is given for the BOOM LOWER operation and the BOOM RAISE operation. Machine operations for a stick operation, heel operation, travel operation, and swing operation are accomplished in the same manner as the boom operation.

When the joystick for the boom is moved to the BOOM RAISE position, pilot oil from pilot control valve (40) flows through pilot line (32) to boom I control valve (26). The pilot pressure shifts the boom I control valve. The oil delivery from the right pump flows to the head end of the boom cylinders in order to perform the BOOM RAISE operation.

When the joystick for the boom is moved to the BOOM LOWER position, pilot oil from pilot control valve (40) flows through pilot line (15) to boom I control valve (26). The pilot pressure shifts the boom I control valve. The pilot oil also flows through pilot line (16) in order to open boom drift reduction valve (17). The return oil from the head end of the boom cylinders flows through the boom drift reduction valve and the boom I control valve to the hydraulic tank. The BOOM LOWER operation is now performed.

Pressure Switch Circuits

Pressure switches (12) and (14) are connected to travel pilot control valve (8). Pressure switch (23) is connected to the straight travel pilot control valve. Pressure switch (59) is connected to pilot control valve (40) and pilot control valve (49). Pressure switch (6) is connected to the control valve for the grapple cylinders. When all of the joysticks and/or travel levers/pedals are in the NEUTRAL position, the pilot oil pressure to the pressure switches is low. Pressure switches (6), (12), (14), (23) and (59) are OFF. The engine and pump controller recognizes the OFF condition of all of the pressure switches. The AEC system is activated in order to lower the engine rpm.

If any of the joysticks and/or travel levers/pedals are moved from the NEUTRAL position, or any grapple control switches are activated, the increased pilot oil pressure is sent to the pressure switches. If pressure switch (6), (12), (14), (23), and/or (59) is ON, the engine and pump controller activates the AEC system in order to increase the engine rpm.

Straight Travel Valve Circuit

When a swing operation and/or implement operation is performed during a travel operation, the increase of pilot pressure in pilot line (58) activates implement/swing pressure switch (59). The implement/swing pressure switch sends an electrical signal to the engine and pump controller. The engine and pump controller energizes straight travel solenoid (24). Pilot pressure now activates straight travel control valve (30). The straight travel control valve maintains straight travel even though there is a swing operation or an implement operation during travel. For more information concerning the operation of the straight travel control valve, refer to Systems Operation, "Control Valve (Straight Travel)".

Swing Parking Brake

When the hydraulic activation control lever is placed in the UNLOCKED position, pilot oil in passage (68) flows through valve (44) and passage (67) to swing parking brake solenoid valve (43). When any of the joysticks are moved from the NEUTRAL position, the increase of pilot pressure in pilot line (58) activates implement/swing pressure switch (59). The implement/swing pressure switch sends an electrical signal to the engine and pump controller. An electrical signal from the engine and pump controller energizes the swing parking brake solenoid valve (43). Pilot oil in line (7) flows to swing parking brake (1). This oil releases the swing parking brakes. For more information concerning the operation of the swing parking brake, refer to Systems Operation, "Pilot Valve (Swing Parking Brake)".

Boom Priority

During combined operations of BOOM RAISE and STICK IN, the pilot oil pressure in pilot line (31) and pilot line (37) activates pressure reducing valve (65) for boom priority. The pressure reducing valve for boom priority allows priority flow to the head end of the boom cylinders during these combined hydraulic operations by disabling the stick II control valve. For more information concerning the pressure reducing valve for boom priority, refer to Systems Operation, "Boom Hydraulic System".

Swing Priority

During a swing operation, pilot oil flows from pilot control valve (49) to pressure reducing valve (64) for swing priority. The pressure reducing valve for swing priority shifts. The pilot oil flow in pilot line (36) from pilot oil manifold (62) is blocked by the pressure reducing valve for swing priority. Most of the left pump delivery flow goes to the swing motor. For more information concerning the pressure reducing valve for swing priority, refer to Systems Operation, "Swing Hydraulic System".

Automatic Travel Speed Change Valve

Pilot oil in passage (66) flows to travel speed solenoid valve (50). When the travel speed switch on the right console is set at the HIGH SPEED position, the travel speed solenoid valve opens. This allows pilot oil to flow through travel speed solenoid valve (61) and through line (42). The oil then flows to displacement change valve (2) for the left travel motor and displacement change valve (3) for the right travel motor. As the displacement change valve operates, the travel speed is maintained at the HIGH SPEED position.

When the travel speed switch on the right console is set at the HIGH SPEED position, the pressure sensors for the pump delivery pressure control the travel speed in accordance with the travel load. For example, low speed during a high load condition and high speed during a low load condition.

For more information concerning the operation of the displacement change valves, refer to Systems Operation, "Displacement Change Valve".

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