320B Excavator Hydraulic System Caterpillar


Pump Control (Main Hydraulic)

Usage:

320B 1CS


Main Pump Regulator




Illustration 1g00630445

Regulator

(1) Slider block

(2) Dowel

(3) Piston

(4) Swashplate

(5) Sleeve

(6) Dowel

(7) Slide plate

(8) Retainer

(9) Link

(10) Spring

(11) Spring

(12) Dowel

Introduction




Illustration 2g00541831

Pump compartment

(13) Line (power shift pressure)

(14) Line (rear pump negative control)

(15) Line (front pump negative control)

The pump regulator controls the output of the pump in the following manner.

  • The electronic controller analyzes the load on the machine and the speed of the engine. The electronic controller then determines the power shift pressure. The electronic controller sends the power shift pressure through line (13) to the regulator. The regulator receives the power shift pressure in the form of a hydraulic signal. The regulator uses this hydraulic signal to control the output flow of the pumps.

  • The regulator receives a hydraulic signal that is called the pump delivery pressure. The regulator uses this hydraulic signal to maintain the same horsepower between the engine and the pump. This is called horsepower control characteristic.

  • When the control levers are in the NEUTRAL position or in the PARTIAL position, the main control valves develop a negative flow control pressure. The pump regulators receive the negative flow control pressure. Negative flow control pressure in line (15) adjusts the output flow of the front pump. Negative flow control pressure in line (14) adjusts the output flow of the rear pump.

Regulator Operation




Illustration 3g00540554

Regulator operation

(16) Spool

(17) Passage

(18) Pilot piston

(19) Passage

(20) Control piston

(21) Negative flow control pressure

(22) Power shift pressure

(23) Mean pump delivery pressure

(24) Front pump delivery pressure

The pump regulator for the front pump and the rear pump are identical in construction and in operation. The front pump regulator will be used in order to describe the operation of the front pump regulator and the rear pump regulator.

Front pump delivery pressure (24) flows to spool (16). Mean pump delivery pressure (23) is the average of the front pump delivery pressure and the rear pump delivery pressure. Mean pump delivery pressure (23) flows through passage (17) to pilot piston (18). Power shift pressure (22) flows through passage (19) to pilot piston (18) .

During constant horsepower flow control, mean pump delivery pressure (23) in passage (17) acts against the shoulder of pilot piston (18). Power shift pressure (22) acts against the right end face of pilot piston (18). Spool (16) and pilot piston (18) now shift in order to control the pump output.

During negative flow control, negative flow control pressure (21) acts against the left end surface of control piston (20). Control piston (20) shifts pilot piston (18) in order to control the pump output.

Horsepower Control Characteristic (Period Before Decreased Pump Stroke)




Illustration 4g00540696

Regulator operation

(1) Slider block

(2) Dowel

(3) Piston

(4) Swashplate

(10) Spring

(16) Spool

(17) Passage

(18) Pilot piston

(22) Power shift pressure

(23) Mean pump delivery pressure

(24) Front pump delivery pressure

(25) Guide

(26) Passage

(27) Passage

(28) Shoulder

(29) Piston chamber

(30) Passage

(31) Passage

(32) Passage

The machine is experiencing a small load condition. Mean pump delivery pressure (23) from passage (17) acts on shoulder (28) of pilot piston (18). Power shift pressure (22) acts on the right end of pilot piston (18). Pilot piston (18) pushes spool (16) and guide (25) against the force of spring (10). When the total force of mean pump delivery pressure (23) and power shift pressure (22) is less than the total force of spring (10), pilot piston (18) remains stationary. Guide (25) is held against the shoulder of the front body. Passage (31) remains closed. Passage (26) opens. Front pump delivery pressure (24) flows through passage (32), passage (27) and passage (30) into piston chamber (29) .

Front pump delivery pressure (24) acts on the right side and the left side of piston (3). The left end surface of piston (3) is larger than the right end surface of piston (3). This creates more force on the left side of piston (3). Piston (3) shifts to the right side. Swashplate (4) is connected to piston (3) through the mechanical linkage of dowel (2) and slider block (1). Swashplate (4) is held at the maximum angle position. The pump maintains the maximum output flow.

Horsepower Control Characteristic (Period After Start of Decreased Pump Stroke)




Illustration 5g00540973

Regulator operation

(3) Piston

(4) Swashplate

(10) Spring

(16) Spool

(18) Pilot piston

(26) Passage

(28) Shoulder

(29) Piston chamber

(30) Passage

(31) Passage

(33) Passage

The machine is experiencing an increased load condition. An increased load on the main pump increases power shift pressure (22) and mean pump delivery pressure (23) .

Power shift pressure (22) and mean pump delivery pressure (23) combine. This combined force acts on shoulder (28) and on the right end surface of pilot piston (18). Pilot piston (18) pushes spool (16) to the left side against the force of spring (10). When the combined force is greater than the force of spring (10), spool (16) shifts. Passage (26) closes and passage (31) opens. Passage (30) and passage (33) are connected. Passage (33) is open to the pump case drain. The pressure in piston chamber (29) becomes less than the mean delivery pressure and piston (3) shifts to the left.

Piston (3) is mechanically connected to swashplate (4). The angle of swashplate (4) decreases.

Swashplate (4) is mechanically connected with sleeve (5) through dowel (6), link (9), retainer (8), dowel (12) and slide plate (7). As swashplate (4) rotates clockwise, sleeve (5) shifts to the left. This closes passage (32). Passage (27) and passage (32) are now closed. Sleeve (5) and spool (16) stop moving. Sleeve (5) and spool (16) are now in a balanced position. Pilot piston (18) and swashplate (4) are held at this balanced position.

Negative Flow Control




Illustration 6g00540767

Negative flow control operation

(34) Negative flow signal pressure port (front pump)

(35) Negative flow signal pressure port (rear pump)




Illustration 7g00540798

Negative flow control operation

(5) Sleeve

(10) Spring

(11) Spring

(16) Spool

(18) Pilot piston

(30) Passage

(31) Passage

(36) Main control valves

(37) Passage

(38) Center bypass passage

(39) Orifice (negative flow control)

(40) Line

(41) Piston chamber

(42) Spring

When the control levers are in the NEUTRAL position the negative flow control pressure minimizes the pump output. When a control lever is partially moved from the NEUTRAL position, the negative flow control allows the pump output flow to gradually increase. Pump output flow will increase depending on the amount of movement of the control levers. This function activates for fine control operations until the center bypass passage in the main control valve is closed.

The oil flow in center bypass passage (38) in main control valves (36) is at a maximum when the control levers are in the NEUTRAL position. When the control levers are partially moved for a fine control operation, part of the main pump oil flows into passage (37). This decreases the oil flow in center bypass passage (38) .

The flow in center bypass passage (38) is restricted by negative flow control orifice (39). Negative flow control pressure (21) develops in line (40). The increase or the decrease of negative flow control pressure (21) depends on the rate of oil flow through center bypass passage (38). Negative flow control pressure (21) is at a maximum when the control levers are in the NEUTRAL position. This maintains the pump output flow to a minimum.

Negative flow control pressure in line (40) flows into the regulator through port (34) and port (35) into piston chamber (41). Negative flow control pressure shifts control piston (20) to the left against the force of spring (11). Control piston (20) shifts pilot piston (18) and spool (16) to the left. Spool (16) shifts to the left. Passage (31) and passage (30) open. Passage (31) and passage (30) are connected to the pump case drain.

When the force of the negative flow control pressure becomes equal to the combined force of spring (11), spring (10) and spring (42) control piston (20), pilot piston (18) and spool (16) stop moving. Front pump delivery pressure (24) overcomes the force of the pressure in piston chamber (29) and piston (3) starts to shift to the left. The angle of swashplate (4) decreases and the pump destrokes. When piston (3) comes into contact with the left side stopper the pump output flow is at a minimum.

Negative flow control pressure (21) continues to decrease as the control lever is moved further from the NEUTRAL position. This causes less force to act against spring (11). Control piston (20) shifts to the right. The angle of the swashplate increases. This upstrokes the pump.

During a fine control operation, the pump output flow is controlled at any rate between the minimum and the maximum. This depends on negative flow control pressure.

Pressure/Flow Characteristic Curves




Illustration 8g00297449

P-Q characteristic curves

(1) Destroked position

(2) Horsepower characteristics

The output characteristics of each pump depends on the following pressures.

  • Main pump delivery pressure

  • Power shift pressure

After a pump starts to operate, each pump has a set of pressure/flow characteristic curves. The pressure/flow characteristic curve represents a set of flow rates for distinct pump circuit pressures. Each point on horsepower characteristic (2) represents the respective flow rate and the respective pressure in order to maintain a constant horsepower output from the pump.

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