330D, 336D and 340D Excavators Hydraulic System – Negative Flow Control System

Introduction

The drive pump and the idler pump receive signal oil pressure from the center bypass passages of the main control valve. This signal oil pressure that is created in the center bypass passages of the main control valve is called negative flow control pressure. Negative flow control pressure flows to the regulators at the drive pump and the idler pump to control the output flow of the pumps. Negative flow control pressure is created during the following machine operating conditions.

• All of the joysticks and travel levers/pedals are in the NEUTRAL position.

• Any of the joysticks and/or travel levers/pedals are partially moved from the NEUTRAL position to perform a fine control operation.

• A boom lower operation is performed alone.

The right body of the control valve receives supply oil from the drive pump. Negative flow control pressure from the right body of the main control valve flows through negative flow control line (13) to the drive pump. The left body of the control valve receives supply oil from the idler pump. Negative flow control pressure from the left body of the main control valve flows through negative flow control line (12) to the idler pump. The negative flow control operation of the drive pump and the idler pump is identical.

Illustration 3 shows the negative flow control operation at the main control valve when all of the control valves are in the NEUTRAL position. When all of the joysticks and the travel levers/pedals are in the NEUTRAL position, the spools of the individual control valves are in the NEUTRAL position. Oil flow to the cylinders and motors is blocked. Center bypass passages (1) and (3) are open.

All of the oil delivery from drive pump (15) flows through center bypass passage (3), passage (4) and negative flow control orifice (9) to return line (2). Negative flow control orifice (9) restricts the oil flow. The pressure in passage (4) increases. Increased negative flow control pressure now flows through passage (10) and negative flow control line (13) to the pump regulator. The negative flow control operation of the drive pump regulator causes the swashplate of the drive pump to move to the minimum angle position. The output flow of the drive pump is decreased due to the increased negative flow control pressure that is created in center bypass passage (3) .

Since center bypass passage (1) is also open, the negative flow control operation of the idler pump regulator is identical to the negative flow control operation of the drive pump regulator.

ReferenceFor more information concerning the negative flow control operation of the main pump regulators, refer to Systems Operation, “Pump Control (Main Hydraulic) (Main Pump Regulator)”.

Illustration 5 shows the negative flow control operation at the main control valve when only the work tool control valve is in the WORK TOOL CLOSE position.

All of the control valves in the left body of the main control valve are in the NEUTRAL position. Center bypass passage (1) is open. All of the oil delivery from the idler pump flows through center bypass passage (1) to negative flow control orifice (7). Since all of the oil delivery from idler pump (14) is restricted by negative flow control orifice (7), negative flow control pressure (PN) in center bypass passage (1) is at maximum pressure. The negative flow control pressure flows through negative flow control line (12) to the idler pump regulator. The negative flow control operation of the idler pump regulator causes the swashplate of the idler pump to move to the minimum angle position. The output flow of the idler pump is decreased due to the increased negative flow control pressure that is created in center bypass passage (1) .

The joystick for the work tool has been moved fully to the WORK TOOL CLOSE position. Pilot oil has fully shifted the work tool control valve. The oil delivery from drive pump (15) flows into the right body of the main control valve. The oil delivery flows through center bypass passage (3) to the work tool control valve. Since the spool in the work tool control valve is fully shifted, center bypass passage (3) is blocked. All of the oil delivery from the drive pump flows to the head end of the work tool cylinder. No oil flows to negative flow control orifice (9) and no negative flow control pressure is created in center bypass passage (3). Since no negative flow control pressure is sent to the drive pump regulator, the drive pump regulator moves the swashplate of the drive pump toward the maximum angle position. The output flow of the drive pump is increased since no negative flow control pressure is created in center bypass passage (3) .

Fine Control Operation

When the joystick for the work tool is in the NEUTRAL position, spool (23) is in the NEUTRAL position. The oil delivery from the drive pump flows through center bypass passage (3) to negative flow control orifice (9). When the joystick for the work tool is partially moved from the NEUTRAL position to perform a fine control operation, pilot pressure (P) enters the control valve at the pilot port. Pilot pressure shifts spool (23) slightly to the left. The movement of spool (23) partially opens passage (24). Center bypass passage (3) is partially blocked.

The oil delivery from the drive pump is now divided into two flow paths. A portion of the oil delivery from the drive pump flows through center bypass passage (3) to negative flow control orifice (9). The remainder of the oil delivery from the drive pump flows through parallel feeder passage (21) and passage (24) to port (22). The oil flow from center bypass passage (3) to negative flow control orifice (9) decreases. The flow resistance through the negative flow control orifice decreases and the negative flow control pressure (PN) in passage (4) decreases. The negative flow control pressure that is sent to the regulator at the drive pump decreases. The pump regulator causes the swashplate of the drive pump to move toward the maximum angle position. The output flow of the pump is increased due to the decrease in negative flow control pressure (PN) .

When the joystick for the work tool is moved to the full stroke position, spool (23) shifts fully to the left. Center bypass passage (3) is now blocked by spool (23). Since there is no oil flow through center bypass passage (24), no negative flow control pressure is created. The swashplate of the drive pump is moved to the maximum angle position. The output flow of the drive pump is maximum. The output flow of the drive pump is now controlled by the constant horsepower flow control.

The ability to modulate the negative flow control pressure by partial movement of the joystick enables fine control of the implements.

Relief Valve (Negative Flow Control)

The following description is given for the operation of the relief valve that is located in the right body of the main control valve. The operation of the relief valve for the negative flow control that is located in the left body of the main control valve is identical.

Relief valve (5) for the negative flow control consists of plug (17), spring (18), relief valve body (19), and valve (20). When any one of the joysticks and/or travel levers/pedals is at the full stroke position, the oil flow through center bypass passage (3) is blocked. No oil flows to the relief valve for the negative flow control.

When all of the joysticks and/or travel levers/pedals are suddenly returned to the NEUTRAL position, all of the output flow from the drive pump flows through center bypass passage (3). The negative flow control pressure in center bypass passage (3) and passage (4) suddenly increases. When the negative flow control pressure becomes higher than the pressure setting of relief valve (5) for the negative flow control, valve (20) shifts to the left against the force of spring (18). Oil in center bypass passage (3) is now allowed to flow past valve (20) into return passage (11) to the hydraulic tank. This flow prevents the hydraulic shock that occurs due to sudden changes in negative flow control pressure.

After the hydraulic shock is relieved by the relief valve for the negative flow control, the force of spring (18) shifts valve (20) to the right. All of the output flow from the drive pump flows through center bypass passage (3), negative flow control orifice (9) and return line (2) to the hydraulic tank.

Negative flow control pressure (PN), that is created in center bypass passage (3), reaches maximum pressure since all of the oil flow is restricted by negative flow control orifice (9). The negative flow control pressure flows to the drive pump regulator. The regulator at the drive pump causes the swashplate of the drive pump to move to the minimum angle position. The output flow of the drive pump is decreased due to the increase in negative flow control pressure (PN) .