M312, M315, M318 and M320 Excavators Hydraulic System Caterpillar

Components

Illustration 1	g00813300

Illustration 1 shows two sectional views of the main pump regulator. The regulator consists of a horsepower control, torque control and load sensing control.

Load Sensing Regulator

Illustration 2	g00813300

The load sensing regulator has priority. The load sensing regulator works as flow control is needed by the user. The flow control is supplied by the pump. The load sensing regulator works with the horsepower regulator in order to regulate the pump flow in relation to demand and in relation to system pressure. The load sensing pressure is generated in the main control valve. Then, the load sensing pressure is directed to the pump regulator. The load sensing pressure is equal to the load pressure from the hydraulic circuit with the highest working pressure. The load sensing pressure is directed to the spring end of the load sensing spool. System pressure from the pump is directed to the other end of the spool. The spring force maintains a constant difference between the two pressures. This pressure difference is the margin pressure. Adjustments to the margin pressure are made with the adjustment screw for margin pressure.

Section A-A shows the upstoke piston which acts on a pinned lever. The pinned lever acts on the right end of the constant horsepower control spool.

Horsepower Regulator

Illustration 3	g00813300

The horsepower spool regulates pump flow in relation to pump pressure and in relation to power shift pressure (PS).

Power shift pressure is a reduced amount of pilot system pressure. The proportional reducing valve (PRV) receives electrical signals from the electronic control unit (ECU). These electrical signals generate the power shift pressure.

In the upper view, the constant horsepower regulator spool is shown from a different angle and the load sensing regulator spool is visible. The power shift pressure piston and the power shift signal are shown.

Torque Control

Illustration 4	g00813300

The torque control moves the constant horsepower curve to a lower value. The PS pressure piston acts directly on the spring and the horsepower spool. The PS pressure is directly related to the power.

The main pump, the control valve and the destroke servo piston are shown as symbols.

The load sensing control contains an orifice with a shutoff valve, that is set to a differential pressure PD - LS of 1200 kPa (174 psi). This minimum pressure ensures a quicker response time during multiple implement operations. If the margin pressure drops below 1200 kPa (174 psi), the shutoff valve closes the tank return line of the LS oil.

Main Pump Regulator Operation

Main Pump Regulator with the Engine Off

Illustration 5	g00813157

Illustration 5 shows the pump regulator, when the engine is off. There is no active implement and all main control valves are in the NEUTRAL position. There is no load sensing signal coming from the control valve. There is no control pressure from the proportional reducing valve. The load sensing spool is forced to the released position by the load sensing spring. The horsepower spool is forced by the torque control spring against the power shift pressure piston. This causes the lever to rotate. Since there is no pressure on the upstroke piston or on the destroke piston, the swashplate is shifted to the maximum angle position by the spring on the upstroke piston.

The following conditions exist when the regulator is in the position that is shown in Illustration 5:

Conditions

Engine ... 0 rpm

LS ... 0 kPa (0 psi)

PD ... 0 kPa (0 psi)

PS1 ... 0 kPa (0 psi)

Flow ... 0 L/min (0 US gpm)

Swashplate ... Maximum angle

Illustration 6	g00813307

Main Pump Regulator In Standby Position

Illustration 7	g00813315

Illustration 7 shows the pump regulator in a standby position. The load sensing regulator and the horsepower regulator work together in order to regulate the pump flow.

The pump servo pistons are connected to the swashplate. Pump output pressure is directed to the upstroke servo piston at all times. The load sensing regulator directs control pressure to the destroke servo piston in order to move the swashplate to the minimum angle for decreased pump flow.

With the joysticks in the NEUTRAL position, the electronic controller sets the power shift pressure to a standby value. The increased power shift pressure works against the horsepower control spring.

Note: Throughout the discussion of the pump regulator operation, the power shift pressure remains constant. Changes in the power shift pressure can cause the pump to stroke.

When all the hydraulic control valves are in the NEUTRAL position, no load sensing signal pressure is directed to the spring side of the load sensing control. With no signal pressure in the spring chamber of the load sensing control, the pump output pressure moves the load sensing spool against the spring force. The load sensing spool opens a passage for the control pressure to the destroke servo piston. The standby pressure acts on the upstroke piston and control pressure acts on the destroke piston. The larger area of the destroke piston moves the swashplate to the minimum angle position.

The force that is exerted by the PS pressure piston on the lever overcomes the force of the actuator piston with the corresponding pivot distance.

The following conditions exist when the regulator is in the position that is shown in Illustration 7:

Conditions

Engine ... Running

Implements ... HOLD

PD ... 2500 to 3000 kPa (360 to 435 psi)

Flow ... Minimum leakage through destroke piston

Swashplate ... Minimum angle

Illustration 8	g00813319

The control valve is in HOLD position with no LS signal to the pump regulator. PS pressure is delivered from the PRV according to the mode selection on the electronic control.

The pump is delivering flow against the closed control valve. The restriction creates pressure on pump output. The load sensing spool is metered to a middle position that is balanced by the pump output and the LS spring. The passage is open for control pressure to the destroke piston. The LS orifice spool is in the open position by pump output pressure. The horsepower spool is pushed by the PS pressure piston against the horsepower spring.

Pump output pressure is applied on the upstroke servo piston and the control pressure is applied on the destroke servo piston.

The force of the destroke servo piston holds the swashplate at "Qmin". Flow is at a minimum in order to replace the leakage oil.

Main Pump Regulator In Upstroke Position

Illustration 9	g00813339

Illustration 9 shows the pump regulator at the start of upstroke that is caused by a increase in the load sensing pressure.

The following conditions cause an increase in flow from the main pump:

• An increase in the load sensing signal

• An increase in power shift pressure from the proportional reducing valve

• A decrease in pump output pressure

When at least one of the hydraulic pilot control valves are activated, load sensing pressure signal is directed to the spring chamber of the load sensing spool. When the load sensing signal pressure increases, the combined force of the load sensing signal pressure and the margin spring becomes higher than the force of the pump output pressure on the load sensing control spool. This unbalanced force shifts the load sensing spool. This connects the passage from the destroke piston to the tank.

When the destroke piston is open to the tank the upstroke piston pushes the swashplate toward the maximum angle. When the upstroke piston moves toward the UPSTROKE position, the mechanical advantage of the actuator piston on the lever pushes the horsepower control spool against the power shift force. This stops the return oil from the destroke piston from returning to the tank and the swashplate stops moving.

Illustration 10	g00813346

When the control lever is moved to the upstroke position, the control valve is open to the cylinder and flow is required. Pressure from the control valve is sent back to the pump regulator as a load sensing signal that moves the load sensing spool in order to open the flow from the destroke servo piston to the tank. LS signal with the spring pushes the LS orifice spool to the closed position.

The horsepower spool is against the spring and the destroke piston. The horsepower spool is open to the tank. The force of the power shift pressure piston is higher than the combined forces of the horsepower spring and the control piston against the lever.

Pump delivery pressure and the upstroke servo spring upstroke the pump. The pump flow increases until the following conditions have been met:

• The pump delivery pressure that acts on the control piston overcomes the force of power shift pressure.

• The horsepower spool rotates the lever.

The increased flow is followed by increased restriction and pressure.

Main Pump Regulator in Destroke Position

Illustration 11	g00813347

Illustration 11 shows the pump regulator at the start of destroking.

The following conditions can cause the pumps to destroke:

• A decrease in load sensing pressure

• A decrease in power shift pressure

• An increase in pump pressure

When the pump begins destroking the pressure pushes the actuator piston. The lever pivots against the force of the power shift pressure. Spring force moves the horsepower control spool against PS pressure. Pump output pressure is directed around the horsepower control spool and the load sensing spool to the destroke piston chamber. The higher pressure in the destroke piston moves the swashplate toward the minimum angle.

As the swashplate moves toward the minimum angle position, the actuator piston moves along the top of the lever. As the actuator piston moves closer to the pivot point of the lever, the mechanical advantage changes and the power shift pressure is able to push the actuator piston and the horsepower spool. The horsepower control spool meters the oil flow of the destroke piston. The swashplate reaches a balance point and flow remains constant until one of the signal pressures changes.

During a destroke condition, the actuator piston moves closer to the pivot point of the lever. As the actuator moves closer to the pivot point, increased pump output pressure is needed to overcome the mechanical advantage of the lever and the power shift pressure.

Initially, the pump destrokes faster. However, as the pump output pressure increases, the destroke rate decreases causing the hydraulic output from the pump to keep a close relation to the torque of the engine.

Illustration 12	g00813350

The control valve is open to the cylinder. The restriction on the cylinder is increasing. This increases the pump supply pressure. The increased pump output pressure moves the load sensing spool against the margin spring. The pump supply opens to the destroke servo piston.

The load sensing orifice spool is open because the difference between the pump output and load sensing signal is higher than the spring force. The horsepower spool is held against the destroke adjustment spring by the power shift pressure.

The destroke servo piston under control pressure has more force than the upstroke piston under pump output pressure. The actuator piston turns the lever against the power shift piston when the pump pressure and the force on the lever is higher.