M312, M315, M318 and M320 Excavators Hydraulic System Caterpillar


Pump Control (Swing)

Usage:

M318 6ES



Illustration 1g00817580

During a swing to the right, the pilot pressure from the swing pilot control enters the pump regulator at the upper right port. The pilot pressure goes through the orifice check valve to the right end of the upper throttle spool piston. The slots in the end of the spool direct pilot pressure to the right side of the actuator piston in order to start upstroking the swing pump.

Pilot pressure from the orifice check valve also goes through internal passages to the spring chamber at the right makeup valve and to the right side of the resolver valve. The resolver valve moves to the left and the pilot pressure is directed to the large shoulder area of both throttle spool pistons and to the swing proportional reducing valve (PRV). As more flow is needed, the upper throttle spool piston moves to the left in order to allow more flow so that the pump will upstroke. The lower throttle spool piston also moves to the left in order to allow more flow so that the pump will upstroke. The lower throttle spool piston will also move to the right in order to open the left side of the actuator piston to the drain. The oil in the left side of the actuator piston flows around the left end of the lower throttle spool piston through the lower orifice check valve. Then, the oil flows to the drain through the swing left pilot control valve.

If a modulated swing is required, a low pilot pressure is maintained by the swing pilot control valve. The two throttle spool pistons do not move in order to open the passage from the resolver valve to the large area of the throttle spool pistons. If a faster swing is needed, an increase in pilot pressure will move the throttle spool pistons to the full OPEN position.

When the pressure in the swing system increases to the pressure cutoff setting, the cutoff valve opens and a portion of the pilot pressure is directed to the tank drain. The swing pump then destrokes back to less flow. If the engine starts to lug during a swing operation, the Electronic Control Module (ECM) will change the electrical signal to the swing pump PRV. The swing pump PRV will open at a lower pressure. The swing pump destrokes in order to reduce the load on the engine.

When the swing operation is stopped, the pilot pressure in the actuator piston is drained to the tank through the slot in the throttle spool piston and the orifice check valve. The pressure decreases to zero and the centering springs on the actuator pistons move the pump swashplate to zero angle in order to stop the oil flow.

During the swing operation, the swing pump is upstroked to direct flow to the swing motor. The swing charge pump is used to keep the closed loop system charged with oil. Flow from the charge pump keeps the swing circuit charged with oil and for cooling and lubrication.

If the swing pilot control valve is returned to NEUTRAL rapidly, the centering springs on the actuator piston move the swashplate to the NEUTRAL position. As the actuator piston moves to the NEUTRAL position, makeup oil flows through the makeup valves and through internal passages to the low pressure side of the actuator piston. The makeup oil is used to help destroke the pump rapidly.

P-Q Characteristic Curves




Illustration 2g00531829

Typical example

P-Q Characteristic Curves

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