Hitachi ZX670 Main Pump (Displacement Variable Assembly)
HPK300 Servo Piston & Variable Swashplate Yoke
1. General Overview
Applicable Model: Hitachi ZX670 heavy-duty excavator (67-ton mining excavator)
Pump Assembly Model: KYB HPK300 tandem main axial piston pump (OEM factory-fitted main pump for Hitachi)
Components: Servo Piston and Variable Swashplate Yoke. As core displacement regulation parts of the main pump, they jointly adjust the swashplate angle to realize variable flow output of the pump, acting as actuators for load sensing control and constant power regulation.
2. Introduction to HPK300 Main Pump
1. Pump Type: KYB HPK300 tandem variable axial piston main pump, original standard configuration for Hitachi ZX670.
2. Application Scenarios: Heavy-duty mining excavation, continuous hammer operation and high-load combined movements, featuring large displacement, high pressure resistance and strong shock tolerance.
3. Core Function: The servo piston pushes the variable swashplate yoke to swing the swashplate and change the plunger stroke, so as to automatically adjust pump output flow to match machine load requirements, balancing fuel economy and power performance.
3. Servo Piston
Definition
Hydraulically driven servo piston installed inside the servo cylinders on both sides of the pump housing. It is actuated by pilot pressure supplied from the PC power valve and LS load sensing valve.
Working Principle
- Pilot high pressure acts on the end face of the servo piston to generate thrust and drive the variable swashplate yoke.
- Arranged in pairs (large-bore and small-bore pistons) on two sides to fine-tune the swashplate angle via push-pull motion.
Common Fault Symptoms
Scuffing, eccentric wear and internal leakage caused by aged seals lead to sluggish pump displacement adjustment, erratic operation, insufficient idle power, engine lugging and insufficient flow output.
4. Variable Swashplate Yoke
(Also known as swashplate rocker arm / yoke lever in industry terminology)
Definition
Mechanical oscillating component hinged to the servo piston pushrod at one end and engaged with the outer groove of the main pump swashplate at the other end. It serves as the intermediate mechanical link to transmit thrust from the servo piston to the swashplate.
Function
Converts linear reciprocating motion of the servo piston into rotary swing of the swashplate to alter its inclination angle:
- Larger swashplate angle = longer plunger stroke = higher pump displacement and delivery flow
- Smaller swashplate angle = reduced displacement for fuel saving at idle speed
Vulnerable Malfunctions
Worn bushing at pivot bores, oversized clearance of clamping grooves and loose hinge points result in excessive variable clearance, swashplate chattering, abnormal pump noise and unstable flow.
5. Coordinated Working Logic of Two Parts
LS load pressure / engine power signal → Pilot pressure output from main pump control valve → Drives linear extension and retraction of Servo Piston → Pulls and pushes Variable Swashplate Yoke to swing → Adjusts HPK300 main pump swashplate angle in real time → Regulates pump displacement and delivery flow to match excavation loads.
6. Matching & Maintenance Notes
1. These parts are exclusive to HPK300 pump and fully interchangeable for ZX670-3 & ZX670-5G full series.
2. Recommended replacement: Servo piston, oil seals and swashplate yoke bushings shall be replaced as a complete set; single-piece replacement tends to cause repeated wear and internal leakage.
3. Typical replacement scenarios: Failed pump displacement regulation, constant high pressure without relief, uncontrollable machine operating speed and abnormal vibration/noise of the main pump.
