Yanmar YB251 YB301 OEM 172122-73100 hydraulic pump for mini excavator
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1. GENERAL IDENTIFICATION
Machine Models: Yanmar YB251, YB251-2, YB301, YB301-2
Yanmar Part Numbers: 172122-73100, 172141-73100, 172141-73101
Pump Configuration: Double Variable Displacement Piston Pump + Integrated Gear Pump
2. PERFORMANCE SPECIFICATIONS
Piston Pump Displacement (P1 & P2): Approx. $34.0$ to $40.0$ $cm^3/rev$ (depending on specific YB sub-model)
Gear Pump Displacement (P3): Approx. $15.0$ to $20.0$ $cm^3/rev$
Rated Operating Pressure: $21.0$ MPa ($3,045$ psi)
Maximum Relief Pressure: $24.5$ MPa ($3,553$ psi)
Input Shaft: Typically a 13-tooth or 15-tooth spline.
3. INTERNAL REPAIR COMPONENTS (ROTARY GROUP) Because these are older machines, rebuilding the existing pump is often more cost-effective than a total replacement. Key parts include:
Cylinder Block (Barrel): The central rotating unit.
Piston Set: 9 pistons with brass shoes per side.
Valve Plate: Must be inspected for "scoring" or circular scratches which cause pressure loss.
Set Plate (Retainer): Holds the pistons against the swashplate.
Cradle Bearings: Essential for the smooth movement of the swashplate during stroke changes.
Full Seal Kit: Includes the front shaft seal (to prevent oil leaking into the engine) and various internal O-rings.
4. FACTORY CALIBRATION SETTINGS
Main Relief Valve: $21.0$ MPa
Travel Relief Valve: $21.0$ MPa
Pilot/Auxiliary Pressure (P3): $3.5$ - $4.0$ MPa
Engine Speed for Testing: $2200$ - $2400$ RPM
5. MAINTENANCE & INSTALLATION PROTOCOL
Case Priming: MANDATORY. You must fill the pump case with clean ISO VG 46 hydraulic oil before the first start. A dry start will destroy the valve plate in seconds.
Suction Line Integrity: On older YB models, the suction hose can become brittle and suck in air (cavitation). Always replace the suction hose if it feels hard or cracked.
Filter Service: Change the return filter and thoroughly clean the suction strainer located inside the hydraulic tank.
Air Bleeding: Loosen the air bleed plug on top of the pump housing until oil flows out without bubbles.