Precautions After Replacing the Hydraulic Pump and Hydraulic Oil in Construction Machinery

Hydraulic pumps and hydraulic oil are essential for construction machinery—even electric excavators rely on hydraulic transmission systems.

Replacing a hydraulic pump is a common task, but many have suffered premature pump failure due to not strictly following procedural requirements. Excluding component quality issues, most problems originate from the hydraulic oil—especially when air is not properly purged from the system, leading to cavitation, dry friction, early wear, and in severe cases, pump failure before the client even pays for repairs. Many technicians have experienced this firsthand.

Practical Observations and Recommendations:

1. Pre-installation Checks:

   • For any repaired or new hydraulic pump (gear pump, vane pump, piston pump—excluding diaphragm pumps), ensure the pump rotates freely without sticking.

   • Fill with an appropriate amount of clean hydraulic oil to pre-lubricate the pump shaft and cover all exposed ports before installation.

2. Post-installation Manual Rotation:

   • Manually rotate the engine and check for system stiffness or binding.

   • Fill the hydraulic oil to slightly above the standard level, since the system is initially empty. The exact level depends on the machine model.

3. Air Purging in the Hydraulic Pump:

   • After filling, thoroughly purge air from the pump. Each machine may have a different method and location for air release, but generally, the highest point on the pump body is used.

   • If unclear, air can also be released via the pump’s return line. During purging, ensure clean hydraulic oil fills the pump casing.

   • For pumps with suction port plugs, open the plug during air release and close it afterward.

   • If available, pressurizing the hydraulic tank with compressed air (≤200 kPa) is an effective method to quickly deliver oil into the pump system. Some machines’ procedures after oil replacement require hydraulic system pressurization for this purpose.

4. Low-Speed Pump Operation:

   • Use the starter to intermittently rotate the hydraulic pump at low speed. Observe the discharge port until the oil is free of visible air bubbles, then close the purge port.

5. Engine Startup and Initial Operation:

   • Start the engine at low idle for 3–5 minutes.

   • Listen for abnormal sounds; if none are present, perform low- and mid-speed operation tests.

   • Gradually increase throttle to test all hydraulic functions.

6. Hydraulic Cylinder Air Purging:

   • Operate each cylinder repeatedly to ~100 mm from the end of stroke, then reverse. Repeat 3–5 times, then perform one full stroke. This typically removes residual air.

   • If cavitation noise persists, repeat the procedure. Investigate and eliminate the air source if necessary before full operation.

7. Hydraulic Oil Replacement Maintenance:

   • During routine oil replacement, avoid opening the pump suction line plug; leaving some old oil in the system prevents initial suction difficulties, especially in older machines.

   • To remove more old oil, retract the hydraulic cylinder rods fully to expel oil from the rodless chamber. Complete removal of all hydraulic oil is impossible without disassembling all components. Replacing approximately half the system’s oil is sufficient for regular maintenance. This limitation explains the necessity of periodic oil changes.

Special Note on Water-Contaminated Oil:

• If hydraulic oil is contaminated with water, replace all the affected oil immediately. Multiple flushes with low-speed operation may be required to remove emulsified oil.

• Used, water-contaminated oil can be dehydrated (e.g., using a transformer oil dehydration unit) and reused if desired, preventing significant financial loss.

Key Warnings:

• Failure to properly purge air from the hydraulic pump can lead to cavitation, excessive wear, or immediate pump burnout.

• Even simple gear pumps or pilot pumps have been destroyed repeatedly when procedures were not followed—this is a real, documented risk, not hypothetical.

Summary:
Strict adherence to procedural steps—pre-lubrication, air purging, controlled initial operation, and hydraulic cylinder venting—is essential to ensure the reliability of a replaced hydraulic pump and hydraulic oil system. These precautions prevent premature failure and protect both the machine and the operator’s investment.

Hydraulic Pump & Oil Replacement – Step-by-Step Flow