Air in Hydraulic Tank of Kobelco SK260-8

[MikePhua]

The Kobelco SK260-8 is a heavy hydraulic excavator (≈ 25.3 tons operating weight) with a powerful auxiliary circuit and large hydraulic capacities.  At about 1,040 hours in service, an operator found air (foam) in the hydraulic tank, with symptoms during travel such as pump whining, machine jerking, and a loud thud from the rear.  Below is a detailed treatment of this problem: what might be going on, what to check, and how to fix/prevent it.

Terminology and System Basics

• Auxiliary Circuit: The hydraulic circuit that powers attachments like thumbs, breakers, or any extra hydraulic tool. When modified manually (valves added or removed), it can introduce potential leak or suction points.
  
• Foaming / Aeration: When air is mixed with hydraulic oil, making it look foamy. Air reduces hydraulic fluid’s ability to transmit power, causes cavitation, erratic motion, pump wear.
  
• Pump Suction Line: The hose or piping that draws oil from the tank into the hydraulic pump. If it leaks or has loose clamps, air can be drawn in.
  
• Pilot Pump: A smaller pump often used to operate control valves or the auxiliary circuits; if fittings or seals here leak, air can enter through its suction/inlet lines.
  
• Relief / Vent Valve on Tank: Many hydraulic tanks have a relief or vent mechanism (sometimes a valve) to allow air escape when oil level changes or during operation. If this vent is blocked or malfunctioning, pressure buildup can force air into the oil or keep air from escaping.
  

Symptoms Observed
From the reported case, the following were noted:

• Thumb (auxiliary circuit) modified (manual shut-off valves).
  
• During travel, pump whining and jerking, loud thud noise from the rear.
  
• Foamy oil in the hydraulic tank when cover removed.
  
• Travel circuits (tracks) produce problem under load; attachment circuits (thumb etc.) seem OK.
  

These symptoms suggest that the main hydraulic flow, especially in travel (track motors etc.), is being disrupted, likely due to air being entrained or a drop in oil suction performance.

Possible Causes
Based on the operator’s description and known system design, potential sources of air in the hydraulic tank include:

• Loose or improperly sealed hose clamps on the pump suction line. Even tiny leaks can pull air in under suction.
  
• Faulty or loose fittings on the pilot pump.
  
• Blocked or malfunctioning relief / vent valve on the hydraulic tank: if air cannot escape, pressure fluctuations cause foaming.
  
• Modified auxiliary circuit (manual shut-off valves) may have introduced additional suction points, leaks, or backflow paths.
  
• Low oil level in tank (if oil drops below pickup / suction point) causing the pump to suck air. Not explicitly stated, but always a risk.
  
• Oil contaminated, or wrong oil viscosity, which can encourage formation of foam or delay settling of air.
  

Diagnostics Steps
To find the root cause, the following checks should be made:

1. Inspect Suction Hose and Clamps
   • Check for loose hose clamps at the pump suction. Tighten if loose.
     
   • Inspect hose for damage, cracks, or shrinkage that could allow air ingress.
     
2. Check Pilot Pump Fittings and Lines
   • Any pilot pump inlet connections should be checked for leaks.
     
   • Ensure fittings are tight and seals are intact.
     
3. Check Vent / Relief Valve on Tank
   • Locate the valve (on top of tank, with allen bolts) and test it: with engine off or low idle, press or open to see if air escapes. If no air, vent may be blocked or stuck.
     
4. Oil Level & Condition
   • Verify oil level in tank is sufficient.
     
   • Sample oil: if contaminated, or contains bubbles or foam after shutdown, likely air ingress.
     
5. Observe Operation Under Load and Warm vs Cold
   • Does foaming / pumping issue happen more when warm or cold? Sometimes, seals are more permissive when hot/worn.
     
   • Test drive/travel circuits separately: stall each track, observe behavior.
     
6. Check Auxiliary Circuit Modifications
   • Because thumb circuit has been modified, check whether the manual shut-off valves are letting in air or if there are additional suction points introduced.
     

Solutions and Repairs
Once cause(s) identified, possible remedies include:

• Tighten or replace any loose suction hoses or clamps. Ensure hose routing avoids sharp bends or points that can chafe or allow leaks.
  
• Replace damaged or worn suction hoses. Use hoses rated for hydraulic suction.
  
• Clean or replace the tank’s vent/relief valve if blocked or non-functional. Ensure it is seated correctly and bolts are tightened.
  
• Correct auxiliary circuit shut-off valves: if they are leaking or do not seal properly, replace or adjust. Possibly redesign if modification introduced extra suction.
  
• Change hydraulic oil and filter if oil is contaminated or if foaming persists. Use proper grade oil.
  
• Ensure correct oil level always maintained, especially before starting heavy travel operations.
  

Prevention Measures
To stop air ingress issues recurring:

• Regular inspection schedule for suction hoses and fittings (every 250 operating hours or sooner under heavy use).
  
• Add a visual window or clear hose section (sight glass) on suction lines where practicable to check for air bubbles.
  
• Ensure modifications to auxiliary circuits are done cleanly, with proper hydraulic fittings and shut-offs rated for the system.
  
• Keep hydraulic fluid clean and free from contamination; change filters as per manufacturer schedule.
  
• Ensure tank vent is kept clean and free of road debris or dirt ingress.
  

SK260-8 Key Specifications for Context
Useful to understand this machine’s scale, pressures, and capacity:

• Operating Weight: ~25.3 metric tons.
  
• Hydraulic Relief Valve Pressure: ~4,975 psi.
  
• Hydraulic Pump Flow Capacity: ~130 gallons per minute (≈ 492 liters per minute) in base configuration.
  

Because of the high flow / pressure, even small air leaks or improper sealing can cause significant symptoms.

Conclusion
Air in the hydraulic tank of a Kobelco SK260-8 leads to foaming, pump whining, jerky travel, and “thud” noises. The likely causes are air ingress through pump suction hose clamps, pilot pump fittings, or a blocked tank vent, perhaps exacerbated by auxiliary circuit modifications. Diagnosing through hose inspection, vent testing, and monitoring oil condition combined with tightening, replacing faulty parts, and ensuring proper maintenance will resolve the issue. Given the machine’s high pressure and flow, even minor leaks or poor sealing can produce large operational impacts, so prevention and early detection are crucial.