Why Is the Komatsu PC210-8 Swing Case Filling with Oil

[MikePhua]

The Komatsu PC210-8 Excavator and Its Hydraulic Legacy
The Komatsu PC210-8 is a mid-sized hydraulic excavator developed by Komatsu Ltd., a Japanese company founded in 1921 and recognized globally for its innovation in construction machinery. The PC210 series has been a cornerstone of Komatsu’s excavator lineup, with the -8 variant introduced in the mid-2000s to meet Tier 3 emissions standards and offer improved fuel efficiency, operator comfort, and hydraulic precision. With tens of thousands of units sold worldwide, the PC210-8 is widely used in road building, utility trenching, and site development.
Equipped with a Komatsu SAA6D107E-1 engine producing around 158 horsepower, the PC210-8 features advanced hydraulic systems including closed-center load sensing and electronic pump control. One critical component is the swing drive system, which allows the upper structure to rotate smoothly. This system includes a swing motor, swing gearbox (or swing case), and a breather cap to regulate internal pressure.
Symptoms of Oil Overfill in the Swing Case
A perplexing issue arises when the swing case fills with oil during operation, causing it to overflow through the breather cap. This typically occurs after several hours of use and may seem to resolve itself by the next day, with the oil level returning to near-normal. Such behavior suggests a dynamic fluid migration rather than a static leak.
Terminology Explained

• Swing Motor: A hydraulic motor that drives the rotation of the excavator’s upper structure.
  
• Swing Gearbox (Swing Case): A gear reduction unit that transmits torque from the swing motor to the swing bearing.
  
• Breather Cap: A vented cap that allows pressure equalization inside the gearbox while preventing contaminants from entering.
  

Probable Cause and Mechanical Implications
The most likely culprit is a worn swing motor seal, which allows hydraulic oil to leak into the swing gearbox. During operation, pressurized hydraulic fluid escapes past the seal and accumulates in the swing case. If the volume exceeds the designed capacity, it forces oil out through the breather.
However, the confusing part is the apparent disappearance of excess oil after shutdown. This can be explained by two phenomena:

• Thermal Expansion and Contraction As the machine cools, oil contracts and redistributes, lowering the visible level.
  
• Backflow into Hydraulic Lines In rare cases, residual pressure in the hydraulic system may draw oil back through the motor into the return lines, especially if check valves are faulty or missing.
  

Another possibility is a cracked swing case, which could allow oil to leak externally. However, this would typically leave visible signs such as wet spots or drips under the machine.
Secondary Damage Risks
If the swing case is repeatedly overfilled, it can lead to:

• Seal Blowout on the Output Shaft Excess pressure may damage the pinion shaft seal, allowing oil to enter the swing gear compartment.
  
• Gearbox Contamination Hydraulic oil lacks the extreme pressure additives found in gear oil, leading to accelerated wear of gears and bearings.
  
• Breather Blockage Oil escaping through the breather can clog its mesh, preventing proper ventilation and increasing internal pressure.
  

Recommended Diagnostic and Repair Steps

• Inspect Swing Motor Seals Remove the swing motor and check the shaft seal for wear or scoring. Replace with OEM-grade seals.
  
• Drain and Measure Oil Volume Compare drained oil volume with manufacturer specifications to confirm overfill.
  
• Check Swing Gear Compartment Remove the inspection cover and verify that no oil has migrated into the gear housing.
  
• Install Pressure Gauge on Hydraulic Lines Monitor pressure during swing operation to detect abnormal spikes or backflow.
  
• Replace Breather Cap if Contaminated A clogged breather can exacerbate pressure buildup.
  

A Story from the Field
In 2022, a contractor in Alberta noticed similar symptoms on a PC210LC-8. After replacing the swing motor seal, the issue persisted. Further inspection revealed that the hydraulic tank had been overfilled, causing excess fluid to seek escape through the weakest seal—at the swing motor. Once the tank level was corrected and the seal replaced again, the problem was resolved.
Conclusion
Oil accumulation in the swing case of a Komatsu PC210-8 is typically caused by a failing swing motor seal, compounded by pressure dynamics and possible hydraulic system anomalies. While the issue may appear intermittent, it poses serious risks to gearbox integrity and overall machine health. Prompt inspection, seal replacement, and system pressure checks are essential to prevent long-term damage and ensure safe, efficient operation.