Hydraulic Seal Ring Issues on the Komatsu PC120-6

[MikePhua]

The Komatsu PC120-6 and Its Hydraulic Legacy
The Komatsu PC120-6 is a mid-sized hydraulic excavator introduced in the 1990s as part of Komatsu’s dash-6 series. With an operating weight around 12 tons and a Komatsu S4D102E engine producing approximately 90 horsepower, the PC120-6 was designed for general excavation, utility trenching, and light demolition. Komatsu, founded in 1921 in Japan, became a global leader in construction machinery by combining mechanical durability with hydraulic precision. The dash-6 series marked a transition toward more refined hydraulic systems and improved operator ergonomics.
The PC120-6 uses an open-center hydraulic system with multiple control valves, pilot-operated joysticks, and a centralized pump group. Its reliability depends heavily on the integrity of internal seals, especially in high-pressure zones like the swing motor, travel motors, and main control valve.
Understanding the Role of Hydraulic Seal Rings
Hydraulic seal rings are critical components that prevent fluid leakage and maintain pressure within cylinders, motors, and valve assemblies. On the PC120-6, seal rings are found in:

• Boom, arm, and bucket cylinders
  
• Swing motor and reduction gear housing
  
• Travel motor assemblies
  
• Control valve spools and end caps
  
• Pump shaft seals and pilot circuit regulators
  

Terminology notes:

• Seal ring: A circular elastomer or composite ring designed to contain hydraulic fluid under pressure.
  
• O-ring: A basic type of seal ring with a round cross-section, used in static and dynamic applications.
  
• Back-up ring: A rigid ring placed behind an O-ring to prevent extrusion under high pressure.
  
• Wear ring: A guide ring that prevents metal-to-metal contact and supports lateral loads.
  

Symptoms of Seal Ring Failure
When a seal ring begins to degrade or fail, operators may notice:

• Hydraulic fluid leaking from cylinder ends or motor housings
  
• Loss of pressure in specific functions (e.g., weak boom lift or slow swing)
  
• Air bubbles in the fluid reservoir
  
• Excessive heat in the hydraulic system
  
• Jerky or inconsistent movement during operation
  

In one case from a contractor in Queensland, a PC120-6 began losing swing torque. After inspecting the swing motor, technicians found a worn seal ring that allowed internal bypass. Replacing the seal restored full swing power and eliminated the fluid loss.
Causes of Premature Seal Wear
Seal rings are designed to withstand thousands of hours of operation, but several factors can accelerate wear:

• Contaminated hydraulic fluid with abrasive particles
  
• Excessive system pressure beyond rated limits
  
• Misalignment of cylinder rods or motor shafts
  
• Improper installation or use of incorrect seal material
  
• Thermal cycling causing expansion and contraction fatigue
  

Older machines like the PC120-6 are especially vulnerable if fluid maintenance is neglected or aftermarket seals are used without proper specification matching.
Best Practices for Seal Ring Replacement
To ensure successful seal ring replacement:

• Identify the exact part number using Komatsu’s parts manual and verify dimensions
  
• Use OEM or high-quality aftermarket seals rated for the machine’s pressure and temperature range
  
• Clean all mating surfaces thoroughly before installation
  
• Lubricate seals with compatible hydraulic fluid during assembly
  
• Use seal installation tools to avoid twisting or stretching
  
• Inspect adjacent components for wear, including bushings and piston rods
  

A technician in Wisconsin developed a habit of photographing each seal stack during disassembly to ensure correct reinstallation. This practice reduced rebuild errors and improved first-pass success rates.
Preventive Maintenance and Fluid Management
To extend seal life and prevent hydraulic failures:

• Replace hydraulic fluid every 1,000 hours or annually
  
• Use ISO 46 or ISO 68 hydraulic oil depending on climate and load
  
• Install magnetic drain plugs and inline filters to capture debris
  
• Monitor system pressure and temperature with diagnostic gauges
  
• Inspect seals during cylinder rebuilds and valve overhauls
  
• Keep a log of seal replacements and fluid changes for each machine
  

A fleet manager in British Columbia implemented a quarterly fluid sampling program across their Komatsu fleet. Over two years, seal-related failures dropped by 40%, and overall hydraulic efficiency improved.
Field Story from a Municipal Excavator Crew
In 2015, a city crew in Ontario noticed a PC120-6 losing travel speed on inclines. After checking the travel motor, they discovered a seal ring had extruded due to overpressure caused by a blocked case drain line. Replacing the seal and clearing the blockage restored normal operation. “It wasn’t the motor,” the operator said, “it was the seal trying to do too much.”
Recommendations for Long-Term Reliability

• Use seal kits with matched O-rings, back-up rings, and wear rings
  
• Avoid mixing seal materials (e.g., nitrile with Viton) unless specified
  
• Train technicians in seal identification and installation techniques
  
• Maintain clean workspaces during hydraulic repairs
  
• Document seal failures and analyze patterns for root causes
  
• Collaborate with Komatsu dealers for updated service bulletins and part revisions
  

Conclusion
Hydraulic seal rings on the Komatsu PC120-6 may be small, but their role is enormous. They hold pressure, prevent leaks, and ensure smooth operation across every function. With proper maintenance, quality parts, and attention to installation detail, these seals can deliver thousands of hours of reliable service. In hydraulic systems, integrity starts at the seal.