Rotary Joint Failures in Excavators: Diagnosing Pressure Loss and Internal Leakage

[MikePhua]

Introduction
The rotary joint—also known as the center swivel or swing joint—is a critical hydraulic component in tracked excavators. It allows hydraulic fluid to pass between the upper structure and the undercarriage while enabling 360-degree rotation. When a rotary joint fails, symptoms often mimic other hydraulic issues, making diagnosis challenging. This article explores a real-world case involving a Daewoo Doosan 140 Solar excavator, where tracking problems persisted despite seal replacement. We’ll unpack the technical anatomy of rotary joints, diagnostic strategies, and field-tested insights.
Understanding the Rotary Joint
The rotary joint consists of stacked internal channels and seals that route hydraulic fluid to travel motors, blade circuits, and auxiliary functions. It must maintain pressure integrity while rotating under load.
Key components include:

• Outer Housing: Fixed to the undercarriage, contains fluid ports.
  
• Rotating Shaft: Connected to the upper structure, rotates freely.
  
• Seal Packs: O-rings and backup rings that prevent cross-port leakage.
  
• Bearing Assembly: Supports axial and radial loads during rotation.
  

Terminology Explained

• Tracking Issue: A problem where the excavator fails to move properly or loses power during travel.
  
• Blanking Hoses: Temporarily capping hydraulic lines to isolate pressure sources.
  
• Case Drain Line: A low-pressure return line that relieves excess fluid from hydraulic motors.
  
• Hairline Crack: A microscopic fracture in metal that may not be visible but can cause pressure loss.
  

Diagnostic Strategy: Pressure Isolation and Flow Testing
In the case of the Doosan 140 Solar:

• Pressure before the rotary joint was measured at 340 bar.
  
• Pressure after the joint dropped to 300–310 bar.
  
• Seals were replaced, but the issue persisted.
  

This suggested internal leakage or a structural fault within the rotary joint itself.
Steps taken:

• Blanking upstream hoses to confirm pump output.
  
• Blanking downstream hoses at the travel motor to isolate the joint.
  
• Visual inspection of seals—no damage found.
  
• Reassembly and retest—pressure loss remained.
  

Field Anecdote: Zebra vs. Horse Diagnosis
A retired mechanic in British Columbia offered a classic diagnostic metaphor: “You’re looking for zebras when you should be looking for horses.” His advice? Check the case drain lines. If oil gushes from them under load, the travel motor may be bypassing internally—mimicking rotary joint failure. This highlights the importance of ruling out downstream components before condemning the joint.
Historical Context: Build Quality and Rotary Joint Vulnerabilities
Rotary joints in older excavators—especially from brands with variable build quality—are prone to wear, corrosion, and cracking. In the 1990s and early 2000s, some manufacturers used softer alloys or inconsistent machining tolerances, increasing the risk of internal leakage. Hairline cracks in the rotary shaft or housing may not be visible but can allow fluid to escape between channels.
Related Case: Swivel Joint Failure in Ireland
A technician in Ireland encountered a similar issue on a Daewoo machine. After replacing seals and confirming pressure loss, they suspected a crack in the rotary joint’s hard parts. Though rare, such failures can occur due to fatigue, impact, or manufacturing defects. The technician ultimately sourced a replacement joint from a salvage yard, resolving the issue.
Best Practices for Rotary Joint Troubleshooting

• Always isolate pressure before and after the joint
  
• Inspect case drain lines for excessive flow
  
• Use dye or ultrasonic testing to detect hairline cracks
  
• Replace seals only after confirming no structural damage
  
• Consider downstream motor or valve faults before condemning the joint
  
• Document pressure readings and test conditions for future reference
  

Conclusion
Rotary joint failures can masquerade as motor or valve issues, making diagnosis a blend of hydraulic logic and mechanical intuition. By methodically isolating pressure zones and inspecting both seals and hard parts, technicians can pinpoint the root cause and avoid unnecessary repairs. In the world of excavators, the rotary joint is a silent workhorse—and when it falters, it demands a sharp ear and a steady hand to restore the rhythm of rotation.