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The Kobelco K907DLC and Its Hydraulic Architecture
The Kobelco K907DLC is a heavy-duty hydraulic excavator built during the late 1990s, known for its robust steel construction and straightforward hydraulic systems. Designed for mass excavation and site development, it features a dual-track drive system powered by hydraulic motors integrated with planetary final drives. Though the machine is no longer in production, many units remain in operation due to their mechanical simplicity and rebuildable components.
Recurring Shaft Seal Failures and Fluid Loss
A critical issue that can arise in older hydraulic excavators like the K907DLC is the repeated failure of the drive motor shaft seal. In this case, the right-side drive motor consistently blew its shaft seal shortly after replacement, resulting in catastrophic hydraulic fluid loss. The fluid would escape through the final drive area, draining the reservoir and rendering the machine inoperable.
This type of failure is not merely a matter of a defective seal. When a new seal fails immediately, it signals a deeper systemic problem—most often related to internal pressure imbalances or component wear.
Understanding the Case Drain System
Hydraulic motors are designed with a case drain line, a low-pressure return path that allows internal leakage to flow back to the tank. This prevents pressure buildup behind the shaft seal. If the case drain becomes restricted, blocked, or overwhelmed by excessive internal leakage, pressure builds up inside the motor housing and forces hydraulic fluid past the shaft seal.
Key components and terms:
The operator replaced the shaft seal, float seal, and shaft bearing, only to have the new seal blow again upon startup. After replacing the return filter and verifying fluid levels, the issue persisted. This pointed to a likely case drain restriction or internal motor damage.
Recommended diagnostic steps:
In this case, the fluid was leaking through the final drive, not externally from the motor housing. This suggests that the shaft seal was failing due to pressure buildup, likely from a blocked or restricted case drain. After cleaning one of the relief valves that was not moving freely, the operator planned to retest the system.
If the problem persists, the next step would be to disassemble the motor and inspect for:
Conclusion
Repeated shaft seal failures in a Kobelco K907DLC are rarely caused by the seal itself. Instead, they point to internal pressure issues—most commonly from a blocked case drain or failed relief valve. Proper diagnosis involves checking flow rates, inspecting filters, and understanding the hydraulic circuit. With careful troubleshooting, costly rebuilds can sometimes be avoided, extending the life of these dependable but aging machines.
The Kobelco K907DLC is a heavy-duty hydraulic excavator built during the late 1990s, known for its robust steel construction and straightforward hydraulic systems. Designed for mass excavation and site development, it features a dual-track drive system powered by hydraulic motors integrated with planetary final drives. Though the machine is no longer in production, many units remain in operation due to their mechanical simplicity and rebuildable components.
Recurring Shaft Seal Failures and Fluid Loss
A critical issue that can arise in older hydraulic excavators like the K907DLC is the repeated failure of the drive motor shaft seal. In this case, the right-side drive motor consistently blew its shaft seal shortly after replacement, resulting in catastrophic hydraulic fluid loss. The fluid would escape through the final drive area, draining the reservoir and rendering the machine inoperable.
This type of failure is not merely a matter of a defective seal. When a new seal fails immediately, it signals a deeper systemic problem—most often related to internal pressure imbalances or component wear.
Understanding the Case Drain System
Hydraulic motors are designed with a case drain line, a low-pressure return path that allows internal leakage to flow back to the tank. This prevents pressure buildup behind the shaft seal. If the case drain becomes restricted, blocked, or overwhelmed by excessive internal leakage, pressure builds up inside the motor housing and forces hydraulic fluid past the shaft seal.
Key components and terms:
- Case Drain Line: A small-diameter hose that routes internal leakage to the tank
- Float Seal: A mechanical face seal between the motor and final drive, designed to keep oil in and contaminants out
- Overload Relief Valve: A pressure-limiting valve inside or near the motor that protects against overpressure
- Return Filter: A low-pressure filter that captures contaminants before fluid returns to the tank
The operator replaced the shaft seal, float seal, and shaft bearing, only to have the new seal blow again upon startup. After replacing the return filter and verifying fluid levels, the issue persisted. This pointed to a likely case drain restriction or internal motor damage.
Recommended diagnostic steps:
- Disconnect the case drain line and run it into a bucket. A healthy motor should produce a slow, steady drip. A rapid flow indicates internal leakage.
- Inspect the case drain filter for metal particles or blockage. Most filters have a bypass valve, but if clogged, they can still cause pressure spikes.
- Remove and inspect the overload relief valves. If one valve is sticky or seized, it can prevent pressure relief and damage internal seals.
- Check for fluid in the drive valve ports on the main control valve. Uneven fluid presence may indicate a stuck valve or backflow issue.
In this case, the fluid was leaking through the final drive, not externally from the motor housing. This suggests that the shaft seal was failing due to pressure buildup, likely from a blocked or restricted case drain. After cleaning one of the relief valves that was not moving freely, the operator planned to retest the system.
If the problem persists, the next step would be to disassemble the motor and inspect for:
- Cracked piston shoes
- Scored cylinder block or port plate
- Warped valve plate
- Excessive wear on the swash plate
Conclusion
Repeated shaft seal failures in a Kobelco K907DLC are rarely caused by the seal itself. Instead, they point to internal pressure issues—most commonly from a blocked case drain or failed relief valve. Proper diagnosis involves checking flow rates, inspecting filters, and understanding the hydraulic circuit. With careful troubleshooting, costly rebuilds can sometimes be avoided, extending the life of these dependable but aging machines.
