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Kobelco’s SK80CS and Its Hydraulic Drive System
Kobelco Construction Machinery, a division of Kobe Steel founded in 1930, has built a reputation for precision-engineered excavators. The SK80CS, part of their compact short-radius series, was designed for urban and utility work where maneuverability and power must coexist. With an operating weight around 8 tons and a hydraulic system pressure exceeding 3,000 psi, the SK80CS uses dual final drive motors to propel its tracks independently.
These motors are typically axial piston units, designed to rotate in a specific direction based on fluid flow. Kobelco’s design philosophy emphasizes modularity—many models use a single part number for both left and right drive motors, simplifying inventory but introducing potential complications during replacement.
Terminology:
- Final Drive Motor: A hydraulic motor mounted at each track, converting fluid pressure into rotational motion. - Axial Piston Motor: A type of hydraulic motor where pistons move parallel to the drive shaft, offering high torque and efficiency.
The Problem of Reverse Rotation
In one case, a new aftermarket final drive motor was installed on the left side of an SK80CS. Despite correct hydraulic line connections—verified against the service manual—the motor rotated in the opposite direction. Forward joystick input caused the track to move backward, and vice versa.
This issue is not uncommon when installing non-OEM motors. While the hydraulic ports may be identical, the internal timing or porting logic can differ. Some motors are built with mirrored internal flow paths, meaning that identical external connections can produce opposite rotation.
Terminology:
- Motor Timing: The internal configuration of valve plates and piston sequencing that determines rotation direction. - Porting Logic: The arrangement of inlet and outlet passages that control fluid flow through the motor.
Why Hydraulic Lines Alone Don’t Guarantee Correct Rotation
In this case, the hydraulic lines were rigid steel tubes, meaning they could only be connected in one configuration. The installer confirmed that the forward/reverse ports were different sizes, eliminating the possibility of accidental cross-connection. However, the new motor had its return port relocated, which was manually adjusted to match the original layout.
Despite these precautions, the motor spun backward. This suggests that the aftermarket unit had a reversed internal timing plate or was designed for the opposite side of the machine. Some manufacturers produce symmetrical motors with reversible timing, while others offer dedicated left-hand and right-hand units.
Terminology:
- Return Port: The hydraulic line that carries low-pressure fluid back to the tank or valve block. - Symmetrical Motor Design: A motor built to operate identically on either side of the machine, often requiring internal adjustments.
Solutions and Workarounds
Several solutions were proposed and tested:
Terminology:
- Pilot Line: A low-pressure hydraulic line that sends control signals to actuators or motors. - Swivel Joint: A rotating hydraulic coupling that allows fluid to pass between the upper and lower structures of the excavator.
Aftermarket Motor Risks and Supplier Accountability
The motor in question was sourced from an aftermarket supplier, possibly a Comer-manufactured unit known for rotation mismatches. While aftermarket parts can offer cost savings, they often lack the precise engineering and documentation of OEM components. In some cases, suppliers fail to disclose rotation direction or compatibility issues, leaving installers to troubleshoot post-installation.
In 2021, a similar issue occurred in a municipal fleet in British Columbia, where five excavators were retrofitted with aftermarket drives. Two units experienced reversed travel, leading to operator confusion and minor site accidents. The supplier later admitted to a batch error in motor timing and offered replacements.
Terminology:
- OEM (Original Equipment Manufacturer): The company that originally designed and produced the equipment or part. - Batch Error: A manufacturing defect affecting multiple units produced in the same production run.
Best Practices for Final Drive Replacement
To avoid similar issues, technicians should follow these guidelines:
Terminology:
- Rotation Chart: A diagram showing fluid flow and resulting motor rotation. - Orientation Mark: A physical indicator on the motor showing correct installation position.
Conclusion
Replacing a final drive motor on a Kobelco excavator may seem straightforward, but subtle differences in internal timing and porting can lead to reversed travel. While hydraulic lines may be correctly installed, the motor’s internal logic determines rotation. OEM motors are typically pre-configured for side-specific use, while aftermarket units may require additional verification. In the end, understanding motor design and supplier practices is key to keeping your excavator moving in the right direction.
Kobelco Construction Machinery, a division of Kobe Steel founded in 1930, has built a reputation for precision-engineered excavators. The SK80CS, part of their compact short-radius series, was designed for urban and utility work where maneuverability and power must coexist. With an operating weight around 8 tons and a hydraulic system pressure exceeding 3,000 psi, the SK80CS uses dual final drive motors to propel its tracks independently.
These motors are typically axial piston units, designed to rotate in a specific direction based on fluid flow. Kobelco’s design philosophy emphasizes modularity—many models use a single part number for both left and right drive motors, simplifying inventory but introducing potential complications during replacement.
Terminology:
- Final Drive Motor: A hydraulic motor mounted at each track, converting fluid pressure into rotational motion. - Axial Piston Motor: A type of hydraulic motor where pistons move parallel to the drive shaft, offering high torque and efficiency.
The Problem of Reverse Rotation
In one case, a new aftermarket final drive motor was installed on the left side of an SK80CS. Despite correct hydraulic line connections—verified against the service manual—the motor rotated in the opposite direction. Forward joystick input caused the track to move backward, and vice versa.
This issue is not uncommon when installing non-OEM motors. While the hydraulic ports may be identical, the internal timing or porting logic can differ. Some motors are built with mirrored internal flow paths, meaning that identical external connections can produce opposite rotation.
Terminology:
- Motor Timing: The internal configuration of valve plates and piston sequencing that determines rotation direction. - Porting Logic: The arrangement of inlet and outlet passages that control fluid flow through the motor.
Why Hydraulic Lines Alone Don’t Guarantee Correct Rotation
In this case, the hydraulic lines were rigid steel tubes, meaning they could only be connected in one configuration. The installer confirmed that the forward/reverse ports were different sizes, eliminating the possibility of accidental cross-connection. However, the new motor had its return port relocated, which was manually adjusted to match the original layout.
Despite these precautions, the motor spun backward. This suggests that the aftermarket unit had a reversed internal timing plate or was designed for the opposite side of the machine. Some manufacturers produce symmetrical motors with reversible timing, while others offer dedicated left-hand and right-hand units.
Terminology:
- Return Port: The hydraulic line that carries low-pressure fluid back to the tank or valve block. - Symmetrical Motor Design: A motor built to operate identically on either side of the machine, often requiring internal adjustments.
Solutions and Workarounds
Several solutions were proposed and tested:
- Swap pilot control lines under the foot pedal to reverse joystick input. This worked but introduced inconsistency between left and right controls.
- Swap control lines at the swivel joint. This was not feasible due to line length constraints.
- Rotate the top casting of the motor to reorient ports. This was impossible due to the motor’s design.
- Contact the supplier for a corrected unit. This was the most recommended path, especially to preserve warranty.
Terminology:
- Pilot Line: A low-pressure hydraulic line that sends control signals to actuators or motors. - Swivel Joint: A rotating hydraulic coupling that allows fluid to pass between the upper and lower structures of the excavator.
Aftermarket Motor Risks and Supplier Accountability
The motor in question was sourced from an aftermarket supplier, possibly a Comer-manufactured unit known for rotation mismatches. While aftermarket parts can offer cost savings, they often lack the precise engineering and documentation of OEM components. In some cases, suppliers fail to disclose rotation direction or compatibility issues, leaving installers to troubleshoot post-installation.
In 2021, a similar issue occurred in a municipal fleet in British Columbia, where five excavators were retrofitted with aftermarket drives. Two units experienced reversed travel, leading to operator confusion and minor site accidents. The supplier later admitted to a batch error in motor timing and offered replacements.
Terminology:
- OEM (Original Equipment Manufacturer): The company that originally designed and produced the equipment or part. - Batch Error: A manufacturing defect affecting multiple units produced in the same production run.
Best Practices for Final Drive Replacement
To avoid similar issues, technicians should follow these guidelines:
- Confirm motor rotation direction before installation, especially with aftermarket units.
- Request technical drawings or rotation charts from the supplier.
- Check for timing plates or orientation marks on the motor housing.
- Avoid modifying pilot controls unless absolutely necessary.
- Document all changes for future service and resale.
Terminology:
- Rotation Chart: A diagram showing fluid flow and resulting motor rotation. - Orientation Mark: A physical indicator on the motor showing correct installation position.
Conclusion
Replacing a final drive motor on a Kobelco excavator may seem straightforward, but subtle differences in internal timing and porting can lead to reversed travel. While hydraulic lines may be correctly installed, the motor’s internal logic determines rotation. OEM motors are typically pre-configured for side-specific use, while aftermarket units may require additional verification. In the end, understanding motor design and supplier practices is key to keeping your excavator moving in the right direction.
