10-18-2025, 02:40 PM
How to Check and Adjust Auxiliary Hydraulic Pressure Relief for a Thumb Attachment
Komatsu PC78MR-6 and Its Auxiliary Hydraulic System
The Komatsu PC78MR-6 is a compact radius excavator designed for urban and utility work, offering a balance of power and maneuverability. With an operating weight of approximately 8 tons and a 55 hp engine, it features a load-sensing hydraulic system and factory auxiliary plumbing for attachments like hydraulic thumbs and breakers. Komatsu, founded in 1921, has sold tens of thousands of PC-series excavators globally, and the PC78MR-6 remains a popular choice for contractors seeking reliability in tight spaces.
The auxiliary hydraulic circuit includes a pressure relief valve to protect the thumb cylinder from excessive force. If improperly set, the thumb may overpower the bucket or vice versa, leading to bent rods, seal failure, or poor performance.
Terminology Notes
Operators installing a hydraulic thumb often find that the bucket curl cannot overcome the thumb, or the thumb clamps too aggressively. This suggests the auxiliary relief pressure is set too high. In one case, a thumb cylinder rod broke due to excessive force during bucket curl—highlighting the need for proper adjustment.
Adjustment Strategy and Field Testing
There are two main approaches depending on valve accessibility:
One technician in Saskatchewan used a hand pump to pressurize the work port from the cylinder side, confirming relief pressure without running the engine. Another operator in Montana found that his factory-plumbed thumb had a changeover valve with its own relief, which didn’t affect thumb pressure as expected. He traced the base-end hose to the main valve and adjusted the inline relief instead.
A mechanic in Wisconsin recommended measuring shim thickness with a micrometer and keeping a log of pressure changes per shim removed. He noted that Komatsu often uses non-adjustable reliefs with internal shims, requiring careful disassembly.
Preventive Measures and Recommendations
Adjusting auxiliary hydraulic pressure relief for a thumb attachment on a Komatsu PC78MR-6 requires understanding the valve type and testing interaction between bucket and thumb. Whether using an external screw or internal shims, the goal is to balance clamping force with safe operation. With careful adjustment and field testing, operators can prevent damage and optimize performance for material handling tasks.
Komatsu PC78MR-6 and Its Auxiliary Hydraulic System
The Komatsu PC78MR-6 is a compact radius excavator designed for urban and utility work, offering a balance of power and maneuverability. With an operating weight of approximately 8 tons and a 55 hp engine, it features a load-sensing hydraulic system and factory auxiliary plumbing for attachments like hydraulic thumbs and breakers. Komatsu, founded in 1921, has sold tens of thousands of PC-series excavators globally, and the PC78MR-6 remains a popular choice for contractors seeking reliability in tight spaces.
The auxiliary hydraulic circuit includes a pressure relief valve to protect the thumb cylinder from excessive force. If improperly set, the thumb may overpower the bucket or vice versa, leading to bent rods, seal failure, or poor performance.
Terminology Notes
- Auxiliary Relief Valve: A pressure-limiting device installed in the auxiliary circuit to prevent overloading the attachment.
- Thumb Cylinder: A hydraulic actuator mounted on the dipper stick, used to clamp material against the bucket.
- Changeover Valve: A diverter that switches flow between breaker and thumb modes.
- Work Port: The hydraulic line that feeds fluid to the base or rod end of a cylinder.
Operators installing a hydraulic thumb often find that the bucket curl cannot overcome the thumb, or the thumb clamps too aggressively. This suggests the auxiliary relief pressure is set too high. In one case, a thumb cylinder rod broke due to excessive force during bucket curl—highlighting the need for proper adjustment.
Adjustment Strategy and Field Testing
There are two main approaches depending on valve accessibility:
- Externally Adjustable Relief Valve:
- Locate the relief valve on the main control valve block.
- Loosen the lock nut and back out the set screw in ¼-turn increments.
- Start the machine and close the thumb against the bucket.
- Curl the bucket inward. If it cannot overcome the thumb, repeat adjustment.
- Stop when the bucket just overcomes the thumb under full curl.
- Locate the relief valve on the main control valve block.
- Shim-Type Relief Valve:
- Disassemble the valve and inspect shim stack.
- Refer to Komatsu specs for shim thickness vs. pressure differential.
- Remove the thinnest shim and reassemble.
- Test bucket vs. thumb interaction and fine-tune as needed.
One technician in Saskatchewan used a hand pump to pressurize the work port from the cylinder side, confirming relief pressure without running the engine. Another operator in Montana found that his factory-plumbed thumb had a changeover valve with its own relief, which didn’t affect thumb pressure as expected. He traced the base-end hose to the main valve and adjusted the inline relief instead.
A mechanic in Wisconsin recommended measuring shim thickness with a micrometer and keeping a log of pressure changes per shim removed. He noted that Komatsu often uses non-adjustable reliefs with internal shims, requiring careful disassembly.
Preventive Measures and Recommendations
- Use a Pressure Gauge: Confirm relief settings during adjustment.
- Label Hydraulic Lines: Prevent confusion between breaker and thumb circuits.
- Check Cylinder Rod Straightness: Overpressure can cause bending.
- Inspect Relief Valve Annually: Look for leaks or wear.
- Document Shim Changes: Maintain service records for future reference.
Adjusting auxiliary hydraulic pressure relief for a thumb attachment on a Komatsu PC78MR-6 requires understanding the valve type and testing interaction between bucket and thumb. Whether using an external screw or internal shims, the goal is to balance clamping force with safe operation. With careful adjustment and field testing, operators can prevent damage and optimize performance for material handling tasks.
