A mining contractor reported that a Kobelco SK200-8 excavator's boom "chattered" (shook) when lowering slowly, especially with a full bucket. The shaking made it difficult to perform fine grading work.
The technician bled the hydraulic system, but the issue persisted. He focused on the "Boom Regeneration Circuit." To increase boom lowering speed, the system uses a "Regeneration Check Valve" that opens when the rod-side pressure is higher than the head-side pressure, allowing oil to flow from the rod side to the head side directly.
He removed the "Regeneration Valve Cartridge." He found the "Check Valve Spring" was broken. The broken spring allowed the check valve to flutter (open and close rapidly) when the flow rate was low (during slow lowering). This fluttering caused the pressure to pulse, resulting in the mechanical chatter.
Additionally, the "Head-Side Port" fitting on the boom cylinder was loose. The loose fitting allowed air to be sucked into the cylinder during the lowering stroke (negative pressure). The air bubbles compressed and expanded rapidly, exacerbating the vibration.
The repair involved replacing the regeneration check valve spring and tightening the cylinder port fitting. The hydraulic oil was topped up, and the cylinder was cycled (extended and retracted) 5 times to purge the remaining air. The boom lowering became smooth and vibration-free.
