A heavy equipment contractor reported that a Hyundai R290LC-9 excavator had weak bucket curl force, taking nearly 10 seconds to curl the bucket from flat to full dig. Additionally, the bucket would slowly drift open (curl back) when carrying a load over the side. The operator reported no other issues with the boom or stick.
The technician first tested the bucket cylinder for drift. With the bucket curled and the engine running, the lever was held in the curl position. The drift stopped, indicating the problem was not the cylinder seals (piston by-pass), but rather the holding valve or control valve. However, when the technician tested the relief pressure at the bucket circuit, it was peaking at only 2,800 psi, well below the system standard of 4,500 psi.
The main control valve was removed and disassembled. The spool for the bucket curl circuit was found to have deep scoring on its lands. The scoring was caused by a microscopic piece of metal debris (likely from a previous hose failure) that had lodged between the spool and the valve bore. The scoring allowed pressurized oil to leak past the spool back to the tank, preventing the system from building full relief pressure.
The bucket cylinder was also removed as a precaution. Upon disassembly, the piston seal was found to be extruded and cracked. This was a secondary issue caused by the erratic pressure spikes from the leaky control valve. The weak curl was due to the valve leakage, and the drift was due to the blown cylinder seal.
The repair required installing a new bucket control spool and honing the valve bore to remove the scoring. A complete new seal kit was installed in the bucket cylinder. The hydraulic tank was drained and magnets were cleaned to prevent future contamination. After reassembly, the bucket curl was crisp and powerful, and the drift was completely eliminated.
