A landscaping contractor reported that a Sany SY75C mini excavator's cabin vibrated heavily when slewing, and the joystick controls felt stiff (high effort) after 20 minutes of operation.
The technician first checked the "Main Pump Pressure," which was normal. He then isolated the "Cabin Dampers" (rubber mounts), which were intact. He focused on the "Swing Joint" (rotary manifold) located in the center of the base machine. He removed the swing joint cover. The joint was hot to the touch. He disassembled the swing joint. The "Lower Needle Bearing" had completely seized on the shaft due to lack of grease.
The seized bearing prevented the upper structure from rotating smoothly. As the swing motor tried to turn the house, the resistance transferred through the rigid swing joint shaft, vibrating the chassis and cabin. Furthermore, the seized bearing had "scored" (scratched deeply) the external surface of the rotary joint. The "Pilot Supply Port" (which routes pilot pressure oil from the pump to the cab joysticks) is machined into this surface. The scoring destroyed the sealing rings on the pilot port, allowing pilot oil to leak to the tank drain.
The repair involved replacing the seized bearing and the damaged swing joint shaft. The rotary sealing rings were replaced. The swing joint grease was repacked. After reassembly, the slewing was smooth and quiet, and the pilot pressure returned to 40 bar, making the joysticks light and responsive.
