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Failure and Root Cause Analysis
A hydraulic excavator model S2800FJ, with 8,664 hours of operation, experienced an issue at a construction site when a dump truck collided with its counterweight. Following the impact, a significant amount of grease was observed leaking from the rotary bearing’s sealing device, accompanied by abnormal noises from the bearing.
During the excavator’s rotation, a squealing noise was noticed when the machine turned forward and to the left. By lifting one side of the track with the bucket and then lowering it, a substantial axial clearance in the rotary bearing was observed. Additionally, significant radial movement occurred when the bucket was extended and placed on the ground.
The rotary gear and reducer were functioning normally.
It was determined that damage to the steel balls and isolation rings within the rotary bearing had increased axial and radial clearances, possibly damaging the sealing device.
Upon disassembling the rotary bearing for inspection, the external ring was connected to the upper structure, while the internal ring was attached to the lower structure. All hydraulic motor oil channels were removed and sealed with clean plastic film.
The upper part of the excavator was carefully lifted with a crane, the bolts of the rotary bearing’s external ring were loosened and removed, and the upper and lower parts of the machine were separated while maintaining balance. Isolation pins were then removed, plugs were taken out, and the steel balls and isolation rings were extracted using wire.
After removing all the steel balls and isolation rings, it was found that only two rings retained their original shape, while the others were crushed into fragments. The steel balls were cleaned with diesel, revealing severe corrosion and deformation on more than 30 balls. Once the external ring was removed and grease was cleaned, damage to the left-front bearing raceway was visible under sunlight.
When the excavator was struck, a significant radial force was applied to the rotary bearing, damaging some of the nylon isolation rings, which are relatively soft. Continued operation exacerbated the damage, increasing the clearance between the inner and outer rings and causing grease to leak through these gaps. This led to insufficient lubrication for the rings, isolation rings, and steel balls, further damaging components and resulting in abnormal sounds and malfunction during rotation.
Repair and Installation of the Rotary Bearing
Rotary Bearing Repair
Replacing the rotary bearing with a new one would have required importation, costing approximately 100,000 RMB, with a long delivery time. Since the excavator was older, it was decided to repair the existing bearing.
The issue was identified early, and the damage to the external rings was not severe. Therefore, the isolation rings, steel balls, and sealing device were replaced.
The original steel balls had a diameter of 28.48 mm, while locally available replacements measured 28.60 mm, 0.12 mm larger. The original rotary bearing contained 136 isolation rings and steel balls. The total length of the balls and rings in the raceways exceeded the original by 16.32 mm (136×0.12). The size of each isolation ring was reduced by 0.12 mm to ensure proper fit within the bearing raceways.
When purchasing steel balls, a selection method was used to ensure a diameter deviation of less than 0.002 mm. Additional isolation rings were prepared for selection during assembly. If the number of rings did not match the original, their size was further reduced until the number of isolation rings and steel balls matched the original configuration and filled the bearing raceway.
Rotary Bearing Installation
First, clean the inner and outer rings of the rotary bearing (including the raceways), steel balls, isolation rings, sealing device, and bolt holes.
Place the external ring over the internal ring to align the raceways. Insert the steel balls and isolation rings alternately into the raceways through the radial openings until the raceway is fully loaded.
Seal the plugs and secure the isolation pins. During the installation of the sealing device, apply sealant to the parts in contact with the sealing groove to ensure proper sealing. Rotate the rotary bearing to check for friction. If friction is detected, make adjustments to ensure smooth rotation. Finally, use a grease gun to fill the raceways and the grease chambers of the rotary gear and large ring.
Using a crane, lift the upper part of the excavator and align the lower stop point of the rotary platform with the external ring of the rotary bearing. Reconnect the upper and lower structures by inserting round steel through the bolt holes and install the bolts, applying Loctite thread-locking adhesive.
Tighten the bolts in a triangular symmetric pattern. Install an initial set of three bolts, followed by a second set of three bolts. Do not fully tighten the bolts initially. Once all bolts are in place, use a torque wrench to tighten each bolt to 650 Nm. After installation, operate the machine and perform various movements, including rotation, until normal operation is achieved.
The machine has been operating for a year since the repair, with no observed issues with the rotary bearing.
