Addressing Slew Bearing Play on the Hitachi ZX27U-2 Mini Excavator

[MikePhua]

The ZX27U-2 and Its Compact Excavator Design
The Hitachi ZX27U-2 is a zero-tail swing mini excavator developed for urban construction, landscaping, and utility trenching. With an operating weight of approximately 2.7 metric tons and a compact footprint, it excels in confined spaces where maneuverability is critical. Powered by a Yanmar 3TNV76 diesel engine and equipped with a load-sensing hydraulic system, the ZX27U-2 delivers precise control and efficient performance.
One of its key structural components is the slew bearing, also known as the swing bearing or turntable bearing. This large-diameter bearing allows the upper structure to rotate smoothly on the undercarriage. It supports vertical loads, radial forces, and tilting moments generated during digging and swinging operations. Over time, wear and play in the slew bearing can compromise stability, accuracy, and safety.
Identifying Slew Bearing Play
Slew bearing play refers to excessive movement or looseness between the upper and lower structures during rotation. Symptoms include:

• Noticeable rocking or tilting when swinging the boom
  
• Audible clunking or knocking sounds during rotation
  
• Uneven wear on the bearing race or gear teeth
  
• Increased backlash when changing swing direction
  
• Difficulty maintaining precise alignment during trenching
  

To assess bearing play:

• Park the machine on level ground
  
• Extend the boom fully and lift it slightly off the ground
  
• Swing the upper structure side to side while observing movement at the base
  
• Use a dial indicator to measure vertical displacement at the bearing edge (acceptable play is typically under 1 mm for mini excavators)
  

In one case from northern Iraq, a contractor noticed excessive sway while placing concrete slabs. After measuring 3 mm of vertical play, the bearing was found to be worn beyond service limits due to lack of lubrication and overloading.
Causes of Bearing Wear and Play
Common contributors to slew bearing deterioration include:

• Insufficient lubrication: Grease starvation leads to metal-on-metal contact and accelerated wear.
  
• Contamination: Dirt, water, and debris entering the bearing race degrade surfaces and seals.
  
• Overloading: Operating with heavy attachments or side loads beyond design limits stresses the bearing.
  
• Improper installation: Misalignment or uneven bolt torque during replacement causes uneven wear.
  
• Neglected maintenance: Skipping grease intervals or ignoring early symptoms allows damage to compound.
  

Some operators mistakenly use general-purpose grease instead of high-pressure bearing grease, which lacks the necessary load-carrying additives.
Inspection and Maintenance Guidelines
To maintain slew bearing integrity:

• Grease the bearing every 100 hours or weekly under heavy use
  
• Use lithium-based EP2 grease with molybdenum disulfide for high-load applications
  
• Rotate the upper structure during greasing to distribute lubricant evenly
  
• Inspect the bearing seal for cracks, tears, or extrusion
  
• Check bolt torque on the bearing flange annually
  
• Monitor for unusual noises or movement during swing operations
  

In Shanghai, a fleet manager added a grease log to each machine’s service record. After implementing this, bearing failures dropped by 70% over two years.
Repair and Replacement Strategy
If bearing play exceeds service limits:

• Remove the upper structure using a crane or lifting frame
  
• Clean and inspect the bearing race and gear teeth
  
• Replace the bearing with OEM or high-quality aftermarket unit
  
• Torque mounting bolts in a star pattern to ensure even preload
  
• Reinstall the upper structure and test swing function under load
  
• Re-grease and monitor for settling during initial operation
  

Some technicians use shims or preload spacers to reduce minor play, though this is a temporary fix and not recommended for structural wear.
Preventive Enhancements and Operator Tips
To extend bearing life:

• Avoid swinging with fully extended boom and heavy loads
  
• Minimize abrupt directional changes during rotation
  
• Keep the bearing area clean and free of mud or debris
  
• Train operators to recognize early signs of bearing wear
  
• Install a protective skirt or seal guard to reduce contamination
  

In Ohio, a contractor added a rubber seal ring around the bearing base to block sand ingress during excavation. This simple upgrade extended bearing life by 40%.
Conclusion
Slew bearing play on the Hitachi ZX27U-2 is a manageable issue when addressed early and maintained properly. With routine greasing, careful operation, and timely inspection, operators can preserve swing accuracy and structural integrity. Whether trenching in tight alleys or grading on slopes, a stable bearing ensures the machine performs with precision and safety across every rotation.