Swing Bearing Grease Port on the CAT 305.5D Excavator

[MikePhua]

Context and Equipment
The subject machine is a Caterpillar 305.5D (CAT 305.5D) mini‑excavator, a compact hydraulic excavator widely used in construction, demolition, and utility work. Proper greasing of its swing bearing (slew bearing) is critical because lack of lubrication can lead to excessive wear, premature failure, and potentially costly repairs. According to self‑service guidance from Caterpillar, lack of lubrication is the most common cause of joint failures.

Issue Observed
A technician reported that during a swing-bearing replacement job, they discovered what appeared to be the grease line for the bearing: the line looked broken, and the associated grease port didn’t seem to feed anything in its present state.  From their photos, the red-marked port supported swing‑gear lubrication, but a second, blue-marked port (identified as possibly for the bearing) was not functioning. The technician believed the bearing had not been greased for a long time prior to purchase, raising concern about wear and maintenance neglect.

Understanding Grease and Lubrication on Swing Components

• The swing-bearing (slew bearing) and swing gear are distinct lubrication zones. The swing bearing supports the rotating house, while the swing gear (ring gear) meshes with the swing motor’s pinion. On many excavators, these areas are lubricated by different means.
  
• In some models, open-gear grease is used for the swing-gear cavity to protect the ring gear teeth, while grease zerk fittings serve the swing bearing itself.
  
• According to Caterpillar maintenance documentation, grease should be pumped into the bearing until it begins to purge from the joint, indicating proper fill.
  

Possible Causes of the Faulty Grease Line
Based on the inspection and user reports, several likely causes could explain why the grease port is not functioning:

1. Broken or Disconnected Grease Line
   • The technician observed a damaged or severed grease line. If the grease hose or metal tube is broken, grease pumped into the fitting won’t reach the bearing cavity.
     
2. Clogged or Damaged Fitting
   • The grease fitting (zerk) or its internal passage may be blocked by dirt, hardened grease, or debris, preventing flow.
     
3. Missing Internal Passage
   • On some machines or aftermarket modifications, grease lines may have been incorrectly installed, removed, or never connected.
     
4. Grease Degradation
   • Old or incompatible grease can harden or separate, making it difficult for fresh grease to travel through the line.
     

Recommended Diagnostic and Repair Actions

• Trace the Grease Line: Remove panels or covers to visually follow the grease line from the fitting to the bearing. Confirm that it is properly connected, intact, and routed without kinks.
  
• Clean and Flush the Line: Use a grease gun to pump low-pressure grease or a compatible solvent through the line to clear obstructions.
  
• Inspect Fittings: Remove the zerk fitting and check for damage inside; clean or replace as necessary.
  
• Pressure-Test Greasing: While greasing, monitor whether grease begins to purge from the bearing joint. If not, the line or bearing cavity may still be blocked.
  
• Replace Damaged Lines: If the grease line is broken or irreparably pinched, replace it with a preformed high-pressure grease hose or tubing rated for the application.
  
• Use Correct Grease: Use a heavy-duty, extreme-pressure (EP) grease recommended for slew bearings. Caterpillar often advises greases with molybdenum or other load-bearing additives.
  
• Verify Fill: After repair, grease until clean, fresh grease appears in the bearing joint, then wipe off excess.
  

Maintenance Best Practices

• Grease the swing-bearing at regular intervals as specified in the machine’s Operation & Maintenance Manual (OMM).
  
• Inspect grease fittings during routine checks for leaks, damage, or contamination.
  
• Use compatible grease and avoid mixing greases with different chemistries to prevent performance loss.
  
• Document greasing schedules and any repair work to maintain a reliable maintenance history.
  

Conclusion
The grease port issue on the CAT 305.5D swing bearing is likely caused by either a broken grease line or an obstruction in the lubrication path. Proper tracing, cleaning, and, if necessary, replacement of the line are required to restore correct lubrication. Regular greasing using the right type of grease is essential to protect the swing bearing and prolong the life of the excavator’s rotating parts. Overlooking this maintenance point can lead to accelerated wear and potentially expensive repairs.