Tearing Down the CAT 6K9879 Carrier Roller: Disassembly, Inspection, and Rebuild Strategy
The CAT 6K9879 carrier roller is a mid-sized upper track roller commonly found on Caterpillar track-type equipment, including dozers and excavators. Its role is to support the track chain from above, maintaining alignment and reducing wear. Over time, carrier rollers can fail due to bearing wear, seal degradation, or shaft scoring. This article provides a detailed guide to disassembling and inspecting the 6K9879 roller, with practical insights from field repairs and rebuilds.
Terminology Notes
- Carrier Roller: A track roller mounted above the track chain, supporting its weight and guiding movement.
- End Cap: A pressed or bolted cover that seals the roller and retains the bearing assembly.
- Bearing Race: The hardened steel surface on which roller bearings rotate.
- Seal Assembly: A combination of metal and rubber components that prevent grease leakage and contamination ingress.
- Shaft Journal: The portion of the roller shaft that contacts the bearing inner race.
Symptoms of Roller Failure
- Excessive noise or vibration during travel
- Grease leakage from end caps
- Visible wobble or misalignment of the track chain
- Heat discoloration on roller surface
- Uneven wear on track pads or links
Disassembly Procedure
Preparation
- Clean the roller exterior thoroughly
- Secure the roller in a bench vise or fixture
- Mark orientation of end caps for reassembly
End Cap Removal
- Use a press or puller to remove the end caps
- Inspect for deformation or scoring
- Retain shims or spacers for measurement
Bearing Extraction
- Remove bearings using a blind bearing puller or press
- Inspect bearing surfaces for pitting or galling
- Measure bearing ID and OD for replacement matching
Seal Removal
- Pry out seal assemblies carefully to avoid damaging housing
- Inspect for hardened rubber, torn lips, or rust trails
- Clean seal grooves and check for corrosion
Shaft Inspection
- Remove shaft and inspect journals for scoring or wear
- Check for straightness using a dial indicator
- Polish minor wear with emery cloth; replace if out of spec
- Housing Evaluation
- Inspect bore for ovality or scoring
- Check for cracks or impact damage
- Clean thoroughly and prepare for reassembly
Field Case: Forestry Dozer in Alberta
A mechanic servicing a D5H dozer noticed excessive track slap and traced the issue to a failed carrier roller. Upon teardown, the roller revealed a seized bearing and a cracked seal lip. The shaft journal was scored from dry operation. He replaced the bearings, seals, and shaft, and added a grease fitting for future service. “It was a two-hour job that saved a $2,000 undercarriage,” he said.
Rebuild Strategy
- Replace all bearings with OEM or high-quality equivalents
- Install new seal assemblies with proper orientation
- Use anaerobic sealant or gasket compound on end caps
- Torque end cap bolts to spec (if applicable)
- Fill roller with recommended grease volume
- Spin test roller for smooth rotation and seal integrity
Preventive Maintenance Tips
- Inspect carrier rollers every 500 hours or during undercarriage service
- Check for grease leakage or dry rotation
- Clean roller surfaces during track washdowns
- Replace seals proactively if operating in mud or abrasive conditions
- Use high-temp, water-resistant grease for longevity
Operator Anecdotes and Practical Wisdom
A contractor in Georgia shared how his excavator’s carrier roller failed during a foundation dig. The roller seized and caused the track to derail twice. After rebuilding the roller and adding a drain plug for future flushing, the issue never returned.
Another story comes from a snowplow operator in Wisconsin. His carrier roller froze during sub-zero operation due to water ingress. He now stores spare rollers indoors and pre-lubes them before winter deployment.
Conclusion: Small Roller, Big Responsibility
The CAT 6K9879 carrier roller may seem like a minor component, but its failure can ripple across the entire undercarriage system. With proper teardown, inspection, and rebuild practices, operators can restore smooth track operation and extend machine life. In the world of heavy equipment, even the smallest roller deserves precision and respect.
