The job looked routine: on a 75-tonne Caterpillar 374F L from 2012, five slewing ring bolts had sheared. The plan: lift the upper carriage off the undercarriage, remove the broken bolts and bolt the machine back together with new ones. A crack in the frame was visible even before disassembly, but only after taking it apart did the full scale of the problem appear.
Dismantling a 75-tonne machine
At this weight everything has to be stable and safe. We positioned the excavator on a hardened yard, and then:
- drained the final drives and removed the drive shafts so the undercarriage could be moved,
- disconnected the lines underneath and raised the swivel joint so the machine would not have to be lifted too high,
- removed the slewing ring bolts and the central lubrication lines,
- prepared supports: wheel rims, steel plates on top and concrete sleepers,
- lifted the upper carriage with slings on a loader and pulled out the undercarriage with a second loader.
The upper carriage was supported high and securely – at the rear, the front and additionally on the bucket – so a welder could work safely underneath.
The scale of the damage
The cracks at the sheared bolts turned out to be the smallest problem. Between the swing motors with their gearboxes, a piece of steel had literally been torn out, and cracks ran in many directions – also between the bolts and at the swing drive mounts. The owner initially planned to weld it himself. However, after the swing drives and swivel joint were removed, the damage proved so extensive that none of several welding companies was willing to put its name to such a repair. Traces of earlier welding attempts were also visible.
The decision: replacing the base plate
The owner decided to replace the entire base plate. Our task was to reassemble the machine so it could be transported safely. We refitted the swing motors and gearboxes (their bolts had sheared too – new ones were fitted), connected the colour-coded lines, and after two months standing started the machine on external power as the batteries were flat. Then we lifted the upper carriage, moved the undercarriage back in, bolted up the slewing ring, fitted the swivel joint and filled the final drives with oil.
Takeaways
- Sheared slewing ring bolts are often a symptom rather than the cause – check the structure around them.
- On machines of this size, a seemingly minor fault can mean a major service operation.
- Not every crack can be welded safely; on load-bearing structures replacement is sometimes the only responsible solution.
Undercarriage removals and slewing ring and bolt replacements are part of our construction machinery service.
Watch the repair video: Why Wouldn't Any Welder Repair This? CAT 374F L
