Case Study: Final drive from a Kubota KX71-3 Compact Excavator

Jan 18th 2018

Kubota KX71-3 Final Drive Problems

Introduction to the Case Study

A final drive had been causing problems for the owner of a Kubota KX71-3 tail swing compact excavator, and he decided to attempt the repairs himself. After struggling for a while and investing quite a bit of money in various replacement parts, he sent it in to have it professionally assessed.

Assessment of the Final Drive

When it arrived, the final drive was disassembled by our team and each part carefully assessed. One immediate issue was with the rotator group, which was not within the final drive itself but rather in a separate bag during shipping. This inevitably causes damage to the rotator group during shipping, and some of the key surfaces were scratched and gouged. With the rotator group, those key surfaces must be kept smooth. In this case, the damage could not be lapped out because of the depth of grooves involved with the affected area. That meant that the damaged parts would have to be replaced.

The second major issue involved replacement of only one of the three planetary gears. A good rule of thumb is this: if there is more than one of something, replace them all at the same time. The original owner noticed damage to one of the gears and replaced only that one out of a set of three.

The original issue resulted from the presence of debris in the final drive that in turn damaged the surface of the gear teeth. The burrs that can form from this damage will eventually come off and continue the damage elsewhere. Scratches and gouges on the gear teeth also compromised the hardened surface of the gears, making additional damage even more likely.

Conclusion

The issue that caused so many problems for this drive was contaminants found in the system; they ate away at key surfaces and key components until there was a catastrophic drive failure. After full disassembly and careful evaluation of each component, it was concluded that the most economical approach would be to purchase a remanufactured drive.

Take-Aways:

Contaminants in the final drive can do serious, expensive damage, so keep track of the condition of the hydraulic fluid and change the gear oil according to the manufacturer’s recommendations.