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CAT Excavator  Final Drives:  The MostCommon Issue

CAT Excavator Final Drives: The MostCommon Issue

Posted by Jim Strong on Dec 7th 2021

The most common issue with Caterpillar mini-excavator final drives might surprise you.

Here are a few other Shop Talk Blog posts you might find helpful:

Floating Face Seals

When track drive motors like those on a Caterpillar 303 CR compact excavator come into the shop, the issues usually involve a particular kind of leak. That leak is usually between the back side of the hydraulic motor and the drive sprocket and is due to a failed face seal.

Face seals, also called floating face seals, mechanical face seals, or duo-cone seals, are responsible for keeping the gear oil in the planetary hub and out of the hydraulic hub. When this seal begins to fail, two things happen: gear oil leaks out of the planetary hub and contamination starts working its way inside

Gear Oil Leak

Gear oil leaks are found between the sprocket and the final drive motor. And while you may not spot them immediately, a good sign that they are present is gear oil levels that are low. In fact, if you find yourself repeatedly topping of the gear oil, then you do have a leak. A simple solution may be to keep topping off the gear oil until you can get the seal fixed, but that isn't guaranteed to work. If the leak accelerates and you let the gear oil levels fall too low, you can end up with dry running that will cause far more costly damage than merely replacing the seals.


If gear oil can leak out, other things can make their way inside your final drive. Our techs will tell you that it isn't uncommon to open up a gear hub that is filled with a disgusting sludge of gear oil, dirt, sand, and other debris. This will do serious (and expensive) damage to the planetary gear system, bearings, and axle contained in the planetary hub. So while you keep topping off the gear oil, you don't know what may be mixing with it inside.

What Face Seal Failure Looks Like

First off, face seals are eventually going to fail. Just like bearings, they are hour rated and have a limited life span. Eventually they will need to be replaced, either when you notice a leak or according to manufacturer recommendations. The downtime and cost associated with replacing a face is going to much less than what happens if leaks lead to contamination.

However, there are things that can accelerate how fast your face seals wear out. One of the main ones relates to the undercarriage of your CAT excavator. If you don't keep the undercarriage and tracks clean, debris is going to pack in tightly. And once it gets packed in tightly around your final drive motor, it can actually push the face seal out of place. As the seal gets pushed out of place, even a little, debris will get wedged in even further and the smooth faces will end up seriously scarred. Every scratch and gouge becomes a way for gear oil to leak out.

One cause of premature failure is related to the excavator undercarriage. If the undercarriage is not kept clean, debris can get packed in so tightly around the drive motor that it pushes the face seal out of place. The smooth mating face of the seal is key to its proper function, and when the face seals are forced out of place then that debris gets wedged between the faces. That abrasive debris can not only forces the faces apart far enough for debris to make its way in, but can scar up the faces and provide additional paths for the gear oil to leak out. This becomes face seal failure.

Preventing Premature Face Seal Failure

On your CAT excavator, the best way to prevent premature failure of your face seals is to keep the undercarriage clean. And this should be a part of your regular maintenance routine. The good news is that not fancy equipment is needed (a hose and broom can do an excellent job) and it shouldn't take very long is you do it regularly.


The most common issue with CAT excavator track drives is face seal failure. You'll first notice it when a leak develops between the sprocket and the final drive. If you're leaking gear oil, don't ignore that leak! It's going to be much cheaper to have the drive motors pulled and their face seals replaced than to deal with the aftermath of leaks and the contamination that will inevitably make its way into your gear hub. And here's something good to know: you can prolong the useful life of your CAT duo-cone seals by keeping the undercarriage clean.

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