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Final Drive Planetary Hub Failure

Final Drive Planetary Hub Failure

Feb 23rd 2023

The planetary hub on your final drive motor endures extremely high forces -- and even though it's built tough, it can fail. Learn what the common failure modes are and how to prevent them.

Final Drive Planetary Hub

Below you can see a typical planetary for a final drive motor. The sun gear is the input gear and it meshes with the three planetary gears. These gears engage the ring gear to turn the final drive motor system.


Here's another viewpoint of the inside of a planetary gear hub for a smaller final drive motor. In this image, it's easier to see how the different gears are meshing together and how they align. It also makes it clear that misalignment of the gears can be very problematic.

Where the gear mesh, there are tremendous forces because of how small that contact area is. Gears like these are made from high-quality steel that has undergone special treatments like case hardening to make them hard enough to resist wear but not so brittle that the teeth break off.

Gear Failure

Tooth Wear

Over time, the teeth on these gears are going to experience wear --  that's unavoidable even with the highest quality gear oil you can find. This is caused as the metal on the gear teeth erodes over time. This leads them to change shape -- and if you open up a gear up and see teeth that look pointy instead of elliptical, that's a sign they need to be replaced very soon.

In fact, here's a drive shaft from a Hy-Dash final drive motor gear hub. If you look closely, you'll see that the splines have almost worn off. The customer sent this in complaining that their drive motor wouldn't spin at all -- and it turns out that it couldn't because there were no splines (teeth) to mesh with!

As the gear teeth change shape, they can't transmit motion and torque as effectively, which eventually affects the performance of your final drive motor. Eventually you'll begin to hear rattling noises and may experience some vibration.

Particle Contamination

There are three ways that contamination becomes a problem in your planetary hub, with the first involving the wear just discussed. Wear loosens tiny flakes and particles of metal from the gear teeth (and the bearings, for that matter). That contaminates the gear oil and speeds up the accelerates further wear. And that leads to even more contamination. Once serious wear begins, it basically speeds up damage to your final drive motor planetary hub and can total it.

Gear Oil

There is also the problem of contaminated gear oil. If you look below, you'll see the aftermath of a failed main seal that allowed dirt and grit to make its way into the planetary hub. Obviously, this is BAD. In fact, having this in your gear hub may be worse than have no gear oil in there at all. This type of failure is why it is so important to replace leaking seals as soon as possible. What may seem like a minor leak on the outside could be doing horrible damage on the inside. 

And another issue with gear oil is allowing it to turn to a sludge. As it ages, lubricants will start to break down chemically in a process called aging. The gear oil loses its ability to protect and lubricate and can even leave a coating on your gears and bearings. Our technicians can open up a drive hub and instantly tell when the gear oil hasn't been changed in a while -- from the smell and the thickness.

Bearing Failure

Even the best quality bearings will eventually wear out. So one way your planetary hub can fail is because of catastrophic bearing failure. As bearings start to wear out, their ability to keep the gears aligned is compromised. And so is their ability to reduce friction as the gears turn. When they eventually reach a certain level of wear, the bearings can start contributing to particle contamination -- and they can be affected by particle contamination, as well. When they start to fail, bearings will making rattling noises and cause the planetary hub to overheat.


Here at Final Drive Parts, we know that if your gear hub stops working your final drive does, too. That's why we encourage you to be aware of these common symptoms potential planetary issues:

  • New vibrations or sounds
  • Skipping
  • Overheating
  • Leaks
  • Check and change your gear oil regularly

The sooner you address a problem, the better!


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