A final drive rarely fails without warning. It fails after a run of small signals that were easy to explain away: a noise that only shows up on a turn, a weep behind the sprocket, a machine that has started pulling slightly to one side.
Listen for grinding, clicking or a screech. Check the gear oil for brass or bronze debris. Look for oil behind the sprocket. Then rule out the cheap causes, loose sprocket bolts, over-tight tracks, worn rollers and hydraulic pressure, before you condemn the unit.
Several cheap faults mimic a failed final drive:
- Loose sprocket bolts, often misdiagnosed as drive failure
- Over-tight tracks, which rob the machine of power and load every mating component
- Worn rollers and idlers creating misalignment and vibration
- Insufficient hydraulic pressure or flow, or a blocked case drain
- A brake that is not releasing, which feels weak and sounds like a screech
What each symptom usually means
| Symptom | Likely cause |
|---|---|
| Grinding or growling | Metal-on-metal contact from worn bearings or damaged gear teeth, usually following contamination or neglected oil changes |
| High-pitched whine or screech | More often hydraulic than gear train: cavitation, fluid starvation, a restriction, or a brake failing to release. Check the hydraulic side first |
| Clicking or knocking, worst on a turn | Loose or broken internal components, damaged bearings or broken gear teeth |
| Popping or banging | Catastrophic internal failure. Stop the machine |
| Oil behind the sprocket | Failed floating face seal |
| Oil level rising above normal | Worn motor shaft seal or bearings, or case pressure pushing hydraulic oil into the gear case |
| Overheating | Lack of lubricant, hydraulic oil entering the gear case, or a clogged case drain filter |
Reading the gear oil
Draining the gear oil tells you more than any other single check, and it costs almost nothing.
- Fine steel particles on the magnetic plug are normal. The plug exists to collect them, and accumulation is expected during break-in.
- Brass or bronze shavings, chips or slivers are not normal. They mean fragments coming off bearing surfaces or gear teeth.
- A burnt smell, or oil that looks like metallic soup, means the gearbox has gone.
- Milky or hazy oil means water has got in, which corrodes components and reduces the oil's lubricating properties. It is a late indicator, not an early one.
Treat what you find as a trend rather than a pass or fail. What matters is whether it is getting worse between checks.
Checking the oil level
- Turn the drive so one plug is at 12 o'clock and the other at 3 or 9 o'clock.
- Clean mud and sand from around the plugs, then remove both.
- At the correct level the oil sits level with the 3 o'clock opening.
- If low, top up through the 12 o'clock opening with SAE 85W/90 or standard gear oil until it runs from the 3 o'clock opening.
- Replace both plugs securely.
Change intervals are machine-specific and vary widely, so follow your operator's manual. Whatever the interval, the condition of the oil tells you more than the hours do.
What kills final drives early
| Cause | Mechanism |
|---|---|
| Contamination | Dirt or metal entering during servicing, through worn seals, or from new fluid that was not clean. Contaminated oil destroys bearings and gear surfaces |
| Running low on oil | Lack of lubricant causes overheating and pitting in the bearings and gears, neither of which is repairable in the field |
| Worn sprockets | Sharp and worn sprockets make the link slip or jump through the teeth, and that shock loading damages the final drive motor |
| Incorrect track tension | Tight track advances wear on every mating component. A detracking machine creates shock loads that can crack hubs or damage motor shafts |
| Debris packed around the hub | Compromises the face seals, letting oil out and contamination in |
When a final drive fails, metal particles do not stay in the drive. They pass into the travel motor and from there into the hydraulic circuit, where they score everything downstream. Case drain filters only strain the larger debris and block quickly.
Before installation, make sure the hydraulic oil is not contaminated with metal particles from the previous drive's breakdown. We strongly recommend a complete change of hydraulic oil and a new hydraulic filter when a final drive is changed. Clean dirt away from the installation area too, particularly around the hoses. Ingress of dirt will cause early failure.
And, in capitals in our own fitting instructions: incorrect piping will result in non-warranty failure. Check the port colour codes before you connect anything.
Ordering a replacement
Astrak supplies final drives for excavators and tracked equipment from 1 to 50 tonnes, with up to 24 months warranty. Many of our drives are manufactured by leading OEM suppliers including Eaton, Doosan, Nabtesco, Brevini and Trasmittal Bonfiglioli.
To identify one we need the machine type, model and serial number, the mounting flange dimensions, hole count and bolt size for both flanges, whether the existing drive is single speed, and a clear photograph of the tag plate. That tag sits on the inward-facing side of the drive, opposite the side the sprocket bolts to, behind a teardrop-shaped cover plate usually held on by three bolts.
The rest of the Terrapart range
Final drives
Excavators and tracked equipment 1 to 50 tonnes, up to 24 months warranty. View final drives.
Hydraulic pumps
Units from Rexroth, Uchida, Hydromatik, Tong Myung and Mottrol, plus Kawasaki K3V and K5V. View pumps.
Swing motors
Stocked for 6 tonne and larger excavators. View swing motors.
Minimum 12-month warranty across the Terrapart range. See Terrapart Hydraulics or swing reducers.
Frequently asked questions
What are the signs of a failing final drive?
Grinding or growling from worn bearings or damaged gear teeth, clicking or knocking that is worst on a turn, metal in the gear oil, oil leaking from behind the sprocket, loss of travel power, the machine tracking to one side, and overheating.
What does metal in final drive gear oil mean?
Fine steel particles on the magnetic drain plug are normal wear debris, and the plug exists to collect them. Brass or bronze shavings, chips or slivers are not normal and point to fragments coming off bearing surfaces or gear teeth. Oil that smells burnt or looks like metallic soup means the gearbox has gone.
Why is my excavator tracking to one side?
Check the cheap causes first: uneven track tension, worn rollers or idlers, loose sprocket bolts. Insufficient hydraulic pressure or flow to one side will do the same thing. Only once those are ruled out is the drive itself the likely cause.
How often should final drive oil be changed?
Intervals are machine-specific and vary widely between manufacturers and models, so your operator's manual is the authority. Whatever the interval, check the level regularly and inspect what comes out when you drain it, because the condition of the oil tells you more than the hours do.
Do I need to change the hydraulic oil when replacing a final drive?
Yes. We strongly recommend a complete change of hydraulic oil and a new hydraulic filter when a final drive is changed. Metal particles from the failed unit pass into the hydraulic circuit and will damage the replacement.
Need a final drive?
Send us your machine model and serial number, or a photograph of the drive tag plate, and we will confirm the right unit and current availability.
Astrak Group develops and manufactures products intended to be compatible with machinery and equipment from various manufacturers. Unless indicated otherwise, Astrak Group has no relationship, commercial or otherwise, with the owners of the trademarks.
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