An excavator throws its track when the track line is wrong: idler misalignment, worn rollers, loose tension, or a worn sprocket. The front idler is the first place to check, because its alignment sets the track line around the front of the machine. Work through the line in order — idler, yoke, rollers, tension, then the track itself — and most throwing machines are back in service the same day.
Related reading: for wear measurement, see our front idler wear guide, and for replacement, see our front idler replacement guide.
Track Alignment Guidance from AFT Parts
- Provider: AFT Parts — front idlers and undercarriage wear components.
- Scope: Track throwing diagnosis and idler yoke alignment
- Works with: CAT, Komatsu, Kubota, Bobcat, Hitachi, John Deere, and more.
- Application coverage: BC steep terrain, roading, construction
- Confirmed against: machine serial number and part number before dispatch.
How Misalignment Causes Track Throwing: The Line, Not the Part
The track follows the line set by the idler, rollers, and sprocket. When one component sits off-line, the track walks toward it until it rides off the flanges and throws.
Misalignment can come from a bent yoke, a worn idler, a shifted roller, or a damaged frame. The throw is the symptom; the alignment is the cause, which is why replacing the idler alone rarely stops a recurring throw.
On a BC machine working steep terrain, a thrown track is also a safety event: the machine loses steering, the track can strike an operator or ground worker, and recovering a 20-tonne excavator on a slope is a job of its own.
Checking Idler Alignment Step by Step, Starting at the Yoke
Check the idler first: inspect the yoke for bends or damage, and confirm the idler sits square in the frame. Then check the track ride — it should sit centred on the idler with even contact.
Measure the gap between the track and the frame on both sides at the front. An uneven gap points to the idler or frame alignment, and a bent yoke is visible as a twist in the mounting face.
Adjusting the Idler Yoke in Small Steps and Re-Checking the Ride
If the idler sits off-line, adjust the yoke within its adjustment range and re-check the track ride. The yoke is the front alignment point of the track line, and small moves are easier to verify than one large move.
Adjust in small steps and run the track between adjustments. A yoke pushed past its range signals frame damage rather than an adjustment problem, and forcing it further risks bending the adjuster.
Rollers and Sprocket: The Rest of the Track Line
Worn or shifted rollers change the track line in the middle of the undercarriage, and a worn sprocket changes it at the rear. Check the full line, not just the front, because a throw can be driven by the middle of the track as easily as by the front.
A track that throws on one side usually has a component on that side off-line. Compare left and right to isolate which position is causing the throw, and measure the same gaps on both sides.
Testing After Adjustment: Straight Runs and Turns
After adjusting, run the machine forward and reverse on level ground and check the track ride. Then test a turn, because turning loads the track line sideways and exposes alignment problems that straight running hides.
If the track stays centred through straight running and turns, the alignment is corrected. A throw that persists after alignment points to a structural issue or to the track itself, not to the components you have already corrected. Repeat the test after a loaded pass as well, because a machine carrying a bucket load rides differently from an empty one.
When Alignment Isn't the Problem: The Track Itself
When the idler, rollers, and sprocket are aligned and the track still throws, check the track itself: stretched chain, wrong size code, or damaged lugs. A worn track throws even on a straight undercarriage.
Confirm the size code and measure the track condition. If the track is near the wear limit, the throw is a replacement signal, not an alignment one, and ordering the correct size code is the fix.
A Field Fault Tree: Diagnosing a Throwing Track in 20 Minutes
The fastest way to work a throwing track is to rule the causes out in order, because the checks build on each other. The table below is the sequence used by field mechanics, and each row ends in a decision rather than a theory.
| Check | What you are looking for | Decision |
|---|---|---|
| Track sag | Sag outside the published range | Re-tension and re-test first |
| Idler yoke | Bent, cracked, or twisted yoke face | Replace yoke or frame part |
| Track ride on idler | Track walks to one side of the flange | Adjust yoke within range |
| Rollers and sprocket | Shifted or worn components off-line | Replace the worn component |
| Track size code | Wrong code or worn chain/lugs | Confirm code and measure wear |
Throwing Tracks on BC Slopes: Speed, Grade, and the Safety Decision
On steep BC sites, travel speed and grade change how the track loads. Descending a slope with the track braking hard, or turning on a grade with the outside track under full load, can throw a marginal track that would hold on flat ground.
The practical rule is to fix the line before the job, not after the first throw: check sag and the idler yoke at the start of the shift on slope work, and keep the machine below the travel speed that makes a marginal track come off.
AFT Parts Expert Views
A thrown track is a line problem. Check the idler yoke first, compare the track ride left and right, then test straight running and turns. If the line is straight and the track still throws, the track itself is the suspect. — AFT Parts Technical Team
Conclusion
Track throwing is a line problem with a front-end suspect: check the idler yoke and alignment first, then the rollers and sprocket, then the track itself. Test straight running and turns after adjustment, and confirm the track size code before replacing parts.
Key Takeaways for Canadian Operators
- Check the idler yoke and alignment first when a track throws.
- Compare the track ride left and right to isolate the cause.
- Adjust the yoke in small steps and test straight and turning.
- Check rollers and sprocket when the front is straight.
- Confirm the track size code and wear condition before replacing.
Questions to Ask Before You Order
- Is the idler yoke bent or the frame damaged?
- Which rollers or sprocket show wear on the throwing side?
- What is the track size code and wear condition?
- Should the machine be checked before slope work starts?
See the Front Idler collection and the 6693238 Bobcat idler for your machine.
Frequently Asked Questions
Why does my excavator keep throwing its track?
The track line is wrong: idler misalignment, worn rollers, loose tension, a worn sprocket, or a stretched track. Check the front idler first, then the full line.
How do I check idler alignment?
Inspect the yoke for damage, confirm the idler sits square, and check the track ride — it should sit centred on the idler with even contact.
Can a worn track cause throwing?
Yes. A stretched or worn track throws even on a straight undercarriage. Confirm the size code and wear condition if alignment is good.
Can a loose track cause throwing without misalignment?
Yes. A loose track can throw even on a straight undercarriage. Check sag before blaming alignment.
Should I check tension before blaming the idler?
Yes. Tension is the cheapest check and the most common cause of throwing. Verify sag before inspecting the idler and yoke.