13/10/2026
A guide to the main excavator undercarriage parts: tracks, sprockets, idlers, track adjusters, track rollers and carrier rollers. It explains what each part does, what to check, and the details you need to identify the right parts for your machine.
The undercarriage is everything below the upper structure that carries an excavator and moves it over the ground. This guide names each of the main excavator undercarriage parts, explains what each one does, and shows what to check before you order replacements. It applies to mini excavators and larger tracked machines alike.
Key takeaways
The undercarriage is the running gear of a tracked machine. The final drive turns the sprocket, the sprocket drives the track, and the rollers and idlers guide the track and carry the machine along it. The track adjuster keeps the whole loop at the right tension.
Because the parts run together, they also wear together. A worn part puts extra load on the parts around it. That is why the undercarriage is treated as one system, not a set of separate parts.
| Part | What it does | What to check |
|---|---|---|
| Rubber track or steel track chain | Puts the machine on the ground and transfers drive from the sprocket | Cuts, exposed cords or steel, chain stretch, loss of tension |
| Sprocket | Bolts to the final drive. Its teeth engage the track links and drive the machine | Sharp, worn teeth or a track that skips |
| Idler | Guides the track around the end of the track frame and keeps it at the right tension | Worn tread or flanges, leaking seals |
| Track adjuster | Sits between the frame and the idler and holds the track at the set tension | Track that will not hold tension, grease leaks |
| Track roller (bottom roller) | Carries the weight of the machine on the track | Worn shells or flanges, rollers that leak or do not turn |
| Carrier roller (top roller) | Supports the top run of the track between the sprocket and the idler | Rollers that do not turn, worn shells, leaks |
Range links go to the Excavator Parts Direct catalog.
Excavators run on one of two track types. Rubber tracks are common on mini excavators and compact track loaders. Larger excavators usually run on steel track chains fitted with steel track shoes. Our guide to rubber tracks vs. steel tracks covers how to choose between them.
A rubber track size is written as width x pitch x number of links, for example 400 x 72.5 x 72W. Check the size molded on your old track, or measure it. Our guide to measuring rubber tracks shows each step.
A steel track chain is made of links joined by pins. The sprocket engages the chain to turn drive from the final drive into forward motion. Excavator track shoes usually have three grousers. Greased chains hold grease around the pin inside the bushing, which reduces internal wear. Greased chains typically last 20 to 30% longer than a dry track chain.
Internal pin and bushing wear shows as chain stretch. To check it, measure the pin centers over four or five links and compare the figure with the new chain specification.
Steel-tracked excavators that work on roads, concrete or finished surfaces can be fitted with rubber pads. Clip-on pads fit quickly and come off again. Bolt-on pads are a semi-permanent conversion. Roadliner pads bolt directly to the track chain.
The sprocket bolts to the final drive. As the final drive turns, the sprocket teeth engage the links in the track, rubber or steel, and move the machine. Every time the machine travels, the sprocket teeth take the load.
Three dimensions decide whether a sprocket fits:
Sharp, worn sprocket teeth can let the track slip or jump, which shock-loads the final drive. Sprocket wear also speeds up in reverse and in higher travel gears. On larger machines, the sprocket may be built from bolt-on segments of three or five teeth, which can be replaced without taking the track off. Our guide on how to tell if a sprocket is worn shows what to look for.
The idler sits at the opposite end of the track frame from the sprocket. It guides the track links and, pushed out by the track adjuster, keeps the track at the correct tension.
The track adjuster fits in the frame between a bulkhead and the idler. It has three main parts:
Tensioning piston
A grease-filled ram. Pumping grease in through the nipple extends it and tightens the track. Opening the grease valve releases it.
Spring pack
Absorbs impacts so shocks from the ground do not go straight into the frame.
Yoke
The fork-shaped end that bolts to the idler.
Common track adjuster faults include failed piston seals, which let the track lose tension, and a broken spring or rod. A cracked spring often goes unnoticed until the idler is changed.
Our advice
Check the track adjuster whenever the idlers are replaced. A worn idler can cause a new adjuster to fail early, and the adjuster is much easier to change while the idler is already off.
Track rollers, also called bottom rollers, run along the underside of the track frame and carry the weight of the machine on the track. Carrier rollers, also called top rollers, support the top run of the track. Rollers come in single-flange and double-flange designs. A double flange gives the track links more guidance.
Tip
Change all the bottom rollers on a track frame together. A single new roller sits higher than the worn rollers around it, takes more of the load and wears faster.
Undercarriage parts are specific to the machine. Before you order, gather these details:
Machine details
Make, model and serial number. Some models were built with more than one undercarriage, and the serial number helps confirm which one you have.
Counts
The number of track rollers and carrier rollers per side, and the number of links in the track or chain.
Track or shoe size
The rubber track size, or the steel shoe width in inches (mm) and the number of grousers.
Markings
Any numbers or letters cast or stamped on the links, rollers, idlers and sprockets.
With these details, our team can check the right parts for your machine. Contact us with what you have, including photos if the markings are hard to read.
Most undercarriage wear comes from how and where the machine works. These habits reduce it:
Our advice
When one part is worn out, inspect the rest before fitting the new one. Replacing worn parts together means one job and one round of downtime, and it protects the new parts. The cheapest part is the one that keeps the machine working.
The main parts are the tracks (rubber tracks, or steel track chains with track shoes), sprockets, idlers, track adjusters, track rollers (bottom rollers) and carrier rollers (top rollers). Steel-tracked machines may also use rubber pads on the shoes.
Track rollers, or bottom rollers, run along the underside of the track frame and carry the weight of the machine on the track. Carrier rollers, or top rollers, support the top run of the track.
A track adjuster sits in the frame between a bulkhead and the idler and keeps the track at the correct tension. Grease pumped into its tensioning piston extends it, and a spring pack absorbs impacts.
Check them every time. Sharp, worn sprocket teeth can let a new track slip or jump and shorten its life. If the sprockets are worn, replace them with the tracks.
Yes, it is best practice. A single new roller sits higher than the worn rollers next to it, takes more of the load and wears faster.
The machine make, model and serial number, the number of track and carrier rollers, the number of links, the track size or shoe width, and any markings on the existing parts.
Related guides
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