Undercarriage is the single largest maintenance cost on a tracked machine. On an excavator or dozer it can account for half of everything you will spend keeping that machine running over its life. It is also the part of the machine most owners understand the least, because it lives underneath, gets covered in mud, and only gets attention when something breaks.
This guide walks through every major undercarriage component, what it does, how it wears, and the signs that tell you it is time to replace it. It applies to excavators, dozers, track loaders, compact track loaders, and mini excavators. Where the machines differ, we say so.
What Is an Undercarriage?
The undercarriage is everything below the machine that lets it move. It carries the weight of the machine, converts engine power into forward motion, and absorbs the punishment of rock, mud, and jobsite debris. On steel-track machines it is a set of components bolted and pinned together. On compact machines it is often a rubber track running over a simpler roller frame.
The parts wear together as a system. That matters, and we will come back to it at the end. Unfamiliar with a term? Our equipment parts glossary covers all of them.
The Track Chain
The track chain is the backbone of the undercarriage. It is a loop of steel links joined by pins and bushings, and the track pads bolt to it. The sprocket drives the chain by engaging the bushings, and the chain carries the machine over the rollers and idler.
How it wears
Every time the sprocket grabs a bushing, a small amount of metal comes off. Over thousands of hours the bushings wear thin and the pins wear inside them. The chain physically gets longer, which is called pitch stretch. A stretched chain no longer matches the sprocket teeth, and that mismatch accelerates wear on everything else.
When to replace
Measure the pitch across several links with the track tensioned. Compare to the original specification. When bushing wear reaches roughly 100 percent of its allowable limit, the chain is done. If you catch it earlier, some chains can have the pins and bushings turned to expose a fresh wear surface, which can extend life at a lower cost. We can help you figure out which case you are in.
Track Rollers
There are two kinds. Bottom rollers sit under the track frame and carry the full weight of the machine as it rolls along the chain. Top rollers, also called carrier rollers, support the chain on its return trip over the top of the frame so it does not sag.
How it wears
Rollers wear on their tread, the surface that rides on the chain rails. Bottom rollers take the most load and wear the fastest, especially the ones nearest the idler and sprocket. Seals can also fail, letting the oil out and dirt in, and a dry roller wears quickly.
When to replace
Measure the tread diameter and compare to spec. Look for flat spots, cracking, or oil weeping from the seals. If one roller has failed and the others are near the end of their life, replace the set. Mixing one new roller into a worn set puts the new one under uneven load.
Idlers
The idler is the large wheel at the front of the track frame, and on some dozers there is one at the rear as well. It guides the chain into the rollers and, together with the recoil spring, keeps the track at the correct tension.
How it wears
The idler wears on its tread and on the center flange that keeps the chain aligned. Uneven wear between the two sides usually means an alignment problem elsewhere.
When to replace
Measure the tread height from the flange. When it wears down to spec, the chain starts riding on the flange and the idler will start damaging the links.
Sprockets
The sprocket is the toothed wheel at the drive end. It is what actually pushes the machine forward by engaging the bushings on the chain. On most excavators it is a single rim. On dozers it is usually a set of bolt-on segments, which is cheaper to replace than the whole hub.
How it wears
Teeth wear into a hooked or pointed shape. A stretched chain makes this much worse because the bushings land in the wrong spot on every tooth.
When to replace
Look at the tooth profile. Sharp, hooked, or noticeably thinned teeth are past due. A rule worth remembering: always replace the sprocket when you replace the chain. A worn sprocket will chew through a new chain in a fraction of its normal life.
Track Pads and Grousers
Track pads, also called track shoes, bolt to the chain and are what actually touches the ground. The raised bar on each pad is the grouser.
Single grouser pads have one tall bar and are used on dozers for traction and pushing power. Double and triple grouser pads have shorter bars and are standard on excavators, where the machine needs to turn and travel without tearing up the ground. Rubber and poly pads bolt over or replace steel pads for work on asphalt, concrete, and finished surfaces.
How it wears
Grousers wear down in height. Pads can also crack or bend, and the bolt holes can elongate if bolts have been running loose.
When to replace
When grouser height drops to spec, traction drops with it. Cracked or bent pads should come off before they damage the chain.
Rubber Tracks
Compact track loaders and most mini excavators run rubber tracks instead of steel chains. A rubber track is a continuous loop with steel cables and embedded metal cores inside the rubber. It runs over rollers and an idler much like a steel chain, and a sprocket drives it through the cores.
How it wears
Tread lugs wear down, the rubber cracks with age and sun exposure, and the internal cables can break if the track is run too loose or hits something hard. Damaged cores or exposed cable mean the track is at the end of its life.
When to replace
Lugs worn smooth, deep cracks that expose cable or cores, or a track that keeps jumping off even at correct tension.
The One Rule That Saves the Most Money
Undercarriage parts wear together, and they are designed to be matched. Putting a new chain on a worn sprocket, or one new roller in a set of worn ones, forces the new part to absorb all of the mismatch. It will wear out early and you will have paid for it twice.
Before replacing any single component, measure the rest. If the chain is at 80 percent and the sprocket is at 90 percent, do the whole undercarriage at once. It costs more today and far less over the life of the machine.
A Ten-Minute Undercarriage Check
You do not need special tools for a first look. Walk the machine and check:
- Track tension. Too tight wears everything faster. Too loose lets the chain slap and jump.
- Sprocket teeth. Hooked or pointed means replacement time.
- Roller and idler treads. Look for flat spots, cracks, and oil leaks at the seals.
- Chain rails. Look for scalloping or a lip along the edge, which means the rollers are cutting into them.
- Pads. Grouser height, cracks, and loose bolts.
- Rubber tracks. Lug depth, cracking, and any exposed cable or core.
If anything looks marginal, take measurements and send them to us with the machine model and serial number. We will tell you what is worth replacing now and what can wait.
Undercarriage Questions We Get Most
How long does an undercarriage last?
Typically 3,000 to 5,000 hours on an excavator, depending on ground conditions, operator habits, and maintenance. Rock, sand, and side-hill work shorten it. Correct track tension and matched components extend it.
What is the most expensive undercarriage part to replace?
The track chain, usually followed by the complete set of bottom rollers. Together they account for most of a full rebuild. That is why catching wear early on the cheaper parts, like the sprocket, protects the expensive ones.
Can you replace just one roller?
You can, but a new roller next to worn ones takes uneven load and wears out early. If the rest of the set is past halfway, replace the set.
Should I replace the sprocket with the chain?
Yes, every time. A worn sprocket will destroy a new chain in a fraction of its normal life.
How do I know if my track is too tight or too loose?
Measure the sag on the top run between the carrier rollers with the machine on flat ground. Your machine manual gives the spec, usually one to two inches on excavators. Outside that range, adjust the track adjuster.
Need Undercarriage Parts?
Industrial Tractor Parts has stocked and built undercarriage since 1956. We carry track chains, rollers, idlers, sprockets, pads, rubber tracks, and hardware for every major manufacturer, and every one of our nine branches has an in-house track press to shorten chains, swap pads, and build complete track groups ready to install.
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Industrial Tractor Parts is an independent aftermarket parts distributor. Caterpillar, Komatsu, John Deere, Case, Kubota, Bobcat, and other manufacturer names are trademarks of their respective owners and are used for identification purposes only. ITP is not affiliated with or endorsed by these manufacturers.
