What are the main parts of an excavator undercarriage?

The industry groups them as four wheels and one chain: drive sprocket, front idler, track rollers (lower), carrier rollers (upper), and the track chain with shoes.

Which undercarriage part wears fastest?

It depends on application. In abrasive ground the chain and shoes wear fastest; in wet conditions seals fail first. Track rollers and idlers usually fail through seal and bearing failure rather than tread wear.

How do I know which undercarriage component is at fault?

Check track tension and alignment first — a failing idler or roller is often a symptom of a misaligned frame rather than a defective part. Replacing the part without fixing the cause repeats the failure.

Are track rollers and carrier rollers interchangeable?

No. A track roller carries the machine's weight along the bottom run; a carrier roller supports the returning chain along the top. They are different parts with different load roles and different mounting, even where their dimensions look similar on a tape measure.

Undercarriage & wear parts

Excavator undercarriage parts diagram: every component, named

The undercarriage is the lowest part of the machine and the highest part of the maintenance cost. This is every component, what it does, and what to measure on each one.

Diagram of excavator undercarriage showing sprocket, front idler, track rollers, carrier rollers and track chain — Four wheels and one chain: component layout

The four running-gear groups

The undercarriage is conventionally divided into four groups — this is the "four wheels and one chain" (四轮一带) grouping used across the industry:

Drive sprocket — at the rear, driven by the final drive; pulls the chain
Front idler — at the front; guides the chain and holds tension
Track rollers (lower rollers) — along the bottom; carry the machine's weight on the chain
Carrier rollers (upper rollers) — along the top; support the returning chain
Track chain and shoes — the chain itself, with shoes bolted to the links

Component by component: what each does and what to measure

Drive sprocket

Function: engages the chain and drives it.
Measure: tooth count, chain pitch, tooth section thickness, bolt pattern (segmented or one-piece).
Wear sign: teeth hook over. Once hooked, it destroys the chain — replace with or before the chain, never after.

Front idler

Function: guides the chain at the front and sets track tension.
Measure: rim diameter, shaft diameter, mounting flange spacing, bolt pattern.
Wear sign: tread wear is measurable; bearing and seal failure is not — look for grease weeping at the hub or a wobble.

Track rollers

Also called: bottom rollers, lower rollers.
Function: carry the machine weight and roll on the chain bushings.
Measure: shaft diameter, outer diameter, overall width, flange spacing, press-fit or bolt-on housing.
Wear sign: a leaking roller is more urgent than a worn one — the seal has failed and grit is entering the bearing.

Carrier rollers

Also called: top rollers, upper rollers, support rollers.
Function: support the chain on its return run; fewer and smaller than track rollers.
Measure: same dimensions as track rollers, but do not assume the two are interchangeable — they are different parts.

Track chain and links

Function: the continuous track; transmits drive to the ground.
Measure: link count, chain pitch, pin and bushing wear.
Wear sign: pitch increases as pins and bushings wear — a stretched chain cannot be tensioned within range.

Track shoes (pads)

Function: the ground contact surface.
Measure: width, thickness, bolt hole count and spacing, grouser height.
Wear sign: grouser height below limit reduces traction before it becomes a fitment problem.

Two things get confused here. Grouser count is one dimension: excavators typically run triple grouser shoes, dozers run single grouser. Material is a separate dimension: on finished surfaces the alternatives are rubber pads bolted over the steel shoes, or rubber tracks — a complete rubber belt replacing the steel undercarriage. Those are two different products, not two names for one.

Neither dimension changes the bolt pattern, but both change what you order. Specify grouser type, material, and bolt pattern separately.

Why this grouping matters for ordering

Each group wears at a different rate and for different reasons. Replacing components without understanding which group is failing — and why — is how the same part gets replaced twice. For example, a failing idler may be a symptom of a misaligned track frame, not a bad idler.

Diagnosing which component is at fault

Our guides cover each in detail: track roller wear limits, idler wheel diagnosis, sprocket replacement, track tension.

Confirming a part

Send machine make, model, full serial number, a photo of the casting mark, and the relevant dimensions. We confirm the part number before quoting: alfrun@gmail.com

Before you order

Send machine make, model, serial number and a photo of the casting mark. We confirm the part number and fitment basis before any price discussion.

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Confirming the part before you order

Measurements get you close. A serial number and a photograph of the casting mark confirm it. Here is exactly what we need, and why each item matters:

What we need to confirm a part

Trademark notice

Machine brand names and part numbers are used only to describe fitment and application. We are not an authorised dealer of any machine manufacturer.

Reviewed by Zhonglei M&E parts desk — export trading partner for undercarriage and wear parts. Last updated 2026-11-13.