Every tracked excavator runs on the same system of parts, and the same terms cause the same confusion: final drive or travel motor, idler or roller, chain or track group. This guide names each component, explains what it does and shows how the parts wear together.
The undercarriage works as one system. The final drive turns the sprocket, the sprocket drives the chain, the idler and rollers guide it, the adjuster keeps it tensioned and the shoes carry it all over the ground. The parts wear together, which is why they are inspected and often replaced together.
The parts at a glance
Track chain
Links joined together by pins. The sprocket engages the links to transfer the forward motion generated by the final drive.
Sprocket
Bolts to the final drive. Its teeth engage the links in the track, rubber or steel, to give the machine motion.
Idler
Guides the link and holds the track at the correct tension, working with the track adjuster behind it.
Rollers
Bottom rollers run on the chain rail beneath the machine. Carrier rollers support the chain on its return run over the top of the frame.
Track adjuster
Sits in the frame between a bulkhead and the idler. Its function is to keep the track correctly tensioned.
Final drive
The hydraulic motor and gearbox that turns the sprocket. A final drive and a travel motor are the same thing.
The drive side: final drive, sprocket and chain
The final drive is where motion starts. It is also where the naming confusion starts, because final drive and travel motor are two names for the same component, and part searches often miss results by using only one of them. If yours is showing symptoms, start with our guide to the signs of failure in a final drive.
The sprocket bolts to the final drive, and its teeth engage the links in the track as the final drive turns. Three things identify a sprocket: the inner diameter, which must match the final drive; the number of teeth, which cannot be changed; and the pitch, which must match the track. Our sprockets are manufactured from cast steel with the tooth rim induction hardened to more than 50RC, and we offer them for machines from 1 to 50 tonnes. On larger machines, sprocket segments come in sections of three or five teeth and can be replaced individually without taking the track off.
The track chain is made up of links joined together by pins, and it is what the sprocket pushes against. Each link has a wide side and a narrow side: the wide side faces the sprocket, the narrow side faces the idler. We supply chains for machines from 3 tonnes at 101.6mm pitch up to 50 tonnes at 228mm pitch, with pitches from 101mm to 260mm across the range.
DuraTrack excavator chain rails are heat treated boron steel hardened to 48-56RC up to 10mm deep, with all pins and bushes hardened to 55-60RC, backed by an 18-month warranty.
Chains up to and including 135mm pitch are dry, because no manufacturer offers a greased version at those pitches. From 140mm pitch upwards all our chains are greased, and greased chains typically last 20-30% longer than a dry track chain.
The guidance side: idler, rollers, adjuster and guides
The idler guides the link and keeps the track at the correct tension. Our cast idler assemblies come complete with heavy-duty seal groups, shafts and brackets, so the assembly is fitted as one unit.
Rollers come in two families. Bottom rollers run on the chain rail under the machine. Carrier rollers support the chain over the top of the frame and come in single flange, double flange and shaft types, with double flange rollers increasing guidance of the link. One rule matters more than any other: change bottom rollers as a set, because a single new roller sits proud of its worn neighbours and increases the wear rate.
The track adjuster, also called the recoil spring assembly, fits in the undercarriage frame between a bulkhead and the idler. It has three integral parts: a tensioning piston, which is a grease-filled ram, a spring pack that absorbs impact, and a fork-shaped yoke that bolts to the idler side-arms or blocks. Tension is set by pumping grease in through a nipple and released by carefully unscrewing the grease valve.
Track guides and guards keep the chain running in line, and they help prevent chains jumping on the sprockets when other components start to wear.
Track adjusters carry a 12-month warranty when purchased and fitted along with a new idler, and 6 months without, because a worn idler can cause a new track adjuster to fail prematurely. It is also far easier to replace the adjusters while the idlers are off.
The ground side: shoes, grousers and the rubber alternatives
The track shoe is the plate that meets the ground, and the grouser is the raised bar on the shoe that grips it. Excavators run triple grouser shoes, dozers run single grouser shoes. We stock triple grouser excavator shoes from 300mm to 1200mm wide for all standard and LGP (low ground pressure) applications.
Always use the narrowest shoe possible with good flotation. Over-wide shoes increase bushing and sprocket wear, increase link, roller and idler wear, loosen pins, bushings and shoe hardware, and can break shoes.
On finished surfaces, the alternatives are rubber tracks, stocked from 180mm to 800mm wide, or rubber pads fitted over the steel undercarriage. We cover that decision in detail in our guide to the advantages of rubber pads on an excavator.
Hardware, and why the grade matters
Undercarriage hardware is graded for its job. Track bolts, segment bolts and split master bolts are 12.9 grade. Sprocket bolts and roller bolts are 10.9 grade. All are forged from alloy steels and heat treated to OEM specification, in metric and imperial sizes, with a bespoke manufacturing service for anything non-standard.
How the parts wear as a system
No undercarriage part wears alone. Digging over the sprockets instead of the idlers increases bush wear and can crack pins and bushes. Material packed between rollers, links and sprocket teeth stops components engaging correctly and accelerates wear on all of them. Chains jumping on sprockets points to worn sprockets, a broken track adjuster spring, offset sprockets fitted the wrong way round, the wrong pitch or worn track rollers.
That is why wear is measured rather than judged by eye. Our free track inspection covers excavators, crushers, dozers and screeners, measuring chain stretch, rail height, bushing, top and bottom rollers, idler and grouser height as percentages against new dimensions, with a visual check of the sprockets, and ends in a detailed wear report with corrective actions.
Frequently asked questions
What are the main parts of an excavator undercarriage?
The track chain, sprockets, idlers, bottom and carrier rollers, track adjusters, track guides, track shoes and the final drives. On machines working over finished surfaces, rubber tracks or rubber pads replace or cover the steel track shoes.
Is a final drive the same as a travel motor?
Yes. Final drive and travel motor are two names for the same component, the unit that turns the sprocket to move the machine. Searching under only one name is a common reason the right part does not come up.
How do I identify which undercarriage parts fit my machine?
We work from the machine manufacturer, model and serial number, the number of links in the track chains, the number of grousers on the shoe and the shoe width in mm, plus any markings on the side of the links, rollers, idlers and sprockets. Send those over and we will confirm the parts.
How do I know which sprocket fits?
Three dimensions decide it: the inner diameter must match the final drive, the number of teeth cannot be changed, and the pitch must match the track. As an example, a 101mm pitch sprocket fits 101mm pitch tracks or 52.5mm pitch tracks.
When should undercarriage parts be replaced?
When measurement says so. Undercarriage wear is assessed as a percentage against new dimensions, covering chain stretch, rail height, bushing, rollers, idler and grouser height. Our free track inspection produces a wear report with corrective actions, so replacement is planned rather than forced by a failure.
Book a free undercarriage inspection
We will measure every wear component against new dimensions and give you a detailed report with corrective actions, so replacement can be planned into a service window rather than forced by a failure.
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.
Need help finding the right part?
Our team is here to help with expert advice and support for your order.