Machinery Insights: 16 Excavator Parts You Need to Know

Knowing your excavator's parts is essential, not just for operation but for long-term maintenance and troubleshooting. As an operator, you might not need the detailed knowledge of a mechanic, but a basic understanding of the machine's components is beneficial. Excavators, though varied in makes and models, share many common parts. These can be broadly categorized into two main sections: the upper structure and the lower structure. In this article, we'll break down these essential components, providing you with the insights needed to navigate and maintain your excavator effectively.

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Lower Structure of a Track Excavator

The lower structure, or the undercarriage, is the foundational section of an excavator. It's the part that makes contact with the ground and enables movement. We'll focus on track excavators, also known as crawler excavators, and explore the five key elements that make up the undercarriage: tracks and track pads, rollers, idlers, sprockets, and the swing drive/slewing ring.

A view of the Kobelco SK220 crawler excavator's lower structure in action on a job site

1. Tracks and Track Pads

The tracks are the backbone of a crawler excavator's mobility. Made of reinforced steel, they consist of track pads or shoes, which vary in size. The outer edges, known as grousers, help the excavator grip the ground during movement. Track bolts link these plates together, forming the entire crawler system. This system is anchored to the excavator through a track frame, similar to an axle. Inside the crawlers, a track chain connects them to the sprocket and idler, ensuring consistent movement.

2. Rollers

Rollers are metal wheels located on the undercarriage, responsible for guiding the chains between the sprocket and idler. There are two types: bottom and top rollers, each playing a crucial role in chain alignment. Some rollers are equipped with rock guards to protect against debris and rocks.

3. Idlers

Idlers are larger than rollers and serve as major guiding wheels within the undercarriage. They work in tandem with rollers and sprockets to maintain the smooth operation of the tracks. Their size and placement are key to the efficient movement of the excavator.

4. Sprockets

Sprockets, akin to giant bicycle sprockets, engage with the excavator track chain links. Their primary function is to move the chain, propelling the machine forward or backward.

5. Swing Drive/Slewing Ring

The swing drive and slewing ring are crucial for the excavator's rotational movement. They enable the upper structure to pivot relative to the lower structure. These components are hydraulically operated and require regular greasing for smooth operation.

Upper Structure of a Track Excavator

The upper structure, or the "house" of the excavator, combines the carbody with the boom-and-arm mechanism, forming the core of the excavator's functionality. We will explore 11 key components that make up this upper structure, moving from rear to front.

A view of the Kobelco SK220 track excavator's upper structure in action on a job site

1. Counterweight

The counterweight is vital for maintaining balance during operation. It prevents the excavator from tipping over when digging. The weight of the counterweight is proportional to the excavator's operating weight. For instance, a CAT 395 excavator with a 94-ton operating weight has a 15.5-ton counterweight. Similarly, a Komatsu PC400 excavator, weighing 43.3 tons, includes a 9.2-ton counterweight.

2. Engine Compartment

More than just housing the engine, this compartment is a hub for several crucial components:

• The fuel tank stores the diesel fuel.

• The engine, typically near the cab for easy maintenance access, is the excavator's powerhouse.

• The hydraulic fluid or oil tank, essential for the hydraulic mechanisms.

• The main control valve, connecting the engine to hydraulic devices, controlling oil pressure.

3. Cabin

The cabin is the control center of the excavator, equipped with essential controls and safety features. Depending on the model, cabs can rotate 360 degrees for maximum visibility. The cabin includes:

• Main control mechanisms vary by model.

• ROPS (Rollover Protective Structure) for safety in rollovers.

• FOPS (Falling Object Protective Structure), a reinforced steel structure protecting against falling debris.

4. Boom Cylinder

This component connects the boom to the carbody and controls the boom's vertical movement.

5. Boom

The boom is the forward extension attached to the cab that supports the arm. It's a pivotal part of the digging mechanism.

6. Arm Cylinder

Linking the arm to the boom, the arm cylinder facilitates the arm's extension and retraction.

7. Arm (Dipper or Stick)

The arm, or dipper, connects to the boom and carries the bucket. It's a key component in positioning the bucket for excavation.

8. Boom/Arm Linkage

This linkage, consisting of pins and bushings, connects the boom and arm. Regular greasing is essential for smooth and squeak-free operation.

9. Bucket Cylinder

Connecting the arm to the bucket, this cylinder allows the bucket to perform digging and dumping actions.

10. Arm/Bucket Linkage

This is the joint mechanism that connects the arm and bucket.

11. Bucket

The bucket is the primary earthmoving component at the end of the arm. Standard excavators come with buckets, but even with different attachments, the basic mechanism remains consistent.

To further enhance your understanding of an excavator's anatomy, we've created a detailed infographic. This visual guide neatly outlines all the components we've discussed, both in the lower and upper structures of a medium KOBELCO SK350 track excavator.

Excavator components: lower and upper structures of a medium KOBELCO SK350 track excavator

FAQs About Excavator Parts

What is the most commonly replaced part of an excavator?

The most frequently replaced parts are typically the tracks and track pads, due to their constant contact with the ground and the wear and tear from movement.

How often should the hydraulic oil be changed in an excavator?

Hydraulic oil should generally be changed every operating hours, but always refer to the manufacturer's guidelines for the specific model you're using.

Can the bucket of an excavator be replaced with other attachments?

Yes, excavators are designed to accommodate various attachments besides buckets, such as breakers, grapples, and augers, depending on the job requirement.

What's the purpose of the counterweight in an excavator?

The counterweight is crucial for maintaining balance. It counterbalances the weight during digging and prevents the excavator from tipping over.

Whether you're a passionate excavator enthusiast, you're an excavator operator looking for more knowledge on your machine, or you're just about to roll up your sleeves and take on an excavator repair, we've created this simple interactive excavator diagram to help you better understand this important construction machine.

Excavator Parts Diagram

Click a part or a label to learn more about the part, its function on an excavator, or to see and search that specific part at H&R.

 

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Cab

The excavator's cab is the central control station for the excavator's operator. In the cab, an operator will have controls for both the movement of the excavator and the manipulation of the arm and attachments of the excavator. Pedals, levers, and joysticks are used in (often complex) combinations to control the excavator. In modern excavators, control and measurement will be augmented with touchscreens, displays, and other forms of feedback and input in the cab. 

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Boom

The boom is a part of the arm and is directly attached to the excavator's house. The length of the boom and the stick define the reach and digging depth of an excavator. Knuckled booms offer side-to-side movement of the arm, hinged booms are divided booms that allow for a new point of articulation, and long reach booms create the ability to extend the area with which an excavator can interact without moving the machine.

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Boom Cylinder

The boom is controlled by hydraulic cylinders pinned to the house and the boom. By extending and contracting the cylinder rods from the cylinder housing the operator is able to lift and lower the boom.

Learn More About Cylinders 

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Stick

A part of the excavator's arm, the stick sits between the boom and the attachments. The far end of the stick incorporates a system for connecting to attachments and allowing for their control from the cab. An excavator could employ a longer stick for added reach or a shorter stick for increased digging power.

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Stick Cylinder

The excavator's stick is controlled by a hydraulic cylinder connected to the boom and the stick. Extending and contracting the stick cylinder will pivot the attachment towards or away from the machine and the operator.

 Learn More About Cylinders 

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Bucket / Attachments

Buckets are the most common type of attachments on an excavator, but excavators today utilize a wide range of attachments for digging, moving items, or interacting with materials in numerous ways.

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Bucket Cylinder

The bucket cylinder controls the action of the bucket and is connected to the stick and the bucket. By extending or retracting the cylinder, the operator can control the attachment. In the case of a bucket, extending the cylinder will initiate a scooping movement and retracting the cylinder will dump the bucket. 

Learn More About Cylinders 

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Linkage

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Linkage (or simply links) are heavy-duty, metal bars that connect attachments (and other parts) to the stick through the use of pins that allow for movement in specific directions. 

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Engine

At its most simple, the engine on an excavator converts energy (generally from diesel fuel) into motion. The engine powers all parts of the machine through the direct output of energy to moving parts, energy conversion to electric parts, or often to output through the hydraulic system.

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Main Hydraulic Pump

The main hydraulic pump on an excavator transforms mechanical energy from the engine into hydraulic energy that is in turn supplied to hydraulic components through hydraulic fluid. Cylinders, travel motors, swing motors, and other hydraulically-powered components all derive their power from hydraulic pumps on the excavator.

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Main Control Valve

By controlling the pressure and direction of hydraulic fluid a hydraulic valve is able to finely dictate a machine's movements and the movements of its attachments. Due to the complexity of a hydraulic system on an excavator, the main control valve is often a complex labyrinth of chambers and connections for the movement of hydraulic fluid and the exchange of energy.

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Slewing Ring

The slewing ring (or slewing ring bearing, turntable bearing, slewing gear) is a horizontally mounted bearing with an attached gear. The bearing connects the upper house with the lower undercarriage and allows each to move separately and, in turn, for the house to spin freely of the undercarriage.

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Swing Drive

The swing drive is directly mounted to the house and consists of a swing motor and a swing gearbox. Energy is converted into high torque output through a gear system and output through a geared shaft that engages with teeth in the slewing ring. Turning of the swing drive shaft results in movement along the path defined by the slewing ring.

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Travel Motor

Located in the undercarriage at the back end of the excavator's tracks, the hydraulic travel motor is paired with a final drive gear box and powers the movement of the tracks.

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Final Drive

On an excavator, a final drive transfers power from the drive train to the tracks using a system of gears designed to reduce speed and increase torque. Most commonly, a final drive is divided into a hydraulic travel motor and a gearbox with gears designed to increase torque as it is applied to the tracks.

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Sprocket

The sprocket is a cogwheel with teeth that connects into and interacts with segments in the excavator's track. As the sprocket rotates it advances the track and propels the excavator forwards or backwards by pulling the track in a direction.

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Front Idler

The front idler on an excavator is a large wheel at the front of the track group that guides the track along its path and, in conjunction with the track springs, sets the tension of the track.

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Track Springs

Track springs (or recoil springs) are large springs designed to absorb impacts and ensure the track is properly tensioned. Most track springs work in conjunction with a grease cylinder to set track tension.

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Bottom Rollers

An excavator has a number of short, rolling cylinders designed to carry the weight of the excavator and to guide the track along its path. These rollers are located inside the track and at the bottom of the undercarriage.

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Track Groups

Excavators are commonly used in a wide-range of ground conditions and must be able to both move in low-traction locations and remain firm and balanced while stationary. The tracks on an excavator are designed to provide ample grip and maneuverability. Tracks can be either metal, rubber, or hybrid. The roughness and pattern of tracks is designed to match the work surface where the excavator will perform its work. While tracks are still most common, increasingly some excavators employ wheels to allow for faster transport and less wear on delicate surfaces like city streets.

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We hope you found this diagram and these simple, short definitions helpful in your excavator knowledge search. As always, if you're searching for a part solution for your own excavator, our expert Parts Specialists are here to help connect you to a part in our deep and ever-expanding inventory or even to search our vast parts network. Just drop us a line or request a quote.

 

Excavator Parts Diagram Image

An excavator diagram shows the location of some of the important parts of an excavator.

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