Hydraulic Excavator Components and Structure Explained: From the Role of Each Section to How to Choose Genuine Parts

When heavy machinery used every day on construction sites develops a fault, it can disrupt the progress of the work.
Problems in the hydraulic system are often difficult to diagnose, and it is not uncommon to be unsure how best to handle repairs or parts replacement.

This article explains the basic structure of a hydraulic excavator and the role of each of its major components.
It also outlines how the hydraulic circuit works, including the hydraulic pump, and explains how to select and order the genuine parts needed for repairs.
Use this guide to deepen your understanding of how the machine works and as a reference for proper maintenance and parts procurement.

The Three Main Sections of a Hydraulic Excavator

A hydraulic excavator can be divided into three main sections.
These sections work together to perform complex tasks such as excavation smoothly and efficiently.

• The upperstructure, which contains the engine and operator’s cab
• The undercarriage, which moves the machine
• The front attachment, which performs digging work

Understanding the role of each section will help you grasp the overall structure of the machine.

The Upperstructure, Which Contains the Engine and Operator’s Cab

The upperstructure is the part of the machine that houses the engine, which generates power, and the operator’s cab.
Located above the undercarriage, it is designed to rotate freely through 360 degrees on a swing bearing.
Inside, it contains components such as the hydraulic pump and the control valve, which regulates the flow of hydraulic fluid.

At the rear, a heavy counterweight is fitted to balance the machine.
This counterweight prevents the machine from tipping forward when it lifts soil or other material.
It is essentially the machine’s operational core.

The Undercarriage, Which Moves the Machine

The undercarriage supports the entire machine and allows it to travel to the required location.
It typically uses crawler tracks, which provide stable movement even on slopes or soft, uneven ground.
This section consists of drive sprockets that transmit power, idlers that maintain track tension, and multiple rollers.

The travel motor rotates in response to hydraulic pressure supplied from the upperstructure, turning the drive sprockets to move the machine forward or backward.
Because this section is in direct contact with harsh ground conditions, mud buildup and component wear are common.
Remove mud regularly and inspect the condition of the parts.

The Front Attachment, Which Performs Digging Work

The front attachment is the section that performs the actual digging and loading work.
It mainly consists of the boom connected to the machine body, the arm attached to the boom, and the bucket at the tip that scoops up soil.
These components are connected by pins like joints, and the hydraulic cylinders mounted at each point extend and retract to create complex movements.
Another key feature is its versatility: by changing the tool at the end, the machine can handle a wide range of tasks, such as breaking concrete or gripping timber.

This is essentially the excavator’s arm, allowing it to adapt flexibly to conditions on site.
Regular greasing is essential to keep it moving smoothly.

How the Hydraulic Circuit and Hydraulic Pump Drive Each Section

The powerful movement of a hydraulic excavator is generated by the hydraulic circuit running throughout the machine.
By using the pressure of flowing hydraulic fluid, it produces enough force to lift heavy soil and material.

When looking at how power is transmitted, it helps to focus on the following three components.

• The hydraulic pump, which converts engine power into hydraulic energy
• The control valve, which controls the direction, flow rate, and other aspects of hydraulic fluid flow
• The hydraulic cylinders and motors, which convert hydraulic energy into mechanical motion

Let’s look at how these components work together to move the machine.

The Hydraulic Pump, Which Converts Engine Power into Hydraulic Energy

The hydraulic pump is the component that converts the engine’s mechanical rotational force into hydraulic energy in the form of pressurized fluid.
Like the heart in the human body, it pumps hydraulic fluid throughout the machine.
Modern hydraulic excavators mainly use variable displacement pumps, which can flexibly adjust the volume of fluid they deliver.

Because they can automatically adjust flow and pressure according to the working load, these pumps allow the machine to operate efficiently while reducing unnecessary fuel consumption.
If the pump develops a problem, the entire machine becomes sluggish, making it one of the most critical components in the excavator.

The Control Valve, Which Controls the Direction and Flow Rate of Hydraulic Fluid

The control valve distributes the high-pressure hydraulic fluid delivered by the hydraulic pump to the appropriate locations.
In response to movements of the control levers in the cab, the internal valves shift to precisely regulate the direction, flow rate, and pressure of the fluid.
Whether the machine raises the boom or swings the upperstructure is determined through this valve.

Multiple valves are integrated into a single block, making it the machine’s “brain” for smooth combined operations.
If oil leakage or internal wear occurs, the machine may no longer respond as commanded, which is why this is such a critical component for accurate operation.

Hydraulic Cylinders and Motors, Which Convert Hydraulic Energy into Mechanical Motion

Hydraulic cylinders and motors are the components that convert the hydraulic pressure delivered from the control valve back into mechanical motion.
A hydraulic cylinder has a cylindrical structure, and when hydraulic fluid enters it, the rod is pushed or pulled in a straight line, powerfully moving the boom and arm.

The travel motor and swing motor, by contrast, use hydraulic pressure to drive pistons and turn that force into rotational motion.
By transmitting the energy they receive efficiently, these components make dynamic movements such as digging into hard ground and traveling steadily possible.
As part of daily inspection, check for oil leaks and address any abnormality as early as possible.

How to Select and Order Genuine Parts for Repair or Replacement

As operating hours accumulate, parts eventually need to be replaced because of seal deterioration and motor wear.
Choosing parts that are not the correct fit can lead to secondary failures, so it is important to follow the proper procurement procedure.

The following four steps will help you arrange the right parts.

• Understand the risks of using low-cost third-party, counterfeit, or imitation parts
• Verify the part using the nameplate serial number rather than the machine model
• Use the manufacturer’s official parts search app or tool
• Identify the required part number and place the order

Let’s go through the process step by step to prevent trouble and restore the machine safely.

Understand the Risks of Using Low-Cost Third-Party, Counterfeit, or Imitation Parts

When replacing parts, low-cost third-party, counterfeit, or imitation products may seem attractive as a way to reduce costs.
However, the durability of such non-genuine parts can vary widely, and metal particles caused by abnormal wear may circulate through the hydraulic circuit and damage valves and other components in a chain reaction.

This can ultimately lead to enormous repair costs and extended downtime.
From the standpoint of long-term safety and overall cost, it is best to use genuine manufacturer parts whose quality and performance are assured.

Verify the Part Using the Nameplate Serial Number Rather Than the Machine Model

When ordering parts, do not rely only on the model number of the hydraulic excavator itself.
Even within the same machine model, specifications may differ depending on the production period, and used machines are often fitted with replacement or modified parts from the past, making the machine information alone unreliable.
To prevent ordering the wrong part, it is essential to check the nameplate attached to the actual component that needs to be replaced.

The nameplate is stamped with the model and serial number specific to that part.
If mud or oil makes the characters difficult to read, wipe the surface clean with a cloth and make sure you read the letters and numbers accurately.

Use the Manufacturer’s Official Parts Search App or Tool

Once you have confirmed the serial number on the part’s nameplate, use the official search tool provided by the parts manufacturer to narrow down the information.
Today, parts lookup systems that can be accessed from a smartphone or computer are widely available.
For example, if the machine uses hydraulic equipment from Kawasaki Heavy Industries, the dedicated Kawasaki Hydraulic Finder is a useful option.

This tool can verify the authenticity of the company’s hydraulic pumps and swing motors used by many construction equipment manufacturers in Japan and abroad.
By simply entering the serial number from the nameplate, you can confirm whether the unit is genuine and quickly view detailed information.

Identify the Required Part Number and Place the Order

Based on the results displayed in the search tool, identify the correct part information for the component that needs to be replaced.
Because the screen shows the exact model code (TYPE) and OEM part number (USE FOR), it helps prevent mistakes such as ordering a different part that looks similar.
Once the information has been identified, provide it to an authorized dealer, confirm the quotation and lead time, and proceed with the order.

The app also includes a function that lets you scan the serial number directly with your camera, saving you the trouble of emailing nameplate photos back and forth and waiting for confirmation.
When arranging parts, it is best to use a tool that allows reliable identification.

Summary: Understand the Structure of a Hydraulic Excavator and Carry Out Proper Maintenance

When replacing components in a hydraulic excavator, it is essential to confirm whether the parts are genuine.

“Kawasaki Hydraulic Finder” is a tool that can identify genuine pumps and motors and provide accurate information from the serial number on the nameplate.
Based on that accurate model information, it also makes it easy to contact an authorized service center with proven testing facilities and repair expertise.
Be sure to make use of it when arranging parts or repairs.

Reviewed by

Ryoji Irie

Manager
After-Sales Promotion Office, Marketing & Sales Group, Precision Machinery & Robot Company, Kawasaki Heavy Industries, Ltd.

《Career History》

After joining Kawasaki Heavy Industries, I spent ten years developing the new K7V pump series as a design engineer, followed by five years at KPM (UK) as an application engineer supporting European OEMs and distributors. I am currently responsible for promoting the after-sales business globally.