Excavators work by using a diesel engine to power hydraulic pumps, which push pressurized oil through a network of hoses and valves to move heavy steel components with precision. Every motion you see on a job site, from the boom rising to the bucket curling into the earth, is driven by this hydraulic system. Understanding the basic loop of engine, pump, valve, and cylinder explains how a single operator sitting in a cab can move tons of dirt in minutes.
The Hydraulic System: Engine to Movement
At the core of every excavator is a closed-loop hydraulic system. The diesel engine spins one or more hydraulic pumps, which draw oil from a reservoir and push it through steel lines and high-pressure hoses. A key detail: the pump doesn’t create pressure on its own. It generates flow. Pressure only builds when that flowing oil meets resistance, like a piston inside a cylinder that’s trying to force a bucket into hard-packed soil. This is why an excavator can handle light grading gently and then ramp up enormous force when it hits rock or concrete, all without the operator changing a setting.
From the pump, oil travels to the control valves, which act as the system’s brain. When the operator moves a joystick, the control valves direct the right volume of high-pressure oil to the right cylinder at the right moment. These valves regulate speed, direction, and force independently for each moving part. That’s what allows an experienced operator to smoothly coordinate several motions at once, swinging the cab while simultaneously raising the boom and curling the bucket.
The hydraulic cylinders themselves are the muscles. When pressurized oil enters one side of a cylinder, it pushes a piston forward, extending the cylinder rod. Send oil to the opposite side and the piston reverses. Each major joint on the excavator’s arm has its own cylinder: one for the boom (the large lower arm), one for the stick or dipper (the upper arm segment), and one for the bucket. The size of these cylinders determines how much force the excavator can generate. On a mid-size machine, the boom cylinder alone can produce tens of thousands of pounds of force.
How the Operator Controls It
An excavator operator sits in the cab with two joysticks and a pair of foot pedals or levers. Each joystick moves in four directions, and each direction controls a different function. The left joystick swings the upper structure (called the house or turret) left and right and moves the stick forward or backward. The right joystick raises and lowers the boom and curls or opens the bucket. This layout is known as the ISO control pattern and is the most common worldwide.
Some machines use an older layout called the SAE pattern, which swaps which joystick controls the boom and which controls the stick. The swing and bucket curl stay on the same joysticks in both patterns. Most modern excavators let the operator switch between the two in the settings menu, which matters because muscle memory is everything when you’re coordinating four simultaneous motions. The foot pedals typically control the tracks, with each pedal driving one side independently, allowing the machine to turn, spin in place, or drive straight.
How the Cab Rotates 360 Degrees
One of the most distinctive features of an excavator is that the entire upper structure can spin continuously in either direction. This is made possible by a large ring-shaped bearing called a slew ring, mounted horizontally between the upper house and the undercarriage. The slew ring does triple duty: it supports the full weight of the cab and arm assembly, handles the sideways forces generated during digging and swinging, and transfers rotational torque.
The rotation itself works through a gear-and-pinion system. When the operator pushes the left joystick to the side, hydraulic fluid drives a swing motor mounted on the upper structure. That motor spins a small pinion gear, which meshes with gear teeth machined into the slew ring. As the pinion turns, it drives the entire house around. The gear ratio is heavily reduced, so the swing motor can spin fast while the house rotates at a controlled, manageable speed. This lets the operator swing a loaded bucket smoothly from the dig face to a waiting dump truck without jerking or overshooting.
Undercarriage and Travel
Below the slew ring sits the undercarriage: a steel frame with two track assemblies. Each track is driven by its own hydraulic motor, typically mounted at the rear of the track frame. The motor turns a drive sprocket, which engages the track chain and pulls it around the idler wheels and rollers. Because each track has an independent motor, the operator can run them at different speeds or in opposite directions. Running both tracks forward at the same speed drives the machine straight. Running one faster than the other creates a gradual turn. Running them in opposite directions spins the excavator on the spot, which is essential for working in tight spaces.
The tracks spread the machine’s weight over a large surface area, which is why a 30,000-pound excavator can work on soft ground that would swallow a wheeled vehicle. Steel tracks with grouser bars provide grip on rough terrain, while rubber tracks (common on smaller machines) reduce damage to paved surfaces.
The Digging Cycle
Watching an excavator dig a trench, you’ll notice the operator repeats a cycle. First, the boom lowers and the stick extends to position the bucket at the dig point. The bucket curls inward, dragging through the soil and filling up. Then the boom lifts to pull the loaded bucket out of the ground. The house swings to one side, the boom extends over the spoil pile or truck bed, and the bucket opens to dump the material. The house swings back to the dig face and the cycle repeats. A skilled operator on a mid-size excavator can complete this cycle in 15 to 20 seconds.
What makes this efficient is that multiple motions happen simultaneously. The boom is already rising while the house starts to swing. The bucket is already curling open before the house finishes its rotation to the dump point. This overlapping of movements is only possible because the hydraulic system can supply oil to several cylinders and motors at once, with the control valves distributing flow based on the operator’s joystick inputs.
Attachments Beyond the Bucket
A standard digging bucket is just the starting point. Excavators have auxiliary hydraulic lines at the end of the stick that can power a wide range of specialized tools, turning a single machine into a multi-purpose platform.
- Hydraulic breakers are impact hammers used to fracture rock, concrete, and asphalt. They connect to a single-direction hydraulic circuit and deliver repeated blows at high frequency.
- Steel shears cut through structural steel beams and plate during demolition, using a bi-directional circuit that opens and closes the jaws.
- Augers (sometimes called post borers) use a hydraulic motor and gearbox to drill holes for posts, piles, or footings. Different auger sizes can be swapped onto the same drive unit.
- Thumbs and grapples clamp onto irregularly shaped objects like boulders, logs, or demolition debris that a standard bucket can’t hold.
- Tilt rotators mount between the stick and the attachment, letting the bucket (or any other tool) tilt side to side and rotate 360 degrees, dramatically increasing precision for grading and slope work.
Each attachment has specific hydraulic flow and pressure requirements. Before connecting an attachment for the first time, the machine’s hydraulic output needs to be measured with a pressure gauge to verify it matches the tool’s specifications. Modern excavators use load-sensing hydraulic systems, where sensors detect how much oil volume and pressure the system needs and adjust the pump output automatically. This keeps the engine from wasting fuel when the attachment demands are light and delivers full power when the work gets heavy.
Electric and Hybrid Models
While diesel engines still dominate, electric excavators are entering the market. Hitachi’s ZX135-7EB, a 13-ton class machine released in early 2025, carries a 198 kWh battery pack and can also plug into three-phase grid power on job sites with reliable electrical connections. In plug-in mode, the machine can run continuously without draining the battery, which solves the runtime limitation that has historically held back electric construction equipment. The hydraulic system itself works the same way whether it’s powered by a diesel engine or an electric motor. The difference is what spins the pump.
For operators and site managers, electric models eliminate diesel exhaust and significantly reduce noise, which opens up work in indoor demolition, urban areas with noise restrictions, and environmentally sensitive sites. The trade-off today is higher upfront cost and the need for charging infrastructure, but the operating cost per hour drops substantially without diesel fuel and with fewer engine maintenance intervals.