How Mini Excavator Hydraulic Systems Work

By Giles Lambertson March 22, 2016 View Profile

Compact excavators would be less awesome without the gallons of fluid coursing through their hydraulic veins. Absent the hydraulics, modern excavators would still clank and grind, using cables, chains and gears to accomplish their work, instead of hum as they pirouette and activate boom-end tools. Operators would still push and pull cast-iron levers through slotted stations instead of tweaking joysticks.

So, thank you, hydraulic systems and the electronics that make you a smart technology — thanks for improving our work lives. Our gratitude extends to the quick hydraulic tool-exchange systems that turn compact excavators into such versatile machines. Yet before a bucket on an excavator can be quickly switched out for, say, a swiveling grapple, a lot has to happen under the hood. It begins with a 15-or-so-gallon reservoir of hydraulic fluid (oil with additives added to reduce the wear on hydraulic components). Many gallons more are suspended in the lines after a system is charged. This is the lifeblood of the enclosed hydraulic system, and it exerts its influence through pressurization.

The pressure relies on the several-centuries-old scientific principle that liquids can’t be compressed. So pumps squeeze fluids in a hydraulic system through small diameter hoses and pipes — pressure in, pressure out — and mechanical force is created. The force of the fluid at the working end of the lines activates pistons or other moveable surfaces, and things begin to occur … a bucket is drawn though soil … a thumb clamps down … a drill begins to spin.
Pumps make it all happen. “The number of pumps varies by machine type,” says Alex Anhalt, excavator instructor and team leader for John Deere Construction and Forestry. “Generally speaking, excavators have higher-flow requirements and combine functions more often; thus they often have more hydraulic pumps.” Depending upon the manufacturer, a compact excavator might have as many as four pumps, all pulling from the same reservoir.

The physics of liquids under pressure are enhanced in latest generation compact excavators. Caterpillar’s E2 models have what Cat calls “high-definition hydraulics,” which utilize streamlined valves that reduce heat, thereby making the fluid more stable, functional and precise in its control. “The more efficient a hydraulic system, the less load it puts on the engine, which also results in less fuel consumption,” says Greg Worley, senior project engineer at Caterpillar.

By alloying electronics and hydraulics, manufacturers are producing ever-smarter compact excavator systems, a trend likely to accelerate. “For the last 10 years or so, most manufacturers had their primary design focus aimed at implementing engine emissions regulations,” says Anhalt. “Now that the Tier 4 Final deadline has come and gone we are able to focus more on next level hydraulics, and the future will likely bring more electronics into the mix. The challenge is marrying the efficiency that electronics can bring without losing the feel customers want.”

The electronic interface with hydraulics already is making things easier. Worley says that Caterpillar mini hydraulic excavators “feature an electronic monitor combined with sensors and solenoids that has improved operator productivity and reduced operating costs. Operators can adjust flow rates for attachments, switch work tools and change the control pattern, all without leaving the comfort of the seat.”

However, as hydraulic systems grow more sophisticated, they also simply grow. A raised hood on an excavator exposes banks of black hoses exiting pumps and heading off in all directions to power machine travel, boom swing, arm movement and attachment functions. How a designer arranges the lines — minimizing right angles and other flow choke points — helps determine the efficiency of the hydraulic system.

“Hose routing is a challenge with compact equipment because of space constraints,” says Worley. “If it becomes an afterthought, it can lead to durability and serviceability issues with the hydraulic system. This is why our engineering team considers hose layout and routing in their design requirements.”

Ultimately, some of the fluid is pumped from under the hood into the metal piping and flexible lines running the length of a boom and arm. Dedicated hydraulic lines for the pistons moving the arm and bucket sometimes are protectively channeled through the framework of the boom; auxiliary lines for attachments more often are affixed to the side of boom and arm, with fast couplings at the working end.

Have sophisticated hydraulics opened the way for more excavator attachments? “Yes, it has broadened the range of types of attachments. More importantly, it has made the attachments easier to use and more efficient,” says Worley.

He cites popular excavator attachments such as rotating and tilting couplers, mowers, mulchers and brush cutters. Some tools are complex and tax a hydraulic system more than others. A thumb on a bucket, for instance, requires minimal hydraulic pressure or flow and connections are easily made. Sometimes more than one auxiliary line is required, as when an attachment requires double-action hydraulic cylinders and variable flow.

“The most difficult attachments are those that require both high flow and high pressure to operate,” Anhalt says. “The higher the pressure gets in a system, the more the flow is reduced. So for example, if you wanted to equip an excavator with a felling head, which has a high pressure and flow demand, you would need to combine flow from multiple pumps. That produces the speed to turn the cutting wheel and the power to cut through a tree. Whereas if you had sufficient flow but not enough pressure, the cutting wheel would spin fast but as soon as it hit the tree, it would stop.”

Hydraulics reduce downtime right up front by speeding the tool attachment process. The days of unbolting and knocking out pins to switch buckets are gone when a machine is fitted with hydraulic quick-coupling fittings, in which retractable pins and wedges are hydraulically activated. The fittings vary in design and are offered both as OEM and aftermarket options. What they have in common is that, except for manually connecting hydraulic fittings to the tool’s hoses, the procedure can be done without climbing down from the cab.

Caterpillar claims its attachment system is the industry leader. Worley describes it as “a hydraulic dual-locking coupler that works with the machine monitor to allow for quick changing of attachments without having to leave the seat. The coupler is locked and unlocked with the push of two buttons and has the capability to sense the pressure on the pins and warn the operator if the attachment isn’t fully connected.”

The latter is important: Injuries and deaths have occurred when an unsecured bucket or other attachment dropped onto someone. To prevent such accidents, mechanical wedges, color-coded locks, automatic safety locking systems and LED warning lights are employed. Manufacturers also recommend that quick-attached buckets be tested before use by embedding bucket teeth in the dirt and applying pressure till the excavator’s tracks begin to lift from the ground.

Cat-305 As new attachments are dreamed up and marketed, Anhalt says they will have marginal impact on an excavator’s hydraulic system. “Base hydraulic systems have not changed much due to new attachments. At the end of the day, all attachments need the same thing — pressure and flow.”

What does change, he says, is how an operator wants to run an attachment. “Do they want to use a foot pedal or a switch, or do they need smooth proportional control that requires solenoid kits? It’s all about which attachment and how the operator wants to run it.”

It’s also all about hydraulics.