For the most part, the basics of a hydraulic system, whether on a compact excavator, tractor loader backhoe or a skid-steer loader, are the same. Hydraulic machines all have the same basic components: the engine, hydraulic pump, hydraulic valve, hydraulic motors, and hydraulic cylinders.
The engine provides power to the hydraulic pump. The hydraulic pump generates a flow of pressurized oil from the hydraulic oil tank to the distribution valve. Utilizing the joystick and foot pedal controls in the cab, the operator is able to signal the valve to direct the hydraulic oil to the appropriate motor or cylinder function.
To run multiple functions simultaneously, such as curling the bucket, raising the boom and swinging the upper carriage, hydraulic motor(s) and multiple cylinders are needed.
Most compact excavators are engineered with multiple hydraulic systems: one system for traveling and one system for the boom, stick, bucket, and swing functions. It’s the design of the hydraulic system that allows the compact excavators to run multiple functions without sacrificing performance and productivity.
“Modern compact excavators are designed with an axial piston pump system,” says Bill Parker, compact excavator product manager for Terex Construction Americas. With this system, pistons within the pump housing pump the oil. But the flow of oil is controlled independently of how fast the pump shaft is rotating. This control of hydraulic oil is referred to as load independent flow division. Hydraulic oil is supplied to the motors and cylinders, per operator commands, without regard to what other functions are being operated. This allows the operator to multi-task without losing power because the oil is able to flow to multiple functions simultaneously. This can only happen because of a load-sensing controller that senses which function has the highest load pressure and then feeds this flow requirement immediately to the pump.
Parker notes that Terex employs a load-sensing system in its compact excavators that permits the axial piston pump to respond to changes in the load without overloading the engine. This saves fuel and extends the life of the machine’s components. With a load-sensing system, the pump senses the operator’s commands and directs the flow of oil to that function, without stopping the other functions. This develops a smoother operation feeling to the user during any operation.
“Flow (or gpm, gallons per minute) controls the speed of the function, and pressure (or psi, pressure per square inch) controls the force of the function,” states Parker. “Hydraulic flow and pressure for compact excavators vary depending on the hydraulic system design and engine horsepower.”
Hydraulic systems on these compact excavators can pump anywhere from 4 to 27 gpm and have a pressure range from 0 to 4,000 psi — with the most common at 3,500 psi. “Knowing the flow and pressure,” continued Parker, “one can get a rough estimate of engine horsepower needed by employing a simple formula: flow x psi ÷ 1714 (conversion factor). For example, take a pump that generates 20 gpm. Multiply 20 gpm times 3,500 psi equals 70,000. When divided by 1714, this equals 40.8 hydraulic gross horsepower.”
The contractor who uses an underpowered compact excavator soon finds out that this formula isn’t just an academic exercise. Matt Sugars, a compact excavator operator with Gaylord Utility Contractors out of Fort Madison, Iowa, knows what it feels like. “We have asked an undersized compact excavator to dig out and pull up a huge boulder from a deep pit in rocky conditions. Did we suffer loss of power? You bet. But that’s just due to the size of the machine. The motor and pump didn’t have enough power. We’re trying to get a job done that was open bid for a certain normal operation but you come up against something tough and you try with what you have. We lost time and profitability on this job because we had to stop and bring in another machine.”
The structural design of a compact excavator can also complement the efficiency of its hydraulic systems. For example, Terex designs its machines with a low center of gravity so that the weight is evenly distributed. A balanced machine enables the operator to quickly change operation speed without the machine bouncing around.