The Three Major Components of Excavators: Engine, Hydraulic Pump, and Distribution Valve

Often when people first encounter excavators, they wonder why these machines don't operate like cars, where the engine powers the vehicle through a gearbox and driveshaft. Instead, excavators utilize an engine to drive a hydraulic pump, which then moves high-pressure hydraulic oil through hydraulic motors and cylinders to facilitate the movement of the machine.

The engine powers the hydraulic pump, which, along with the hydraulic piping, hydraulic motor, and hydraulic cylinder, carries out hydraulic transmission. The control valve manages the hydraulic control. Today, I listed some basic knowledge about the excavator hydraulic pump to help everyone understand it better and to reinforce my own understanding of this product.

Hydraulic Pumps：

Hydraulic pumps convert mechanical energy into hydraulic pressure energy. Commonly seen in excavators and loaders, there are two main types of hydraulic pumps: gear pumps and piston pumps.

Similarities: Both types operate by altering the volume to generate pressure in the fluid.

Differences: The structure and volume displacement differ. In gear pumps, the fluid volume changes between two gears. In piston pumps, the fluid volume is altered within each piston cylinder.

In medium and large excavators, it's common to combine piston and gear pumps into a single hydraulic pump assembly. The main pump, usually a piston pump, supplies high-pressure hydraulic oil to the hydraulic travel motor, rotation motor, and cylinders. The pilot pump, typically a gear pump, supplies oil at lower pressure to the distribution valve.

Gear Pump:

Gear pumps function by the rotation of two meshing gears creating a sealed moving volume. These pumps are fixed displacement pumps, often used in low-precision, low-pressure controls.

Advantages: Simple structure, easy to manufacture, low cost, compact size, lightweight, good self-priming capabilities, insensitive to oil contamination, and reliable operation.

Disadvantages: High flow and pressure fluctuations, high noise levels, and non-adjustable displacement.

Gear pumps are widely used in various low-pressure systems. Initially used in low-pressure systems due to their lower capabilities, modern gear pumps can achieve pressures around 25 MPa and are often employed in machinery where high pressure is not crucial.

Piston Pump:

Piston pumps rely on the reciprocating motion of pistons to generate power. These variable pumps are used for high-precision, high-pressure control.

Advantages: High working pressure (typically 20–40 MPa, up to 1000 MPa), compact structure, high efficiency, and easy flow adjustment.

Disadvantages: High cost, minimal pulsation, variable displacement, and poor self-priming capabilities.

Piston pumps are commonly used in high-pressure systems and construction machinery.

Operation Principles

When the engine drives the transmission shaft (as shown on the left side of the diagram), the connecting rod pushes the piston to reciprocate within the cylinder, while also rotating the piston and cylinder assembly. The distribution plate remains stationary.

During operation, the cam and piston spring cause the piston to move up and down, pumping oil. When the cam lobe passes, the piston spring pushes the piston downward, creating a vacuum in the pump chamber above the piston. The hydraulic oil enters the pump chamber through the inlet hole as the piston reaches its lowest point, marking the end of the intake phase.

As the camshaft rotates to the raised part of the cam, the piston spring compresses and the piston moves upwards, pressurizing the oil. Some oil returns to the upper chamber through the oil hole. Once the piston covers the inlet hole, the small gap between the piston and cylinder liner seals the space above the piston, rapidly increasing the oil pressure in the pump chamber. When the pump pressure exceeds the outlet spring pressure, the high-pressure oil is pushed through the main control valve into the cylinder.

The process of oil supply and its cessation is controlled by the alignment of the piston's slotted grooves with the return holes in the cylinder liner, allowing the low-pressure oil path to connect with the middle hole and slotted grooves in the piston head. As the piston continues to rise and the cam lobe passes, the spring pushes the piston down, starting the next cycle.

The K3V pump by Kawakita is widely used in excavators. Excavator main pumps generally use a dual pump system, with a rated pressure of 34 MPa and a momentary peak pressure of 39 MPa. The higher efficiency and volumetric performance of piston pumps over gear pumps is why they are preferred in high-demand applications. Most hydraulic excavators today use a swash plate-type variable axial piston pump. Typically, these dual pumps are connected via a common shaft, also known as a through-shaft pump. Some excavator through shaft pumps may also incorporate a small gear pump as a servo pump (pilot pump), significantly simplifying the excavator's layout.

This introduction covers some basic knowledge of hydraulic pumps, giving a preliminary understanding of the three major components of excavators. More detailed information on hydraulic components, as well as excavator engines and distribution valves, will be shared in future articles.