Hydraulic Pump

A hydraulic pump is the central drive element of a hydraulic system. It converts mechanical energy—usually from an electric motor or internal combustion engine—into hydraulic energy by pumping hydraulic fluid and pressurizing it. The resulting flow rate and pressure supply the connected actuators, such as cylinders and motors, with the required power.

Basics and Operation of a Hydraulic Pump

Every hydraulic pump operates on the positive displacement principle. In this process, a mechanically driven component increases the volume of a working chamber, creating a vacuum that draws fluid from the reservoir. The chamber then contracts again, displacing the fluid and forcing it under pressure into the line. This cycle repeats continuously, producing a steady or pulsating flow, depending on the design.

Fixed-Displacement and Variable-Displacement Pumps

A basic distinction is made between pumps with a fixed displacement volume and those with a variable displacement volume. Fixed-displacement pumps always deliver the same amount of hydraulic oil per revolution. The flow rate can only be adjusted by changing the speed of the drive motor. Variable-displacement pumps, on the other hand, adjust their displacement volume during operation, allowing the flow rate and pressure to be continuously adapted to current demand. This reduces throttling losses and saves energy, especially in applications with highly fluctuating power requirements.

Displacement and Efficiency

The displacement volume describes the amount of oil that a hydraulic pump displaces per revolution. It is the key parameter for flow rate. Volumetric efficiency indicates how much of the theoretically delivered volume actually reaches the pressure port, since internal leakage losses reduce the usable flow. Modern axial piston pumps achieve volumetric efficiencies of over 95 percent, while simple gear pumps range from 80 to 90 percent. The overall mechanical-hydraulic efficiency also takes friction losses into account and is decisive for the system’s energy balance.

Types of Hydraulic Pumps

The choice of design depends on pressure level, flow rate, noise requirements, and cost. The four main types differ significantly in their characteristics and applications.

Gear Pumps

Gear pumps are the simplest and most cost-effective design. Two meshing gears rotate inside a housing and transport the hydraulic oil through the spaces between the teeth from the suction side to the discharge side. External-tooth variants dominate the market, while internal-tooth variants operate more quietly and with lower pulsation.

Typical operating pressures range from 63 to 300 bar; in special designs, they can reach up to 350 bar. Volumetric efficiency ranges from 80 to 90 percent. Gear pumps are robust, resistant to contamination, and suitable for applications with constant flow requirements, such as lubrication systems, cooling circuits, or simple control circuits in mobile machinery. Their disadvantages include pulsating flow and a limited pressure range.

Vane Pumps

Vane pumps, also known as rotary vane pumps, use an eccentrically mounted rotor with radially movable vanes. The rotary motion causes the chambers between the vanes to expand and contract, thereby drawing in and displacing oil. The result is a virtually pulsation-free flow with low noise levels.

The operating pressure typically ranges from 70 to 175 bar, occasionally up to 200 bar. The volumetric efficiency is between 85 and 95 percent. Vane pumps are primarily used in machine tools, presses, and steering systems, where quiet operation and consistent flow are required. Variable-displacement models allow the flow rate to be adjusted to meet specific needs.

Axial-piston pumps

Axial piston pumps are the most versatile and powerful design. Several pistons—usually five to eleven—are arranged parallel to the drive shaft and move within a rotating cylinder block. In the inclined-disc design, the pistons slide on an inclined disc; in the inclined-axis design, the cylinder block itself is angled relative to the drive shaft.

These pumps cover operating pressures ranging from 160 to 700 bar; high-pressure versions can even reach up to 1000 bar. Volumetric efficiency ranges from 90 to 98 percent. Their strength lies in the combination of high pressure, variable displacement, and excellent controllability. Axial piston variable displacement pumps with load-sensing control adjust pressure and flow rate to meet the needs of the load and significantly reduce energy consumption. Typical applications include excavators, cranes, injection molding machines, and wind turbines.

Radial-piston pumps

Radial piston pumps arrange their pistons in a star pattern around an eccentric shaft. Each piston draws in fluid from the inside and discharges it outward. This design is characterized by extremely high pressures, which can also reach 700 bar and beyond. The volumetric efficiency ranges from 90 to 95 percent.

Radial piston pumps are particularly suitable for applications requiring high pressures at low speeds, such as foundry presses, offshore equipment, or test benches. Their compact design and high pressure resistance make them the top choice when extreme loads are involved.

Comparison of Design Types

Selection Criteria for the Right Hydraulic Pump

Selecting a hydraulic pump requires a systematic evaluation of the requirements of the overall system. Several factors interact and influence one another.

Pressure and Flow Rate

The required operating pressure determines the basic design. Gear pumps and vane pumps are suitable for pressures up to 200 bar; above 400 bar, piston pumps are the dominant choice. The flow rate must meet the maximum demand of all consumers; however, with variable-displacement pumps, only the actual instantaneous demand needs to be supplied. This lowers the installed power and reduces losses.

Fluid and Operating Conditions

The viscosity, temperature, and purity of the hydraulic oil significantly influence the choice of pump. High temperatures reduce viscosity and increase internal leakage losses, while low temperatures thicken the oil and make the suction process more difficult. In harsh environments with dust, moisture, or a risk of explosion (ATEX zones), specially protected pumps are required. The drive speed must be matched to the pump, as excessively high speeds cause cavitation and excessively low speeds result in insufficient flow.

Energy Efficiency and Life-Cycle Costs

The purchase cost of a hydraulic pump is only part of the overall bill. Energy consumption, maintenance costs, and service life all factor into the life-cycle costs. A variable-displacement pump with load-sensing control is more expensive to purchase than a fixed-displacement pump, but often pays for itself within a few years due to lower energy consumption. This effect is particularly pronounced in systems with long downtimes or partial-load operation.

Causes of Failure and Maintenance

Hydraulic pumps are robust machines, but they are sensitive to certain operating conditions. The most common causes of failure can be traced back to a few core issues.

Contamination of the hydraulic oil

Solid particles such as metal abradive, dust, or seal debris cause the majority of all pump damage. They wear down sliding surfaces, damage seals, and increase internal leakage losses. Water in the oil leads to corrosion and emulsion formation, which impairs lubricity. Compliance with the cleanliness class specified in ISO 4406 and regular filter changes are the most effective preventive measures.

Cavitation

Cavitation occurs when the pressure on the suction side drops below the vapor pressure of the oil. Gas bubbles form and collapse abruptly in the pressure zone, leading to material erosion on the surfaces. Causes include an oil level that is too low, clogged suction filters, suction lines that are too small, or incorrect oil viscosity. A metallic whining noise from the pump is a typical warning sign.

Overpressure and Overheating

Continuous operation at the pressure limit places excessive strain on seals, bearings, and the housing. The resulting frictional heat increases the oil temperature, which lowers the viscosity and accelerates the oxidation of the oil. Any temperature increase above 55 degrees Celsius doubles the rate at which hydraulic oil degrades. Reliable overpressure protection and adequate cooling are therefore essential.

Standards

EN ISO 4413 specifies the general safety requirements for hydraulic systems and their components, including pumps. It defines requirements for design, installation, operation, and maintenance and is incorporated as a harmonized standard into the Machinery Directive 2006/42/EC. ISO 8426 specifies test methods and dimensions for positive displacement pumps and motors. ISO 4406 classifies particle contamination in hydraulic oil, thereby providing an important reference value for the pump’s operational reliability.

Trends and Developments

Hydraulic pumps continue to evolve, driven by energy efficiency requirements and digitalization. Variable-displacement pumps with variable-speed drives and load-sensing control reduce energy consumption by 50 to 80 percent compared to conventional systems with throttle control. Condition-monitoring systems measure pressure, temperature, and vibrations in real time, enabling predictive maintenance before a failure occurs. Integrating sensors directly into the pump transforms a passive component into an intelligent system that monitors and reports its own condition. Electrohydraulic designs combine electric drives with hydraulic power transmission, paving the way for more compact, efficient, and cleaner systems.