In modern industrial automation, the hydraulic pump is one of the most widespread and essential components. Every time a machine needs to lift, press, move or transmit force with precision and continuity, it is very likely that behind it there is a hydraulic circuit powered by a pump. But how does a hydraulic pump work? And what are the most commonly used types in different industrial sectors?
In this article we will examine in detail the operating principle of hydraulic pumps, the main categories (gears, vanes, pistons), their fields of application and some practical criteria to guide the selection. A guide designed for technicians, buyers and designers who truly want to understand what lies inside a hydraulic circuit.
Before the advent of hydraulics, power transmission relied almost exclusively on mechanical and electrical systems. With the introduction of hydraulic circuits, starting from the first industrial applications during the twentieth century, it was possible to achieve a significant qualitative leap: greater power density (large forces generated with compact components), fine and adjustable control of motion and pressure, and reliability even under severe operating conditions. Positive displacement pumps are at the heart of this system.
The hydraulic pump is the heart of every hydraulic circuit. It is a device that converts mechanical energy, supplied by an electric motor or internal combustion engine, into hydraulic energy, i.e. a flow of pressurized fluid.
The principle is simple: the pump draws oil from the reservoir and pushes it towards the actuators (cylinders, motors, valves). It is important to emphasize a frequently misunderstood concept: it is not the pump that directly generates pressure, but the load imposed by the system that converts flow into pressure. The pump provides flow; the system converts that flow into useful pressure.
From a design standpoint, hydraulic pumps belong to the category of positive displacement machines: at each working cycle, they displace a well-defined volume of fluid (called displacement), ensuring continuity and precision in motion. They can be fixed displacement, with constant flow at a given speed, or variable displacement, capable of adapting the delivered flow to the actual demand of the circuit.
The operation of a hydraulic pump is based on the ability to create a continuous flow of fluid within the circuit. The pump shaft, driven by an electric or internal combustion motor, operates the internal components (gears, vanes or pistons) generating variable volume chambers.
These chambers open and close cyclically: during the suction phase the chamber volume increases, creating a vacuum that draws the fluid from the reservoir; in the discharge phase the volume decreases, pushing the oil into the circuit. The pressure, as mentioned, develops only when the flow meets the resistance of the actuators.
This mechanism, apparently simple, allows for precise control of movements, speeds and forces. In the representation of hydraulic schematics, ISO 1219 symbols are used, which describe the function of the component (fixed or variable displacement, type of control, connections) without going into internal design details.
Not all hydraulic pumps work in the same way: depending on the design principle and intended use, there are variants that differ in performance, cost, efficiency and level of complexity. In general, we can distinguish three main families (gear pumps, vane pumps and piston pumps), each with distinctive characteristics that make them more suitable for specific sectors and applications. Knowing their differences is essential for choosing the most appropriate solution for your operational needs.
Gear pumps are the simplest and most widespread type, used wherever robustness, compactness and low costs are required. Their structure is based on two gears that, rotating in opposite directions, trap the fluid between the teeth and the housing, pushing it towards the outlet. Depending on the configuration, different variants can be distinguished.
External gear pumps are the most common: economical and compact, they offer good flow rates at medium-low pressures (typically 150-250 bar). The main limitation is higher noise levels and decreasing efficiency as pressures increase. They are widely used in agricultural machinery, small mobile systems and auxiliary circuits.
Internal gear pumps, instead, are distinguished by their gerotor geometry: an internal gear drives the external one, ensuring a smoother and quieter flow, with better efficiency. They are the ideal choice when reliability and lower noise need to be combined, for example in industrial presses or systems where acoustic comfort is important.
There are also versions with helical gears, where the angled teeth reduce flow pulsations and vibrations, improving flow continuity.
Vane pumps are based on a rotor equipped with sliding vanes, which move inside an eccentric ring. When the rotor turns, the vanes slide radially due to centrifugal force, creating variable volume chambers that draw in and compress the fluid.
The great versatility of this solution has led to the development of two main families. Fixed displacement vane pumps always maintain the same amount of fluid per revolution, ensuring a constant and regular flow. They are valued for their quietness and for their stability, characteristics that make them ideal for machine tools and many standard industrial systems.
Variable displacement vane pumps, instead, allow adjusting the eccentricity of the ring and therefore the delivered displacement. This results in superior energy efficiency and reduced consumption, because the pump provides only the flow that is actually needed. They are therefore very common in applications involving variable work cycles, such as plastic molding or industrial systems that need to adapt to changing loads.
Piston pumps represent the most advanced technological tier, designed for high-pressure applications and maximum efficiency. The operation is based on multiple pistons arranged inside a cylinder: with alternating movement, these pistons draw in and compress the fluid extremely effectively.
The pumps axial piston have pistons arranged parallel to the drive shaft. This group includes both swash plate and bent axis versions: both offer adjustable flow rates and are capable of reaching high pressures (350–400 bar and above). They are widely used in earthmoving machinery, large industrial plants, presses and contexts where it is essential to ensure continuity and power even under heavy loads.
The pumps radial piston, instead, have pistons arranged radially around a central axis. This configuration allows for very high torques and great reliability, making them particularly suitable for pressing systems, high-pressure testing and extrusion machines.
The greater design complexity and higher cost compared to other families are offset by remarkable durability and superior performance, especially where high pressures and maximum energy efficiency are required.
|
Type |
Max pressure |
Efficiency |
Cost |
Noise level |
Typical applications |
|
External gears |
150–250 bar |
Medium |
Low |
High |
Agricultural machinery, mobile systems, auxiliary circuits |
|
Internal gears |
200–300 bar |
Medium-high |
Medium |
Medium |
Industrial presses, systems with acoustic requirements |
|
Fixed displacement vanes |
140–210 bar |
Medium |
Medium |
Low |
Machine tools, standard industrial systems |
|
Variable displacement vanes |
200–250 bar |
High |
Medium-high |
Low |
Plastic molding, variable load cycles |
|
Axial pistons |
350–400+ bar |
Very high |
High |
Medium |
Earthmoving, naval, large systems |
|
Radial pistons |
400–700+ bar |
Very high |
High |
Medium |
Pressing, high-pressure testing, extrusion |
Hydraulic pumps are used in a wide range of sectors, thanks to their ability to generate high forces, controlled movements and reliable work cycles. Their versatility allows them to be integrated into both mobile machinery and fixed industrial systems, covering very different needs.
In summary, any sector that needs concentrated power, controlled movements and repeatable work cycles finds a reliable solution in hydraulic pumps. The choice of type (gears, vanes or pistons) always depends on the balance between costs, performance, pressure requirements and specific application needs.
The choice of pump cannot be random: it is the result of a careful analysis that crosses several technical and operational parameters. The main criteria to consider are:
For further reading on common mistakes and practical advice, also read our Guide to choosing a hydraulic pump.
What is the difference between a fixed displacement pump and a variable displacement pump?
The fixed displacement pump always delivers the same displacement per revolution: the pressure varies with the load and is limited by a safety valve. The variable displacement pump adjusts the displacement based on the circuit demand, typically through a pressure compensator, reducing energy waste and fluid overheating.
What is a pressure compensator?
It is a control device that, when the system pressure reaches the set value, acts on the pump reducing its displacement to near zero. In this way, the pump delivers only the flow strictly necessary to maintain the set pressure, limiting energy losses.
How long does a hydraulic pump last?
The lifespan depends on multiple factors: operating load, rotational speed, fluid filtration class (ISO 4406 standard), operating temperature and proper maintenance. With clean fluids, controlled temperature and respected maintenance intervals, the useful life is measured in many thousands of operating hours.
What pressures do the different pump families reach?
Approximately: gear pumps reach about 170–280 bar; vane pumps up to about 275 bar (but quieter); axial piston pumps between 280 and 400 bar; radial piston pumps up to 700 bar and above. The exact values vary depending on the series and manufacturer.
How is a pump represented in hydraulic schematics?
ISO 1219 symbols are used, which describe the function of the component (fixed or variable displacement, type of regulation, connections) and the relationships with the circuit. The schematic indicates "what the component does", not how it is built internally.
Why is a pressure relief valve needed even with variable pumps?
The pressure relief valve is a safety protection: it intervenes in case of transient peaks or anomalies, preventing the pressure from exceeding the structural limits of the circuit. Even with variable displacement pumps, keeping it is good design practice.
What is the quietest hydraulic pump?
The vane pumps and internal gear pumps are generally the quietest types, thanks to their geometry that reduces flow pulsations and mechanical vibrations.
The hydraulic pump is the heart of every hydraulic circuit: the performance, efficiency and reliability of the entire system depend on its correct selection. Understanding how a hydraulic pump works and what types exist is the first step towards making an informed choice.
On Fluid-Hub you can find a wide range of hydraulic pumps and related components, with detailed technical data sheets and technical support for selection. Explore our pump catalog from the leading manufacturers: Casappa and Marzocchi for gear pumps, Berarma and Veljan for vane pumps, Bosch Rexroth and Kawasaki for piston pumps. Also discover the product guides on our blog to find the most suitable solution for your operational needs.
ISO 1219-1:2012 - Fluid power systems and components — Graphical symbols and circuit diagrams
CETOP (Comité Européen des Transmissions Oléohydrauliques et Pneumatiques)
ASSOFLUID / Federtec — Italian Association of Manufacturers and Operators in the Hydraulic and Pneumatic Sector