Hydraulic Pumps

Wherever large forces have to be transmitted under control, hydraulics form the technical backbone. Whether in industrial hydraulic presses or in mobile working machines on a construction site, the hydraulic pump is the central component of every hydraulic system, supplying the energy that all downstream components rely on.

Unlike pumps that move a medium from one place to another, a hydraulic pump converts mechanical energy in the form of speed and torque into hydraulic energy, that is, into flow and pressure. The hydraulic oil is not the pumped product here but the energy carrier, which is converted back into mechanical motion by hydraulic cylinders or hydraulic motors. Technically, the hydraulic pump belongs to the positive displacement machines. It delivers a defined fluid volume per revolution and thereby generates a flow rate, from which the operating pressure builds up against the resistance of the connected consumer.

How Hydraulic Pumps work

Regardless of their design, all hydraulic pumps operate on the positive displacement principle. Hydraulic fluid is drawn from a tank on the suction side, transported through sealed chambers and finally delivered into the hydraulic system via the pressure side. The displacement components take different forms depending on the design, such as the gears of a gear pump, the pistons of an axial piston pump or the vanes of a vane pump, but the basic process remains the same.

The delivery process can be divided into three steps.

1. Suction, an expanding working chamber creates a vacuum through which hydraulic oil flows in from the tank
2. Transport, the trapped oil volume travels within a sealed chamber towards the pressure port
3. Displacement, the working chamber contracts again and forces the oil into the system via the pressure side

It is essential to understand that the pump initially delivers flow only. The operating pressure builds up where this flow meets resistance, that is, at the connected hydraulic cylinder or hydraulic motor. If the pressure side is blocked completely, pressure rises very quickly, which is why pressure relief valves perform an indispensable safety function in every hydraulic system.

Design variants of Hydraulic Pumps

Hydraulic pumps are manufactured in various designs that differ in pressure level, efficiency, controllability and robustness. Selecting the right hydraulic pump depends on the required operating pressure, the necessary dynamics, the oil cleanliness and the acceptable noise and cost limits.

• External gear pumps. Two meshing gears carry the oil in the tooth gaps along the housing wall. Robust, compact and comparatively insensitive to contamination, and almost always designed as fixed displacement pumps. Suitable for applications with constant pressure and speed levels, frequently used in mobile hydraulic power units. The external gear pump is the most widely used design.
• Internal gear pumps. An externally toothed pinion runs inside an internally toothed ring gear. This makes delivery smoother and considerably quieter than with an external gear pump, while maintaining high pressure resistance. The preferred choice in stationary industrial hydraulics where quiet running is required.
• Gerotor pumps. Designed for the low pressure range and typically used in cooling, filtration and lubrication circuits. Thanks to standardised flange and shaft variants, they can be flexibly mounted onto axial piston, vane or internal gear pumps.
• Axial piston pumps. Several pistons arranged parallel to the drive shaft displace the oil, built either in swash plate or bent axis design. High efficiency and mostly available with variable displacement. Suitable for the medium and high pressure range in both industrial and mobile hydraulics.
• Radial piston pumps. The pistons are arranged radially and perpendicular to the drive shaft, usually in an odd number such as 5, 7 or 9. As a result, no two pistons ever reach the same dead centre simultaneously, which reduces pulsation and vibration and ensures smooth running. This design achieves very high pressures with precise operating behaviour and is therefore ideal for applications with particularly high force requirements.
• Vane pumps. Vanes guided in a rotating rotor trap the oil in chambers and carry it to the pressure side. They provide a smooth, low pulsation flow and are available as both fixed and variable displacement pumps.
• Hand pumps. The hand pump generates pressure through manual lever actuation rather than a motor and therefore requires no electrical power supply. Hand pumps are used wherever work is only occasional or has to be carried out without a power supply, for example for the emergency actuation of hydraulic systems during a power failure, for pressure testing hydraulic systems or on mobile clamping and lifting tools.

Cutting across these designs is the distinction based on displacement volume. Fixed displacement pumps such as most gear pumps deliver a fixed volume per revolution, and their flow rate can only be influenced via the drive speed. Variable displacement pumps, typically axial piston pumps, adjust the displacement volume to match the load situation, which significantly reduces throttling losses, energy consumption and heat generation.

A further selection criterion is the direction of rotation. Many hydraulic pumps are designed for one direction of rotation only, while variants with reversible rotation are available for hydrostatic drives or changing installation positions. Which combination of design and control principle is the right one is decided by the specific load profile of the application.

Advantages of Hydraulic Pumps

Hydraulic drives have become established in industrial and mobile hydraulics because they combine properties that other drive technologies do not offer in this combination.

• High power density in a small installation space. Hydraulic drives make it possible to realise very high power output in confined spaces, which is decisive in mobile machinery and tight installations.
• Precisely controllable force transmission. The generated pressure is distributed evenly throughout the system, allowing the force at the hydraulic cylinder to be regulated and controlled with great sensitivity. Starting under load from standstill is also possible without difficulty.
• Constant flow rate independent of back pressure. Because a defined volume is delivered per revolution, the flow rate remains largely stable even when system pressure fluctuates. This produces reproducible motion sequences and predictable cycle times.
• Simple overload protection. Pressure relief valves reliably protect the pump, lines and consumers against impermissible pressures. The protection is constructively straightforward and works without complex control technology.
• High robustness and long service life. Hydraulic systems tolerate harsh ambient conditions, severe load changes and continuous operation. With sound design and regular maintenance, pump and system achieve correspondingly long service lives.
• Demand based energy efficiency. With variable displacement pumps, the system operates only at the power actually required instead of throttling off excess flow. This pays off particularly with strongly fluctuating load profiles.

These strengths are matched by design related limitations that should be taken into account when selecting a system. Hydraulic systems require high manufacturing precision and corresponding investment, incur ongoing costs for hydraulic oil and filtration, and react sensitively to temperature fluctuations, since these change the viscosity of the oil. Very fast motion sequences are also only possible to a limited extent hydraulically because of the frictional resistance in the lines, an area where pneumatics have the advantage.

Applications of Hydraulic Pumps

Hydraulic pumps are used wherever large forces have to be moved in a controlled manner and steered precisely. A basic distinction is made between stationary industrial hydraulics and mobile hydraulics in moving machinery.

• Industrial and manufacturing technology. In hydraulic presses for generating high forming forces and in machine tools for feed drives and spindle drives, where high force and fine controllability have to come together. Internal gear and vane pumps are frequently used here.
• Plastics processing. In injection moulding machines, hydraulics handle the opening and closing of the moulds via hydraulic cylinders. The constant flow ensures reproducible cycle times in series production.
• Materials handling and intralogistics. In forklift trucks and pallet trucks, the compact design of hydraulic drives enables high lifting forces in limited space. Cost effective gear pumps are the standard here.
• Construction machinery. In excavators, dump trucks and cranes, hydraulic pumps are the heart of the working hydraulics. Changing load conditions make variable displacement pumps in axial piston design particularly economical here.
• Agriculture and forestry. Tractors and harvesting machines use hydraulics for attachments, steering and hydrostatic travel drives, mostly powered by combustion engines.
• Workshop and maintenance. For straightening, pressing and pulling components, a hand pump with a connected cylinder is often sufficient, without the need to set up a complete power unit.
• Marine applications. On car ferries, hydraulic systems open and close the bow and stern doors, where high forces have to be transmitted reliably and continuously under maritime conditions.
• For extending and retracting landing gear and for adjusting flaps and slats, applications with the highest demands on operational reliability and power density.

In all applications, the interplay of required force, movement speed and load change frequency determines which hydraulic pump design and which control principle make economic sense. For the specific design, oil temperature, viscosity range and the cleanliness requirements of the system are decisive alongside pressure and flow rate.

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