Introduction To: Hydraulics and Hydraulic Components

From winches and cranes, through to pumps and steering gears to watertight doors- hydraulic systems are used in many applications on board of vessels. Hydraulics is a very interesting subject and learning the basics of hydraulic components and how a hydraulic system works will be beneficial to all maritime buyers. Being able to identify the different components and their basic characteristics will help buyers better understand hydraulic products and make the right purchase decisions.

This Introduction To: Hydraulic Systems and Hydraulic Components is the first article, which will be followed by a series of useful and easily explained buyer guides, in which we will cover the most important hydraulic vessel components in detail.


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Introduction to Hydraulics

WHAT: What is Hydraulics? What is a hydraulic system?

Hydraulics is a drive technology in which fluids are used to transfer the energy from e.g. an electric motor to an actuator and as a result, drive hydraulic machinery. A hydraulic system is any system that uses the pressure of a liquid to perform mechanical work, which as we will learn further, can be either linear or rotary.


HOW: How does a hydraulic system (hydraulic circuit) work?

Pascal’s Law is fundamental to the principle of hydraulics. According to Pascal’s Law, when a fluid is enclosed and pressure is applied at any point, the pressure will be distributed and applied equally in all directions of the vessel. Liquids are also virtually incompressible – their volume cannot be reduced. Based on those two principles: a hydraulic system can be used to multiply force – a smaller force on one piston (called also a master piston) can result in a larger force on another piston with a greater area (or: a slave piston).

If the slave piston’s area is 4 times greater than the master piston’s, the force at the slave piston will be 4 times greater, in spite of the same pressure applied. To illustrate that, imagine a brake system in your car. When your foot applies pressure to the brake pedal (master piston), the force is multiplied and able to stop a vehicle moving. That same simple principle is used in many other industrial applications, including maritime.

Each hydraulic system (hydraulic circuit) consists of at least three basic parts:

WHY: Why are hydraulic systems used?

Hydraulic systems are common thanks to their multiple advantages, some of which are:

- easy force multiplication: a smaller piston can move another piston with a surface many times greater

- high power density: the mass of a hydraulic drive is several times smaller than the mass of a comparable power electric drive

- simplicity & flexibility: few components are needed to build a system, allowing machinery to move and one component can perform multiple tasks

- automatic lubrication, thanks to the hydraulic fluids used

- control (motion control, speed control)

In short, hydraulics is one of the most efficient ways to multiply force, handle heavy loads, or create accurate, repetitive (linear or rotary) motion.


WHERE: Where is hydraulics used?

Hydraulic systems are used for many applications. Some hydraulic machinery/systems examples are:

  • hydraulic brakes of cars motorcycles and other vehicles
  • construction site machinery: dump trucks, excavators, loaders, cranes
  • certain types of hydraulic winches
  • ship’s cargo systems, stabilizers, engine rooms, cranes
  • hydraulic presses and in manufacturing plants
  • aircraft landing gear, flaps, brakes, etc.

Hydraulics vs. pneumatics

These two types of power circuits are similar in some ways as they both use a contained medium to create mechanical energy, they also use similar terminology and components. They both need an actuator, a pump to initiate flow, and a set of valves to control it.

The main difference between these two power circuits is the medium used to transform power. While a hydraulic system uses the pressure of a liquid to perform mechanical work, the pneumatics uses compressed air.

Pneumatics calls for the use of a compressor, which is not the case with hydraulics. Hydraulics, on the other hand, needs a tank/ reservoir to store fluid, while in pneumatics air can be directly drawn from the atmosphere. Gases are easily compressible, which means there can be a delay in the movement, while fluids are virtually incompressible and delay does not take place.

Generally speaking, pneumatics are used in lower pressure applications, and hydraulics in higher pressure ranges, often 30-70x higher than a pneumatic circuit. As such, hydraulics comes into its own when higher forces are required.

Both systems have pros and cons when it comes to certain aspects of their usage, e.g. contamination and safety.


Types of hydraulic systems

Hydraulic systems can typically be electric or diesel-powered. Depending on the type of fluid used inside the system, they can be mineral oil-based, synthetic, biodegradable, or water-based (water-glycol or seawater). Hydraulic systems can also be divided into an open-loop or closed-loop hydraulic systems. An open (loop) hydraulic system is a system where the hydraulic fluid is returned into the tank at the end of a cycle. A closed (loop) system is where the hydraulic fluid stays in the system's loop without returning to the tank.

Now, let’s look at the common components making part of a complete hydraulic system.

Introduction to Hydraulic System Components


Hydraulic Pumps: What is a hydraulic pump? How does a hydraulic pump work?

Hydraulic pumps supply fluid to the components in the hydraulic system – they are there to transfer the mechanical power into hydraulic energy. They do it first by creating a vacuum at the pump inlet, which causes the hydraulic fluid from the system’s reservoir to enter the pump. Then, by applying mechanical force to the fluid, it forces it into the hydraulic system.

Hydraulic pumps can be divided into fixed displacement pumps (those that can only pump a constant amount of fluid with each revolution) or variable displacement pumps, which contain moving components and able to increase or decrease the pump’s displacement per revolution.

Types of hydraulic pumps

The main types of hydraulic pumps are gearvane and piston pumps, and they come in many different executions.

Hydraulic gear pump types

Gear Pumps

Gear pumps cause fluid flow by carrying it around the teeth of two equally sized meshing gears. The two common types/variations are internal gear pumps and external gear pumps. As the name implies, external gear pumps which use two external spur gears, while internal use one external and an internal spur, where teeth face inwards.

Gear pumps are fixed displacement pumps, which means that they pump a constant amount of fluid with each revolution. They are also known for being more suitable in pumping high viscosity fluids, therefore they are also common in chemical installations. Gear pumps tend to be cheaper, simple but durable.

Hydraulic vane pump types

Vane Pumps

Vane pumps consist of a rotor with vanes, which is connected with the driveshaft and turns inside of a cam ring. As the rotor turns, the vanes extend or retract, following the inner surface of the cam ring. Vane pumps cause fluid flow by carrying it between vanes. They create a good vacuum, perform well with low viscosity fluids, but are not suitable for high viscosity fluids. The disadvantage of a vane pump is that as the vanes and the inner cam ring surface are in constant contact, components can be subjected to higher amounts of wear. Vane pumps can either be fixed displacement or variable displacement pumps.

Hydraulic piston pump types

Piston Pumps

They are commonly used hydraulic pumps, mostly applied as variable displacement pumps, they are highly efficient and can produce comparatively little noise. Piston pumps come in different executions, some of which are axial piston pumps, bent axis piston pumps, and radial piston pumps.

Inside an axial pump’s body (housing) we will find a drive shaft to which a swashplate is attached at an angle, multiple pistons housed inside a piston block, attached to a swashplate. All that is closed off with a port plate with inlet and outlet ports. Bent axis piston pumps are a variation of axial piston pumps, where pistons are arranged at an angle.

A radial piston pump consists of a rotor (piston block), and pistons enclosed in a cam ring. Radial pumps work differently to axial pumps in a way that the pistons move (radially) away from the shaft (in and out) instead of inline (parallel) with the shaft, as it is the case with axial pumps.

Piston pumps can either be fixed displacement or variable displacement pumps. They are more expensive than gear pumps or vane pumps, but have a longer life, especially when working under high pressures (and the oil cleanliness levels are maintained).

Hydraulic actuators: What is an actuator? What are the types of actuators?

We just said that hydraulic pumps are used to transfer mechanical power into hydraulic energy. Now, a hydraulic actuator is where the hydraulic energy is converted back to mechanical energy, of course with multiplied force. Actuators can either be hydraulic cylinders or hydraulic motors:


What is a hydraulic cylinder?

A hydraulic cylinder, or a linear hydraulic motor, is a mechanical actuator that converts the energy of the hydraulic fluid into unidirectional, linear force through a linear stroke; they are able to give pushing and pulling forces.

A hydraulic cylinder typically consists of the following components: a cylinder barrel (body), piston, piston rod, dirt wiper, and multiple rings and seals. They can be mounted to machinery with the use of flanges, reunions, or clevises.
Hydraulic cylinder types

Types of hydraulic cylinders

They are two types of hydraulic cylinder and they are single-acting or double-acting cylinders.

A single-acting cylinder is a cylinder with only one fluid port, where the fluid acts on only one side of the piston. The other side uses a momentum/ mass, a loaded spring, or compressed air in order to move the cylinder back (so-called: retraction stroke).

In a logical manner, a double-acting cylinder is one, where fluid can act on both sides of the piston. Such a cylinder has two fluid ports and fluid entering a particular port will either extend or retract the cylinder rod. In this system, a directional control valve is necessary to direct the fluid back to the cylinder and cause a retraction stroke.

What is a hydraulic motor?

A hydraulic motor converts hydraulic energy into rotary motion and work. Rotary motion is the difference setting a hydraulic motor apart from a hydraulic cylinder.

Types of hydraulic motorsHydraulic motor types

The three most common types of hydraulic motors are vane motors, gear motors, and piston motors, which does sound very much like the hydraulic pump types. The way they are built and how they work is similar, but they are generally not interchangeable - many hydraulic pumps cannot be used as hydraulic motors because they cannot be used in reverse (back-driven).

Hydraulic motors can be either fixed displacement motors (when the motor speed is constant) or variable displacement motors (varying motor speed). Fixed displacement motors provide constant torque and speed for a given flow while variable displacement motors provide variable torque and speed. 

So, explained in simple words, the difference between a hydraulic cylinder and a hydraulic motor is that while the cylinder gives a linear force through a linear stroke hydraulic motors are used for rotary movements. Whether you choose one or the other depends on the application and desired outcome movement of the actuator.


Hydraulic accumulators: What is an accumulator? How does an accumulator work?

Hydraulic accumulator typeshydraulic accumulator is part of the hydraulic system used to store energy. Its functions are maintaining the system’s pressure, developing fluid flow, absorbing shocks and noise, compensating for fluid leakages, and performing a shutdown of a system when needed. An accumulator should be a very respected hydraulic component by virtue of the fact that it is a pressure vessel.

What are the different types of hydraulic accumulators?

Accumulators are divided into types according to the means used to separate the gas and fluid chambers; these are the diaphragm (membrane), bladder, and piston accumulators. They also might be spring-loaded accumulators, weight loaded accumulators or gas loaded accumulators.

For more detailed information about those hydraulic accumulators, go to: 

Buyer’s Guide: Finding the correct Hydac bladder accumulator spare parts


Hydraulic Valves: What is a hydraulic valve? What are the types of hydraulic valves?

Hydraulic valve typesHydraulic valves are also important for a safe and well-functioning hydraulic system. They can be grouped based on their functions:

  • Pressure control valves (PC Valves) whose function is to regulate the pressure of the fluid and releasing any excess pressure. They are vital in preventing unsafe bursting and leakages caused by high-pressure spikes.
  • Flow control valves (FC Valves) which are responsible for controlling and adjusting the fluid flow rates, and therefore the speed of an actuator
  • Direction control valves (Directional control valves or DC Valves) whose function is to route the fluid.

Valves in each of these categories have different construction and can be actuated pneumatically, hydraulically, electrically, mechanically, or simply manually. A common way to control a valve is electromechanically with the use of solenoids.

What is a hydraulic solenoid?

Hydraulic solenoids are usually used in hydraulic valves and pumps, where they control the pressure or flow of hydraulic fluid. A solenoid is sort of a spiral electromagnet, the purpose of which is to generate a controlled magnetic field, which can create a desired mechanical force used to e.g. shut off, release, dose, or distribute fluids.

A solenoid valve is simply a valve operated electromechanically with the use of solenoids. Solenoids are commonly used on valves as they offer fast and safe switching, high reliability, long service life, low control power, and compact design. Various AC & DC control voltages are available for solenoid valves.

Multiple solenoid valves can be installed together on what is called a manifold.

What are hydraulic manifolds?

hydraulic manifold could be compared to a switchboard in an electrical circuit. They are components made of multiple hydraulic valves (pressure/flow/solenoid), which are consolidated into one device, typically of an aluminum/steel construction. Hydraulic manifolds are used as a compact means of offering maximum functionality in a very small space, regulating and distributing fluid flow to the system, as required. They allow for control of how much fluid flows between components of hydraulic machinery.

Types of hydraulic manifolds

Manifolds are available as complete mono-blocks, or modular blocks, where a couple of blocks are needed to make up an entire system, or customized manifold blocks where size, design, and material can be engineered to suit a particular application.

What are manifold ports? What are the manifold port types?

Ports are openings in the manifolds where the inlet (supply) and outlet connections are made. The port connections are expressed in many different executions (eg BSP, SAE, Metric, NPT, etc). Manifolds can be simple or complex. The number of supply ports defines the number of independent fluid supplies that can be interfaced, while the number of outlet ports the number of outlets in the system.


Hydraulic Filtration: What is a hydraulic filter? What are the types of hydraulic filters?

Hydraulic filter typesFilters can be positioned in different places in the hydraulic system, but their role is always to remove all unwanted particles from the hydraulic fluid and prevent damages of the system's components. When placed between the reservoir and the pump they are used to protect the pump but can also create cavitation issues if not sized or monitored correctly. When placed between the pump and the control valve they are used to protect the control valve and other components from pump failure debris.

There are many types of filters and they can be classified according to the location of the filter, filtering method, size of pores, and the amount of oil filtered. The most common filter types are suction filters (placed in the suction life before the pump), pressure filters (placed between the pump output and the following component), return filters (placed in between the control valve and reservoir).

You can also encounter a self-contained filter system placed independently from the hydraulic system. These are called off-line filters. It is important to know, that even brand new hydraulic oil is generally regarded as unsuitable for immediate use and requires filtration before use in a hydraulic system. It is therefore important to filter it first using the off-line filters, or a transfer pump with adequate filtration. The advantage of an off-line filter system is that they operate independently – even if the hydraulic system is off.

Another type of filter found on a hydraulic system is an air breather, which is an important air filter installed on the tank. They protect the system from dirt ingress (preventing any sand, dust, or water contaminants from entering).

Hydraulic filter element typesWhen it comes to filters in general, the pore size of a filter is expressed in microns. Different micron size filters are used depending on what the minimum manufacturer’s cleanliness level requirements are for the hydraulic system. The viscosity grade and temperature of the hydraulic oil in the system also plays an important role in filter sizing/ selection.

Filter elements: What is a hydraulic filter element?

A filter consists of a filter housing (outside cover) and the actual filter element. Housings aren’t exchanged often unless they are damaged. It is the inside filter element which needs regular replacements.

A common mistake that some users make when ordering filters is providing the part number of the filter housing when actually having the intention to purchase just the filter elements. The part number of the filter element itself can generally be found on the filter housing’s label too, but it can also be found in the maker’s datasheets.

For more information on understanding the popular HYDAC's part coding system, the specifications behind these part numbers and tips on how to match filters with the correct filter elements see also our handy and easily explained Buyer’s Guide to: Hydac filters and filter elements part coding

Hydraulic clogging indicator types

Clogging Indicators: What is a clogging indicator?

Some hydraulic filters are equipped with clogging indicators. Those small, but essential components are very handy in notifying users about the need to exchange a filter element before they get blocked and bypass. Some inexpensive and simple clogging indicators require periodic visual inspection, while some more advanced ones can be connected to a control system and send a warning or even system shut down on sensitive or safety-critical installations.


Hydraulic tanks: What is a hydraulic reservoir (hydraulic tank)?

A big reservoir, or a hydraulic tank, is there to store the hydraulic fluid, but it is not its only function. Thanks to its large surface it also transfers heat from the system to the surrounding environment, and thanks to any installed filters on it, filters the fluid from contaminants, air, and moisture. Furthermore, a reservoir also performs the very important function of allowing the hydraulic oil to settle and release any entrapped air before it re-enters the pump suction line.

A traditional hydraulic reservoir contains the following components: a filler where a hydraulic liquid is added, a drain plug where it can be removed, a pump inlet, return line, thermometer, and a sight glass. It can also have a strainer and a baffle plate which is a barrier inside the tank, that separates fluid entering the tank from fluid meant to exit the system via the pump. Traditionally, the top of the hydraulic tank is where the pump and the electric motor are installed, although many different configurations exist.


Hydraulic hoses: What is a hydraulic hose?Hydraulic hoses

When flexibility matters, instead of hydraulic tubes and pipes, hydraulic hoses are applied. Hydraulic hoses are high-pressure hoses made of rubber and steel layers, generally fitted with steel fittings at the ends. When choosing hoses, the hose bend radius must also be taken into consideration.

Types of hydraulic hoses

The most common types of hydraulic hoses are wire braided hoses and spiral hoses. Braided hoses are designed to withstand high pressures and their advantage is flexibility (higher bend radius). They can have one or more layers of steel wire braids on the inside layers. Spiral hoses are less flexible (lower bend radius) heavy-duty hoses, more suitable for impulse applications.

The right hydraulic hoses for every application can be chosen by selecting the right diameter size, pressure, temperature, and fluid compatibility ratings.

Hydraulic fittings

Hydraulic fittings: What are hydraulic fittings and connections

Hydraulic system components are connected with a set of hydraulic fittings preventing leaking or loss of pressure. Fittings can be female-threaded or male-threaded usually, two components with a female-threaded port are connected with the use of a male-threaded adapter.

The role of hydraulic fittings is to connect securely components with ports of same/ different sizes, standards or to allow a required orientation of components (position the components straight or at an angle).

Hydraulic seals: What is a hydraulic seal

Small as they might seem, they are indispensable components of every hydraulic system. These non-metallic, flexible components are used to block or separate hydraulic fluids, confining them and allowing them to be converted into mechanical energy. Except for preventing leakages, hydraulic seals inside a cylinder must also withstand high pressures, extreme temperatures, and transverse forces. Seals can be static or dynamic (exposed to movements, such as cylinder piston seals).

Examples of hydraulic seals (based on the material they are made of) are polyurethane seals, rubber seals, Viton seals, NBR seals, and PTFE seals.

When all these hydraulic components are installed and safely connected, the system can be filled with hydraulic fluid and put into operation.

Hydraulic instruments & switches

What also makes an important part of any hydraulic system are different diagnostic testers and monitors, flow meters and indicators, sensors and alarms, level and temperature gauges, and test points. They are all there to help monitor the condition of the system, prevent damages, and breakdowns.

They range from simple hydraulic see-through circulation sights which make it possible to make a manual visual check, to in-line sensors (testers and monitors) used for measuring and reporting key values, such as the fluid’s flow rate, pressure, temperature, or quality. Hydraulic column level indicators allow checking whether fluid levels are sufficient. Hydraulic test pointsin-line test points are small diagnostic connecting devices used to monitor pressure. Hydraulic pressure sensors, indicators, and displays convert pressure measurements into an analog electrical output signal while hydraulic pressure switches track fluid pressure levels and respond to variances in pressure by opening/closing electrical contacts.

Hydraulic fluids: What is a hydraulic fluid?

Although not a typical component, hydraulic fluid is indispensable. As we already mentioned, the main function of hydraulic fluid in a hydraulic system is to provide energy transmission, but it is also responsible for lubrication, heat transfer, and contamination control.

Hydraulic fluid can be petroleum-based, water-based, or synthetic. Using a wrong fluid, or mixing different types of hydraulic fluid can have a terrible effect on the system, so it is vital to always select the right one, having the optimum viscosity, viscosity index, oxidation stability, and wear resistance characteristics as advised by the system’s maker.

See Also: Storage Handling Marine Lubricants

HPUs: What is a hydraulic power unit?

Hydraulic power units (a hydraulic power pack, or, an HPU) is a self-contained system which produces the flow needed to drive motors, cylinders, and other components of a hydraulic system. The load that a consumer (cylinder or motor) sees creates the pressure seen by the HPU. It is a system made of separate components that we already discussed above.

Hydraulic power units come in different sizes, also in compact designs. They are sold as complete installations containing the power section (an electric motor with a hydraulic pump), together with a set of valves, couplings, manifold blocks a tank, etc.


We have just covered not only what hydraulics is, where hydraulics is used, but we also took a closer look at typical hydraulic systems. We listed which are all the basic components used on hydraulic systems and we learned what their role is what are their most common types.

We hope that this introductory article was useful to all buyers and that you now have a better understanding of the hydraulic components you are about to purchase.

If you would like to improve your knowledge of hydraulics even further, please do not forget to follow us on LinkedIn and subscribe to our newsletter, as we will shortly publish new and more detailed buyer guides to most common hydraulic components.

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