Hydraulic systems are found in a huge variety of applications and environments from small assembly machinery or security gates through to piling rigs, theme park rides, supersonic aircraft and the bascules on London’s Tower Bridge.

Tower Bridge BasculesWhen it was built, Tower Bridge was the largest and most sophisticated bascule bridge ever completed (“bascule” comes from the French for “see-saw”). These bascules were operated by hydraulics, using steam to power the enormous pumping engines. The energy created was stored in six massive accumulators, as soon as power was required to lift the Bridge, it was always readily available. The accumulators fed the driving engines, which drove the bascules up and down. Despite the complexity of the system, the bascules only took about a minute to raise to their maximum angle of 86 degrees.

Today, the bascules are still operated by hydraulic power, but since 1976 they have been driven by oil and electricity rather than steam.


The use of hydraulics enables the operator to achieve significant work (lifting heavy loads, turning a shaft, drilling precision holes, etc.) with minimum effort through the application of Pascal’s Law, which states that the: “Pressure applied to any part of a confined fluid transmits to every other part with no loss. The pressure acts with equal force on all equal areas of the confining walls and perpendicular to the walls.” Because hydraulic fluid is nearly incompressible, it is able to transmit power instantaneously.

In addition to hydraulic fluid, the main components that make up a hydraulic system are the reservoir, pump, valve(s) and the actuators, (the motor, cylinder etc.) Looking at each of these in turn:

Reservoir: The reservoir holds a volume of hydraulic fluid and allows any solid contaminants to settle at the bottom of the reservoir while transferring heat from the system, and helping air and moisture to be released from the fluid.

Pump: The hydraulic pump converts mechanical energy into hydraulic energy by moving, or transmitting, the hydraulic fluid. There are several types of hydraulic pumps including gear, vane and piston (see our Hydraulic Pumps page in our Technical Library for more information). In all cases, the role of the hydraulic pump is to displace fluid volume against a resistant load or pressure.

Valves: Hydraulic valves are used to start, stop and direct the flow of hydraulic fluid in the system.

Actuators: Hydraulic actuators come at the end of the process, where the hydraulic energy is converted back to mechanical energy. This can be done through using a hydraulic cylinder which converts hydraulic energy into linear motion and work, or a hydraulic motor which converts hydraulic energy into rotary motion and work.

If you are looking for more general information on hydraulics then we have more for you within our reference and hydraulic products pages. The following articles may be of interest too:

How hydraulic cylinders work

The hydraulic cylinder consists of a cylinder barrel, in which a piston connected to a piston rod moves back and forth. The barrel is closed on each end by the cylinder bottom (also called the cap end) and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber (cap end) and the piston rod side chamber (rod end). The hydraulic pressure acts on the piston to do linear work and motion.

Flanges, trunnions, and/or clevisses are mounted to the cylinder body. The piston rod also has mounting attachments to connect the hydraulic cylinder to the object or machine component that it is pushing.

A hydraulic cylinder is the actuator or “motor” side of the system. The “generator” side of the hydraulic system is the hydraulic pump which brings in a fixed or regulated flow of oil to the bottom side of the hydraulic cylinder, to move the piston rod upwards. The piston pushes the hydraulic oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is approximately zero, the force on the piston rod equals the pressure in the hydraulic cylinder times the piston area (F=PA).

The piston moves downwards if oil is pumped into the piston rod side chamber and the hydraulic oil from the piston area flows back to the reservoir without pressure. The pressure in the piston rod area chamber is (Pull Force) / (piston area – piston rod area).


A hydraulic cylinder (hydraulic actuator, hydraulic ram) consists of the following parts:

Cylinder Barrel: The cylinder barrel is mostly a seamless thick walled forged pipe that must be machined internally. The cylinder barrel is ground and/or honed internally.

Cylinder Bottom or Cap: In most hydraulic cylinders, the barrel and the bottom are welded together. This can damage the inside of the barrel if done poorly. Therefore some hydraulic cylinder designs have a screwed or flanged connection from the cylinder end cap to the barrel (see “Tie Rod Cylinders” below). In this type the cylinder barrel can be disassembled and repaired in future.

Cylinder Head: The cylinder head is sometimes connected to the barrel with a sort of a simple lock (for simple cylinders). In general however, the connection is screwed or flanged. Flange connections are the best, but also the most expensive. A flange has to be welded to the pipe before machining. The advantage is that the connection is bolted and always simple to remove. For larger hydraulic cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a huge problem as well as the alignment during mounting.

Piston: The piston is a short, cylinder-shaped metal component that separates the two sides of the cylinder barrel internally. The piston is usually machined with grooves to fit elastomeric or metal seals. These seals are often O-rings, U-cups or cast iron rings. They prevent the pressurized hydraulic oil from passing by the piston to the chamber on the opposite side. This difference in pressure between the two sides of the piston causes the cylinder to extend and retract. Piston seals vary in design and material according to the pressure and temperature requirements that the hydraulic cylinder will see in service. Generally speaking, elastomeric seals made from nitrile rubber or other materials are best in lower temperature environments while seals made of Viton are better for higher temperatures. The best seals for high temperature are cast iron piston rings.

Piston Rod: The piston rod is typically a hard, chrome-plated piece of cold-rolled steel which attaches to the piston and extends from the cylinder through the rod-end head. In double rod-end hydraulic cylinders, the actuator has a rod extending from both sides of the piston and out both ends of the barrel. The piston rod connects the hydraulic actuator to the machine component doing the work. This connection can be in the form of a machine thread or a mounting attachment such as a rod-clevis or rod-eye. These mounting attachments can be threaded or welded to the piston rod or, sometimes, they are a machined part of the rod-end.

Rod Gland: The hydraulic cylinder head is fitted with seals to prevent the pressurized hydraulic oil from leaking past the interface between the rod and the head. This area is called the rod gland. It often has another seal called a rod wiper which prevents contaminants from entering the hydraulic cylinder when the extended rod retracts back into the cylinder. The rod gland also has a rod bearing. This bearing supports the weight of the piston rod and guides it as it passes back and forth through the rod gland. In some cases, especially in small hydraulic cylinders, the rod gland and the rod bearing are made from a single integral machined part.

A hydraulic cylinder should be used for pushing and pulling only. No bending moments or side loads should be transmitted to the piston rod or the cylinder. For this reason, the ideal connection of a hydraulic cylinder is a single clevis with a spherical ball bearing. This allows the hydraulic actuator to move and allow for any misalignment between the actuator and the load it is pushing.

Hydraulic motor technology

A hydraulic motor (or hydraulic actuator) is used to convert the kinetic or potential energy (hydraulic pressure and flow) of a fluid (hydraulic oil) into mechanical energy and rotation, the hydraulic motor being the rotary counterpart to the hydraulic cylinder.

Hydraulic motors, hydraulic pumps and hydraulic cylinders can be combined together into hydraulic drive systems. When one or more hydraulic pump is coupled to one or more hydraulic motor, a hydraulic transmission is then created with hydraulic fluid being used under pressure to drive machinery.

The useful power in any hydraulic drive system is calculated as a product of flow and pressure, minus any inefficiencies. Therefore, when selecting a hydraulic motor and/or hydraulic pump for a specific application the relationships between flow, displacement, speed, torque and pressure, and the influence of any inefficiencies must be taken into account.

Hydraulic motors have many applications including: winches and crane drives, wheel motors for military vehicles, self-driven cranes, excavators, conveyor and feeder drives, mixer and agitator drives, roll mills, drum drives for digesters and kilns, shredders for cars, tyres, cable and general recycling, drilling rigs and trench cutters.