The basic concepts of hydraulics: definition, fields of application, design and main problems

Hydraulics is a technique for transmitting mechanical energy using pressurised fluid (oil). A typical application involves two energy conversions in order to make mechanical energy available at the shaft of a hydraulic motor (torque) or on the piston rod of a cylinder (force).The mechanical energy available at the shaft of a prime mover (electric, internal combustion) is processed by a working machine (pump) which pressurises the working fluid (hydraulic oil), which is then processed by a power machine (motor, hydraulic cylinder) to be converted back into mechanical energy. This dual conversion enables the transfer and regulation of energy, which is achieved by means of special valves positioned between the pump and the actuator.

The working fluid is energised in the form of pressure and flow rate, which determine the operation and performance of the hydraulic system. Flow rate is directly proportional to the speed (linear/rotational) of the actuator, and pressure to the force (force/torque) of the actuator. The regulation of flow rate and pressure via specific valves therefore allows for the regulation of the mechanical energy available at the actuator’s output. All this enables the creation of systems, even those of high power, that are flexible and offer extensive regulation capabilities.

To put it simply, the main components of a hydraulic system are: reservoir, power unit, valve block, piping, oil conditioning system (mainly filtration systems and cooling units), and actuators (both linear and rotary).

The fields of application are vast and range from earth-moving machinery to industry and the naval sector. In this regard, reference is made to tractors, presses, cranes and a ship’s heavy lifting systems. In all cases where there is a need to transfer mechanical power from a primary unit and then to be able to regulate it, hydraulics is widely used.

The design of a hydraulic system begins with defining the load(s) and the required mechanical characteristics, in order to then define the power unit (hydraulic power unit) and the control system (valve block).

More specifically, a hydraulic power unit is defined as the system comprising the oil reservoir, the motor-pump unit, the fluid conditioning system and the accessories (level and temperature gauges, pressure gauges and other sensors). Following the path of the oil along the pump’s delivery line, one will find pipes, valve blocks and actuators. The combination of the hydraulic power unit with pipes, valve blocks and actuators is defined as a hydraulic system. All components of a system must be appropriately selected or sized, taking into account the requirements of the specific application.

The challenges associated with a hydraulic system are manifold, although all aspects relate to the objective of reducing operating costs and extending the service life and maintenance intervals of a hydraulic system.

In recent years, designers have been focusing on aspects such as energy efficiency, reliability and fluid conditioning (primarily filtration).

Another particularly important aspect is noise reduction. This example presents a study (Senatore A., Cardone M., Buono D., and Balsamo F., 2008, “Experimental Analysis of a Ship Stabilisation Hydraulic System”, in Ovidius University Annals of Mechanical, Industrial and Maritime Engineering) carried out on a hydraulic system used to operate stabilising fins for pleasure craft. The stabilising fins are designed to reduce the boat’s oscillations, both whilst underway and at anchor, in order to improve passenger comfort on board. This purely experimental article highlights, amongst other things, the influence of an accumulator on the operation and performance of a hydraulic system.