Hydraulic actuators convert fluid pressure to mechanical energy that is used to drive output members in heavy and high-speed industrial equipment. Highly incompressible fluids, especially oils are used in the cylinders to ensure that pressure applied at the input is instantaneously transmitted. The operation of these machines primarily relies on Pascal’s principle which states that pressure applied at any part of an enclosed fluid is transmitted equally and undiminished throughout the fluid body in all directions. Additionally, the pressure acts on the interior parts of the encasing vessel at right angles. A simple hydraulic actuator consists of a cylinder, drive, and control unit (Zhang, 2008). Other components include the accumulator, pump, fluid tank, filter and heat exchange system.
Figure 1
Figure 1 shows a simple model of a hydraulic actuator. The pump draws water from the fluid tank and supplies it to the piston space in the cylinder. Pressure-build-up in the cylinder produces a mechanical force thus pushing the piston forward. This force is transmitted to the external components via a piston rod (Harries, 2003). The piston can be returned to its original position by hydraulic action or gravity.
The above illustration only depicts the basics of hydraulic actuators. To fully understand the components of modern actuators, it is essential to analyze a more complex system. Figure 2 below shows a hydraulic actuator made up of a double acting cylinder and a single piston rod.
Figure 2
1; Pressure Control Function – Maintains safety in the system by keeping pressures below the upper limit
2; Cooler – Reduces the temperature of oil thus preventing oxidation.
3; Strainer – Removes foreign particles that can damage the hydraulic pump and filters.
4; Fixed displacement pump – pumps oil or the hydraulic fluid from the fluid tank to the hydraulic cylinder.
5; Heater – Increases the temperature of oil to maintain its quality and consistency (Sch{\"o}nfeld, 2012). During winter, the oil becomes thicker and sluggish as a result of low temperatures, therefore necessitating heat supply to restore normal properties.
6; Water trap – Removes water from the hydraulic system. Presence of water compromises the quality of oil.
7; Filter – removes foreign particles that that can contaminate and block the hydraulic system.
8; pressure indicator – This component is also called the pressure gauge, it is used to take pressure readings of the hydraulic system to decide the appropriate control action.
9; Accumulator – a container that store oil or the hydraulic fluid under pressure. The stored fluid acts as a reservoir that supplement the system during fluctuations in the fluid tank.
10; Fluid Reservoir – Contains the hydraulic fluid.
11; Directional control valve – Directs the flow of fluid throughout the hydraulic system through an ON/OFF switch functionality.
12; Hydraulic cylinder [Single acting cylinder] - The component where hydraulic power is converted to linear mechanical force and motion.
In an outward stroke, the fluid is cooled and passed through the strainer after which it is sucked by the pump. From the pump, the fluid is passed progressively through the heat exchanger, water trap, filter and the accumulator (Durfee, et al., 2009). After the accumulator is filled, the fluid goes through a directional control valve unit. The directional control valve directs the fluid into the empty end of the hydraulic cylinder thus pushing the piston outwards. At the same time, the fluid that initially occupied the rod end compartment returns to the fluid tank. To retract the piston, in an inward stroke, the directional control valve directs the pump discharge into the rod end port hence pushing the piston inwards (Yao " Reedy, 2000). The fluid that initially occupied the opposite side of the cylinder is discharged back into the fluid tank.
References
Durfee, W., Sun, Z. " Van de Ven, J., 2009. Fluid power system dynamics. Center for Compact and Efficient Fluid Power.
Harries, D. A., 2003. Hydraulic actuation systems. s.l.:Google Patents.
Sch{\"o}nfeld, J., 2012. Analogy of hydraulic, mechanical, acoustic and electric systems. Applied Scientific Research, Section A, pp. 417-450.
Yao, B. a. B. F. " Reedy, 2000. Adaptive robust motion control of single-rod hydraulic actuators: theory and experiments. IEEE/ASME Transactions on mechatronics, pp. 79-91.
Zhang, Q., 2008. Basics of hydraulic systems. s.l.:CRC Press.
