Hydraulic systems are power-transmitting assemblies employing pressurized liquid to transmit energy from an energy-generating source to an energy-use area. All hydraulic systems depend on Pascal’s law, named after Blaise Pascal, who discovered the law. This law states that pressurized fluid within a closed container-such as cylinder or pipe-exerts equal force on all of the surfaces of the container.
In actual hydraulic systems, Pascal’s law defines the basis of the results which are obtained from the system. Thus, a pump moves the liquid in the system. The intake of the pumps connected to a liquid source, usually called the tank or reservoir. Atmospheric pressure, pressing on the liquid in the reservoir, forces the liquid into the pump. When the pump operates, it forces liquid from the tank into the discharge pipe at a suitable pressure.
The flow of the pressurized liquid discharged by the pump is controlled by valves. Three control functions are used in most hydraulic systems: (1) control of the liquid pressure, (2) control of the liquid flow rate, and (3) control of the direction of flow of the liquid.
The liquid discharged by the pump in a fluid-power system is directed by valves to a hydraulic motor. A hydraulic motor develops rotary force and motion, using the pressurized liquid as its energy source. Many hydraulic motors are similar to pumps, except that the motor operates in a reverse manner from a pump.
Where linear instead of rotary motion is desired, a cylindrical tube fitted with a movable piston, called a hydraulic cylinder, is often used. When the piston is moved by the pressurized fluid, the piston rod imparts a force or moves an object through a desired distance.
Restricting the movement of the piston in a hydraulic cylinder, as when the piston carries a load, creates a specific pressure relationship within the cylinder. The surface area of the piston face is said to contain a specific number of square inches. The pressure of the pressurized liquid, multiplied by the piton area, produces an output force, measured in pound, at the end of the piston rod.
The speed of movement of the piston rod depends on how fast the pressurized fluid enters the cylinder. Flow into the cylinder can be directed to either end, producing either a pushing or pulling force at the piston rod end. A seal around the rod where it passes through the cylinder end prevents leakage of the liquid.
Directional control of the piston depends on which end of cylinder the liquid enters. As pressurized liquid enters one end of the cylinder, liquid must be drained from the other end. The drained liquid is led back to the reservoir. In a pneumatic system using air, the air in the exhausting end of the cylinder is vented to the atmosphere.
Directional-control valves, also called two-way, three-way, four-way, etc. , are named in accordance with their basic function. Pressure-control and simple restrictor valves are usually two-way valves. They provide ON or OFF service. A three-way valves may perform several functions, all associated with the three-ports in the valve. For example, the power or pressurized liquid from a pump in a tractor may be sent to the hydraulic system serving the tractor’s front-end loader. Or the three-way valve may send the pressurized liquid to a hydraulic motor driving a feed conveyor while the front-end loader is not being used.
Three-way valves may also be used to direct pressurized fluid to a single-acting hydraulic cylinder. As the three-way valve is actuated (operated) it can stop the pressurized flow to the cylinder. Further, the same valve can divert liquid from the cylinder to the reservoir, so the cylinder can retract by gravity or return springs and assume its original position.
