Hydraulic System: Function, Components, Advantages & Drawbacks  

Hydraulic systems are built in such way that they are able to move large loads by controlling a high-pressure fluid in distribution lines and pistons with mechanical or electromechanical valves. A typical hydraulic system such as one illustrated below consists of a pump used to deliver high-pressure fluid, a pressure regulator to limit the pressure in the system, valves to control flow rates and pressures, a distribution system composed of hoses or pipes, and linear or rotary actuators.

The pump is the actual source of mechanical power, and it is physically separate from the cylinder actuator. Practically, this is important in the sense that, only the cylinder needs to be mounted at the place where the motion is required; the pump can be located elsewhere. This allows a relatively small component such as a hydraulic cylinder to provide far more power than a similarly sized actuator, which must have the motor attached.

Hydraulic Pumps

In an active hydraulic system, a pump is used to create the hydrostatic pressure. A hydraulic pump is usually driven by an electric motor such as a large AC induction motor or an internal combustion engine. Typical fluid pressures generated by pumps used in heavy equipment such as construction equipment and large industrial machines are in range of 1000 psi to 3000 psi. The hydraulic fluid selected should have the following characteristics:

• Good lubrication to prevent wear in moving components e.g. between pistons and cylinders.
• Corrosion resistance.
• Incompressibility to provide rapid response.

Most hydraulic pumps act by positive displacement, which means that they deliver a fixed volume of fluid with each cycle or rotation of the pump. The three main types of positive displacement pumps used in hydraulic systems are: gear pumps, vane pumps, and piston pumps.

A diagram of a gear pump, which displaces the fluid around the housing between teeth of meshing gears, is shown below:

The gear pump above consists of two meshed gears in the housing. As the gears rotate, the fluid is trapped in the little spaces between the teeth and the housing (both top and bottom) and is conveyed from the inlet to the outlet. The mesh between the gears in the center is tight enough so that no fluid moves through either way at that point.

Vane pump, illustrated in figure 1.2 below, displaces the fluid between vanes guided in rotor slots riding against the housing and vane guide. The vane guide supports the vanes from one side of the housing to the next and is constructed to allow the fluid to pass. The output of the displacement can be varied with a constant motor speed by moving the shaft vertically relative to the housing.

A vane pump, as illustrated in the diagram above, consists of an offset rotor in housing with retractable vanes. The spring-loaded vanes push out and seal against the housing wall. Since there is more fluid between the vanes in the top half of the housing than in the bottom, there is a net transfer of fluid from the inlet to the outlet. In some designs, the position of the rotor is adjustable. The more offset the rotor axis, the more fluid is pumped. Such a pump is referred to as a variable displacement pump.

An axial piston pump uses small pistons reciprocating back-and-forth to pump the fluid as shown in the figure below:

The pump consists of a rotating cylinder and a metal ring called a swash plate (which doesn’t rotate). The cylinder contains a number of small pistons that do the actual pumping. One end of each piston rides against the swash plate. Because the swash plate is at an angle to the cylinder, each piston is forced to move in-and-out with each rotation of the cylinder. By changing the angle of the swash plate, the quantity of fluid pumped per revolution is changed.

Table below lists and compares the general characteristics of the different hydraulic pump types.

Table 1: Comparison of pump characteristics

Pump Type	Displacement	Typical Pressure (psi)	Cost
Gear	Fixed	2000	Low
Vane	Variable	3000	Medium
Piston	Variable	6000	High

Pressure Regulator

Because positive displacement hydraulic pumps provide a fixed volumetric flow rate, it is necessary to include a pressure relief valve, called a pressure regulator, to prevent the pressure from exceeding design limits. A simple example of a pressure regulator is the spring-ball arrangement shown below:

In reference to the above figure, when the pressure force exceeds the spring force, fluid is vented back to the tank, preventing a further increase in pressure. The threshold pressure, or cracking pressure, is usually adjusted by changing the spring’s compressed length and thus its resisting force.

Hydraulic Valves

Generally, they are two types of hydraulic valves: the infinite position valve that allows any position between open and closed to modulate flow or pressure, and the finite position valve that have discrete positions, usually just open and closed, each providing a different pressure and flow condition.

The common types of fixed position valves are check valves, poppet valves, spool, and the rotary valves. The check valve allows flow in one direction only. The poppet valve is a check valve that can be forced to open to allow reverse flow.

A spool valve consists of a cylindrical spool with multiple lobes moving within a cylindrical casing containing multiple ports. The spool can be moved back and forth to align spaces between the spool lobes with the input and output ports in the housing to direct high-pressure flow to different conduits in the system. This is illustrated in the figure below:

The spool is balanced in each position because the static pressure is the same on the opposing internal faces of the lobes. Thus, no force is needed to hold a position. In the left position, port A is pressurized, and port B is vented to the tank; and in the right position, port B is pressurized and port A is vented to the tank. To move the spool between positions and overcome the hydrodynamic forces linked with changing the momentum of flow, an axial force is needed, from either an actuator or manual control lever.

The aforementioned spool operation is limited to two positions (ON and OFF). We can achieve continuous operation by using a proportional valve, one whose spool moves a distance proportional to a mechanical or electrical input such as a lever or an adjustable current solenoid, thus changing the rate of flow and varying the speed and force of the actuator. When the spool position is controlled by electrical solenoids, the proportional valve is referred to as the electrohydraulic valve.

Hydraulic Actuators

A common type of hydraulic actuator is the hydraulic cylinder, usually with a piston driven by the pressurized fluid. A cylinder can be single acting, where it is driven to and held in one position by pressure and returned to the other position by a spring or by the weight of the load, or double acting, where pressure is used to drive the piston in both directions.

Accumulators

An accumulator, which is connected into the hydraulic system, is type of spring-loaded storage tank for hydraulic fluid, and is used to serve two functions:

• Acts as a low-pass filter to remove pressure pulsations from the pump.
• Stores extra fluid for high-demand times when the actuator requires fluid at a faster rate than the pump can actually supply.

A Complete Hydraulic System

A diagram below, using standard symbols of a complete simple hydraulic system, which includes the tank, filter, pump, accumulator, pressure-control valve, directional control valve and the cylinder.

Advantages and Shortcomings of Hydraulic Systems

Hydraulic systems have the following advantages:

• Hydraulic systems can generate extremely large forces from very compact actuators.
• They can also provide precise control at low speeds and have built-in travel limits defined by the cylinder stroke.

The shortcomings of hydraulic systems include:

• They require a large infrastructure that includes a high-pressure pump, tank and distribution lines, this may be costly to set up.
• There is likelihood for fluid leaks, which are undesirable in a clean environment.
• Possible hazards associated with high pressures such as ruptured line.
• Vibration.
• Noisy operation.
• Maintenance requirements.

Because of these drawbacks posed by the hydraulic systems, electric motor drives are often the preferred choice. Nonetheless, in large systems, which require extremely large forces, most often than not, hydraulics provide the only alternative.

Also read:

• Directional Control Valves: Function & Principle of Operation
• Pneumatic Actuators
• Pneumatic System Components: Types & Functions