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In operation, the flow will enter the priority flow divider from port B. From Sullivan, J. Fluid Power: Theory and Application, 2nd ed. This action will begin to close the priority outlet port A and open the secondary outlet B. When the flow rate is below the designed priority flow rate, the spool will be all the way to the right, the secondary outlet will be closed, and the priority outlet will be wide open.

The proportional-type flow divider follows the same principle as the prior- ity flow divider, except that two orifices are used and the spool is normally spring-loaded to a particular flow split ratio. Observing the directional control valves described in Section 1. However, several of the control mechanisms used for directional control valves, like a solenoid, detent lever, hydraulic pilot, and so forth, only allow the valve to move to specific positions.

Directional continuous control valves with mechanical control are well known in mobile hydrau- lics where the position of the command lever is defined by a human operator based on his or her own observation of the position or velocity of the cylinder or motor.

Valve technology with continuous electrical input started with the servo-valves in the early s [21]. Another notable event was the development of the proportional directional control valves in the late s [22]. Encompassing technological principles from both these valve types, new products are being offered on the market, such as servo-proportional valves [23]. Regardless of their commercial identification or construction principle, according to ISO [17] and ISO [18] these are electrically-modulated hydraulic flow control valves, since they provide a degree of proportional flow control in response to a continuously variable electrical input signal.

The first stage pilot stage is composed of either a jet pipe valve or flapper-nozzle valve driven by a torque motor a permanent magnet, variable reluc- tance actuator.

The second stage is a spool valve, its position being fed back in order to place the torque motor armature at the null position. Other methods of position feedback are the spring-centered spool, direct position feedback or hydraulic follower, and electric feedback using a position transducer [24].

Frequently, the spool slides into a sleeve where the ports were machined. The relative position between the spool lands and sleeve ports then determines the flow control orifices. The same solu- tion is adopted for directly operated valves with electrical feedback, driven by a linear force motor. Advances in the manufacturing process and changes in the user requirements have led to changes in the construction details. For example, pilot-operated servo-valves like that shown in Figure 1. In both cases, the objective was to obtain the same functional characteristics as servo-valves—that is, the continuous control of flow direction and rate, but with a distinct mechanical design.

The proportional valves are controlled by proportional solenoids, which, unlike the torque motor and linear force motor, do not comprise a permanent magnet and the force is provided in only one direction for any current polarity. The metering notches on the spool, as shown in Figure 1. However, they are not machined on all valve designs. Valve designs with spool-sleeve mounting are also available with both smaller machining toler- ances and radial clearances.

The servo-proportional valve designation has also been used by valve manu- factures for these construction solutions [23,26,27]. The valve behavior can be described through the composition of two parts—with feedback or without feedback. The first block corresponds to the transformation of the input voltage into spool displacement.

The second one refers to the output flow rate as a consequence of the spool displacement and the pressures in the supply P , return T , and working A and B ports of the valve Figure 1. In essence, the valve amplifier controls the current applied to each proportional solenoid or to the pair of coils of a torque motor or linear motor.

According to electromechanical principles, this current produces a force or torque that is transmitted to a valve element. In the case of a pilot-operated servo-valve, as shown in Figure 1. The pressure difference makes the resting spool change its position, which is fed back to the pilot valve. In directly-operated valves, as shown in Figure 1. The parameter values of Equation 1.

Comparing these curves with the general response time of a second-order system Figure 1. Using Equation 1. The valve catalogs also inform the response time defined according to ISO [17] and shown in Figure 1. The approximate calculation of the natural frequency based on the response time is carried out using Equation 1. Another way to present the valve dynamic response is through a frequency response diagram Bode diagram , where it is possible to extract directly the values of the natural frequency and damping ratio [16].

The second block in Figure 1. By applying the concepts related to Equation 1. By combining Equations 1. The nominal flow occurs when the valve is operating with nominal voltage, that is, with the nominal opening. It is important to observe that for some valves the nominal flow is specified at a partial pressure drop DpP—A and this must be multiplied by two to allow the flow coefficient calculation.

Constructive aspects of the directional control valves, like different center position arrangements Figure 1. The accumulators used in hydraulic systems can be grouped into three categories: weight-loaded or gravity type, spring-loaded type, and gas-loaded type [31] Figure 1.

The weight-loaded type consists of a cylinder with a piston where a mass is attached to its top. The gravitational action on the mass creates a constant fluid pressure, irrespective of the flow rate and fluid volume in the cylinder chamber. The spring-loaded accumulator simply uses the spring force to load the piston. When the fluid pressure increases to a point above the preload force of the spring, fluid will enter the accumulator to be stored until the pressure reduces.

In this type of accumulator, the fluid pressure varies with the piston position and, consequently, with the fluid volume in the accumulator.

The gas-loaded accumulator can be either without separation between liquid and gas, a piston type or a bladder and diaphragm type, as shown in Figure 1. In the gas-loaded accumulator, an inert gas, such as dry nitrogen, is used as a pre-charge medium. In operation, this type of accumu- lator contains the relatively incompressible hydraulic fluid and the more readily compressible gas.

When the hydraulic pressure exceeds the pre-charge pressure exerted by the gas, the gas will com- press, allowing hydraulic fluid to enter the accumulator. The reservoir should be sized to both afford adequate fluid cooling and to enclose a suf- ficient volume of oil to permit air bubbles and foam to escape during the residence time of the fluid in the reservoir.

Commonly, the reservoir is sized to hold at least three times the volume of fluid that can be supplied by the pump in one minute. The reservoir depth must be adequate in order to assure that during peak pump demands, the oil level will not drop below the pump inlet level.

Moreover, the pump should be mounted below the reservoir so that a positive head pressure is available at all times. This is critical when water-based hydraulic fluids are used, as these fluids can have a higher mass density as well as a much higher vapor pressure than mineral-oil-based fluids. Sight gauges are normally used to monitor the fluid level and a cleanout plate is provided to promote cleaning and inspection. A breather system with a filter is also provided to admit clean air and to maintain atmospheric pressure as fluid is pumped into and out of the reservoir.

With water-based hydraulic fluids, a pressurized reservoir is recommended. Special breather caps can be installed to vent between 0. From Norvelle, F. This is an important feature to have so that when the reservoir is cooling down, no appreciable vacuum develops in the reservoir. This feature will minimize pump cavitation upon start-up and also prevent a possible reservoir implosion.

Recent trends in industrial manufacturing are to compact machines and equipment in order to economize materials, energy consumption, and required space. A reduction in the size of fluid power systems is encouraged in order to conserve energy and reserve oil. It is somewhat inevitable in designing these systems to minimize the size of the oil reservoir, meaning that the bubbles entrained in the oil may not be removed effectively during the fluid sojourn time in the reservoir.

As mentioned above, in order to remove bubbles big vessels are generally used, but it takes a long time to eliminate minute bubbles from fluids by flotation alone. Another solution is the device shown in Figure 1. Due to the difference in centrifugal forces created in the swirl flow, the bubbles tend to move toward the central axis port B where they are collected and ejected through the vent port port X.

The hydraulic fluid is expected to create a lubricating film, thereby keeping precision parts separated. Particulate contaminants can break this film, cause erosion on the surfaces or even block the relative movement. Consequently, the hydraulic component life expectancy is reduced, impairing its performance or even causing its complete failure. The contaminants in hydraulic systems come from several sources, such as the degradation of the circuit components, the external environment, the circuit assembly, and from the new hydraulic fluid which can have a standard contamination level below the system requirements.

The removal of particulate matter and silt from a hydraulic fluid is performed by filters that can be installed at different locations in the hydraulic circuit, characterizing the following types of fil- tration: suction, pressure, return and off-line filtration [35,36]. Suction line filtration: Suction filters are located before the suction port of the pump and provide pump protection against fluid contamination Figure 1.

Some may be inlet strainers, submersed in the fluid. Others may be externally mounted. Some pump manufacturers do not recommend the use of a suction filter. Pressure line filtration: Pressure filters are located downstream of the pump Figure 1.

They usually produce the lowest system contamination levels to assure clean fluid for sensitive high- pressure components and provide protection of downstream components from pump-generated contamination. Return line filtration: In most systems, the return filter is the last component through which fluid passes before entering the reservoir Figure 1. Therefore, it captures wear debris from system working components and particles entering through worn cylinder rod seals before such contami- nants can enter the reservoir.

Return lines can have substantial pressure surges, which need to be taken into consideration when selecting filters and their locations. The relatively low cost and the cleanli- ness of the fluid suctioned by the pump are factors that make the use of these filters attractive. Re-circulating or off-line filtration: Off-line filtration consists of a hydraulic circuit with at least a pump and its prime mover and a filter. These components are installed off-line as a small subsystem separate from the working lines or can be included in a fluid-cooling loop Figure 1.

As with a return line filter, this type of system is best suited to the maintenance of overall cleanliness, but does not provide specific component protection. An off-line filtration loop has the added advantage of being relatively easy to retrofit on an existing system that has inadequate filtration. Also, the filter can be serviced without shutting down the main system. The circuits shown in Figure 1. In general, the systems can incorporate multiple filtration techniques, using a combination of suc- tion, pressure, return, and off-line filters.

A fourth function is related to fluid storage and conditioning. This function is required because the fluid must be available for the energy trans- mission, and since the fluid is continuously in contact with the hydraulic components its proprieties must be controlled.

Fluid proprieties such as viscosity, mass density, vapor pressure, contamination, gas solubility, and bulk modulus change the physical relations modeled by the continuity equation, and conserva- tion of energy, among others. Therefore, besides causing component degradation, the modifying of physical proprieties also changes the hydraulic system behavior. Throughout the chapters of this Handbook the proprieties of different fluids that are used in hydraulic systems are analyzed as well as their effect on the life and behavior of the components.

Luis Alberto Galaz Mamani for his assistance with the preparation of the figures shown herein. Linsingen, I. Belan, H. International Organization for Standardization, ISO - Fluid Power Systems and Components — Graphic symbols and circuit diagrams — Part 1: Graphic symbols for conventional use and data-processing applications, Switzerland, 2nd ed.

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