PRESSURE CONTROL VALVES 1.Discuss the function of the pressure control valves in hydraulic power systems. The control of hydraulic power in hydraulic power systems is carried out by means of control valves. The control requirements are imposed by the function of the system. The parameters of the mechanical power delivered to the load are managed hydraulically by controlling the pressure, flow rate, or the direction of flow. 2.In a sketch, draw the classification, the function and the symbol of pressure valves that are used in hydraulic power transmission systems Direct-Operated Relief ValvesPilot Operated Relief ValvesPressure Reducing ValvesSequence valveS 3.Describe briefly the operation of the direct-operated pressure relief valve illustrated in Figs.: 5.1, 5.2 and 5.4. Relief valves are connected with high-pressure and return lowpressure lines. They are used to limit the maximum operating pressure in the high-pressure lines. The relief valve consists mainly of a poppet, loaded by a spring. The poppet is pushed by the spring to rest against its seat in the valve housing. The spring pre-compression force is adjusted by a spring seat screw or by inserting distance rings. The poppet is subjected to both the spring and pressure forces. The poppet rests against its seat as long as the pressure force, FP PAP, is less than the spring force, Fx kxo. The two forces are equal when the pressure reaches the cracking pressure, Pr. For further increases of pressure, the poppet is displaced and the oil flows from the high pressure line, P, to the return line, T. 4.Draw schematically a pilot-operated relief valve and describe its operation briefly (fig. 5.5). The pilot-operated relief valve consists of a main valve (1) loaded by a spring (2). The valve is designed with a large diameter of the main poppet (1) and a small stiffness of the main valve spring (2), which decreases the override pressure. The spring is pre-compressed. But the pre-compression force can be overcome by a small pressure difference (about 410) bar between the input pressure, P, and the spring chamber pressure, Ps.The operation of the main valve is controlled by installing a pilot stage (3). The pilot valve is a direct-operated relief valve, connected to the input high-pressure line through the two nozzles, Nand N2. The diameters of these nozzles are usually less than 1 mm. The flow rate passing through them is very small. The direct-operated relief valve (3) is used to impose an upper limit to the spring chamber pressure (2). The pilot stage has small dimensions and a very stiff spring. However, the override pressure is of negligible value in this valve, due to the very small flow rate.When the supply pressure is less than the cracking pressure of the pilot stage, its poppet is seated and the pressures in the valve input line and spring chamber, C1, are equal. At this condition, the pressure forces acting on the main poppet (1) are compensated. The spring (2) acts to close the main valve. When the pressure, P, becomes greater than the pilot valve cracking pressure, the pilot poppet valve opens, and the pressure in chambers C1and Cbecomes equal to the relief pressure of the pilot stage, Pr. As the pilot valve flow rate increases, the dynamic depression in nozzles 

N1and N2results in a sufficient pressure difference (– Ps) between the inlet chamber and the chamber C1. The main valve poppet moves to allow the fluid to flow to the return line. The pilot-operated relief valves allow for great flow rate with smaller override pressure, compared with the direct-operated relief valves.5.Discuss the unloading function in a pressure relief valves and how we can obtain it in pilot operated valves. The pilot-operated relief valves can be used for system unloading. When we are not doing any work or when we are waiting, if we have a normal relief valve, the system has the maximum pressure, so there is a high consumption. In order to avoid that, we can use a system like the one in the next Figures. We use a directional control valve, controlled electrically, and when we activate it, the flow rate passes through there to the tank, with no need to activate the relief valve, and therefore, with no pressure in the system and no consumption.6.Discuss the application of the hydraulic pressure reducing valves.Pressure reducers are used when a subsystem (a cylinder, for example), operates at a lower pressure than the one of the main system.7.Explain briefly the operation of the direct-operated hydraulic pressure reducer illustrated in Figs. 5.11 and 5.13. The pressure in the exit port (A) is connected to the control chamber, at the right hand side of the spool, via the line (6). It acts on the spool, against the spring (3). If the pressure in the exit port (A) is less than the value corresponding to the spring pre-compression force, the spool shifts to its extreme righthand position. The pressure line (P) is then connected to the exit port (A). The pressure in the port (A) increases and the force acting on the spool increases. When this force overcomes the spring force, the spool moves to the left and the connection (P-A) is throttled. At the final position, the spool lands separate the line (A) from both the pressure and tank lines, except for the radial clearance. If the pressure increases to values greater than that preset at the spring, the spool moves further to the left. This spool displacement connects the line (A) to the tank, which decreases the pressure in the line (A).8.Discuss briefly the meaning of the information given in figure 5.12. In the previous figure the flow characteristics of a pressure reducer for different preset pressure levels. If the exit pressure, PA, is increased, the spool moves to the left, allowing the fluid to flow from port (A) to the tank line (T), creating a negative flow rate. In this way, the valve acts as a pressure reducer and as a relief valve for line (A). A built-in check valve (2) allows for the free flow from line (A) to line (P) whenever needed.9.Describe the functions of pressure sequence valves in hydraulic systems. The sequence valves are used to create a certain sequence of operations according to the pressure level in the system. The pressure in this chamber acts on the spool against the spring force. If the pressure forces overcome the spring force, the spool displaces to the left connecting line (P) to (A). The valve can be externally controlled through port (B). In this case, the connection of port (P) with the control chamber should be blocked. Optionally, the valve is equipped with a check valve to allow for free reverse flow. The sequence valves are also called multifunction valves and are used in various configurations to control sequencing, braking, unloading, load counter balancing, or other functions.

CHECK VALVES 1.Which are the differences between direct operated and pilot operated check valves?Some applications, such as the hydraulic locking of hydraulic cylinders, require the installation of check valves. In certain operating modes of these systems, it is recommended to open the check valve to allow free fluid flow in both directions. The pilot-operated check valves are designed to fulfil this requirement. In comparison with the direct operated check valves in which the fluid can flow only in one direction.2.Discuss briefly the function of pilot and direct operated check valves.Spring-Loaded Direct-Operated Check Valves: The direct-operated check valves consist of a simple poppet valve with a poppet loaded by a spring (see Fig. 5.49). The poppet rests against its seat, obstructing the direction from (B) to (A). It allows the fluid flow in the direction (A) to (B) if the pressure difference (PA – PB) is greater than the cracking pressure Pr, defined as the pressure diference which produces a pressure force equal to the spring force. The cracking pressure is usually less than 10 bar for the check valves. Direct-Operated Check Valves Without Springs: Some applications require a very low cracking pressure. In this case, the check valve is designed with springs of very low stiffness or even without springs (see Fig. 5.50). These valves operate with a cracking pressure less than 0.2 bar. Their symbol is drawn without the spring.  Pilot-Operated Check Valves Without External Drain Ports: Some applications, such as the hydraulic locking of hydraulic cylinders, require the installation of check valves. In certain operating modes of these systems, it is recommended to open the check valve to allow free fluid flow in both directions. The pilot-operated check valves are designed to fulfill this requirement (see Figs. 5.51 and 5.53).These valves allow the fluid to flow in one direction (A to B) and are piloted to allow for the reverse flow (from B to A). These pilot-operated check valves consist of valve housing (1), main poppet (2), spring (3), pilot piston (4), and an optional decompression poppet assembly (5). In the checked direction (B to A), the main poppet (2) and the decompression poppet (5) are seated by the spring (3) and by the pressure in port (B). When the pilot pressure is applied to the port (X), the pilot piston (4) moves to the right. The decompression poppet (5) opens first, followed by the main poppet (2). This design permits the rapid and smooth decompression of the fluid. Figure 5.52 shows an example of the application of a pilot-operated check valve for the hydraulic locking of a cylinder position.3.Analyze the hydraulically piloted check valve (Figure 5.51) and describe briefly its function.Figure 5.54 shows a double pilot-operated check valve of sandwich plate design. This valve provides leak-free closure of the two actuator ports (A2 and B2) during the idle periods. Free flow (from A1 to A2 or B1 to B2) is permitted, while the flow in the opposite direction is not allowed. The flow (from A1 to A2 or B1 to B2) applies a pressure force to spool (1), which moves to the left (or the right), unseating the opposite poppet (2). The oil now flows from (B2 to B1 or A2 to A1). To ensure that the poppet valves seat correctly, the ports of the DCV (A1 and B1) should be drained. They should be connected to the tank when the DCV is put in its neutral position. 4.Draw the symbols of all type of check valves. –Spring loaded direct operated check valve-Direct operated check valve without springs-Pilot operated check valve with internal drains-Pilot operated check valve with external drains-Double pilot operated check valve-Mechanically piloted check valve 

FLOW VALVES 1.What are the main types of Flow Control Valves? And what are the main differences between them? The next figure represents the classification of the main types of flow control valves:Throttle valves: They are pressure-dependent. The flow rate through them changes with the variation of the pressure difference. Therefore, the simple throttle valves do not control precisely the fluid flow rate. They can be viscosity dependent or viscosity independent (also called sharp edged throttle valves)Pressure compensated valves: They are pressure independent, they give the same flow rate, no matter the variation of the pressure difference of the system. There are series pressure compensated flow control valves, parallel pressure compensated flow control valves, and flow dividers. 2.Explain the operation of the pressure-compensated flow control valve illustrated in figure 5.59. Figure 5.59 gives the hydraulic circuit of a system incorporating a series pressure-compensated flow control valve FCV, also called two way FCV. The valve consists of a sharp-edged throttle and a pressure compensator connected in series. The pressure compensator is installed downstream of the throttle. It consists of a spool valve loaded by a spring. The pressure difference across the main throttle (P1 – P2) acts on the spool by the force Fp = As(P1 – P2), against the spring force Fx. The compensator keeps a constant pressure drop, ΔPt, across the main throttle. Typically, the value of the pressure difference is selected in the range 4 to 10 bar. In the steady state, this pressure difference produces a force equal to the spring force. The two-way flow control valve operates as follows:In the steady state, the pressure difference across the main throttle reaches its required value, ΔPt. The pressure and spring forces are in equilibrium and the spool gets in its steady-state position. If the pressure difference is increased, P1 – P2 > ΔPt, the flow rate increases. Simultaneously, the spool moves downward, against the spring, to decrease the area of the spool valve restriction. The flow rate through the main throttle decreases and so does the pressure difference across the main restriction. Afterward, the valve again reaches the steady state, where P1 – P2= ΔPt.If the pressure difference is decreased, P1 – P2 < ΔPt, the flow rate decreases. The pressure force acting on the compensator spool becomes less than the spring force. The spring pushes the spool upward, increasing the restriction area of the spool valve. The flow rate through the main throttle and the pressure difference (P1 – P2) increase until they reach the required steady-state values.The pressure compensator acts constantly to compensate for the effect of the variation of supply and load pressures. In the steady state, the spool of the pressure compensator is in equilibrium.3. In what kind of applications is it convenient to use throttle valves? and the Flow Control Valves?The throttle valves are used to restrict the fluid flow in both directions.The flow control valves are used when the fluid rate is going to be controlled. 11. What are flow dividers used for?The flow dividers are used to divide the fluid flow rate into two or more parts: either equal parts or by a certain division ratio. The two main classes of flow dividers are displacement and spool types. Displacement flow dividers consist of two or more hydraulic motors mounted on the same shaft, rotating at the same speed.