CIRCUIT DESINGSummaryThe selection of hydraulic components for use in a given application is determined by their ability to meet the required specification within the desired cost framework. A variety of components can be arranged to fulfil a given function by using different circuit configurations as the fluid power system designer has the freedom, within the constraints set by the preferences of the machine builder and/or the user, to select components of his choice.This freedom makes it difficult to summarise circuit on design however, the designer need to be able justify the circuit on the basis of technical considerations. This chapter therefore describes and, where applicable, evaluates variety of circuit options that can be used for the range of functions generally encountered in the application of fluid power systems.1. IntroductionTo a very large degree the main function of hydraulic circuits is to control the flow to one or several actuators as required by the application. There are, however, a variety of methods for controlling flow, some of which act indirectly by using pressure as the controlling parameter.The circuits discussed in this chapter include: Directional control and valve configurations. Velocity controls with constant supply pressure. Velocity controls with load sensing. Variable displacement pump controls. Hydrostatic transmissions. Load control. Contamination control.2. Pressure and FlowHydraulic systems provide flow from the pump that is directed to one or more actuators(motors) at a pressure level that satisfies the highest demand. Where a single output is being driven the pump pressure will float to the level demanded by the load. However, even for such simple systems the method that is employed to provide variable flow needs to be evaluated in order to ensure that best efficiency is obtained. In circuits with multiple outputs this aspect can be more difficult to evaluate.For operation at pressures and flows that are lower than the required maximum values the efficiency of the system will depend on the type of pump being used (i.e. fixed or variable displacement). This can be represented diagrammatically as in Figure 1.For fixed displacement pump system it is clear from Figure 1 that excess pump flow will have to be returned to the reservoir so that the power required by the pump is greater than that being supplied to the load. The level of inefficiency incurred is dependent on the ratio between the pressure required by the load and that at the pump outlet which can be controlled at the maximum level by the relief valve or at lower pressures by various types of bypass valves.Figure1 Flow and pressure varlotionFor variable displacement pumps the generation of excess flow can be avoided. However, the lever of pump pressure will depend on the method that is used for controlling the displacement but clearly there is scope for achieving much higher efficiencies than with fixed displacement pumps.Each of these control methods will require a particular circuit design employing components that have been described in the previous chapters.3. Directional controlValves used for controlling the direction of the flow can be put into fixed positions for this purpose but many types are frequently used in a continuously variable mode where they introduce a restriction into the flow path.3.1 Two position valvesA four-way valve with two positions for changing direction of the flow to and from an actuator is shown in Figure 2. For supply flow, Q, the actuator velocities will be:Extend UE=Q/Ap; Retract UR=Q/AAHere, the actuator areas are Ap for the piston and AA for the annulus or rod end the actuator. Hence, URUE as ApARAny external forces (F) that are acting on the actuator rod must be in opposition to the direction motion. For reversing force applications it will be necessary to apply restrictor control which will be discussed later in the chapter. These forces will create a supply pressure that is = F/Ap or F/AAFigure2 Two position four-way valveThree-way valves are used in applications where only one side of the actuator needs a connection from the supply. A typical example for this is the operation of the lift mechanism on a fork lift truck, as shown in Figure 3 where the actuator is lowered under the action of the weight.Figure3 Two position three-way3.2 Three position valvesThree position valves have a third, central position that can be connected in different configurations. These variants are described.Closed Centre Valves (Figure 4)Closed centre valves block all of the four ports. This prevents the actuator from moving under the action of any forces on the actuator. The supply flow port is also blocked which may require some means of limiting the supply pressure supply pressure can be made by appropriate pump controls or