Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 76

A fixed orifice is a hydraulic element characterized by a specific throat area Ω_O; in a variable orifice, the area Ω_O can vary according to the instantaneous geometric configuration.

Figure 4.2 gives an example of a needle valve, which is an adjustable orifice. The characteristics of both fixed and variable orifices are represented in Figure 4.4. The relationship between pressure drop and flow rate for turbulent flow conditions is parabolic, and the curves show the trend for different orifice areas. The component working conditions pertain only to the first (positive flow) and the third quadrant (negative flow). The figure also highlights the symbols used to indicate both the fixed and the variable orifice cases.

It is important to remark the square root dependence between Δp and Q highlighted in Figure 4.4. Because of this dependence, in order to double the flow across the orifice, it is necessary to increase the pressure by four times.

In hydraulic control valves, variable orifices are often used to represent the positions and the connections implemented by the valve. Figure 4.5 shows the example of two proportional valves, one is a two‐position two‐way (a) and the other one is a three‐position four‐way (a). In both cases, each square represents the possible configurations of the port connections obtained by the valve, and the continuous lines above and below the symbols indicate that the orifice area of every connection can be continuously varied.

The valve symbol includes different details when compared to the representation with basic orifices. For instance, in Figure 4.5, the information about the closed configuration (all ports completely closed Ω_O = 0) is not provided by the basic orifice symbols.

Figure 4.4 Orifice equation plotted in a (p, Q) layer for different area openings.