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4-Way Directional Valve

Four-port three-position directional control valve

  • Library:
  • Hydraulics (Isothermal) / Valves / Directional Valves

Description

The 4-Way Directional Valve block represents a directional control valve with four ports and three positions, or flow paths. The ports connect to what in a typical model are a hydraulic pump (port P), a storage tank (port T), and a double-acting actuator (ports A and B). Fluid can flow from the pump to the actuator via path P-A or P-B and from the actuator to the tank via path A-T or B-T—depending on the working side of the actuator.

Typical Valve Setup

In the default configuration, one valve position corresponds to the P-A and B-T flow paths maximally open and the P-B and A-T flow paths maximally closed (position I in the figure). Another valve position corresponds to the inverse configuration, with P-B and A-T maximally open and P-A and B-T maximally closed (position II). The third valve position corresponds to all flow paths maximally closed (position III). A translating spool serves as the valve control member and determines the position that the valve is in—I, II, III, or in between.

Valve Positions

Physical signal port S controls the spool displacement. In the default configuration, a zero displacement signal corresponds to valve position III. A positive displacement signal shifts the spool toward valve position I. A negative displacement shifts the spool toward valve position II. The spool displacement acts indirectly by setting the spool position relative to each flow path—a length known here as the orifice opening. The orifice opening in turn determines the opening area of the respective flow path.

Orifice Openings

The orifice opening of a flow path depends partly on its opening offset—the orifice opening of a flow path at zero spool displacement. The block models only the effects of the opening offsets. An offset can be due to a change in distance between ports or spool lands—the thick disks built into the spool to obstruct flow. It can also be due to a change in the thicknesses of the spool lands. The orifice openings are computed separately for each flow path in terms of the respective opening offset:

hPA=hPA0+x

hPB=hPB0x

hAT=hAT0x

hBT=hBT0+x

where:

  • hPA, hPB, hAT, and hBT are the orifice openings of the P-A, P-B, A-T, and P-B flow paths. The orifice openings are computed during simulation.

  • hPA0, hPB0, hAT0, and hAT0 are the orifice opening offsets of the P-A, P-B, A-T, and P-B flow paths. The opening offsets are specified in the Valve opening offsets tab.

  • x is the spool displacement relative to what in the zero-offset case is a fully closed valve. The spool displacement is specified through physical signal port S.

The figure shows the effects of the opening offsets on the orifice openings. Plot I corresponds to the default configuration with both opening offsets equal to zero. Plot II corresponds to a valve with both opening offsets greater than zero and plot III to a valve with both opening offsets smaller than zero. These cases are similar in behavior to zero-lapped (I), underlapped (II) and overlapped (III) valves. The valve schematics to the right show what the offset might look like. The circle highlights the offset in path P-B.

Zero (I), Positive (II), and Negative (III) Opening Offsets

An underlapped valve is always partially open and allows some flow at all spool displacements. An overlapped valve is fully closed over an extended range of spool displacements and requires longer spool travel to open. The table summarizes the opening offsets for zero-lapped, underlapped, and overlapped valves. Other configurations are possible—e.g., with one opening offset positive and the other negative.

Valve LappingOpening Offsets
Zero-lapped (default)All zero
UnderlappedAll positive
OverlappedAll negative

Opening Areas

The Model parameterization setting determines the calculations used for the opening areas of the flow paths—or, in the Pressure-flow characteristic case, the volumetric flow rates. The calculations are based on orifice parameters or tabulated data sets specified in the Model Parameterization tab. The block uses the same data for all flow paths if the Area characteristics parameter in the Basic Parameters tab is set to Identical for all flow paths and different data otherwise. Model parameterizations that you can select include:

  • Maximum area and opening — Specify the maximum opening area and the corresponding orifice opening. The opening area is a linear function of the orifice opening,

    AAT=AAT,MaxhAT,MaxhAT+ALeak,

    where A is the opening area and h the orifice opening of a given flow path. The subscript Max refers to a fully open orifice and the subscript Leak to a fully closed orifice—one with internal leakage flow area only. The figure shows a plot of the linear function A(h).

  • Area vs. opening table — Specify the opening area at discrete orifice openings as a 1-D lookup table. The opening area is computed for a given orifice opening by interpolation or extrapolation of the tabulated data. The figure shows a conceptual plot of the tabulated function A(h).

  • Pressure-flow characteristic — Specify the volumetric flow rate at discrete orifice openings and pressure differentials as a 2-D lookup table. The opening area is computed for a given orifice opening and pressure differential by interpolation or extrapolation of the tabulated data. The figure shows a conceptual plot of the tabulated function q(h, p).

Flow Regime and Internal Leakage

Volumetric flow rates are computed analytically in the Maximum area and opening and Area vs. opening table parameterizations. The calculations are based on additional block parameters such as the flow discharge coefficient and account for the effects of flow regime—laminar or turbulent. Regime transition occurs at the specified critical laminar flow ratio or critical Reynolds number.

The Maximum area and opening and Area vs. opening table parameterizations also account for a small internal leakage area even in the fully closed state. The leakage area ensures that portions of the hydraulic network do not become isolated when a flow path is closed. Isolated, or "hanging", network portions affect the computational efficiency of the model and can cause simulation to fail.

The effects of flow regime and internal leakage are assumed to be reflected in the tabulated flow rate data specified directly in the Pressure-flow characteristic parameterization.

Valve Configurations

The opening offsets are by default zero. This configuration corresponds to a valve with all flow paths closed in the neutral position (III in the Valve Positions schematic). Many other configurations exist. You can model a specific configuration by setting the opening offsets as shown in the table. All opening offset parameters are in the Valve opening offsets tab of the block dialog box

Basic 4-Way Directional Valve Configurations

ConfigurationValve opening offsets

  • Between ports P and A < 0

  • Between ports P and B < 0

  • Between ports A and T < 0

  • Between ports B and T < 0

  • Between ports P and A > 0

  • Between ports P and B > 0

  • Between ports A and T > 0

  • Between ports B and T > 0

  • Between ports P and A < 0

  • Between ports P and B < 0

  • Between ports A and T < – Maximum valve opening

  • Between ports B and T < – Maximum valve opening

  • Between ports P and A < 0

  • Between ports P and B < 0

  • Between ports A and T > 0

  • Between ports B and T > 0

  • Between ports P and A > 0

  • Between ports P and B < 0

  • Between ports A and T > 0

  • Between ports B and T < 0

  • Between ports P and A < 0

  • Between ports P and B < 0

  • Between ports A and T > 0

  • Between ports B and T < 0

  • Between ports P and A < 0

  • Between ports P and B < 0

  • Between ports A and T < 0

  • Between ports B and T > 0

  • Between ports P and A > 0

  • Between ports P and B > 0

  • Between ports A and T < 0

  • Between ports B and T < 0

  • Between ports P and A > 0

  • Between ports P and B < 0

  • Between ports A and T < 0

  • Between ports B and T < 0

  • Between ports P and A < 0

  • Between ports P and B > 0

  • Between ports A and T < 0

  • Between ports B and T < 0

  • Between ports P and A < 0

  • Between ports P and B > 0

  • Between ports A and T < 0

  • Between ports B and T > 0

Structural Component Diagram

The block is a composite component with four Variable Orifice blocks driven by a single physical signal. Blocks Variable Orifice P-A and Variable Orifice P-B represent the P-A and P-B flow paths. Blocks Variable orifice A-T and Variable orifice B-T represent the A-T and B-T flow paths. The physical signal is specified through Connection Port block S.

The Orifice orientation block parameters are set so that a positive signal acts to open Variable Orifice P-A and Variable Orifice B-T while closing Variable Orifice A-T and Variable Orifice P-B. A negative signal has the opposite effect—acting to open Variable Orifice A-T and Variable Orifice P-B while closing Variable Orifice P-A and Variable Orifice B-T.

Valve Structural Diagram

Assumptions

  • Fluid inertia is ignored.

  • Control member loading due to inertial, spring, and other forces is ignored.

  • All valve orifices are assumed identical in size unless otherwise specified.

Ports

Input

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Physical signal input port for the control member displacement.

Conserving

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Hydraulic (isothermal liquid) conserving port associated with the pressure supply line inlet.

Hydraulic (isothermal liquid) conserving port associated with the return line connection.

Hydraulic (isothermal liquid) conserving port associated with the actuator connection port.

Hydraulic (isothermal liquid) conserving port associated with the actuator connection port.

Parameters

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Basic Parameters

Choice of different or identical flow path opening characteristics. Select Different for each flow path to specify flow path parameters or tabulated data separately for each flow path.

Parameterization of the valve model. Options include:

  • By maximum area and opening — Specify the maximum orifice opening and opening area. The opening area varies linearly with the spool displacement specified at physical signal port S.

  • By area vs. opening table — Specify the flow path opening area at discrete orifice openings as a 1-D lookup table. The opening area is computed by interpolation or extrapolation of the tabulated data.

  • By pressure-flow characteristic — Specify the flow path volumetric flow rates at discrete orifice openings and pressure differentials. The flow rate is computed by interpolation or extrapolation of the tabulated data.

Method of computing values inside the tabulated data range. The Linear method joins adjacent data points with straight line or surface segments with generally discontinuous slope at the segment boundaries. Surface segments are used in the 2-D lookup table specified in the Pressure-flow characteristic model parameterization.

The Smooth method replaces the straight segments with curved versions that have continuous slope everywhere inside the tabulated data range. The segments form a smooth line or surface passing through all of the tabulated data points without the discontinuities in first-order derivatives characteristic of the Linear interpolation method.

Dependencies

This parameter is active when the Model Parameterization parameter is set to By area vs. opening table or Pressure-flow characteristic.

Method of computing values outside of the tabulated data range. The Linear method extends the line segment drawn between the last two data points at each end of the data range outward with a constant slope.

The Nearest method extends the last data point at each end of the data range outward as a horizontal line with constant value.

Dependencies

This parameter is active when the Model Parameterization parameter is set to By area vs. opening table or Pressure-flow characteristic.

Ratio of the actual and theoretical flow rates through the valve. This parameter depends on the geometrical properties of the valve. Values are usually provided in textbooks and manufacturer data sheets.

Dependencies

This parameter is active when the Model Parameterization parameter is set to By maximum area and opening or By area vs. opening table.

Total area of internal leaks in the completely closed state. The purpose of this parameter is to maintain the numerical integrity of the fluid network by preventing a portion of that network from becoming isolated when the valve is completely closed.

Dependencies

This parameter is active when the Model Parameterization parameter is set to By maximum area and opening or By area vs. opening table.

Select the parameter to base the laminar-turbulent transition on. Options include:

  • Pressure ratio — Flow transitions between laminar and turbulent at the pressure ratio specified in the Laminar flow pressure ratio parameter. Use this option for the smoothest and most numerically robust flow transitions.

  • Reynolds number — The transition occurs at the Reynolds number specified in the Critical Reynolds number parameter. Flow transitions are more abrupt and can cause simulation issues at near-zero flow rates.

Dependencies

This parameter is active when the Model Parameterization parameter is set to By maximum area and opening or By area vs. opening table.

Pressure ratio at which the flow transitions between the laminar and turbulent regimes. The pressure ratio is the fraction of the outlet pressure over the inlet pressure.

Dependencies

This parameter is active when the Model Parameterization parameter is set to By maximum area and opening or By area vs. opening table and the Laminar transition specification parameter is set to Reynolds number.

Maximum Reynolds number for laminar flow. This parameter depends on the orifice geometrical profile. You can find recommendations on the parameter value in hydraulics textbooks. The default value, 12, corresponds to a round orifice in thin material with sharp edges.

Dependencies

This parameter is active when the Model Parameterization parameter is set to By maximum area and opening or By area vs. opening table and the Laminar transition specification parameter is set to Reynolds number.

Model Parameterization: Maximum area and opening

Orifice opening at which the opening area of a flow path is a maximum. The maximum opening is the same for all flow paths when this parameter is exposed. For the orifice opening definitions, see Orifice Openings.

Dependencies

This parameter is active when the Area characteristics parameter is set to Identical for all flow paths and the Model parameterization parameter is set to Maximum area and opening.

Maximum cross-sectional area of a flow path. The maximum opening areas are the same for all paths when this parameter is exposed. For the opening area calculations, see Opening Areas.

Dependencies

This parameter is active when the Area characteristics parameter is set to Identical for all flow paths and the Model parameterization parameter is set to Maximum area and opening.

Control member position at which the P-A opening area is equal to the P-A, maximum opening area parameter value.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Maximum cross-sectional area of the P-A flow path. The maximum opening areas are different for each flow path when this parameter is exposed. For the opening area calculations, see Opening Areas.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Control member position at which the P-B opening area is equal to the P-B, maximum opening area parameter value.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Maximum cross-sectional area of the P-B flow path. The maximum opening areas are different for each flow path when this parameter is exposed. For the opening area calculations, see Opening Areas.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Control member position at which the A-T opening area is equal to the A-T, maximum opening area parameter value.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Maximum cross-sectional area of the A-T flow path. The maximum opening areas are different for each flow path when this parameter is exposed. For the opening area calculations, see Opening Areas.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Control member position at which the B-T opening area is equal to the B-T, maximum opening area parameter value.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Maximum cross-sectional area of the B-T flow path. The maximum opening areas are different for each flow path when this parameter is exposed. For the opening area calculations, see Opening Areas.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Maximum area and opening.

Model Parameterization: Area vs. opening table

Array of orifice openings used to construct the opening area 1-D lookup table for all flow paths. The opening characteristics are identical for all flow paths when this parameter is exposed. The number of elements in the array determines the number of elements in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Identical for all flow paths and the Model parameterization parameter is set to Area vs. opening table.

Array of flow path opening areas corresponding to the specified orifice openings. The opening characteristics are identical for all flow paths when this parameter is exposed. The number of elements in the array must match the number of elements in the Opening vector parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Identical for all flow paths and the Model parameterization parameter is set to Area vs. opening table.

Array of P-A orifice openings used to construct the opening area 1-D lookup table for the P-A flow path. The number of elements in the array determines the number of elements in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Array of P-A opening areas corresponding to the specified P-A orifice openings. The number of elements in the array must match the number of elements in the P-A, opening vector parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Array of P-B orifice openings used to construct the opening area 1-D lookup table for the P-B flow path. The number of elements in the array determines the number of elements in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Array of P-B opening areas corresponding to the specified P-B orifice openings. The number of elements in the array must match the number of elements in the P-B, opening vector parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Array of A-T orifice openings used to construct the opening area 1-D lookup table for the A-T flow path. The number of elements in the array determines the number of elements in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Array of A-T opening areas corresponding to the specified A-T orifice openings. The number of elements in the array must match the number of elements in the A-T, opening vector parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Array of B-T orifice openings used to construct the opening area 1-D lookup table for the B-T flow path. The number of elements in the array determines the number of elements in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Array of B-T opening areas corresponding to the specified B-T orifice openings. The number of elements in the array must match the number of elements in the B-T, opening vector parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Area vs. opening table.

Model Parameterization: Pressure-flow characteristic

Array of orifice openings used to construct the volumetric flow rate 2-D lookup table. The number of elements in the array determines the number of rows in the table. The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Identical for all flow paths and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of pressure differentials used to construct the volumetric flow rate 2-D lookup table. The number of elements in the array determines the number of columns in the table. The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Identical for all flow paths and the Model parameterization parameter is set to Pressure-flow characteristic.

Matrix with the volumetric flow rates corresponding to the specified orifice openings and pressure differentials. The number of rows must match the number of elements in the Opening vector, s parameter. The number of columns must match the number of elements in the Pressure differential vector, dp parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Identical for all flow paths and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of P-A orifice openings used to construct the P-A volumetric flow rate 2-D lookup table. The number of elements in the array determines the number of rows in the table. The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of pressure differentials used to construct the volumetric flow rate 2-D lookup table for the P-A flow path. The number of elements in the array determines the number of columns in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Matrix with the volumetric flow rates through the P-A flow path at the specified control member displacements and pressure differentials. The number of rows must match the number of elements in the P-A, opening vector, s parameter. The number of columns must match the number of elements in the P-A, pressure differential vector, dp parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of control member displacements used to construct the volumetric flow rate 2-D lookup table for the P-B flow path. The number of elements in the array determines the number of rows in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of pressure differentials used to construct the volumetric flow rate 2-D lookup table for the P-B flow path. The number of elements in the array determines the number of columns in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Matrix with the volumetric flow rates through the P-B flow path at the specified control member displacements and pressure differentials. The number of rows must match the number of elements in the P-B, opening vector, s parameter. The number of columns must match the number of elements in the P-A, pressure differential vector, dp parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of control member displacements used to construct the volumetric flow rate 2-D lookup table for the A-T flow path. The number of elements in the array determines the number of rows in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of pressure differentials used to construct the volumetric flow rate 2-D lookup table for the A-T flow path. The number of elements in the array determines the number of columns in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Matrix with the volumetric flow rates through the A-T flow path at the specified control member displacements and pressure differentials. The number of rows must match the number of elements in the A-T, opening vector, s parameter. The number of columns must match the number of elements in the A-T, pressure differential vector, dp parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of control member displacements used to construct the volumetric flow rate 2-D lookup table for the B-T flow path. The number of elements in the array determines the number of rows in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Array of pressure differentials used to construct the volumetric flow rate 2-D lookup table for the B-T flow path. The number of elements in the array determines the number of columns in the table.

The array must increase from left to right but the intervals between the array element values need not be uniform. There must be at least two elements for Linear interpolation and three elements for Smooth interpolation.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Matrix with the volumetric flow rates through the B-T flow path at the specified control member displacements and pressure differentials. The number of rows must match the number of elements in the B-T, opening vector, s parameter. The number of columns must match the number of elements in the B-T, pressure differential vector, dp parameter.

Dependencies

This parameter is active when the Area characteristics parameter is set to Different for each flow path and the Model parameterization parameter is set to Pressure-flow characteristic.

Valve Opening Offsets

Orifice opening of the P-A flow path at zero spool displacement. Specify a positive offset to model an underlapped valve or a negative offset to model an overlapped valve. The default value of 0 corresponds to a zero-lapped valve.

Orifice opening of the P-B flow path at zero spool displacement. Specify a positive offset to model an underlapped valve or a negative offset to model an overlapped valve. The default value of 0 corresponds to a zero-lapped valve.

Orifice opening of the A-T flow path at zero spool displacement. Specify a positive offset to model an underlapped valve or a negative offset to model an overlapped valve. The default value of 0 corresponds to a zero-lapped valve.

Orifice opening of the P-B flow path at zero spool displacement. Specify a positive offset to model an underlapped valve or a negative offset to model an overlapped valve. The default value of 0 corresponds to a zero-lapped valve.

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