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Servovalve , Electrohydraulic Flow Control
An electrically commanded, flow-control valve, which is capable of continuous control.
Hydraulic Amplifier
A fluid valve device which acts as a power amplifier, such as a sliding spool, a nozzle flapper, or a jet pipe with receivers.
Stage
A hydraulic amplifier used in a servovalve. Servovalves may be single-stage, two-stage, three-stage, etc.
Output Stage
The final stage of hydraulic amplification used in a servovalve
Port
A fluid connection to the servovalve, e.g., supply port, return port, control port.
Two-Way Valve
An orifice flow control component with supply and one control port arranged so that action is in one direction only (from supply to control port).
Three-way Valve
A multiorifice flow-control component with supply, return and one control port arranged so that valve action in one direction opens supply to control port and reversed valve action opens the control port to return.
Four-Way Valve
A multiorifice flow-control component with supply, return, and two control ports arranged so the valve action in one direction opens supply to control port #1 and opens control port #2 to return. Reversed valve action opens supply to control port #2 and opens control port #1 to return.
Torque Motor
The electromechanical transducer commonly used in the input stages of servovalves.
Input Current
The current to the valve, expressed in ma, which commands control flow.
Rated Current
The specified input current (expressed in ma) of either polarity to produce rated flow. The particular coil connection (differential, series, or parallel) must be specified in conjunction with the rated current. Rated current does not include null bias current.
Quiescent Current
A DC current that is present in each valve coil when using a differential coil connection. The polarity of the current in the coil being in opposition such that no electrical control exist.
Electrical Quiescent Power
the dissipation required for differential operation when the current through each coil is equal and opposite in polarity.
Electrical Control Power
The power dissipation required for control of the valve. Control power is at maximum with full input signal and is zero with zero-input signal. It is independent of the coil connection (series, parallel, or differential) for any conventional two-coil operation. For differential operation, the control power is the power consumed in excess of the electrical quiescent power. This power increase is a result of the differential current change.
Total Electrical Power
The sum of the instantaneous control power and the quiescent power, expressed in mw.
Coil Impedance
The complex ration of coil voltage to coil current. It is important to note that the coil impedance may vary with signal frequency, signal amplitude and other operating conditions due to back emf generated by the moving armature.
Coil Resistance
The dc resistance of each torque motor coil, expressed in ohms.
Polarity
The relationship between the direction of control flow and the direction of input current.
Dither
A low amplitude, relatively high frequency periodic electrical signal sometimes superimposed on the servovalve input to reduce threshold. Dither is expressed by the dither frequency (Hz) and the peak -to-peak dither current amplitude(ma).
Control Flow
The flow through the valve control ports, expressed in cis or gpm. Control flow is referred to as No-Load Flow when there is zero load-pressure drop. Conventional test equipment normally measures no-load flow.
Rated Flow
The specified control flow corresponding to rated current and specified load pressure drop. Rated flow is normally specified as the no-load flow.
Flow Curve
A graphical representation of control flow versus input current. This is usually a continuous plot of a complete cycle between plus and minus rated values.
Normal Flow Curve
The locus of the midpoints of the complete cycle flow curve, which is the zero hysteresis flow curve. Usually valve hysteresis is low, such that either side of the flow curve can be used for the normal flow curve.
Flow Gain
The slope of the control flow versus input current curve in any specific operating region, expressed in cis/ma or gpm/ma. Three operating regions are usually significant with flow-control servovalves: (1) the null region, (2) the region of normal flow control and (3) the region where flow saturation effects may occur. Where this term is used without qualification, it is assumed to mean normal flow gain.
Rated Flow Gain
The ratio of rated flow to rated current, expressed in cis/ma or gpm/ma.
Flow Saturation region
The region where flow gain decreases with increasing input current.
Flow Limit
the condition wherein control flow no longer increases with increasing input current. Flow limitation may be deliberately introduced within the servovalve.
Symmetry
the degree of equality between the normal flow gain of one polarity and that of the reversed polarity. Symmetry is measured as the difference in normal flow gain of each polarity, expressed as percent of the greater.
Linearity
The degree to which the normal flow curve conforms to the normal flow gain line with other operational variables held constant. Linearity is measured as the maximum deviation of the normal flow curve from the normal flow gain line, expressed as percent of rated current.
Hysteresis
The difference in the valve input currents required to produce the same valve output during a single cycle valve input current when cycle at a rate below that at which dynamic effects are important. Hysteresis is normally specified as the maximum difference occurring in the flow curve throughout plus or minus rated current, and is expressed as percent of rated current.
Threshold
the increment of input current required to produce a change in valve output, expressed as percent of rated current. Threshold is normally specified as the current increment required to revert from a condition of increasing output to a condition of decreasing output, when current is changed at a rate below that at which dynamic effects are important.
Internal Leakage
The total internal valve flow from pressure to return with zero control flow (usually measured with control ports blocked), expressed in cis or gpm. Leakage flow will vary with input current, generally being a maximum at the valve null (null leakage).
Pressure Gain
The rate of change of load pressure drop with input current at zero control flow (control ports blocked), expressed in psi/ma (bars/ma). Pressure gain is usually specified as the average slope of the curve of load pressure drop versus current between +- 40% of maximum load-pressure drop.
Null Region
The region about null wherein effects of lap in the output stage predominate.
Null
The condition where the valve supplies zero control flow at zero load-pressure drop.
Null Pressure
The pressure existing at both control ports at null, expressed in psi (bars).
Null Bias
The input current required to bring the valve to null, excluding the effects of valve hysteresis, expressed as percent of rated current.
Null Shift
A Change in null bias, expressed as percent of rated current. Null shift may occur with changes in supply pressure, temperature, and other operating conditions.
Lap
In a sliding spool valve, the relative axial position relationship between the fixed and movable flow-metering edges with the spool at null. For a servovalve, lap is measured as the total separation at zero flow of straight line extension of the nearly straight portions of the normal flow curve, drawn separately for each polarity, expressed as percent of rated current.
Zero Lap
The lap condition where there is no separation of the straight line extension of the normal flow curve.
Overlap
The lap condition which results in a decreased slope of the normal flow curve in the null region.
Underlap
the lap condition which results in an increased slope of the normal flow curve in the null region.
Frequency Response
The complex ratio of output flow to input current as the current is varied sinusoidally over a range of frequencies. Frequency response is normally measured with constant input current amplitude and zero load pressure drop, expressed as amplitude ratio and phase angle. Valve frequency may vary with the input-current amplitude, temperature, supply pressure, and other operation conditions.
Amplitude Ratio
The ratio of the control-flow amplitude to the input-current amplitude at a particular frequency divided by the same ratio at the same input-current amplitude at a specified low frequency (usually 5 or 10 Hz). Amplitude ratio may be expressed in decibels where db = 20 log 10 AR.
Phase Lag
The instantaneous time separation between the input current and the corresponding control-flow variation, measured at a specified frequency and expressed in degrees (time separation in seconds x frequency in Hz x 360 deg per cycle).
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PUMPS
The building block of any hydraulic system is the pump . The four most common designs are the vane, gear, gerotor and piston. All are well suited to common hydraulic uses with the piston design best suited for higher pressures. The variable displacement type is particularly well suited in circuits using hydraulic motors where variable speeds and the ability to reverse is needed.
OUTPUT
The Output of a hydraulic pump (gallons per minute, or GPM) is related directly to it's operating speed. The pressure of a pump is determined by it's manufactured capablilities.
HORSEPOWER
The horsepower required to drive a pump is dependent on both pressure and output in GPM. The higher the pressure, or greater the volume, the more Horsepower(HP) required. As a rule of thumb, a 1000 PSI(pounds per square inch) pump will require 1 Horsepower(HP), for the first gallon per minute and 3/4 HP each gallon per minute after that. Doubling the pressure or output volume will require 4 time the input HP.
The math: Input HP= GPM x PSI / 1714
CONTROL VALVES
The ways you can harness the power of a hydraulic system is through the use of control valves . The three basic types of control valves are the tandem center type (see below), the open center (motoring spool) type, and the closed center type.Both the tandem center and the closed center types are available in a three way or four way valve. BY opening or closing valves, you can control how much or in what direction a hydraulic piston moves. Valves can also control many pistons working with each other at the same time.
TANDEM CENTER VALVES
Tandem center valves when in a neutral position by passes the flow of hydraulic oil to the return line. This is used to hold the cylinder/piston in position with no load on the pumps. When this system is the pump is running constantly to keep a ready supply of hydraulic oil, but as long as the cylinder/piston is not in operation the pump is working under no pressure or load. This system keeps wear and tear on the pump down to a minimum.
OPEN CENTER VALVES
Open center valves are the same as the tandem center, except that in the neutral position all lines are connected back to the reservoir. The primary use of this system is to prevent "shock" loading when the valve is placed in neutral. This takes pressure off the motor. This system is used in situations where the operating device needs to be moved by hand.
CLOSED CENTER VALVES
This type of valve is used in a hydraulic system where the valve blocks the flow of oil from the pump into an accumulator. The accumulator (see below) is used to store the oil under pressure. This valve takes the pressure off the pump and in neutral locks the cylinder in place with no load on the pump.
PRESSURE RELIEF VALVE
A pressure relief valve is a safety device and is required on all hydraulic systems. Once adjusted, the pressure relief valve opens whenever the pressure goes beyond the value set and allows oil to flow back to the reservoir.
UNLOADING VALVE
The Unloading valve is installed in systems using accumulators (below). The function of this valve is to "unload" the pump of pressure until such time as a device on the system, such as a cylinder, actually begins operation. In this way, the pump is allowed to operate without load (pressure) until it is needed. The unloading valve is not intended to replace the pressure relief valve. In fact, the pressure setting of the unloading valve is much lower than the setting on the pressure relief valve.
ACCUMULATORS
An accumulator is used to store hydraulic oil, under pressure, to pressurize the system while the pump is unloaded. This oil is also used to supplement the power pump output during times of heavy use or for limited operations when the pump is not working. Accumulators also dampen surges within the hydraulic system.
HYDRAULIC CYLINDERS
Hydraulic cylinders transform the pressure and oil flow in a hydraulic system into work or mechanical force. They are used where linear motion is required to move something. Cylinders are ususally double-acting, that is, oil under pressure can be applied to either side of the piston to provide movement in either direction. Single acting cylinders are sometimes used where the weight of the load is used to return the cylinder to the closed position.
CYLINDER THRUST
The push of a cylinder in pounds:
Thrust = Piston Area (in.) x Pump Pressure (PSI) or
Thrust = bore diameter squared x .78 x PSI
HYDRAULIC MOTORS
Hydraulic motors are another important piece of the hydraulic system. However, instead of a cylinder (force moving linear) the motor uses hydraulic pressure to rotate. In terms of how it's built, a motor is like al pump. But, when it's operated oil enters the motor and turns the shaft. The speed of a hydraulic motor is dependent on the amount of oil supplied by the pump and the torque is dependent on the amount of pressure supplied.
RESERVOIRS
The size of a reservoir will depend on the capacity of the hydraulic system, as well as what is required by the system. The reservoir should contain a large volume of oil and should provide ample oil to the pump.
PIPING
When you are connecting up a hydraulic system, use the tubing or pipe that is capable of handling heavy pressures and loads required by the hydraulic system. Pipes should have a minimum number of bends and fittings, should be securely fastened and and clean. Iron pipes are not recommended because they have particles that will flake off and contaminate a system.
CLEANLINESS
The most important part of a hydraulic system is how clean it should be. Any extreme pressure system is extremely vulnerable to dirt, particles and other matter that contaminate the close tolerances necessary for any hydraulic system. All pipes, fittings, and other components must be extremely clean before use.
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