Moog 768-578A Valve

HOW TO SELECT A SERVO OR PROPORTIONAL VALVE
DETERMINE THE REQUIRED VALVE FLOW RATE AND
FREQUENCY RESPONSE
KEY PARAMETERS FOR SERVO OR PROPORTIONAL
VALVE SELECTION
a) In order to compensate for unknown forces, size the actuator area to produce a stall force 30% greater than the desired force
to the supply pressure available.
Supply Pressure Servovalve and ServoJet® Valves are intended to operate with
constant supply pressure and require continuous pilot flow to maintain the hydraulic bridge balance.The supply pressure should
be set so that the pressure drop across the valve is equal to onethird of the supply pressure.The flow capacity should include the
continuous pilot flow to maintain the hydraulic bridge balance.
Direct Drive Valve performance is constant no matter what the supply pressure.Therefore, they are good in systems with fluctuating
supply pressures.
Standard Moog Inc. valves will operate at supply pressures from 200 to 3,000 psi. Optional valves for 50 to 5,000 psi
operation are available. Refer to individual valve specifications.
A=
1.3 FR PS
where:
A = actuator area (in2)
FR = force required to move the load (lb) at maximum velocity, ref. key parameters
PS = supply pressure (psi)
Refer to the NFPA standard cylinder bore and rod sizes and select the area closest to the result of the above calculations.
b)From the maximum required loaded velocity and the actuator area from the above calculation, determine the valve loaded
flow and the load pressure drop.
where:
QL = loaded flow (in3/sec)
XL = maximum required loaded velocity (in/sec)
QL = AXL
PL =
Type of Fluid Moog Inc. valves operate most effectively with fluids that exhibit
a viscosity of 60 to 450 SUS at 100˚F. Due to the Servovalve operating range of -40˚F to 275˚F, care should be taken to assure fluid
viscosity does not exceed 6,000 SUS. In addition, fluid cleanliness is of prime importance and should be maintained at ISO DIS 4406
Code 16/13 max, 14/11 recommended. Consult the Moog Inc.
Filtration and Valve Series catalogs for recommendations.
Fluid compatibility with material used in the construction of valves must be considered. Contact the factory for specific
information.
where:
PL = load pressure drop (psi)
FR A
c) Compute the no-load flow.
PS
PS - PL
QNL = QL
Force Requirements In most applications, a portion of the available supply pressure
must be used to overcome some force. Since valve flow ratings are given as a function of pressure drop across the valve, total force
requirements must be known in order to determine what portion of the supply pressure is available to be dropped across the valve.
Total force is the summation of all individual forces that occur due to the static or dynamic configuration of the system.
where:
QNL= no-load flow (in2/sec)
d)Determine the valve rated flow at 1,000 psi valve drop for Servovalves and 150 psi valve drop for Proportional Valves.
Increase by 10% for margin.
10% pad
QR = 1.1
(
QNL 3.8
)
in3/sec to gpm conversion
where:
QR = Servovalve rated flow (gpm) at 1,000 psi drop or Proportional Valve
rated flow at 150 psi drop
FR = FL + FA + FE + FS FR FL FA FE FS
e) For open-loop control, a valve having a 90˚ phase lag at 3 Hz or higher, should be adequate.
f) For closed loop control of systems utilizing electrical feedback, calculate the load natural frequency using the equations in this
brochure under “Load Resonant Frequency”.The optimum performance will be achieved if the Servovalve 90˚ phase point exceeds
the load resonant frequency by a factor of three or more.
g) With a calculated flow rate and frequency response, reference the Valve Selection Table on page 3 for valve selection. Any
Servovalve that has equal or higher flow capacity and response will be an acceptable choice. However, it is preferable not to oversize the Servovalve flow capacity as this will
needlessly reduce system accuracy.
h)Consult individual data sheets for complete valve performance parameters.
where:
= total required force (lb) = force due to load (lb) = force due to acceleration (lb)
= force due to external disturbance (lb) = force due to seal friction (lb)
Force Due to a Load Force due to a load FL can be an aiding or resistive component,
depending upon the load’s orientation and direction of travel.
Consideration has to be taken when computing FL to ensure the proper external friction coefficients and resolved forces are used.
WL RESISTIVE LOAD
PISTON EXTENDING
4
WL AIDING LOAD
PISTON RETRACTING