MOOG 771, 772, 773 Series Servo Valves

PRODUCT OVERVIEW

The 77X Series flow control servo valves are throttle valves for 3 and

preferably 4-way applications. They are a high performance, 2-stage

design that covers the range of rated flows from 4 to 57 l/min (1 to

15 gpm) at 210 bar (3000 psi) valve pressure drop per spool land.

The output stage of the valve is a closed center, four-way sliding spool.

The pilot stage is a symmetrical double-nozzle and flapper, driven by

a double air gap, dry torque motor.

Mechanical feedback of spool position is pro vid ed by a cantilevered

spring. The valve design is simple and rugged for de pendable, long life

op era tion. These valves are suitable for electrohydraulic position, speed,

pressure or force control systems with high dynamic response

requirements. The 77X is ideally suited for applications where superior

dynamics are a must.

Intrinsically safe valve versions are available for use in applications with

potentially hazardous environments. Specific models are certified to FM,

ATEX, CSA, TIIS and IECEx standards.

FEATURES AND BENEFITS

The 77X Series is proven technology that performs reliably in machines

where high performance, stability and accuracy are required.

Moog’s Mechanical Feedback Valves are designed to provide high reliability

and long service life.

DESCRIPTION OF OPERATION

The 77X Series Flow Control Servo Valve consists of a polarized electrical

torque motor and two stages of hydraulic power amplification. The motor

armature extends into the air gaps of the magnetic flux circuit and is

supported in this position by a flexure tube. The flexure tube acts as a seal

between the electromagnetic and hydraulic sections of the valve.

The 2 motor coils surround the armature, one on each side of the flexure

tube.

The flapper of the first stage hydraulic amplifier is rigidly attached to the

midpoint of the armature. The flapper extends through the flexure tube and

passes between 2 nozzles, creating two variable orifices between the nozzle

tips and the flapper. The pressure controlled by the flapper and nozzle

variable orifice is fed to the end areas of the second stage spool.

The second stage is a conventional four-way spool design in which output

flow from the valve, at a fixed valve pressure drop, is proportional to spool

displacement from the null position. A cantilevered feedback spring is fixed

to the flapper and engages a slot at the center of the spool.

Displacement of the spool defects the feedback spring which creates a force

on the armature/flapper assembly. Input signals induce a magnetic charge

in the armature and cause a deflection of the armature and flapper. This

assembly pivots about the flexure tube and increases the size of one nozzle

orifice and decreases the size of the other.

This action creates a differential pressure from the one end of the spool 

displacement causing a force in the feedback wire which opposes the original

input signal torque. Spool movement continues until the feedback wire force

equals the input signal force.