MOOG 78 series Moog family of Mechanical Feedback (MFB) Servo Valves

PRODUCT OVERVIEW

The 78 series is part of the Moog family of Mechanical Feedback (MFB)

Servo Valves. They are throttle valves for 3 and 4 way applications.

This series has a high performance, two-stage design that covers a

range of rated flows from 75 to 150 l/min (20 to 40 gpm) at 35 bar

(500 psi) valve drop per spool land. These valves are intended for

position, speed, pressure or force control applications that require high

dynamic response. The 78 series MFB offers high dynamics, high

resolution and low hysteresis due to its low friction double nozzle pilot

stage.

The design is simple and rugged for dependable, long life operation.

The output stage is a closed center, 4 way sliding spool. The pilot stage

is comprised of a symmetrical, double nozzle dry torque motor. The 2nd

stage spool position is controlled by a carbide tipped feedback wire.

The carbide ball on the end of the feedback wire is a mandatory design

requirement that ensures high accuracy, reliable operation and long

service life. All of our servo valves are known for high accuracy and

reliable operation even in the harshest industrial applications.

This family of valves is considered a flow control servo valve with an

analog interface, but does not contain integrated electronics. The

options in this series include standard and high response, special null

(spool) cuts, seal materials and connectors. Intrinsically safe and

flameproof designs for use in hazardous environments are also

available with specific models certified to FM, ATEX, CSA and TIIS

(Asian) standards.

All of our valves are backed by Moog Global Support, our promise to

provide world-class repair and maintenance services delivered by our

trained technicians. Each valve possesses a long life design, controlled

by proven servo valve technology that has an outstanding history of 60

years of meeting the motion control needs of our customers. All of this

makes Moog servo valves the best choice for your hydraulic motion

control requirements.

FEATURES AND BENEFITS

The 78 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. The current technology reflects over 60 years of

experience of servo control in some of the world’s most demanding

environments.

DESCRIPTION OF OPERATION

The 78 Series Flow Control Servo Valve consists of a polarized electrical

torque motor and two stages of hydraulic power amplification.

The torque 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 torque 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 pilot pressure is controlled by the

flapper/nozzle variable orifice and is in turn fed to the end areas of the

second stage spool. This action creates a differential pressure from one

end of the spool to the other and results in spool displacement. The spool

displacement causes 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.

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 cantilever feedback spring is fixed to

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

Displacement of the spool deflects the feedback spring