Moog G78K Series Installation And Operation Instruction

1. INTRODUCTION

This manual provides instructions and procedures necessary to install,
operate and troubleshoot the Moog Inc. Series 78K or G78K series intrinsic
safety protected industrial serovalve. The (G)78K series valves are electrical
equipment for hazardous areas with intrinsic safety or non-incendive protection.
The approved hazardous location markings include:
1G Ex ia IIC/IIB T4 Ga KEMA 02ATEX1015 X
II 3G Ex ec II T3 Gc KEMA 02ATEX1016 X
0344 per ATEX directive 2014/34/EU
Ex ia IIB/IIC T4 Ga IECEx KEM 10.0041X
Ex ec IIC T4 Gc IECEx KEM 10.0041X
per IECEx certification scheme.
The G78K servovalves are also approved by FM, CSA, and TIIS for hazardous
locations. They are intended for directional, position, velocity, pressure and
force control in hydraulic control systems that operate with mineral oil based
fluids. Others on request.

2. OPERATION

The Moog Inc. Series (G)78K Electrohydraulic Servovalve 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 member. The flexure
tube acts as a seal between the electromagnetic and hydraulic sections of the
valve. The two 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 two 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 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 which creates a force on the armature/flapper
assembly.
Input signal induces a magnetic charge in the armature and causes 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 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.
CAUTION
DISASSEMBLY, MAINTENANCE, OR REPAIR OTHER THAN IN ACCORDANCE WITH THE
INSTRUCTIONS HEREIN OR OTHER SPECIFIC WRITTEN DIRECTIONS FROM MOOG WILL
INVALIDATE MOOG'S OBLIGATIONS UNDER ITS WARRANTY AND YIELD THE INTRINSICALLY
SAFE PROTECTION PERMIT NULL AND VOID.
(G)78K Series Installation and
Operation Instruction
Electrohydraulic Servovalve
Intrinsic Safety Protected
ELECTROHYDRAULIC VALVE CUT-AWAY
Magnet
Coil
Armature
Nozzle
Spool
Filter
Control Port B
Figure 1 Moog Series (G)78K
Upper Polepiece
Flexure Tube
Flapper
Lower Polepiece
Feedback Wire
Inlet Orifice
Control Port A
Tank
Pressure