Potential Trouble
Servo valve does not follow input
command signal. (Actuator or components
are stationary or creeping slowly).
High threshold. (Jerky, possible oscillatory
or “hunting” motion in closed loop
system).
Poor response. (Servo valve output lags
electrical command signal).
High Null Bias, (High input current
required to maintain hydraulic cylinder or
motor stationary).
Probable Cause
1. Plugged inlet filter element.
1. Plugged filter element.
1. Partially plugged filter element.
1. Incorrect null adjustment.
2. Partially plugged filter element.
Remedy
1. Replace filter element.
1. Replace filter element.
1. Replace filter element.
Check for dirty hydraulic fluid in system.
1. Readjust null.
2. Replace filter element and check for
dirty hydraulic fluid in system.
9. TROUBLESHOOTING CHART
The following troubleshooting chart list potential troubles encountered, probable causes, and remedies.
10. FILTER ASSEMBLY REPLACEMENT
a.
Remove the socket head cap screws that retain the end caps to the body
using an Allen wrench, 4 mm for 760N or 5 mm for 78N. Gently pull or
pry the end caps from the body.
b.
Remove and discard the orings from the end caps.
c.
Remove the filter retainer plugs and inlet orifices from both sides of the
body. The filter will come out with one of the inlet orifices. Note: A
2-56 screw will thread into each piece so it can be pulled from the body
cavity. These parts may be interchanged from side to side.
d. Remove and discard the O-Rings from the retainer plugs and inlet orifices.
e.
Discard the used filter.
f.
Visually inspect all parts for damage or contamination.
g.
Lubricate and install new O-Rings on the inlet orifices and retainer plugs.
h.
Remove the new filter from the sealed package being very careful not to
introduce contamination into the ID of the filter.
i.
Install the new filter on an inlet orifice and insert it into the filter bore.
Install the other inlet orifice and the retainer plugs. Note: These parts
should go in most of the way without excessive use of force. The end
caps will push them fully into place.
j.
Lubricate and install end cap to body O-Rings. Carefully position the end
caps on the servo valve. Re-install the end cap screws and torque them
to 46 in-lbs for 760N, or 90 in-lbs for 78N.
11. FUNCTIONAL CHECKOUT AND CENTERING
a. Install servo valve on hydraulic system or test fixture, but do not connect
electrical lead.
b. Apply required system pressure to servo valve and visually examine for
evidence of external leakage. If leakage is present and cannot be rectified
by replacing O-Rings, remove the discrepant component and return for
repair or replacement.
Note:
If the system components are drifting or hardover, adjust
the mechanical null of the servo valve.
c. Connect electrical lead to servo valve and check phasing in accordance
with system requirements.
12. AUTHORIZED REPAIR FACILITIES
If servo valve continues to malfunction after all recommended corrective
action procedures are performed, defective valve should be returned to
Moog for repair. Moog does not authorize any facilities other than Moog or
Moog subsidiaries to repair its servo valves. It is recommended you contact
Moog at (716) 652-2000 to locate your closest Moog repair facility. Repair by
an independent (unauthorized) repair house will result in voiding the Moog
warranty and could lead to performance degradation or safety problems.
13. DECLARATION OF MANUFACTURER
An EU Declaration of Conformity according to Council Directive
2014/34/EU is supplied with each servo valve.
Figure 3
Inlet Orifice Assemblies
(one each end of body)
End Plate
Filter Tube
Orifice Assembly
End Cap-Body
O-Rings
Filter Plug
Filter Plug
O-Ring
Inlet Orifice O-Ring
(Note: 760N shown, 78N similar)
Moog • 78N/760N Series Operation Instruction • RevG 08/18
3
Table 2. Replacement Parts
Series
760N Series
78NxxxC
Part Description
Qty.
Part Number
Filter Replacement Kit
1 B52555RK004K001 B52555RK052K001
Inlet Orifice to Body Oring (I)
2
-42082-059
-42082-189
Filter Plug to Body Oring (I)
2
-42082-060
-42082-012
End Cap to Bushing ID Oring (I) 2
-42082-042
–
End Cap to Body Oring (I)
2
-42082-001
-42082-007*
Filter Tube (I)
1
-23020
C39486-005-060
Base Orings
4
-42082-022
-42082-021
(I) Included in Filter Replacement Kit
*Not included in 78NxxxD or later kit. P/N B52555RK208K001
