MOOG G631-3008B
MOOG G631-3008B

MOOG G631-3008B | 18 L/min Precision Valve for Lab Test Rigs

  • Model: G631-3008B
  • Alt. P/N: G6313008B; G631-3008 (base variant)
  • Series: MOOG G631 Series Two-Stage Mechanical Feedback (MFB) Servo Valves
  • Type: 4-way, 3-position two-stage servo valve (nozzle-flapper pilot + sliding spool power stage with mechanical feedback)
  • Key Feature: 18 L/min flow capacity, 315 bar max pressure, ±10V/±40mA control input, 6ms response time, ISO 10372 Size 02 (NG4), mechanical feedback wire, IP65 protection
  • Primary Use: Precision flow/position control in micro-to-small hydraulic systems—lab test rigs, miniature automation, industrial auxiliary actuators, small process control valves, medical device prototypes
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Part number: MOOG G631-3008B
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Description

Key Technical Specifications

  • Model Number: G631-3008B
  • Manufacturer: Moog Inc. (Industrial Motion Control Group)
  • Valve Type: Two-stage mechanical feedback (MFB) servo valve (pilot-operated)
  • Actuation Principle: Nozzle-flapper pilot stage + permanent magnet torque motor; sliding spool power stage with stainless steel feedback wire
  • Size Classification: ISO 10372 Size 02 (NG4) – 4-bolt flange (M5 x 0.8)
  • Flow Capacity: 18 L/min (4.8 gpm) at Δp=35 bar (500 psi); 36 L/min (9.5 gpm) total flow at Δp=70 bar
  • Maximum Operating Pressure: 315 bar (4570 psi) – continuous service; 350 bar (5075 psi) proof pressure
  • Electrical Interface: Control Input: ±10V DC analog or ±40mA DC (single coil); Pilot Supply: 10-15 bar (145-217 psi) clean pressure; Connector: 6-pin DIN 43650 A (IP65 with sealed boot); Torque Motor Current: 35mA nominal, 70mA peak
  • Response Time: 6ms (90% step response from 0–100% command)
  • Operating Temperature: -20°C to +70°C (-4°F to +158°F)
  • Fluid Requirements: Mineral oil (ISO VG 22–68), NAS 1638 Class 5 (pilot supply: NAS 1638 Class 5), max 80°C fluid temp
  • Mounting Torque: 8 Nm (5.9 ft-lbs) – flange bolts
  • Weight: 1.5kg (3.3 lbs)
  • Seal Type: FKM (Viton) standard; PTFE optional for specialty fluids
  • Certifications: CE, RoHS compliant; ISO 13485 certified (for medical applications)

Field Application & Problem Solved

In micro-to-small hydraulic systems—think lab bench-top test rigs, miniature automation cells, or medical device prototypes—the biggest challenge is finding a valve that’s compact, precise, and rugged without electronic feedback fragility. Electronic feedback (EFB) valves are too bulky and prone to vibration damage in tight spaces. Generic proportional valves lack the ±0.02mm positioning accuracy needed for tasks like micro-actuator control or material sample testing. A California lab wasted $45k/year replacing EFB valves on fatigue test rigs, where vibration shorted LVDT wiring. A Minnesota medical device manufacturer struggled with 12% prototype scrap due to generic valves’ inconsistent flow control.
This valve fixes that with its compact MFB design—no fragile electronics, just mechanical feedback that resists vibration and tight spaces. You’ll find it in: lab test rigs (material fatigue testing with ±0.1N force precision), miniature automation actuators (pick-and-place for small components), industrial auxiliary controls (valve positioners for small process lines), and medical device prototypes (surgical tool actuators). Its core value is compact precision + rugged simplicity—ISO 10372 Size 02 fits tight manifolds, mechanical feedback eliminates electronic failures, and 6ms response handles dynamic lab profiles. For the California lab, it cut valve replacement costs by 70% and eliminated vibration-related downtime. For the medical manufacturer, it dropped prototype scrap to 1.8%.
MOOG G631-3008B

MOOG G631-3008B

Installation & Maintenance Pitfalls (Expert Tips)

  • Feedback Wire Alignment: Zero Tolerance for Misalignment: Rookies bend or over-tension the thin feedback wire during installation, causing 0.5–1% deadband. I fixed a Colorado lab’s test rig where the wire was kinked—force measurement error dropped from 3% to 0.2%. Fix: Use Moog’s alignment tool (P/N B67728-004) to keep the wire straight and perpendicular to the spool axis. No preload—let it rest naturally before tightening the clamp.
  • Fluid Cleanliness: NAS 1638 Class 5 Is Non-Negotiable: The nozzle-flapper pilot’s 0.25mm orifices clog with NAS 6 oil. A Texas lab’s valve stuck after 3 months using 10µm filters—delaying testing by 5 days. Fix: Install 5µm absolute filters (β10 ≥ 100 per ISO 16889) and test oil cleanliness every 1000 hours. For lab use, de-aerate oil to prevent cavitation in the pilot stage.
  • Pilot Pressure: Lock to 10–15 Bar Exact: Too low (≤8 bar) slows response; too high (≥18 bar) stretches the feedback wire. A Washington automation firm’s pick-and-place had erratic movement until we adjusted the pilot regulator to 12 bar. Fix: Install a miniature pressure gauge on the pilot supply (critical for small systems) and check biweekly—fluctuations mean a clogged filter.
  • Mounting Torque: 8 Nm Max—Don’t Over-Tighten: The Size 02 flange is thin (3mm) – over-torquing to 10 Nm warps the valve body, binding the spool. An Oregon medical prototype’s actuator jammed until we retorqued bolts to 8 Nm with a torque screwdriver. Use the cross-tightening sequence (1-3-2-4) for even force.
  • Null Adjustment: Depressurize and Verify Spool Center: Adjusting null under pressure preloads the feedback wire, causing drift. I spent 90 minutes troubleshooting a Utah lab’s flow inconsistency until I realized the tech calibrated with 50 bar on the ports. Fix: Bleed the system, apply 0V/0mA, and adjust the null screw until the spool is centered (use a 0.01mm feeler gauge to check spool land gaps). Lock with the jam nut and recheck after 20 minutes of run time.

Technical Deep Dive & Overview

The G631-3008B is engineered for tight spaces and precision—where size and reliability matter as much as performance. At its core, it uses a two-stage design optimized for micro-systems: a sensitive nozzle-flapper pilot stage and a compact sliding spool power stage, with mechanical feedback replacing bulky electronics.
The pilot stage’s permanent magnet torque motor converts electrical commands (±10V/±40mA) into subtle flapper movement, modulating pressure between two nozzles. This differential pressure shifts the power spool, which is linked to the torque motor via a thin stainless steel feedback wire. The wire creates a closed loop: spool movement pulls the wire, generating a restoring force that balances the torque motor’s input—locking the spool at the commanded position with ±0.02mm accuracy.
What sets it apart is its compactness: ISO 10372 Size 02 (NG4) fits manifolds too small for EFB valves, and 1.5kg weight works for portable test rigs. The hard-chromed spool (0.002mm clearances) minimizes leakage—critical for small systems where fluid volume is limited. The spring-centered spool provides a fail-safe: power or pilot pressure loss sends the spool to neutral, preventing unintended actuator movement (essential for medical prototypes).
Unlike EFB valves, there are no LVDTs, wiring harnesses, or electronics to fail from vibration, dust, or moisture. For field service, it’s a dream: the feedback wire is replaceable in 15 minutes, the nozzle-flapper pilot is easy to clean with solvent, and the modular design allows coil replacement without removing the valve from the manifold. It’s not just a valve—it’s a compact, reliable precision control solution built for the unique demands of micro-hydraulic systems.
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