I. Model Meaning (by each digit)

D631: 631 Series Two-stage Mechanical Feedback Servo Valve

Z: Standard Non-Explosion-proof Type (General Industrial Environment)

557: Flow Code → 55 L/min @ Δp=35 bar; The last digit 7 = High Response / High Performance Customization (Higher Frequency Response, Optimized Valve Core) Moog Inc.

A: External Pilot Supply Oil, FPM (Fluorine Rubber) Sealing, Low Leakage Valve Core

Sentence: Z557A = Non-Explosion-proof + 55 L/min Ultra-Large Flow + External Pilot + FPM Sealing + High Response Customization

II. Core Parameters (Δp=35 bar / Each Side)

Rated Flow: 55 L/min (Upper Limit of 631 Series Flow) Moog Inc.

Maximum Working Pressure: 315 bar

Frequency Response (−3 dB, 90°): ≈ 55–65 Hz (10 Hz higher than 556A, High Response Version) Moog Inc.

Step Response (100%): ≈ 25–30 ms (Faster) Moog Inc.

Hysteresis: ≤ 0.25% (Better than Standard 556A) Moog Inc.

Resolution: ≤ 0.08% (Higher Precision) Moog Inc.

Zero Position Leakage: ≈ 0.40 L/min (Low Leakage Customization, Lower than 556A)

Pilot Supply Oil: External (Requires X Port) Moog Inc.

Explosion-proof Grade: None (Z = Non-Explosion-proof)

Sealing Material: FPM (Fluorine Rubber), High Temperature Resistance (−20 ℃ ~ +120 ℃)

Filter Requirements: NAS 6–7 / ISO 4406 <14/11 Moog Inc.

III. Comparison of D631Z557A with 556A/556B Table

Project D631Z557A (55 L/min external guide high-response FPM) D631Z556A (55 L/min external guide standard FPM) D631Z556B (55 L/min external guide standard NBR)

Rated Flow Rate 55 L/min 55 L/min 55 L/min

Frequency Response 55–65 Hz (High Response) 45–55 Hz 45–55 Hz

Step Response 25–30 ms 30–35 ms 30–35 ms

Hysteresis / Resolution 0.25% / 0.08% 0.3% / 0.1% 0.3% / 0.1%

Zero-Position Leakage ≈0.40 L/min (Low Leakage) ≈0.50 L/min (Medium Leakage) ≈0.60 L/min (Standard)

Sealing / Temperature FPM (-20~+120℃) FPM (-20~+120℃) NBR (-20~+80℃)

Price Baseline + 35% (Highest) Baseline + 30% Baseline + 25%

In one sentence: Z557A is the 55 L/min "external guide + FPM seal + high response low leakage" top-spec version: highest frequency response, lowest leakage, best accuracy, high temperature resistance, and the most expensive, suitable for ultra-large flow + high temperature + high dynamic closed-loop scenarios.

Four. Features and Applications Characteristics

✅ 55 L/min Ultra-high Flow Rate: Compatible with heavy-duty actuators from Moog Inc.

✅ High Response Customization (7): Frequency response increased by 10 Hz, faster step response, suitable for high-speed closed-loop systems from Moog Inc.

✅ External Pilot (A): Independent pilot oil supply, flexible system design from Moog Inc.

✅ FPM Seal (A): Resistant to high temperatures and corrosion, reliable in harsh conditions

✅ Low Leakage Valve Core: Zero leakage at the zero position as low as 0.40 L/min, energy-saving and efficient

✅ Non-explosion-proof Z-Type: Suitable for ordinary industrial environments, cost lower than explosion-proof K-Type

Typical Applications

Ultra-high Flow Rate and High-Speed Closed Loop: Heavy-duty high-speed presses, large injection molding machines for high-speed injection, die-casting machines for high-speed injection

High-Temperature and High-Dynamic Conditions: Metallurgical hot rolling lines, glass forming machines, high-temperature testing equipment (+100℃)

High Precision and Large Load Synchronization: Multi-cylinder synchronous drive, large platform high-speed positioning, vibration test bench

External Pilot Energy-saving System: High-pressure main oil circuit + low-pressure pilot oil supply, energy-saving during long-term operation from Moog Inc.