I. Model Meaning (Sequentially)

D631: 631 Series Two-stage Mechanical Feedback Servo Valve

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

558: Flow Code → 55 L/min @ Δp=35 bar; The last digit 8 = Standard Response / Standard Valve Core (Conventional Frequency Response, General Type)

A: External Pilot Supply, FPM (Fluorine Rubber) Sealing, Standard Leak Valve Core

In one sentence: Z558A = Non-Explosion-proof + 55 L/min Ultra-Large Flow + External Pilot + FPM Sealing + Standard Response General Type

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

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

Maximum Working Pressure: 315 bar

Frequency Response (−3 dB, 90°): ≈ 45–55 Hz (Standard Response, Same Level as 556A)

Step Response (100%): ≈ 30–35 ms

Hysteresis: ≤ 0.3%

Resolution: ≤ 0.1%

Zero-Position Leakage: ≈ 0.50 L/min (Standard Leakage, Same as 556A)

Pilot Supply: External (Requires Connection to X Port)

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

III. Comparison of D631Z558A and 556A/557A (The Three Brothers of 55 L/min) Table

Project D631Z558A (55L external guide standard FPM) D631Z556A (55L external guide standard FPM) D631Z557A (55L external guide high response FPM)

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

Frequency Response 45–55 Hz (standard) 45–55 Hz (standard) 55–65 Hz (high response)

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

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

Zero Position Leakage ≈0.50 L/min (standard) ≈0.50 L/min (standard) ≈0.40 L/min (low leakage)

Sealing / Temperature FPM (−20~+120℃) FPM (−20~+120℃) FPM (−20~+120℃)

Price Baseline + 30% (standard) Baseline + 30% (standard) Baseline + 35% (high configuration)

In one sentence: Z558A = Z556A, same specification, same performance, same price, only the last digit of the model name differs (8 = standard type); Z557A is the high response high configuration version.

Four. Features and Applications Characteristics

✅ 55 L/min ultra-high flow rate: Suitable for heavy-duty actuators

✅ Standard response (8): 45–55 Hz frequency response, meeting the needs of most common closed-loop scenarios

✅ External pilot (A): Independent pilot oil supply, flexible system design

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

✅ Standard leak valve core: Zero leakage at 0.50 L/min, universal energy-saving balance

✅ Non-explosion-proof Z-type: Ordinary industrial environment, cost lower than explosion-proof K-type

Typical applications

General closed-loop with ultra-high flow rate: Heavy-duty presses, large injection molding machines for injection, die-casting machines for injection

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

High-precision conventional dynamics: Multi-cylinder synchronous drive, large platform positioning, vibration test bench (standard frequency response requirements)

External pilot energy-saving system: High-pressure main oil circuit + low-pressure pilot oil supply, energy-saving during long-term operation