I. Model Meaning (Sequentially)

D631: 631 Series Two-stage Mechanical Feedback Servo Valve

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

556: Flow Code → 55 L/min @ Δp=35 bar; The last digit 6 = Special Customization (Zero Position / Gain Fine-Tuning + Medium Leakage)

B: External Pilot Supply, NBR (Nitrile Butadiene Rubber) Sealing, Standard Leak Valve Core

Sentence: Z556B = Non-Explosion-proof + 55 L/min Ultra-Large Flow + External Pilot + NBR Sealing + Custom Zero Position

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

Rated Flow: 55 L/min (631 Series Ultra-Large Flow Gear)

Maximum Working Pressure: 315 bar

Frequency Response (−3 dB, 90°): ≈ 45–55 Hz (Typical Value for Ultra-Large Flow)

Step Response (100%): ≈ 30–35 ms

Hysteresis: ≤ 0.3%

Resolution: ≤ 0.1%

Zero Position Leakage: ≈ 0.60 L/min (Standard Leakage, Slightly Higher than 556A)

Pilot Supply: External (Requires Connection to X Port)

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

Sealing Material: NBR (Nitrile Butadiene Rubber), General Temperature / General Hydraulic Oil

Temperature Range: −20 ℃ ~ +80 ℃ (Upper Limit of NBR Sealing)

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

III. Comparison of D631Z556B with 556A/407C Table

Project D631Z556B (55 L/min external guide standard seal) D631Z556A (55 L/min external guide FPM seal) D631Z407C (40 L/min internal guide custom)

Rated Flow Rate 55 L/min 55 L/min 40 L/min

Guide Method External (X port) External (X port) Internal (P port)

Sealing Material NBR (−20~+80℃) FPM (−20~+120℃) NBR

Zero Position Leakage ≈0.60 L/min (standard) ≈0.50 L/min (custom leakage) ≈0.35 L/min (low leakage custom)

Frequency Response 45–55 Hz 45–55 Hz 50–60 Hz

Price Basis + 25% Basis + 30% Basis + 25%

In one sentence: D556B is the 55 L/min "external guide + standard NBR seal + custom zero position" version: the flow rate is the same as 556A, the temperature range is normal, the leakage is slightly higher, and the price is 5% lower than 556A, suitable for large flow + normal temperature + external guide system.

Four. Features and Applications Characteristics

✅ 55 L/min Ultra-high Flow Rate: 631 series has a maximum flow limit, suitable for heavy-duty actuators

✅ External Pilot (B): Independent pilot oil supply, high-pressure main valve + low-pressure pilot energy-saving design

✅ NBR Seal (B): General hydraulic oil compatibility, low cost, reliable at normal temperatures

✅ Special Customization (6): Zero gain / offset fine-tuning, balancing flow rate and accuracy

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

Typical Applications

Ultra-high Flow Rate at Normal Temperature Closed-loop: Heavy-duty presses, large injection molding machines for mold locking / injection, die-casting machines

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

Conventional Working Conditions High Precision: 55 L/min type cylinders / motors, normal temperature (<80℃), general hydraulic oil

Metallurgy / Heavy Machinery (Non-Hot Sections): Thickness rolling mill lowering, large roller system synchronous drive