I. Explanation of Model Numbers

D631: 631 Series Two-stage Mechanical Feedback Servo Valve

Z: Non-explosion-proof, standard industrial environment

558: 55 L/min (Δp = 35 bar); Last digit 8 = Standard Response / General Valve Core

B: External Pilot Supply + NBR (Nitrile) Sealing (Normal Temperature Type)

Sentence: Z558B = 55 L/min Standard Response + External Supply + Normal Temperature NBR Sealing + Economy Type

II. Core Parameters (Δp = 35 bar)

Rated Flow: 55 L/min

Maximum Pressure: 315 bar

Frequency Response (−3dB/90°): 45–55 Hz (Standard Response)

Step Response: 30–35 ms

Hysteresis: ≤0.3%

Resolution: ≤0.1%

Zero Leakage: ≈0.50 L/min (Standard Leakage)

Pilot Method: Independent Supply from External X Port

Sealing: NBR Nitrile, Temperature -20 to +80°C

Filter: NAS 6–7 / ISO 4406 14/11

III. Comparison of 558A / 558B (Same Flow Rate, Same Standard Response) Table

Parameters: D631Z558A, D631Z558B

Flow rate: 55 L/min, 55 L/min

Frequency response: 45–55 Hz (standard) 45–55 Hz (standard)

Pilot: External, External

Sealing: FPM fluororubber (−20~+120℃), NBR nitrile (−20~+80℃)

Zero leakage: 0.50 L/min, 0.50 L/min

Price: Higher (high-temperature sealing), More economical (normal-temperature sealing)

Operating conditions: High-temperature, harsh oil conditions, General use at normal temperature, cost priority

Summary: 558A is more expensive for high-temperature applications; 558B is an economical option for normal temperature, with high性价比.

Four. Overview of the entire series of five brothers (Z556/557/558)

Z556A： 55L， standard response， external pilot， FPM， general use at high temperature

Z556B： 55L， standard response， external pilot， NBR， economical at normal temperature

Z557A: 55L, high response (55–65Hz)， external pilot， FPM， high configuration at high temperature

Z557B： 55L， high response (55–65Hz)， external pilot， NBR， high configuration at normal temperature

Z558A： 55L， standard response， external pilot， FPM， standard at high temperature (same as 556A)

Z558B： 55L， standard response， external pilot， NBR， standard at normal temperature (same as 556B)

Summary：

556/558： Standard response (45–55Hz)， general use， cheap

557： High response (55–65Hz)， high precision， fast dynamics

A： FPM high-temperature； B： NBR normal temperature

Five. Application scenarios (Z558B)

Heavy-duty presses， large injection mold locking / injection， die-casting machines

Metallurgical conventional rolling， large oil cylinder synchronization， platform positioning

Normal temperature (≤80℃)， ordinary hydraulic oil， standard dynamic requirements

External pilot， desire to control cost， stable and reliable closed-loop system