The ZDR 6 D is a direct-operated, sandwich-plate pressure reducing valve designed for modular hydraulic stack assemblies conforming to ISO 4401-03-02-0-05 interface standards. Its primary function is to maintain a constant secondary pressure in a specific actuator circuit, independent of fluctuations in the primary system pressure, provided the primary pressure remains higher than the set secondary pressure. The valve utilizes a spring-loaded spool design to modulate flow, effectively throttling the fluid path to achieve the desired pressure drop. Due to its compact footprint and standardized porting, it is frequently integrated into multi-axis machine tool manifolds and mobile hydraulic pilot control circuits where precise pressure regulation is required for secondary functions.
For complete selection guidelines and cross-reference documentation on this product line, please consult our Rexroth equivalent hydraulic valves guide.
Operational Principles and Mechanical Configuration
The ZDR 6 D operates on the principle of force balance between the secondary pressure acting on the spool face and the adjustable compression force of the control spring. When the secondary pressure falls below the spring setting, the spool is held in the open position, allowing flow from the P-port to the A-port. As the pressure at the A-port rises to the set point, the spool shifts against the spring, restricting the flow path until a state of equilibrium is reached.
The valve is available in multiple pressure adjustment ranges, typically controlled via a threaded spindle with a locknut or a hand knob. To ensure stability, the valve features a pilot-operated drain line (Y-port) that must be routed directly to the tank to prevent backpressure from interfering with the spool’s reference pressure.
Technical Specifications
The following table outlines the critical performance parameters for the ZDR 6 D series.
| Parameter | Specification |
|---|---|
| Interface | ISO 4401-03-02-0-05 |
| Max. Operating Pressure (P-port) | 315 bar |
| Max. Flow Rate | 50 L/min |
| Pressure Adjustment Ranges | 25, 50, 100, 200 bar |
| Fluid Temperature Range | -30°C to +80°C |
| Viscosity Range | 10 to 800 mm²/s |
Fluid Contamination and ISO 4406 Compliance
Spool silting is the primary failure mode for direct-operated pressure reducing valves. Given the tight radial clearances between the spool and the valve body, even minor particulate ingress can lead to spool sticking or erratic pressure regulation. To maintain operational integrity, the hydraulic fluid must adhere to a minimum cleanliness class of 20/18/15 according to ISO 4406. Filtration systems should be sized to prevent the accumulation of metallic fines or oxidation products within the valve’s internal pilot galleries.
Elastomer Compatibility and Seal Selection
The ZDR 6 D is supplied with specific seal kits depending on the application environment and fluid type. Standard NBR (Nitrile Butadiene Rubber) seals are suitable for mineral oil-based fluids (HLP/HLPD) within standard temperature ranges. However, for applications involving synthetic esters or phosphate ester-based fire-resistant fluids (HFD), FKM (Viton) seals are mandatory. Users must verify chemical compatibility with the specific hydraulic medium to prevent elastomer swelling, hardening, or degradation, which can lead to internal leakage and loss of pressure control.