Shijiazhuang Laiye Imort& Export Trading Co., Ltd. is one of the leading manufacturers and suppliers of da pilot unloading valve rexroth substitute in China, also supports customized service. If you're going to wholesale high quality da pilot unloading valve rexroth substitute made in China, welcome to get pricelist from our factory.
How DA Pilot Unloading Valve Works – The Pilot Secret
Unlike a simple relief valve, an unloading valve has a pilot piston, a main spool, and a separate check valve. Here is the sequence:
Pump runs, system pressure below set point – The main spool is held closed by spring force. Pump flow goes to the system (port P to port A).
Pressure reaches set value – Pilot oil pushes the piston, opening the main spool. Flow from P now goes to tank (port T). The pump unloads. A small check valve inside keeps the system pressure from draining back through the valve.
System pressure drops to reset point – The pilot spring closes the spool. Pump flow returns to the system.
The critical detail: the unloading valve does not regulate pressure like a relief valve. It is either fully closed or fully open. That means no throttling loss during unloading – just a small pressure drop (3‑5 bar) across the open spool.
One common mistake: using an unloading valve without a check valve on the system side. If the check valve leaks, the system pressure will bleed back through the unloading valve and the pump will cycle rapidly. Our DA includes a hardened poppet check valve rated for zero leakage (under 1 ml/min at 315 bar).
Technical Specifications – The Numbers That Count
| Parameter | NG6 | NG10 |
|---|---|---|
| Max pressure (P, A) | 315 bar | 315 bar |
| Max back pressure (T) | 30 bar | 30 bar |
| Flow rating – unloading | 40 L/min | 120 L/min |
| Flow rating – system (A port) | 40 L/min | 120 L/min |
| Pressure ranges (springs) | 10‑100, 20‑200, 30‑315 bar | Same |
| Hysteresis (standard) | 5‑8% | 5‑8% |
| Hysteresis (‑H version) | 15‑20% adjustable | 15‑20% adjustable |
| Reset pressure (standard) | 85% of set | 85% of set |
| Check valve leakage | ≤ 1 ml/min at 315 bar | ≤ 2 ml/min at 315 bar |
| Temperature range | -20 to +80°C (NBR); up to +120°C (FKM) | Same |
The body is forged steel for both sizes (no cast iron – high cycling can crack castings).
Port threads: G1/4 or SAE 6 for NG6; G1/2 or SAE 10 for NG10.
All adjustment screws have a friction washer and a brass‑tipped locknut.
Durability-Focused Materials & Construction
Main spool: Hardened 20MnCr5 steel (Ra 0.2 µm), DLC coated on sealing areas.
Pilot piston & seat: Matched & lapped set; sintered bronze seat press-fitted into body.
Spring: Shot-peened 54SiCr6, pre-set. Cycle drift ＜3% @ 200,000 cycles.
Check poppet: 440C stainless steel (58 HRC), lapped to bronze seat.
Pilot orifice: Removable 0.8 mm laser-drilled orifice, easy cleaning/replacement. Custom sizes available.
Seals: NBR + backup rings for dynamic parts. FKM/HNBR/Viton optional for special media.
FAQ
Q: Can I use this valve to unload two pumps simultaneously
A: Yes, but with a caution. If you connect two pump outlets to the same unloading valve, the higher pressure pump will unload first, then the lower pressure pump will see back pressure from the open valve. The system may not unload both completely. Better to use two separate unloading valves, or a single pilot‑operated unloading valve with a shuttle valve. Contact us with your circuit – we will draw a recommended arrangement.
Q: My valve unloads but the system pressure does not drop to the reset point. The pump stays unloaded. What happened?
A: This usually means the check valve is stuck open. Debris may be holding the poppet off its seat. Remove the check valve cartridge (hex plug on the side of the body). Clean the poppet and seat with solvent. Reassemble. If the problem returns, the poppet or seat may be worn – order a replacement check cartridge (part DA‑CK).
Q: What is the difference between an unloading valve and a pressure switch with a solenoid valve?
A: A pressure switch plus a solenoid valve costs less upfront but has slower response (the solenoid takes 30‑50 ms to open). An unloading valve is purely hydraulic – it responds in under 10 ms and never needs electrical wiring. For accumulator charging or high‑low pump systems, a DA pilot unloading valve is simpler and more reliable. Only use a solenoid if you need remote control.
Q: How do I adjust the unloading pressure correctly without a pressure gauge?
A: You need a gauge. Connect a pressure gauge to the system side (port A). Run the pump. Slowly turn the adjustment screw clockwise until the valve unloads – you will hear the pump sound change and see the gauge drop to near zero. Then turn the screw counter‑clockwise in small increments until the pump loads again at your desired pressure. Lock the nut. Without a gauge, you are guessing.
Q: What is the maximum back pressure allowed on port T (tank line)?
A: For standard DA, maximum tank line back pressure is 30 bar. If your return filter or heat exchanger creates more back pressure, the main spool may not close fully, causing continuous unloading. For higher back pressure (up to 100 bar), order our DA‑HP version with a modified spool spring.
Q: Can I convert a standard DA to the adjustable hysteresis version (DA‑H)?
A: No – the DA‑H has a different pilot spring cartridge with an additional adjustment screw. However, we sell the cartridge separately. You can remove the standard pilot section and screw in the DA‑H cartridge. The body is the same. Send us your valve serial number and we will confirm compatibility.
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