The service life and reliability of servo valves are closely tied to the contamination level of the working fluid. Impure fluid can, at minimum, impair product performance and shorten the valve’s lifespan, or, in severe cases, render the product inoperable. Therefore, users must pay special attention to the contamination level of the system’s working fluid. To ensure proper operation, hydraulic systems using servo valves must adhere to the following guidelines:
1.1 Thorough Cleaning of Hydraulic Systems with Servo Valves
Newly installed hydraulic system pipelines or those replacing existing pipelines should be cleaned according to the following steps:
A. Pre-assembly Cleaning: After pre-assembling the pipelines, disassemble them for acid pickling and phosphating.
B. Pipeline Flushing: After reassembly, flush the pipelines. During flushing, do not install the servo valve. Instead, use a flushing plate at the servo valve’s mounting seat. If the system allows, a directional valve can be installed to simultaneously clean the working pipelines and actuators. Fill the reservoir with cleaning oil (use low-viscosity dedicated cleaning oil or hydraulic oil of the same grade). Start the hydraulic source and run the system for flushing, ideally actuating all system components to dislodge contaminants. During flushing, gently tap the pipes, especially at welds and joints, to help remove rust and debris.
Regularly check the filter during flushing. If clogging occurs, replace the filter cartridge promptly. Paper, chemical fiber, or powder metallurgy filter cartridges must not be cleaned and reused; other materials may be reusable depending on their condition. Continue flushing until the oil contamination level meets requirements or no contamination is visible on the filter cartridge. Drain the cleaning oil, clean the reservoir (using dough or putty to remove fixed particles is recommended; avoid using cotton, hemp, or synthetic fiber cloths), and replace or clean the filter. Then, inject new oil into the reservoir through a 5–10 µm filter. Start the hydraulic source and flush for an additional 24 hours, then replace or clean the filter to complete the pipeline cleaning process.
1.2 Filtration Requirements Before Servo Valve Inlet
A full-flow, non-bypass filter with a nominal filtration accuracy of at least 10 µm must be installed before the servo valve’s oil inlet. The internal filter within the servo valve is a coarse filter designed only to catch larger contaminants that may occasionally pass through. It should not be relied upon as the primary defense against contamination. Filter accuracy depends on the servo valve type:
- Nozzle-flapper valves require an absolute filtration accuracy of 5–10 µm (NAS1638 Class 5–6).
- Jet-pipe valves require an absolute filtration accuracy of 10–20 µm (NAS1638 Class 7–8).
1.3 Recommended Oil Cleanliness for Hydraulic Systems Using Jet-Pipe Electro-Hydraulic Servo Valves
For long-term operation, the hydraulic oil cleanliness should meet GB/T14039-2002 level -/15/12 (equivalent to NAS1638 Class 6). For general use, the cleanliness should not be worse than GB/T14039-2002 level -/18/15 (equivalent to NAS1638 Class 9).