What is the design purpose of the cracking pressure for the built-in bypass valve of HYDAC filter elements?

HYDAC return filter elements come with built-in bypass valves as standard, pre-set via springs to specified cracking differential pressures. Designed around three core goals: system safety, equipment protection and uninterrupted operation, such bypass structures are indispensable safety components for low-pressure filters.

Primarily, the bypass keeps oil passages unobstructed once the filter is severely clogged. As contaminants accumulate, differential pressure rises steadily. When hitting the factory-set cracking pressure (normally 0.3 MPa to 0.35 MPa), the valve opens automatically to divert hydraulic oil past the filter media and avoid full flow blockage. Complete flow cutoff would stop return oil from flowing back to the tank and cause sharp backpressure spikes, resulting in sluggish cylinder movement and overheated pumps in mild cases, or pipe rupture and pump breakdown in severe cases. By bypassing excess pressure at preset values, the design eliminates shutdown risks caused by full clogging.

Secondly, it protects filter cartridges from rupture or deformation under excessive pressure. Without bypass protection, full clogging would subject filter media and inner support cages to the entire differential load. Sustained high pressure may tear glass fiber layers or crush inner support screens. Fragmented filter debris contaminates hydraulic fluid and invades precision parts such as valve spools and plungers, leading to valve seizure or cylinder bore scoring. The fixed cracking pressure diverts excess flow once the threshold is reached to contain pressure within safe limits, preventing filter damage and subsequent secondary system contamination.

Thirdly, it enables reliable cold startup and stable operation under abnormal working conditions. HYDAC’s rated flow and differential pressure ratings are calibrated based on ISO VG46 hydraulic oil at a standard viscosity of 32 mm²/s. At low ambient temperatures, oil viscosity rises sharply, pushing the initial differential pressure of new filters close to bypass setpoint; partial bypass opening facilitates smooth oil circulation and prevents pump cavitation and abnormal noise during cold startup. Besides, sudden flow surges or brief overloads may temporarily lift differential pressure to the opening value, where the bypass diverts excess flow to cushion hydraulic shocks and sustain continuous machine operation.

However, bypass activation means unfiltered fluid circulates into the system. Manufacturers therefore set bypass cracking pressure below the filter’s alarm differential pressure: maintenance alerts trigger first at the warning threshold to prompt element replacement, and bypass only opens if maintenance is delayed. This arrangement strikes a balance between non-stop system running and sustained hydraulic fluid cleanliness.