Question
The manual only says to replace the element according to operating conditions. How often is that in practice, and is there a measurable criterion?
Two Criteria: Running Hours and Differential Pressure
For a standard hydraulic power unit, plan replacement by running hours, but treat differential pressure as the primary indicator. When the gauge approaches the alarm value or the clogging indicator trips, the element has reached its dirt-holding limit and should be changed at once. If pressure drop rises very quickly, the system is generating abnormal wear debris or contamination is entering, so the source must be traced as well.
Role of Oil Analysis
Periodic particle counting shows whether the element is doing its job. Cleanliness persistently out of specification while pressure drop stays low suggests the bypass is open, a seal is missing or the element rating does not match the requirement. High pressure drop with clean oil suggests an over-fine rating or high viscosity at low oil temperature.
Two Cases Where Time Alone Is Not Enough
- New or overhauled equipment: running-in generates metallic particles, so change the element earlier than the normal interval.
- Light-duty equipment: running hours accumulate slowly, but the oil may have absorbed moisture or oxidised, so follow oil analysis rather than the calendar.
Points to Note When Replacing
Clean the filter housing and sealing face, check that the bypass valve is free, confirm the new gasket is not twisted and that the element is installed the right way round, then vent the housing and observe pressure drop. Paper elements must not be washed and reused; metal mesh elements may be back-flushed according to material, but should be scrapped if pressure drop stays high after cleaning.
Common Misconceptions
Assuming that a finer rating is always better does not hold: an over-fine element clogs faster, raises pressure drop and may keep the bypass open. Select the rating specified for the equipment and keep oil cleanliness stable, which is what actually extends component life.
Differences by Filter Position
The suction filter protects the pump inlet and should not be over-fine, otherwise the pump cavitates. The pressure filter sits downstream of the pump and must withstand high differential pressure, so media and core strength are critical. The return filter has the largest dirt-holding capacity and intercepts most of the contamination returning to the tank.
- Suction: prioritise flow capacity with coarse filtration.
- Pressure line: verify rated differential pressure and collapse pressure.
- Return line: size by dirt-holding capacity to avoid frequent changes.
What the Replacement Record Should Contain
Record the date, machine number, element part number and rating, differential pressure at change, oil analysis result and the reason for replacement. After a few cycles this data gives a realistic interval for that machine instead of a generic hour count.
Practical Advice
For workshops with mixed machine types, build a cross-reference table listing machine, element number, rating and quantity per machine, then set safety stock from it. With the table in place, issuing and stocktaking no longer depend on memory, and wrong-fit installation is avoided.
References
- GB/T 3766-2015 Hydraulic fluid power — General rules and safety requirements for systems and their components
- GB/T 7935-2005 Hydraulic fluid power — General technical requirements for hydraulic components
- GB/T 14039-2002 Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles
- GB/T 17446-2012 Fluid power systems and components — Vocabulary
