1. Filter Types and Functions

(1) Classification by Function

① Contamination Control Type:

Achieves and maintains the recommended cleanliness level of the system medium. Whether for refueling, high-pressure, return, or bypass systems, it should be installed in a position that filters out contaminants to the greatest extent possible, and is usually equipped with a bypass valve.

② Protection Type:

Provides individual protection for contamination-sensitive elements. It is directly located upstream of the protected filter, equipped with a high-pressure differential filter element (without a bypass valve) and a 6.9 Bar differential pressure alarm, and installed on the pressure line.

③ Air Breathing Apparatus:

Protects the system from contaminated air entering the system. It is installed on the air breathing line of the fuel tank or on the fuel tank cover.

(2) Classification by Installation Location

① Suction Line Filter:

Not recommended. To avoid the risk of pump cavitation, do not install overly fine filters in the suction line.

② High-Pressure Filter (Pump Outlet):

Function: Prevents pump failure and damage, contamination of downstream components, and provides protection for wear-sensitive components.

Requirements: Must withstand system pressure (including pulsating pressure), and withstand pressure and flow pulsations and spikes.

Precautions: The system must be shut down when replacing the filter element (unless a dual filter or bypass system is installed). It is less sensitive to pressure drop than low-pressure return or bypass filters.

③ Return Oil Filter:

Function: Prevents contaminants in the system from entering the oil tank through the return oil line.

Requirements: Must withstand alternating flow impacts (especially when the system drives linear actuators such as hydraulic cylinders).

Arrangement: Pipeline type and box-mounted type. Precautions: When not using a dual filter, the system must be shut down before replacing the filter element; the maximum flow rate at the application point (not the total flow rate of the system pump) must be known, as the flow rate may be amplified due to hydraulic cylinders, accumulators, etc., so avoid selecting an undersized filter.

④ Bypass Circulation / Oil Filter Cart:

Advantages: Filtering continues uninterrupted even when the system is shut down; operation is relatively independent of the main system; the filter can be placed in an easily replaceable location (no system shutdown required for replacement); oil can be added to the system tank via the bypass circulation system.

Precautions:

(1) An additional filter is required, and space must be considered for the placement of additional pumps, valves, filters, etc.

(2) If the pump flow rate is less than 10% of the system volume, the system filtration is insufficient, and the flow rate needs to be increased.

(3) Generally not used alone; a filter element still needs to be installed on the main system circuit.

(4) Cannot directly protect specific system components.

⑤ Protective Filter:

Function: Prevents catastrophic failure, provides additional protection for specific components, and protects the filter element.

Working Principle: When the protective filter element becomes clogged, the system back pressure increases, eventually opening the system relief valve to directly drain the hydraulic oil back to the tank. (The system relief valve setting pressure must be lower than the crushing pressure of the protective filter element; for example, the relief valve setting pressure for a 210 bar high-pressure differential filter element must be lower than 210 bar.)

Installation Location: Upstream of the protected hydraulic component, on a pressure pipeline or pressure circuit.

Configuration Requirements: No bypass valve; equipped with a 6.9 bar alarm level differential pressure alarm; fitted with a high-pressure differential filter element (typically a 210 bar crushing-grade filter element).

Features: Not the finest system filter; its primary purpose is to protect downstream components rather than remove contaminants.

2. Factors for Selecting Filter Housings and Filter Elements

(1) Core Requirements: No failure, appropriate filter element lifespan, and required cleanliness level.

(2) Media Compatibility

(3) Filtration Effect: The media can be fully filtered during normal use.

(4) Filter Element Replacement Instructions: An alarm should be triggered when the filter element needs to be replaced.

(5) Filter Housing Selection: Simplify customer installation as much as possible and consider the space required for filter element removal.

3. Key Data and Details to Collect for Filter Selection

（1） Pressure Data:

Actual pressure at the application point (including peak pressure), pressure variation range and frequency; oil viscosity doubles when pressure exceeds 42MPa.

（2） Temperature Data:

Actual temperature of hydraulic oil or lubricating oil. If there is low temperature, it needs to be clarified whether it is for a short or long period (e.g., whether the equipment operates at low temperatures for extended periods in winter); the viscosity of most mineral oils is closely related to temperature, the lower the temperature, the higher the viscosity and the greater the resistance, which may lead to undersized filters.

（3） Flow Rate Data:

Actual maximum flow rate at the application point, whether there are severe flow pulsations or flow shocks (excessive flow pulsation can easily damage the filter bypass valve; undersized filters are prone to static electricity problems because non-metallic oils quickly wash away non-metallic materials such as glass fibers in the filter media).

（4） Other:

For vehicle-mounted systems, vibration should be considered (it can easily cause parts to fall off); the specific gravity of the medium used.