Hydraulic Filter

Every minute, approximately one million particles that are larger than 1 micron (0.001 mm or 1 μm) can enter a hydraulic system. These particles can cause damage to hydraulic system components because hydraulic oil is easily contaminated. The wear of hydraulic system components is dependent on this contamination, and the existence of metal parts in hydraulic system oil (iron and copper are particularly powerful catalysts) accelerates its degradation.

Hydraulic filters remove dirt and particles from fluid in a hydraulic system.

A hydraulic filter helps to remove these particles and clean the oil on a continuous basis. The performance for every hydraulic filter is measured by its contamination removal efficiency, i.e. high dirt-holding capacities. Almost every hydraulic system contains more than one hydraulic filter.

Suction Filter

The suction filter provides protection to the hydraulic pump from particles larger than 10 microns. You should use a suction filter if there is any likelihood of pump damage due to larger particles or pieces of dirt. For example, this may occur when it is difficult to clean the tank or if several hydraulic systems use the same tank for oil supply.

Characteristics of a suction filter:

  • Low cost

  • Servicing difficulty, since mounting is below fluid level

  • Grade of filtration: Coarse Filtration

    • 25 to 90 μm, using stainless steel filter mesh

    • 10 μm, using paper

    • 10 to 25 μm, using glass fibre

  • Equipped with bypass check valve, opening pressure very low (anti-cavitation)

Note: Suction filters have to be mounted well below the oil level of the tank in the suction line of the pump.

Pressure Line Filter

The pressure line filter, also referred to as a high pressure filter, is most commonly used in a hydraulic system. This type of filter is also equipped with a bypass check valve. When a pressure line filter is installed directly in back of the pump, it acts as the main filter for the complete flow and protects hydraulic components against wear.

Characteristics of a pressure line filter:

  • Medium cost (depending on size and excess pressure)

  • High grade of filtration

  • Easy use of clogging indicators

  • Grade of filtration: Finest Filtration 

    • 25 to 660 μm, using stainless steel filter mesh

    • 1 to 20 μm, using paper , glass fibre and polyester

  • Equipped with bypass check valve that opens at 7 bar (maximum)

Note: A pressure line filter acts as a safety filter when installed in front of an endangered component (e.g. servo control valve). To ensure maximum functionality of these critical components, the normal practice is that the pressure line safety filter must be fitted as close as possible to the component it is protecting (e.g. in the housing bores).

Return Line Filter

Almost every hydraulic system uses a return line filter. It's designed to mount directly onto the tank cover. Therefore, you can easily replace filter element(s) when needed; it's perfect for easy maintenance. Select your return line filter based on the maximum flow of the hydraulic system.

Characteristics of a return line filter:

  • Low cost

  • Very easy servicing

  • No downtime because it incorporates duplex filters

  • Grade of filtration: Fine Filtration

    • 40 to 90 μm, using stainless steel filter mesh

    • 10 μm, using filter paper

    • 10 to 25 μm, using glass fibre

  • Equipped with a bypass check valve that opens at 2 bar (maximum)

Note: To prevent the hydraulic oil from foaming in the tank - in all operating conditions - the return flow must come from below the fluid level in the tank. As a general rule, the distance between the bottom of the tank and the end of the pipeline should be more than 2 to 3 times the pipe diameter.

Bypass Filtration

Modern hydraulic systems use bypass filters as main flow filters (i.e. system filters or working filters). It usually consists of a bypass unit complete with pump, filter and oil cooler. Bypass filters are also used in mobile hydraulics where they are connected to the pressure side of the system. A flow control valve ensures a constant flow with low-flow pulsation.

Characteristics of a bypass filter:

  • High costs; however, high returns in terms of improving component lifetime and considerable slow-down of the ageing process of the hydraulic fluid

  • Very high grade of filtration: 0.5 μm

  • Silt removal from the fluid

  • Flow through bypass filter is completely free of pressure shocks

  • Possibilities of offline filtration

Note: Bypass filters allow very dense hydraulic filtration by removing even the smallest dirt particles. Silt would otherwise degrade the dopes, which are added to the hydraulic oil to form a protecting layer for the moving parts in the system. In general, the entire hydraulic system fluid can be flushed through the bypass filter within 10 to 12 hours, which is approximately 1% of the main flow.

Fillers and Breathers

You should use breathers or fillers when the air compresses or expands due to increasing/decreasing levels of fluid in the tank. The function of a breather is to filter the air flowing in and out of the tank. Most breathers are also designed to work as fillers. The filler must be easy to reach and should be equipped with a filling strainer.

Breathers are currently considered to be the most important components for filtration in hydraulic systems. A large amount of ambient contamination enters hydraulic systems via unsuitable ventilation devices. Other constructive measures, such as the pressurisation of oil tanks, are usually uneconomic when viewed with respect to the highly effective breathers available today.

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