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We were discussing hydraulic actuators, hydraulic flow control valve, pressure relief valve, direction control valve in our previous post. 

Today we will start here another important topic in hydraulic system i.e. Strainer and filter with the help of this post. Further, we will classify the filters and we will also see the working of each type of filter in detail in our next post.

So let us come to the main topic

As we know very well that in order to secure the smooth and correct operation of each component of hydraulic system, hydraulic fluid must be clean as much as possible. There is one specific unit of measurement for contamination level in hydraulic fluid i.e. NAS value. 

We will discuss the topic related with determination of NAS value and significance of NAS value, in a hydraulic unit, later in detail. 

Hydraulic system must be operated with clean hydraulic fluid as according to some estimate, 75% of all hydraulic system failure occurs due to contamination problems.

Why hydraulic fluid will be contaminated?

There are lots of reasons for contamination of hydraulic fluid, but let us see here some examples to secure the brief idea for the contamination of hydraulic fluid. 

With continuous operation of hydraulic system for completing the desired tasks, there will be normal wear of hydraulic valves, hydraulic pumps and other hydraulic components. As a result of such normal wear of hydraulic components, there will be produced tiny metal particles which will be measured in microns and these tiny metal particles will be mixed with hydraulic fluid and will cause increase in the contamination level of hydraulic fluid. 

Hydraulic fluid will also be contaminated due to entry of foreign particles in to the hydraulic fluid or in to the hydraulic system. 

Foreign particles may enter in to the hydraulic system during pouring of new hydraulic fluid in hydraulic reservoir. 

There are some more examples of sources of hydraulic fluid contamination and these are as listed here

1. Builtin foreign particle or dirt
2. Foreign particle already present during initial charging of hydraulic fluid
3. Foreign particles may enter in to the hydraulic system through loose air-breather cap and cylinder oil seals
4. Rust and paint flakes of reservoir may also mix with the hydraulic fluid. Therefore after preparing the reservoir it is very important for sandblasting of reservoir and also selection of suitable paints for painting of reservoir.
Therefore strainers and filters are used in hydraulic units as filtering device to remove the foreign particles and tiny metal particles from hydraulic fluid and provide clean hydraulic fluid as much as possible to the system. 
In simple, we can say that strainers and filters are used in hydraulic unit to secure the hydraulic fluid free from contamination.


Strainer is basically used in hydraulic system to remove the large foreign particles from hydraulic fluid. 

Strainer will provide less resistance to the flow of hydraulic fluid as compared to resistance provided by the filter to flow of hydraulic fluid.

Screening action of a strainer will be less effective as compared to the screening action of a filter.
Strainer will be basically installed at the inlet of the hydraulic pump and will be inside the hydraulic fluid in reservoir.

A strainer will have one metal frame which will be wounded or wrapped by fine mesh wires to provide the screening action for removing the large foreign particles.


Filter is basically used in hydraulic system to remove the quite small tiny foreign particles from hydraulic fluid. 

A filter will provide high resistance to the flow of hydraulic fluid as compared to resistance provided by the strainer to flow of hydraulic fluid. 

Screening action of a filter will be quite good and effective as compared to the screening action of a strainer.

Filter might be installed at the inlet of the hydraulic pump, in return line, in pressure line, in reservoir or at any other location as per OEM instructions.

There are basically two types of filters used in hydraulic system as mentioned here.
Full flow filters
Proportional flow filters

So we have seen here the importance of filtering action and we have also differentiated the strainers and filters.

We will discuss full flow filters and proportional flow filters in our next post.
Do you have suggestions? Please write in comment box.


Hydraulic maintenance manual (Industrial hydraulic control)
Image Courtesy: Google

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