Hydraulics and Hydraulic Tools


The article is written for persons who wish to get a detailed knowledge about hydraulic filters and filter elements.


This will enable them to select the best filter for their use.

Selection of Hydraulic Filter

The selection of filter is based on the clearances between the mating parts as well the clearances between the valves in the system. The valve clearances being finer, they tend to dictate the filter pore size rather than the clearances between the moving parts.


 The valves also require special filters placed just before the valves to ensure proper performance. The filters placed before the valves are typically 1 micron filter. These filters are costly and difficult to replace without shutting down the system. Hence to keep system operative for a long time the filtering size of 2 to 5 microns are used in many places where the valves require filters of 1 micron size.

Size rating of Hydraulic Filters

When ordering the filter, one has to pay attention to check if the size requirement is absolute, mean or the nominal. The absolute rating of filters means that no spherical particle greater than the specified size will pass through the filters. This way they tend to be denser than the other two categories and are costly. The mean size gives you the average size of dirt that will be caught in the filter and in the Nominal rating there is a possibility that some of 10% material escaping the filter.

Types of Hydraulic Filters

The filters may also be classified as surface filters and depth filters. The surface filters have a relatively thin filtering material and all the dust particles are caught on the filter surface. Some of the surface filters can be reused depending on the mechanical construction and the properties of the filtering element.

The depth filters on he other hand are made of layers of fabric and depend on the thickness of the filter pads so that the dust passing through the first layer may be caught in the subsequent layers. This means that the filter pads are inherently disposable and cannot be reused.

Mechanical construction of filters

The mechanical construction of filter is fairly standardized for hydraulic fliter application and has an outer covering made of aluminum and a top and bottom support. Between the top and bottom support the filter element is put in. To ensure the mechanical strength of the filtering elements, the perforated metal grills are placed at the inlet and out let areas of filter and the filtering element is sandwiched between the two support perforated sheet metal. The outer and inner supporting material has to be strong enough to withstand the differential pressure across the filter without distortions that will open up additional path for oil to pass through.

Filtering material of Hydraulic Filters

Most of the filters use non woven filtering elements but some of them also use woven material, ceramic material, and porous metallic media. Some of filters use charged metallic and magnetic media for final filtering. Commonly   5-micron filter medium is made up of   of (woven or non-woven) organic or inorganic fibers bonded by epoxy resin. Stainless steel fibers may also be used in the filtering elements and more often than not this is used only in the 15 to 25 micron range and is generally re-usable.

Selection of Filters

The manufacturer of hydraulic equipment generally recommends the filter to be used and in case the particular model is not available, the manufacturer might suggest another model or can even get one designed made for the use in the equipment. The cost of filter being a minor matter in the effective use of hydraulic systems, the manufacturer’s opinion can be taken as final and no additional market explorations be made for this purpose.