multitude of shapes, sizes, micron ratings, and construction materials.
Hydraulic filters provide inbuilt protection and minimize hydraulic system breakdowns that are quite often caused by contamination. Efficient filtration helps prevent system failure and makes a significant contribution to low cost of ownership. These are fitted for both low-and high-pressure hydraulic
applications. The filter used in a hydraulic system should be subjected to periodic and routine cleaning and maintenance. The life of a filter in a hydraulic system depends primarily on the system pressures, level of contamination, and nature of contaminants.
Construction and Working of Hydraulic Filter
Construction of a typical hydraulic filter is shown in Figure 5.10 along with its important parts. It works the same as a pneumatic filter. Hydraulic fluid entering from an inlet port is entered into the filter body and bound to
pass out only from the filter element. After passing out from the filter element, all the impurities which were present in the hydraulic fluid gets trapped in the filter element and clean hydraulic fluid is obtained at outlet of
the hydraulic filter. Hydraulic filters are designed to tolerate higher pressures then in case of pneumatics, i.e., why they are sometimes called as heavy duty
filters.
Types of Filters
In a hydraulic system, there are two main types of filters that are frequently used. These are:
1. Surface filters: These are simple screens used to clean oil passing through the pores. The dirty unwanted particles are collected at the top
surface of the screens when the oil is passed.
2. Depth filters: These are thick walled filter elements through which the oil is made to pass retaining the undesirable foreign particles. The capacity of depth filters is much higher than surface filters as much finer materials have a chance of being arrested by these filters.
Filters can also be classified as:
1. Full-flow filter: In a hydraulic system it is necessary that all the flow must be through the filter element. So, the oil must enter the filter element at its inlet side and get sent out through the outlet after crossing the filter element fully. In case of full flow filtration, the filter is sized to accommodate the entire oil flow at that part of the circuit.
2. By-pass filter: At times, the entire volume of oil need not be filtered and thus, only a portion of the oil is passed through the filter element. The main portion is directly passed without filtration through a restricted passage.
Filter Location
The location of a filter in the hydraulic system is of much significance. Various locations are used to arrange a filter either in the return line or in the pressure line. A filter may be used either in the intake side of the pump or in the outlet side of the pump. Generally, for a filter, the preferred locations are:
Return line filter: It may reduce the amount of dirt ingested through the cylinder and seals from reaching the tank.
Intake filter: These are fitted before the pump so that they can prevent random entry of contaminants like large chips into the pump and thus prevent damage to it.
Pressure filter: A pressure filter is used sometimes at the pump outlet to
prevent entry of contaminants generated in the pump, into other components like valves, etc. and thus help in avoiding the spread of such
undesirable elements into the whole system. This will thus protect valves, cylinders, etc.
Final control filter: To keep the large debris out of a component that can cause the component to fail through inbuilt arrangement or by
additional protective design.
Filtering Material and Elements
The general classes of filter materials are mechanical, absorbent inactive, and absorbent active.
1. Mechanical filters contain closely woven metal screens or discs. They generally remove only fairly coarse particles.
2. Absorbent inactive filters, such as cotton, wood pulp, yarn, cloth, or resin, remove much smaller particles; some remove water and watersoluble contaminants. The elements often are treated to make them sticky to attract the contaminants found in hydraulic oil.
3. Absorbent active materials, such as charcoal and fuller’s earth (a claylike material of very fine particles used in the purification of mineral or vegetable-base oils), are not recommended for hydraulic systems.