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      Example 2.3 Solid contamination of a hydraulic oil according to the ISO Standard

      Determine the contamination level, in terms of particles per milliliter of fluid, of a hydraulic oil with a code 20/16/13 according to the ISO 4406.

Given:

      The ISO 4406 fluid specification, ISO 20/16/13.

      Find:

      The particle count, per milliliter of fluid.

      Solution:

      The answer is straightforward, after understanding Tables 2.7 and 2.8. In particular, per each ml the given fluid has:

       from 5000 to 10 000 particles larger than 4 μm;

       from 320 to 640 particles larger than 6 μm; and

       from 40 to 80 particles larger than 14 μm.

Hydraulic component manufacturers usually specify the maximum tolerable contamination level for the working fluid. An indicative list, adapted from [11, 28], is reported in Table 2.10. Obviously, the choice of the filter(s) to use in a hydraulic system should be based on the unit with the most stringent requirements. The units with stringent filtration requirements are typically the most expensive components of the system; therefore, preventing their failure is also justified from an economical point of view. One should consider that the performance of the filter degrades with time, and it is based on the dirt holding capacity. It is common to require the replacement of the filter elements between 1000 and 10 000 hours, depending on the application and filter parameters.

Table 2.10 Suggested cleanliness code for typical hydraulic components according to ISO 4406 [11, 28].

      Source: Assofluid [11] and Parker Hannifin [28].

	Low pressure (<70 bar)	Medium pressure (between 70 and 130 bar)	High pressure (>130 bar)
Pumps
Gear pumps	21/18/16	20/17/15	20/17/14
Vane pumps fixed displacement	20/18/15	19/17/14	18/16/13
Vane pumps variable displacement	19/16/14	18/15/13	17/14/13
Piston pumps fixed displacement	19/17/15	18/16/14	17/15/13
Piston pumps variable displacement	18/16/14	17/15/13	17/14/12
Actuators
Vane motors	20/18/15	19/17/14	18/16/13
Axial piston motors	19/17/15	18/16/14	17/15/13
Radial piston motors	20/18/16	19/17/14	18/16/14
Orbital motors	21/19/17	20/18/15	19/17/14
Cylinders	20/18/15	20/18/15	20/17/14
Valves
Check valves	20/18/15	20/18/15	20/18/15
Pressure and flow control valves	19/17/14	19/17/14	19/17/14
Directional control valves – on/off	20/18/15	20/18/15	19/17/14
Directional control valve – proportional	18/15/13	17/15/12	17/14/11
Servo valves	17/14/12	16/13/11	16/12/10
Cartridge valves	20/17/15	19/17/14	19/16/13

      The reader should know that there are different types of filter available on the market: each one can apply a different filtration technology, have a particular construction type, or integrate other accessories. Illustrating the details and the operating features of the different types of filters is beyond the scope of this chapter. A high‐level description of the available options for filters is provided in [11, 28].

      2.8.2 Filter Placement

      It is important to comment on the application of filters in hydraulic circuits, since there are different alternative options. In general, filters can be classified based on their location in the circuit:

      1 Suction filters (Figure 2.15) are placed between the reservoir and the pump inlet, which is usually the first component of the hydraulic system crossed by the hydraulic fluid after the tank.

      2 High‐pressure filters (Figure 2.16) are placed right after the pump outlet, before the working fluid can reach the control elements and the actuators of the hydraulic system.

      3 Return filters (Figure

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