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The hydraulic components most affected by solid contaminants are those where mechanical parts are in relative motion with minimum clearances. This is the case of hydraulic pumps and motors, linear actuators, and hydraulic control valves. Ideally, a thin film of fluid is maintained in the gaps between the moving parts to avoid solid‐to‐solid contact. In presence of solid contaminant, the particles can cause erosion or abrasion or can even block the relative motion, as shown in Figure 2.14. The figure shows different particle sizes to make evident that the most dangerous particles are those of a size comparable with the gap height. Particles of the smallest size have high chances of passing through the gap without causing significant damages to the solid surfaces. Particles bigger than the gap height will not enter the gap, although they may cause erosion at the gap entrance regions. Finally, particles similar in size to the gap height will likely enter the gap and engage with frequent contacts with the surfaces. In the worst case they can get stuck between the mechanical parts and leave the gap region after causing severe abrasion damages.

When selecting the features of a hydraulic filter, it is important to keep in mind the geometrical clearances of the components used in each system. Table 2.7 gives a general reference on the typical clearances within hydraulic components. To provide the reader with a tangible reference for these values, it is worth mentioning that the diameter of a human hair ranges from 50 to 150 μm and the diameter of a grain of salt is from 80 to 200 μm. In inches, 10 μm corresponds to 0.000 39 in. Considering that a human cannot see objects smaller than 20–40 μm, the most dangerous particles cannot be seen by a naked eye!

Figure 2.14 Solid particles entering the clearances of a hydraulic control valve.

Table 2.7 Typical clearances in hydraulic components.

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

Component	Clearance [μm]
Vane pumps (vane tip)	0.5–1
Gear pumps (side plates)	0.5–10
Piston pumps (piston to bore) (valve plate)	5–40 0.5–5
Servo valves (spool to sleeve)	1–4
Directional valves	2–20
Cylinders	50–250

ISO standard 4406 [29] defines a method for quantifying the solid contamination level of a hydraulic fluid. The standard is based on the counting, through proper experimental procedures, of the number of particles per milliliter of fluid. The contamination level is expressed by three numerical codes, which are determined according to the convention defined in Table 2.8.

Based on the number of particles counted in each size range, the corresponding codes are assigned according to Table 2.9.

Table 2.8 Codification of the fluid cleanliness according to ISO 4406, with the codes of Table 2.6.

Source: ISO 4406:2017 [29].

First code	Second code	Third code
Number of particles per milliliter larger than 4 μm	Number of particles per milliliter larger than 6 μm	Number of particles per milliliter larger than 14 μm

Table 2.9 Code for the cleanliness level of a hydraulic fluid according to ISO 4406.

Source: ISO 4406:2017 [29]

Particles per milliliter	ISO code
More than	Up to
80 000	1 600 000	24
40 000	80 000	23
20 000	40 000	22
10 000	20 000	21
5000	10 000	20
2500	5000	19
1300	2500	18
640	1300	17
320	640	16
160	320	15
80	160	14
40	80	13
20	40	12
10	20	11
5	10	10
2.5	5	9
1.3	2.5	8
0.64	1.3	7
0.32	0.64	6
0.16	0.32	5
0.08	0.16	4
0.04	0.08	3
0.02	0.04	2
0.01	0.02	1
0.005	0.01	0