Hydraulic Fluid Power. Andrea Vacca

Hydraulic Fluid Power - Andrea Vacca


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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.

Schematic illustration of the solid particles entering the clearances of a hydraulic control valve.

      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

      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

      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
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