Sifting is a separating procedure where the falling particles are exposed to a contrary flow [SALM07, p. 134]. If the flow rate meets the falling velocity of a certain particle size, these particles hover. Smaller particles stay in the flow, coarser particles decant.
One method is to suspend the particles in an aerosol beam and measure the movement time of the particles between two points of known distance apart [UAMA09]. If a fluid is used as medium, the process is called elutriation [SALM07, p. 137]. Air forms the medium for wind sifting and the time of flight defines the mass and grit size.
2.9.1.3 Sedimentation
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Stokes’ law can be used for evaluating the particle size distribution (Eq. 2.20) [ANSI77]. Sedimentation or Stokesian methods work with a stationary medium and the particles are in free fall [SALM07, p. 138]. Sedimentation methods are usually used for grading finer grit, such as in ANSI B74.10 [ANSI77].
v settling velocity in (cm/s) g gravity acceleration in (980 cm/s2) r effective particle radius in (cm) p density of the particles in (g/cm3) d density of the settling medium in (g/cm3)
П viscosity of the settling medium in [g/(s cm)]
The FEPA 43 and DIN ISO 8486-2 standards define a calibrated US-Sedimentometer. The water in the sedimentation tube should be thermally controlled. In an automated system, light barriers can measure the sedimentation height per time [TOPA10]. This data is then used to calculate the particle size distribution with Stokes’ law. Another method is to apply abrasive grits in a carrier medium onto the surface of a rotating fluid ring.
Stokesian methods work best for spheres. Flat particles and needles experience a greater drag per unit mass than spheroidal particles. As consequence, they settle in a viscous medium as fast as smaller round particles and will therefore cause oversizes in the grading. Especially for fine finishing operations such as lapping and polishing, these oversizes can ruin the workpiece quality. [DAVI74]. Overall, irregular, less
blocky grit shapes can present severe problems in size grading [HERB81]. In addition, sedimentation can be a very time consuming method (more than 24 h for very fine particles) [UAMA09]. The operator has to constantly tap the settling tube during the test to insure even packing and level settling [UAMA09].