Below about four microns, ferrous wear debris stops behaving like a solid and starts behaving like part of the fluid. Gravity does not remove it. A settling tank does not remove it. It circulates with the mud, passes through every clearance in the system, and keeps cutting. This stage is only about establishing that it is there. Nothing is being sold.
Drop steel wear debris into drilling mud and watch. The coarse swarf a ditch magnet is built for hits the bottom in minutes. The fine fraction, the abrasive that actually cuts pumps, motors and MWD tools, is still hanging in the fluid weeks later. Change the fluid and the timescales move, but the ordering never does.
Compressed view raises every vertical rate to the power 0.75 so all five classes move visibly in one window. Real settling spans 40,000:1 — at any single playback speed the coarse fraction would hit the floor before the fines had twitched. Ordering, ratios and Brownian dominance are all preserved; only the spread is squeezed. The true drop times in the readout are always the real Stokes values. Switch to True Stokes for uncompressed physics.
Stokes' law governs how fast a particle falls through a viscous fluid. The driving force is buoyant weight, which grows with the cube of diameter. The resisting force is viscous drag, which grows only with diameter. What survives is a settling velocity proportional to d².
That exponent is the whole story. Cut a 100 micron chip down to 2 micron and you have not made it 50 times slower. You have made it 2,500 times slower. Twenty-eight minutes becomes forty-eight days.
Halve a particle and it falls four times slower. Halve it again and you are sixteen times slower than where you started. By the time you reach the sizes that fit inside a running clearance, the settling velocity has collapsed to something measured in microns per second, and every other force acting on the particle is larger.
Fluid molecules bombard the particle from every direction. The impacts do not cancel exactly, so the particle performs a random walk. Displacement follows the Stokes–Einstein relation and grows with the square root of time.
Below roughly 1 micron this random wander exceeds the distance gravity pulls the particle down in the same second. The particle is not descending with a wobble. It is wandering, with a slight downward bias.
A water-based mud is thixotropic: leave it still and it builds a gel. To fall, a particle must generate enough shear stress to break that gel. Its buoyant weight spread over its own cross-section gives the stress available.
A 2 micron steel particle presses down at about 0.086 Pa. A typical water-based mud gels at 2.4 to 14 Pa. It is short by a factor of thirty to a hundred and sixty.
So when circulation stops, the fine fraction does not slowly sink to the bottom overnight. It is locked in place exactly where it was.
The Stokes number compares a particle's response time to the timescale of the flow around it. A value well below 1 means the particle cannot deviate from the fluid: it follows every eddy, every turn, every acceleration.
A 2 micron steel particle in drilling mud has a Stokes number of about 6 × 10⁻⁸. It is, to eight decimal places, a tracer. Wherever the mud goes, it goes — down the drill string, through the bit, up the annulus, over the shakers and back to the pump.
This is why "let it settle out in the pits" is not a strategy. The fluid is never still long enough for the physics of settling to matter.
Abrasive wear happens when a hard particle enters a running clearance and is dragged across both surfaces under load. Too large and it cannot enter — it is excluded, or it jams and is crushed. Too small and it passes through without touching both faces. The damaging band is the one that just fits.
That band sits almost exactly where settling collapses. Below is the same size ladder measured against the clearances it has to get into. Drag the slider to see which components a given particle size can enter.
Clearance ranges are typical published values for the component classes shown and vary by make, condition, load and temperature. They are indicative of the band, not a specification for any particular machine.
Steel ρ = 7,800 kg/m³ · mud ρ = 1,200 kg/m³ (10 ppg) · plastic viscosity 30 cP · 60 °C. Brownian displacement from the Stokes–Einstein relation, RMS over one second. The Brownian ÷ gravity column is the ratio of random wander to gravitational fall: above 1.0 the particle is being shaken about faster than it is sinking.
Buoyant weight spread over the particle's cross-section, against the shear stress a mud gel resists.
A 2 micron particle exerts roughly 1/100th of the stress needed to move through a gelled mud.
Same steel, same two-metre drop, three fluids. Viscosity and density move the absolute numbers; nothing moves the ordering.
Even in water — a hundredth the viscosity of mud — a half-micron particle still takes over a week.