Friction is several things sharing one name
The distinct mechanisms lumped under a single word.
Friction is one word covering at least four different physical mechanisms: the resistance to starting motion from rest, the resistance to keeping something sliding, the much smaller resistance to something rolling, and the resistance a fluid film offers as it shears. Treating all four as one interchangeable property is the mistake that turns routine maintenance into an unexpected failure.
A bearing cleaned thoroughly and carefully reassembled failed within a week, and the cause traced back to exactly this confusion. The cleaning had stripped away the thin fluid film doing most of the work, leaving dry surfaces that looked, at a glance, like the same bearing that came apart.
Four mechanisms under one word
Friction is the general name for all of them. Static friction resists a stationary object being set into motion, arising from microscopic surface irregularities interlocking wherever two surfaces touch, and it is typically larger than the friction that follows once motion begins. Sliding (or kinetic) friction resists surfaces already moving past each other; it is generally smaller, and for many dry surface pairs it barely changes with sliding speed. Rolling friction is far smaller again, coming mostly from the tiny elastic flattening of a round surface under load, with no surface interlocking involved. Viscous friction, from a fluid film sheared between two surfaces, grows with speed and fades to almost nothing at very low speed, where the other three barely notice speed at all.
One heavy box, four ways to move it
Pushing a heavy box across a bare floor from a dead stop takes a real shove, more than it takes to keep the box sliding once it is underway, and that is the gap between static and sliding friction felt directly through the arms. Put the box on wheeled furniture sliders and the effort drops sharply, because the box now rolls. Float the same box on a thin, maintained film of oil or air and it would hardly resist a gentle nudge, yet would push back harder the faster it was shoved, since a fluid film's resistance climbs with speed in a way none of the others does.
How a clean bearing ran dry
A bearing running well depends on a thin, continuous fluid film keeping its moving surfaces apart, its low, speed-dependent viscous friction standing in for the far larger and more damaging friction of metal sliding directly on metal. The difference is large. A full oil film can cut the friction of steel sliding on steel by a factor of a hundred or more, which is why a bearing stripped of its film and never relubricated can generate enough heat and wear to fail within days.
Cleaning a bearing thoroughly enough to strip away every trace of that film, without deliberately reapplying a proper replacement, leaves it far from any neutral, friction-free state. It trades its intended low viscous friction for dry sliding friction, and the change is invisible to anyone who assumes a clean, freely spinning bearing must be working well. The bearing spun freely by hand straight after cleaning because a freshly cleaned dry surface, with nothing yet ground into it, still starts out smooth. That made the coming failure easy to miss. The one check that mattered was whether the correct fluid film had been restored, and a quick spin by hand could never answer it.
The practical fix is simple once the distinction is understood: never return a lubricated surface to service in a cleaned, dry state without deliberately restoring the film it depends on.
Parts built to run dry
Not every part relies on the same mechanism. A brake pad or a friction clutch is designed around dry sliding friction, and the fluid film a bearing depends on would ruin it, since those parts need the higher, more predictable resistance a dry surface provides. The useful question before any cleaning or maintenance step is which of the four mechanisms a part is supposed to be running on, since whether it currently feels smooth or rough says little about that.
The rest of this set takes each mechanism in turn: starting friction against sliding friction, rolling against sliding, and the ways a bearing fails. Each is a closer look at one of the four introduced here, in enough detail to show what the difference costs or saves in a working machine.