Chapter ix · The Universal Control & Rotation
A driveshaft that is 0.1 mm off-axis is fine sitting still. At 6000 rpm it will shake your car apart.
Most of the controls you have met answer one part to itself or to a static datum. Runout is different: it is the control for things that spin. It always answers to a datum axis — often a coaxial pair like A-B, two bearing journals that together define the line the part actually turns about. Wheel-hub runout is why a recall once pulled tens of thousands of cars off the road for a shimmy nobody could feel below sixty miles an hour.
The measurement is brutally physical. Set the part on its datum axis, rest a dial indicator against the surface, and turn the part one full revolution. The needle swings; the full indicator movement (FIM, historically TIR) — high reading minus low reading — is the runout. No math, no derived points. Just the sweep.
Circular runout measures the sweep within one cross-section. The indicator stays put; the part spins. It bundles out-of-roundness and off-axis wobble into a single slice's reading.
Total runout sweeps the whole surface at once — the indicator traverses along the length as the part turns. It catches taper and surface profile that a section-by-section check would miss.
Crucially, runout is a composite control. A circular-runout reading is not pure roundness and not pure coaxiality — it is both, fused. Spin the section below: dial in some lobing (out-of-roundness) and some eccentricity (off-axis), and watch a single TIR number absorb both errors at once.
- indicator now
- mm
- circular runout (TIR)
- mm
- sweep this rev
One thing runout never touches: size. The indicator reads geometry — where the surface is and how it moves — not how big the diameter is. A shaft can be dead-on diameter and still fail runout, or be runout-perfect and out of size. Those are separate clauses, separate inspections. Our bracket’s bolt holes never needed runout — they don’t spin; but the moment a feature rotates, like the bushing the bracket pivots on, runout is the only control that watches it turn.
And total runout is not circular runout repeated down the part. A series of perfect circular slices can still describe a cone. Only the full-length traverse of total runout sees the taper — which is exactly why a wheel hub or a sealing journal so often calls for it.
Circular vs total: what each one can and can't catch
Picture a shaft that is perfectly round in every slice but slightly tapered — a gentle cone. Walk a circular-runout indicator down it section by section and every reading can pass: each individual circle is true. The part still leaks, still rubs, because the surface as a whole is not a cylinder.
Total runout closes that gap. Because the indicator traverses the length while the part spins, its single FIM bounds roundness, coaxiality, taper, and surface profile of the controlled feature simultaneously — the most complete rotational control in the standard.
For the advanced reader → Concentricity is gone — so how do you say ‘coaxial’ now?
This is a real 2009 → 2018 migration question. Y14.5-2009 had concentricity (and symmetry) to control the derived median points of a feature about a datum axis. Both were error-prone and miserable to inspect, so Y14.5-2018 removed them — leaving twelve characteristics, not fourteen.
If an old print said "concentricity," you now restate the coaxial requirement with one of three surviving controls, depending on intent: runout if the part rotates and you care about the surface sweep; position (often at a basic ⌀0 with an axis interpretation) if you care where the feature's axis lives; or profile if you want to bound the whole surface. Concentricity's derived-median-point meaning has no direct replacement — and that is the point: nobody could measure it reliably anyway.
Spin the Part, Read the Dial
A shaft is perfectly round in every slice but tapers gently along its length — a cone. You run a CIRCULAR runout check at several stations. What happens?
A circular-runout indicator sweeps one section and shows 0.05 mm FIM. What is that 0.05 actually measuring?
An old 2009 print calls out CONCENTRICITY on a rotating sealing journal. Under Y14.5-2018, what’s the cleanest restatement?
Runout is the most honest control in the language: it does not model the part or hunt for median points. It just spins the thing, watches the needle, and reads the truth off a dial.
That single sweep is what stands between a smooth driveshaft and a steering wheel that buzzes apart at highway speed.
You now know all twelve. Time to read a real drawing — the one that an hour ago was noise. Chapter x.