Runout Tolerances

Runout tolerances are the GD&T controls that limit how much a rotating feature wobbles about its datum axis — how true a shaft’s diameter runs as it turns, how evenly a face spins. They are the everyday controls of the round part: the shaft, the pulley, the bearing journal, the seal surface, anything that rotates about an axis and must run smoothly against a mating part or a stationary reference. Runout is read with an indicator on a rotating part, which makes it one of the most directly measurable tolerances in the system, and it comes in two forms — circular runout and total runout — that control different amounts of the feature. This entry sets out the two runout tolerances, what each one checks, how runout relates to the other controls of round and coaxial features, and what it asks of the machining of a rotating part.

Wobble about the axis

Runout is defined against the feature’s datum axis — the axis established by the datum features the call-out names, usually the centres or the main diameter of the part — and it measures the feature’s departure from true rotation about that axis. When a part rotates about its datum axis, every point of a controlled surface should sweep a circle centred on that axis; runout is the indicator’s reading as the part turns, the total movement of the surface towards and away from the axis. The wobble it catches is a blend of the errors this group has treated separately: a surface that is not perfectly round contributes to runout, a surface whose axis is not the datum axis contributes to runout, and a feature that leans or tapers contributes as the indicator travels. Runout does not separate those causes; it measures their combined effect on the running of the part, which is exactly what a bearing or a seal experiences — not roundness alone, not position alone, but the total truth of the surface as it turns.

The two runout tolerances

The system splits runout by how much of the feature it examines. Circular runout is checked at a single circular element — one cross-section of the surface — and it limits how far that element may move towards and away from the datum axis as the part rotates one full turn. The indicator sits at one point on the surface, the part rotates, and the total swing of the reading is the circular runout at that element; the call-out is met if every circular element, checked at any point along the feature, stays within its limit. Circular runout is the control that makes a shaft’s diameter run true at any section, and it is the one most often specified, because it is quick to check and covers what most running surfaces need. Total runout examines the whole surface at once: the indicator travels over the entire feature — along a length, and over a face if the face is controlled — while the part rotates, and the total movement of the reading across the whole sweep is the total runout. Total runout therefore also catches the feature’s taper, its straightness and its change of diameter along its length, and it is the tighter, more demanding control, specified where the whole surface must run true together.

Runout and its neighbours

The machinist reads runout against the neighbouring controls because they overlap and the drawing chooses between them by what the part actually does. A circular runout of a diameter is a combined check of the section’s roundness — the form tolerance of this group — and of how the diameter’s axis sits relative to the datum axis; if the section is round and its centre lies on the datum axis, the runout is small. Total runout adds the feature’s straightness and taper to that check. Runout differs from concentricity — the median-point control of the location family treated in this group’s entry on position and location — in what it measures and how: concentricity judges balance through the feature’s median points and is hard to verify, while runout is read directly with an indicator and catches the errors that actually make a part run rough. Runout is the control designers reach for when the question is does it run true, and it is why runout, rather than the rarer median-point controls, carries most rotating work.

Measuring runout

Runout is one of the few geometric tolerances measured directly, and the reading is the measurement. The part is mounted so it can rotate freely about its datum axis — held between centres, on a mandrel through a bore, or in a chuck when the chuck is the datum — and an indicator is brought to the controlled surface. For circular runout the part is turned one full revolution and the indicator’s total swing is the runout at that element, repeated at whatever points along the feature the inspection calls for; for total runout the indicator is swept across the whole controlled surface while the part turns, and the greatest total movement of the needle over the entire sweep is the runout. The surface plate, the centres and the indicator are the classic kit of measuring a feature, and the reading carries no arithmetic — the number on the indicator is the tolerance verdict. Because the result depends on rotating the part about its true datum axis, the inspection reproduces the setup: the centres or the datum surfaces the part was made on.

Making a part run true

The runout tolerances ask the machinist for a part that is true about its axis, and the answer is almost always in the setup. A shaft that must meet a runout tolerance is turned, and where it is tight, ground, about the very axis the drawing names as the datum: turned between centres so that the centres establish the axis and every diameter is cut concentric with it, or finished in one clamping so that no re-chucking shifts the work. A face controlled for runout is machined square to the axis in that same setup, because a face that leans reads as runout when the part spins. The discipline is the discipline of the turning centre: the work runs true in the setup or it never will, and re-locating the part between operations — out of the chuck, into a fixture, back again — invites the very wobble the tolerance forbids. Cut true about its datum axis, the part meets its runout at the indicator; cut in setups that shift the axis, it fails no matter how round each surface is on its own. That is the whole lesson of runout, and the reason the drawing’s runout call-out is, for the round part, the setup instruction in its clearest form — the GD&T language telling the machinist that the part must not merely be shaped and placed, but must run true, as one, about the axis it will turn on.