13.3 Runout vs Position, Profile, and Form

13.3 Runout vs Position, Profile, and Form

Quick Answer: Runout controls the surface to a datum axis in one composite reading; position controls the axis or center of a feature of size; profile controls a contour to a true profile. Runout is restricted to surfaces of revolution (cylinders/cones) referenced to a datum axis and bundles coaxiality with form. The Senior exam tests whether you can pick the right control for the function: assembly -> position, contour -> profile, rotating surface -> runout, pure shape -> form.

What Each Control Actually Constrains

The four controls are often presented as alternatives for the same surface, but they constrain different things:

• Runout constrains the surface itself to a 2D (circular) or 3D (total) zone defined relative to a datum axis. It is a composite control: the reading combines coaxiality of the feature's axis with the datum axis AND form errors of the surface. It does not separate the two.

• Position constrains the axis or center plane of a feature of size to lie within a cylindrical or parallel-plane zone located relative to the datum reference frame. Position says nothing directly about the surface's form; form is controlled separately (e.g. by a size tolerance under Rule 1, or by a form callout).

• Profile of a line/surface constrains the surface to lie within a zone bounded by two offset curves/surfaces equidistant from a true profile. Profile can control size, form, orientation, and location simultaneously when used with datum references, and it is the only one of the four that can control a non-revolution contour (e.g. an airfoil, a free-form surface).

• Form (circularity, cylindricity, flatness, straightness) constrains the surface's shape to itself, with no datum reference. Form alone does not control coaxiality or location.

Why Runout Is the Composite Choice

Runout's strength is its composite nature. For a rotating shaft where the service requirement is 'the outer surface must run true to the bearing journals,' runout directly tests that requirement in one reading. Position would require you to control the axis of the outer surface (a derived feature) AND separately control its form - two callouts instead of one, and the composite reading that actually matters in service is never directly checked.

This is why runout is the natural choice for:

• Pulley belt surfaces that must run true to the bore
• Gear pitch cylinders that must run true to the mounting bore
• Motor shaft outputs that must run true to the bearing journals
• Pump shaft seal journals that must run true to the bearing seats

In each case the function is 'run true to an axis,' and runout measures exactly that.

When Runout Cannot Be Used

Runout is restricted to surfaces of revolution - cylinders and cones - referenced to a datum axis. It cannot be used for:

• Flat surfaces (a shoulder face perpendicular to the axis needs orientation - perpendicularity - or profile)
• Non-revolution surfaces (a keyway side, a flat on a D-shaft, a spline flank needs profile or orientation)
• Parts that do not rotate (the datum axis concept is meaningless if there is no axis of rotation)
• Surfaces with no datum axis available (runout always requires a datum reference that establishes an axis)

A Senior question that shows a runout callout on a flat shoulder or on a rectangular pad is testing exactly this restriction.

Decision Table

Worked Scenario: Gear on a Shaft

A helical gear is pressed onto a shaft. The gear's pitch cylinder must be concentric with the shaft's two bearing journals (A and B) so the gear meshes without radial lash. The designer is choosing between three callouts on the gear's finished OD: a 0.05 position tolerance to A-B, a 0.05 profile of a cylinder to A-B, or a 0.05 total runout to A-B.

Analysis.

• Position to A-B would control the axis of the gear OD relative to A-B, but says nothing about the gear OD's form. A gear that is eccentric but perfectly round, and a gear that is concentric but out-of-round, could both pass position - yet the second causes mesh error.
• Profile of a cylinder to A-B would control the surface to a true cylindrical profile and can bundle form, orientation, and location, but profile of a cylinder is unusual for a simple revolution and the inspection is more involved than runout.
• Total runout to A-B directly measures 'does the gear OD run true to A-B' in one sweep, bundling coaxiality with the journals AND cylindricity of the OD. This is exactly the service requirement, inspected in one reading.

The correct Senior-level choice is total runout to A-B. The service function is 'run true to the journals,' and total runout tests that directly and economically. Position would under-control (no form), and profile would work but is overkill for a simple cylinder where runout is purpose-built.

If instead the surface were an involute gear tooth flank - not a surface of revolution - runout would be invalid and profile of a surface (the standard gear-tooth control) would be the right answer. The pivot is whether the surface is a revolution referenced to an axis.

Senior Selection Traps

The exam pairs a function description with four candidate callouts. The traps to recognize:

• A runout callout on a flat surface - always wrong, choose orientation or profile.
• A runout callout with no datum axis - always wrong, runout requires a datum axis.
• A position callout where the service function is 'run true to an axis' and form also matters - under-controls; choose runout.
• A form callout where coaxiality to a datum also matters - form is datum-free; choose runout or position.
• A profile callout on a simple cylinder that could be runout - works but is overkill; the exam usually prefers the purpose-built control.

Mastering these tradeoffs is the runout portion of the Senior exam: roughly 5% of the 150 questions, almost all of them application-level selection.