How do I verify gear concentricity and runout when the gear mounts on a shaft?

Question

Accepted Answer

Verify gear concentricity and runout from the functional datum

Measure gear concentricity and runout from the same bore, shaft journal, or locating face that controls the real assembly—not from a convenient but unrelated surface. Agree the datum scheme, fixture, indicator or measuring-center method, rotation condition, and drawing limits before production. Good tooth geometry cannot compensate for an eccentric mounting reference, because assembly runout changes the mesh in service.

Build the check around assembly reality

Define the datum

Identify the bore, pilot, shaft journal, face, keyway, or spline feature that actually locates the gear. The inspection report should name that reference and the drawing revision.

Separate the measurements

Check radial and axial runout of the relevant mounting features separately from tooth-related measurements. Record instrument, fixture, sampling position, and any assembly condition that affects the result.

Review the system

If runout is high, review the blank, bore or shaft feature, fixture clamping, mounting method, and mating part. Do not assign the cause to tooth cutting without evidence.

Example: mounted-gear inspection request

For a gear installed on a shaft, ask for the drawing revision, functional datum, runout characteristic and limit, measuring method, fixture condition, and whether the reading is taken before or after any heat treatment and finishing. PairGears’ manufacturing-process overview similarly treats datum control and runout as process-chain controls.