Runout (TIR)

A shaft can have exactly the right diameter and still wobble. Runout is the number that catches it. On rollers, pulleys, shafts and motor rotors a small wobble becomes vibration, uneven wear and early bearing failure.

How runout is checked

The part is supported on its bearing journals, between centers or in vee blocks. A dial indicator is set with its tip touching the surface, and the part is turned slowly by hand. The needle moves as the surface passes under it. The total movement from the lowest reading to the highest is the TIR.

The reading is the combined effect of out-of-round and an off-center axis, so it is a practical check rather than a pure measurement of either one. Where a drawing needs finer control it uses GD&T symbols to name the datums the part must turn about.

Radial and axial runout

Radial runout is measured with the indicator tip pushing straight in on the outside diameter or inside diameter, and it shows how far the surface wobbles across the axis. Axial runout, sometimes called face runout, is measured with the tip on a flat face, and it shows how far the face moves along the axis as the part turns.

A pulley face or a flange that runs out axially will not seat flat against its mate, and a shaft that runs out radially will load its bearings unevenly. Both are checked from the same datum, usually the bearing journals or the bore.

Why it matters on rollers, pulleys and shafts

Anything that turns at speed is sensitive to runout. A print roller with an off-center face marks the paper, a conveyor pulley with a wobble tracks the belt to one side, and a motor shaft with a bend shortens the life of the bearings and the seals on it.

When a shaft is built up or turned during a repair, the finished journals have to turn true to the ones that were not touched, so the part is set up and checked from the original bearing surfaces. If your print calls out a runout value, send the full drawing with the quote request, and put the callout in the note if the part is a sample with no print. The tolerance entry covers how limits like this are written.