How do I measure shaft run-out on an electric motor?

Run-out is the change in position of a monitored surface relative to a given axis over one full revolution. In electric-machine repair, technicians check radial run-out, axial (face) run-out, the coaxiality of fitted surfaces, shaft straightness, eccentricity, ovality, and the run-out of a mounted coupling half, a commutator, slip rings, a fan, or the rotor core.

Total indicated run-out (TIR) is the difference between the indicator’s maximum and minimum readings over one full turn. An indicator reading does not always equal the actual bend in the shaft’s axis — it describes the movement of a specific surface relative to the chosen datum.

These terms are often confused, but they describe different defects.

• eccentricity — the offset of a surface’s geometric center from the axis of rotation; for a perfectly round surface, TIR ≈ 2 × eccentricity;
• ovality — the difference between a surface’s diameters in different directions within one cross-section, which can shift the indicator reading even when the mean axis is correctly positioned;
• shaft bend — a deviation of the geometric axis from a straight line, often producing run-out that grows gradually toward the middle or the free end;
• misaligned fits — a situation where the axes of individual cylindrical surfaces do not coincide, even though each one may be fine on its own.

Common causes include a mechanical impact, a dropped rotor, improper transport, the motor jamming, the rotor touching the stator, a bearing failure, overload, incorrectly removing or fitting a coupling half, local overheating, thermal distortion, incorrect welding or weld overlay, a shaft crack, and sag in a long rotor under its own weight.

The need for a check may be signaled by high vibration at running speed, unstable balancing results, one or both bearings running hot, bearings failing quickly, coupling run-out, seal leakage, an uneven air gap, or shaft-end wobble visible to the eye.

Testing may call for a dial indicator (0.01–0.001 mm graduation), a digital or lever-type indicator, a magnetic stand, vee-blocks, roller rests, lathe centers, a balancing machine, a micrometer, an inside micrometer, a laser measuring instrument, and eddy-current displacement probes.

Before measuring, clean the shaft of grease and dust and inspect it for corrosion and mechanical damage. An indicator riding over a scratch, a burr or a pit will register a local irregularity, not the axis’s true geometric run-out — so never grind the surface freely before measuring, since that can erase important evidence of a defect.

The result always depends on the datum the shaft turns against — centers, vee-blocks, roller rests, or the machine’s own bearings. The right datum has to match the purpose of the check: to verify a coupling seat’s coaxiality with the bearing journals, reference the bearing fits; to assess overall shaft straightness, centers are typically used; to assess how the assembled rotor actually behaves, measure it in its own bearings or on a balancing machine.

Measuring on centers is convenient for checking straightness and journal coaxiality, but depends on the condition of the center holes, and a long rotor can sag under its own weight. Measuring on vee-blocks is simple and accessible, but sensitive to ovality of the base journals and to contamination. Measuring in the motor’s own bearings shows how the assembled machine actually behaves, but the result can include the bearings’ internal clearance and does not always distinguish shaft bend from a bearing-assembly defect.

Mount the indicator on a rigid base, as close as possible to the measuring point, with the plunger perpendicular to the surface and enough preload applied. A plunger set at a significant angle to the direction of travel introduces cosine error. For radial measurements, aim the plunger at the shaft axis; for face measurements, keep it roughly parallel to the axis.

Turn the shaft slowly, without jerks, in one direction, without pressing on it radially or creating axial movement. Avoid turning the shaft by pushing right next to the measuring point — hand pressure can elastically shift a long shaft or a rotor within its bearing clearance.

1. 01Mount the shaft on the chosen supports — centers, vee-blocks, rollers, or its own bearings.
2. 02Clean the measuring surface of contamination and debris.
3. 03Set up the indicator with the plunger aimed radially at the axis.
4. 04Apply preload and zero the dial.
5. 05Slowly rotate the shaft through a full 360°.
6. 06Record the maximum and minimum readings and the angular position of each.
7. 07Calculate TIR as the difference between the maximum and minimum readings.
8. 08Repeat the measurement at least twice to confirm repeatability.
9. 09Move the indicator and repeat the measurement at other cross-sections along the shaft.

A single result cannot reveal the shape of a deformation. It is best to measure at both bearing journals, next to them, near the edges of the core, at the rotor’s middle, and at the coupling and fan seats. Changes in amplitude and phase along the shaft reveal the point of maximum bend, a local deformation, the eccentricity of a single fit, or misalignment between surfaces.

The 0° mark on the shaft end must stay the same at every cross-section — this lets you compare the phase of run-out along the whole shaft. If the maximum falls at roughly the same angular position at every section, that points to an overall bend; if the maximum jumps sharply in angle between neighboring surfaces, misalignment or a local machining error is more likely.

Axial run-out is measured on a face surface, with the indicator plunger set roughly parallel to the shaft axis. It can result from the face not being perpendicular to the axis, a distorted shoulder, contamination, a burr, or rotor end-float. It is especially important to check the face run-out of bearing-fit shoulders, coupling-half faces, flanges, slip rings and commutators.

If the rotor has axial play, it can drift back and forth during a face measurement — to stabilize it, apply a small, steady axial force in one direction, or use the machine’s own thrust arrangement, without reversing the direction of rotation.

A long or heavy rotor can sag elastically under its own weight, so a horizontal measurement can include both the actual residual bend and elastic sag from weight and support deflection. For accurate measurements of large rotors, engineers use sag calculations, additional steady rests, special roller supports, or measurement on a balancing machine — a steady rest must never mask the actual bend or force additional straightening.

Run-out describes a geometric deviation of a surface or axis and is measured while slowly turning the shaft by hand. Imbalance describes an uneven mass distribution about the axis of rotation and shows up dynamically — it is found on a balancing machine or through vibration. All combinations are possible: a shaft with run-out but a statically balanced rotor; a rotor with imbalance and little geometric run-out; or both defects present at once.

There is no universal allowable value for every electric motor — the tolerance depends on shaft diameter and length, rotational speed, motor power, bearing type, coupling design, seal type, air gap, and the manufacturer’s requirements. The main sources of criteria should be the shaft drawing, repair documentation, the manufacturer’s specifications, and the standards for that machine type — never rely on one arbitrary figure for every motor.

• surface contamination or a scratch under the plunger tip;
• a magnetic stand on a thin or movable part, or a poorly secured indicator;
• a wrong plunger angle and excessive measuring force;
• play in the stand’s joints or in the bearings, and axial movement;
• uneven hand rotation and deflection under the rotor’s own weight;
• ovality of the datum surfaces, damaged centers, and roller-rest run-out;
• temperature changes and vibration from nearby equipment.

Repair may be needed if run-out exceeds the specified tolerance, a bend is confirmed, fits are misaligned, the coupling shows excessive run-out caused by the shaft, the air gap becomes uneven, or bearings keep failing. Depending on the design and the defect, mechanical or thermomechanical straightening, grinding, restoring a fit, fitting a repair sleeve, metal spraying, weld overlay, or manufacturing a new shaft may be used.

Straightening is a demanding operation: the wrong force can crack the shaft, damage a fit, or cause it to bend again. Before straightening, map the run-out, locate the plane of the bend, and check the shaft for cracks. When run-out is large or appeared suddenly, a crack must be ruled out with magnetic-particle, dye-penetrant, ultrasonic, or eddy-current testing — especially at fillets, keyways, threads and weld-overlay zones. Straightening a shaft with an undetected crack is dangerous.

• the rotor touching the stator, or a sharp rise in vibration;
• a metallic grinding sound or rapid bearing heating;
• a destroyed seal or a shifted coupling;
• dangerous axial movement or oil leakage;
• vibration protection tripping repeatedly;
• a visible wobble in the shaft.

• measuring over a dirty surface, or resting the indicator on a moving cover;
• turning the shaft in jerks or pressing on it by hand;
• drawing a conclusion from a single point without a multi-section map;
• confusing TIR with the actual axis deflection, or ignoring ovality;
• balancing a rotor instead of repairing a bent shaft;
• fitting new bearings onto an unchecked shaft;
• straightening a shaft without prior crack inspection, especially after an accident or an impact;
• adopting an arbitrary universal tolerance without checking that machine’s documentation.

• How do I test the rotor?
• How do I test the bearings?
• How do I align an electric motor?
• How is rotor balancing done?
• Why does an electric motor vibrate heavily?
• How do I inspect a shaft for cracks?Coming soon
• How is a shaft straightened?Coming soon

EPR (Elektropromremont) performs comprehensive inspection, repair and testing of shafts and rotors in industrial electric machines.

The scope of work includes:

• measuring radial and face run-out and mapping run-out along the shaft;
• checking straightness, fit coaxiality, ovality and taper;
• shaft crack inspection — magnetic-particle, dye-penetrant and ultrasonic testing;
• checking keyways and the coupling-half fit;
• mechanical and thermomechanical shaft straightening;
• restoring bearing fits, metal spraying and weld overlay;
• manufacturing new shafts;
• dynamic balancing and laser alignment.