Understanding Shaft Runout in Vibration Analysis

Definition: What is Runout?

Runout is a general term for imperfections in a rotor that produce a once-per-revolution (1x) signal, even when the rotor is turning at a very low speed where dynamic forces like unbalance are negligible. It is a measurement of the total variation or deviation of a rotating surface from a perfect circle, relative to the shaft’s true centerline. A key challenge in vibration analysis is that runout can look exactly like unbalance in the vibration data, but it is not a mass-related problem and therefore cannot be fixed by balancing.

Types of Runout: A Critical Distinction

It is crucial to distinguish between the two main types of runout:

1. Mechanical Runout

Mechanical runout is a true physical or geometric imperfection of the shaft. It means the shaft surface is not perfectly round or is not perfectly centered on its axis of rotation. Common causes include:

• Out-of-roundness: The shaft journal is slightly oval or has other shape imperfections from machining.
• Eccentricity: A component, like a pulley or gear, is machined or mounted slightly off-center relative to the shaft’s centerline.
• Bent or Bowed Shaft: A permanent bend in the shaft will cause its surface to move in and out relative to a fixed point as it rotates.

Mechanical runout can be measured directly using a dial indicator while slowly rotating the shaft by hand.

2. Electrical Runout

Electrical runout is not a physical defect but rather a measurement error that occurs exclusively with non-contact eddy current proximity probes. These probes work by creating a magnetic field and sensing changes in the shaft’s surface. If the shaft’s surface has localized variations in its magnetic or electrical properties, the probe will produce a fluctuating signal even if the shaft-to-probe gap is perfectly constant.

Causes of electrical runout include:

• Variations in Material Permeability: A localized spot of magnetism on the shaft can create a strong 1x signal. This can happen if the shaft is accidentally magnetized, for example by a magnetic-base dial indicator.
• Changes in Surface Finish: Scratches, dents, or tool marks in the “viewing area” of the probe.
• Inconsistent Material Composition: Variations in the alloy or metallurgical properties of the shaft material.

Electrical runout is invisible to a dial indicator but is a significant source of error in turbomachinery vibration monitoring.

Why Runout is a Problem for Diagnostics and Balancing

The signal generated by both types of runout occurs at 1x the shaft’s running speed, which is the same frequency as unbalance. This creates a major problem:

• It can be mistaken for unbalance: An analyst might see a high 1x vibration peak and incorrectly diagnose it as unbalance, leading to unnecessary and ineffective balancing attempts.
• It interferes with balancing: The runout signal adds to the true unbalance signal. To perform an accurate balance, the runout component must be measured and vectorially subtracted from the total vibration signal to isolate the true dynamic response.

Runout Compensation: The Slow-Roll Vector

To solve this problem, analysts use a technique called runout compensation. This is a critical step in the analysis of any machine monitored with proximity probes.

1. Slow Roll: The machine is operated at a very low speed (typically 200-500 RPM), where centrifugal forces from unbalance are insignificant.
2. Measure the Slow-Roll Vector: The 1x vibration vector (amplitude and phase) measured at this low speed is almost entirely due to runout. This is called the “slow-roll” or “runout” vector.
3. Subtract the Vector: This slow-roll vector is then stored and vectorially subtracted from the 1x vibration vector measured at the machine’s high operating speed.

The result is the runout-compensated 1x vector, which represents the true dynamic motion of the shaft due to unbalance and other rotordynamic forces. This compensated value is what should be used for accurate diagnostics and for calculating balance correction weights.

← Back to Main Index