What Is Shaft Runout? Definition, Types, Effects & How to Measure It

Introduction

Shaft runout is a critical yet often overlooked factor that can significantly impact the performance and reliability of mechanical seals. In pump systems, mixers, and rotating equipment, excessive runout is one of the leading causes of seal leakage and premature failure.

As a trusted name in sealing solutions, QMSeals emphasizes the importance of understanding shaft runout, its types, its effects on mechanical seals, and how to measure it properly. This guide provides a detailed breakdown to help engineers, technicians, and plant operators maintain optimal sealing performance.

What Is Shaft Runout?

Shaft runout refers to the deviation of a rotating shaft from its ideal circular rotation path. In simple terms, it is how much the shaft wobbles or moves off-center while spinning. Ideally, a shaft should rotate perfectly around its centerline, but due to manufacturing tolerances, wear, or misalignment, this doesn't always happen.

Runout is a dynamic condition that directly affects the components connected to the shaft, especially mechanical seals, bearings, and couplings.

Types of Shaft Runout

There are two main types of shaft runout, both of which can affect mechanical seals differently:

1. Radial Runout

Definition: The side-to-side movement of the shaft as it rotates.

Cause: Bent shafts, eccentric machining, or bearing wear.

Effect: Causes the seal faces to separate intermittently, leading to leakage and uneven wear.

2. Axial Runout (or Endplay)

Definition: The forward-and-backward movement of the shaft along its axis.

Cause: Improper axial preload on bearings or thermal expansion.

Effect: Creates axial displacement between seal faces, reducing sealing efficiency.

Common Causes of Shaft Runout

• Worn bearings or housings
• Bent or damaged shafts
• Poor machining tolerances
• Thermal expansion
• Improper installation or alignment
• Vibration and unbalanced loads

For more detailed insight, refer to our blog on 10 Reasons for Mechanical Seal Failure, which discusses how runout is a leading contributor to seal issues.

How Shaft Runout Affects Mechanical Seals

1. Increased Leakage

Runout causes the rotating and stationary seal faces to lose full contact, allowing process fluid or gas to escape.

2. Accelerated Seal Wear

Uneven rotation puts more stress on certain areas of the seal faces, leading to premature wear or cracking.

3. Vibration and Noise

Excessive shaft movement increases vibration, affecting not just the seal but also the bearings and motor performance.

4. Seal Face Separation

Intermittent contact caused by runout can cause dry-running conditions, generating excessive heat and damaging the seal.

5. Reduced Seal Life

Even small amounts of runout can significantly reduce mechanical seal lifespan, leading to more frequent maintenance and downtime.

How to Measure Shaft Runout

1. Using a Dial Indicator

A dial indicator is the most common and accurate tool for measuring shaft runout.

Steps:

Mount the dial indicator on a fixed surface near the shaft.

Place the tip of the indicator against the shaft surface (for radial) or shaft end (for axial).

Rotate the shaft manually (360 degrees).

Record the total indicator reading (TIR).

Acceptable Tolerance:
For most pumps and sealing systems, radial runout should be below 0.002 to 0.005 inches (0.05 – 0.13 mm).

2. Laser Alignment Tools

These offer more advanced diagnostics and are especially useful in high-precision equipment or critical applications.

Tips to Minimize Shaft Runout in Seal Applications

Inspect shafts during installation for straightness and surface finish.

Use high-quality bearings and ensure proper bearing preload.

Avoid over-tightening coupling bolts which can warp the shaft.

Ensure proper alignment during equipment installation.

Use cartridge seals that tolerate slight misalignments better than component seals.

Schedule routine maintenance to detect early signs of wear or imbalance.

Explore more on seal installation in our blog How to Install a Mechanical Seal Properly.

How QMSeals Helps You Combat Runout Issues

At QMSeals, we engineer mechanical seals to withstand challenging conditions, including moderate shaft runout. Our cartridge seals, multi-spring designs, and bellows-type seals are manufactured with runout compensation in mind.

We also offer technical support and troubleshooting, helping customers identify and resolve seal failures caused by alignment and runout issues.

Learn more about Different Mechanical Seal Types for Centrifugal Pumps and how they can handle operational challenges like shaft movement.

Conclusion

Understanding shaft runout is crucial for maximizing the performance and life of your mechanical seals. Whether it’s radial or axial runout, ignoring this factor can lead to increased downtime, costly repairs, and seal replacement.

By monitoring shaft conditions and using the right seal design, you can avoid premature failures and keep your systems running smoothly. For seal solutions that perform even under tough conditions, QMSeals is your trusted partner.