Explaining the Basics of Bearing Preload

Bearings are crucial components in the mechanical world, helping rotating equipment run smoothly and efficiently.

Preloading a bearing is an engineering strategy designed to optimize its performance.

But what exactly is bearing preload, and why is it so important?

This blog post covers the basics of bearing preload.

It discusses the benefits and effects of bearing preload on the lifespan and performance of bearings in various applications.

Understanding Bearing Load and Preload

Before we delve into the specifics of bearing preload, let's clarify what we mean by bearing load.

What is Bearing Load?

Bearing load refers to the forces exerted on a bearing during operation.

These forces can be static or dynamic.

The load affects how the bearing operates, its service life, and its performance.

Bearings can handle specific loads, exceeding these limits can cause them to break down sooner.

What is Bearing Preload?

Bearing preload is a force acting between the rolling elements and bearing rings.

It operates without any external load and acts as negative internal clearance within the bearing.

The Importance of Bearing Preload

Bearing preload can eliminate the clearance in standard ball bearings , such as the 608 Bearing.

Most radial ball bearings have built-in bearing clearance for free movement.

Without preload, this space can cause the rolling elements to slide instead of roll and may lead to race misalignment.

Preloading keeps the bearing parts in contact, removing any looseness and making the structure more rigid. 

It offers several advantages, such as:

Increased Rigidity and Accuracy

By preloading a bearing, you effectively increase its rigidity.

This is particularly beneficial in applications where precise movements are crucial.

A preloaded bearing keeps a more stable position, improving the system's overall accuracy.

Reduced Vibration and Noise

Vibrations can be detrimental to bearing performance.

Preloading reduces vibrations by keeping the bearing components in constant contact, which lowers noise and extends the machinery's lifespan.

Optimal Load Distribution

Preloading helps distribute the load more evenly across the rolling elements.

This uniform distribution can prevent uneven wear and tear, leading to a more reliable and longer-lasting bearing.

Challenges of Bearing Preload

While bearing preloading is beneficial, it's not without its challenges.

Incorrect preload can lead to issues such as increased heat generation, reduced efficiency, and premature failure.

• Excessive Preload: Too much preload causes excessive friction and heat, damaging the bearing lubricant and even the bearing. This can lead to thermal expansion issues, higher torque, fatigue, and premature wear.
• Insufficient Preload: Not enough preload results in excessive vibration, noise, and play within the bearing assembly. This leads to imprecise operation, fretting of the raceways, and faster wear.

How Much Preload?

Choosing the right preload for a bearing system is crucial.

It depends on factors like bearing size, required stiffness, torque, lifespan, and loading conditions.

Preload Scenarios:

Fig 1. No play or interference fit. All rolling elements are loaded, causing excessive wear and heat.

Fig 2. Standard play in the free state. Low rigidity, with rolling elements sliding or skidding.

Fig 3. Proper preload applied. Ensures system rigidity, reduced vibration, and longer bearing life. 

Applying the correct preload is essential for optimal performance, providing system rigidity, reduced vibration, and extended bearing life.

How to Preload a Bearing?

Be precise when preloading a bearing because both under-preloading and over-preloading can cause problems.

Here are the methods for preloading bearings:

Fixed Preload

Achieve fixed preload by manufacturing the bearing with a predetermined amount of force applied internally.

Create this preload by slightly offsetting the dimensions of the races or by using springs.

Use fixed preload in applications where operating conditions are relatively stable and predictable.

Adjustable Preload

Adjust preload to suit various needs, offering flexibility for different purposes.

Achieve this adjustment through techniques such as using shims, threaded nuts, or wave springs.

Adjustable preload is ideal for situations where operating conditions are variable or not well known in advance.

Types of Preload

Radial Preload

Applicable to cylindrical roller bearings due to their design.

Fig 4 Preload cylindrical roller bearings radially

For tapered bore bearings:

• Drive the inner ring onto its tapered seat.
• Create an interference fit, expanding the inner ring for the necessary preload.

Use internal clearance gauges to set preload accurately.

Axial Preload

• Used for thrust ball bearings and cylindrical roller thrust bearings.
• Single row angular contact ball bearings and tapered roller bearings require axial preload. Mount them with a second bearing in either a back-to-back or face-to-face arrangement.
• Deep groove ball bearings can also be axially preloaded if they have greater radial internal clearance (C3 or C4) to create a contact angle.

Preload for Angular Contact Ball Bearings

Angular contact ball bearings require preload to ensure constant contact between the balls and raceways. This can be achieved through:

Methods of Preloading

Axial Displacement:

• Mount bearings in sets (back-to-back or face-to-face).

Fig 5 Back to Back

Fig 6 Face to Face

• Displace one bearing ring axially.
• Consider interference fit and operating conditions.

Spring Preload:

• Use springs to apply a constant axial load.
• Accommodates thermal expansion and maintains preload.
• Not ideal for high stiffness or variable load directions.

Considerations

• External Loads: External axial forces can alter preload, risking skidding and failure. Adjust preload or use different contact angles to prevent this.
• Adjustment: Use spacers to fine-tune preload.

Consult engineering services for optimal preload methods to ensure bearing performance and longevity.

Applications of Bearing Preload

Preloading bearings is a common practice in many industries.

Preload is essential to prevent skidding in high-speed applications.

This is especially true during rapid starts, stops, or under light or no-load conditions.

Bearing preload is also crucial for increasing system rigidity and positional accuracy in most high-precision applications.

Here are a few applications where bearing preload is essential:

Automotive Industry

Adjust the wheel bearings before using cars.

This process ensures that the wheels can turn smoothly and support the weight of the vehicle without wearing out quickly.

Aerospace

The aerospace industry requires extremely high levels of precision and rigidity.

Bearings in airplanes and spaceships are pre-set to comply with these rigorous demands.

Machine Tools

Bearings for machine tool spindles use bearing preload for better accuracy and stability, resulting in high-quality, precisely made parts.

Conclusion

Preload on bearings is a crucial factor to consider in any application that utilizes bearings.

Engineers and technicians can improve machinery efficiency and longevity by understanding bearing preload and its impact on bearing performance.

Whether you're working with fixed or adjustable preload, it's crucial to apply the correct amount to avoid potential complications. 

Proper bearing preloading improves your equipment and prevents issues.

At LILY Bearing, we offer a wide range of bearings to fit any application.