Bearing Preload: Why and When to Apply It

Bearing Preload: Why and When to Apply It

Preload is one of the most critical—yet often misunderstood—adjustments in precision machine maintenance. By intentionally creating a permanent axial or radial load within a bearing system, engineers can eliminate internal clearance, increase rigidity, and improve rotational accuracy. However, applying too much preload can shorten bearing life, while too little leads to vibration and skidding. This article explains what bearing preload is, why it matters, when to apply it, and how to select the correct preload for your application.

Understanding Bearing Preload: Definition and Purpose

Bearing preload is the deliberate introduction of a load into a bearing system that remains even when the machine is at rest. Unlike operational loads, which vary during service, preload is constant and positive. Its primary purposes are:

**Eliminating internal clearance** – Rolling elements and raceways maintain constant contact, preventing sliding and impact.

**Increasing bearing rigidity** – A preloaded bearing has a higher stiffness, meaning less deflection under external loads.

**Improving rotational accuracy** – Shaft runout and axial play are minimized, which is essential for precision spindles.

**Reducing noise and vibration** – With no loose clearance, the bearing operates smoothly and quietly.

Preload is typically expressed in newtons (N) or pounds-force (lbf), and it directly influences the contact angle in angular contact ball bearings. For example, in a 7005 series angular contact bearing (25 mm bore), light preload is approximately 30–90 N, medium preload is 90–190 N, and heavy preload can exceed 300 N. These values are not arbitrary—they are chosen based on bearing stiffness and allowable heat generation.

Why Preload Matters: Rigidity, Accuracy, and Lifespan

Rigidity and Deflection Control

In machine tool spindles, robotic arms, and servo drives, even a few micrometers of deflection can ruin a finished part. Preload increases the bearing's load-deflection curve by shifting the operating point into the stiffer region. For instance, a standard deep groove ball bearing with zero clearance can have a radial stiffness of 50–100 N/µm, while a properly preloaded angular contact bearing can reach 200–500 N/µm. This improved rigidity is essential for maintaining tool position under cutting forces.

Rotational Accuracy and Axial Stability

For high-speed applications, such as CNC machining or precision grinding, preload prevents the rolling elements from skidding and maintains a consistent contact angle. Skidding at high speeds can cause smearing and premature failure. Additionally, preload eliminates axial float, which is critical for applications like ball screws and lead screws, where positional accuracy is paramount.

Effects on Bearing Life

It is a common misconception that preload always reduces bearing life. Light preload actually extends life by ensuring even load distribution and preventing rolling elements from sliding. However, excessive preload increases friction, raises operating temperature, and accelerates fatigue. A general rule from bearing manufacturers: for every 10°C rise in bearing temperature, the rated life (L10) decreases by approximately 50%. Therefore, proper preload selection is a balance between rigidity and thermal management.

When to Apply Preload: Application Guide

Not every bearing application needs preload. Below is a practical guide to help you decide.

Application Type	Preload Needed?	Recommended Bearing Type	Typical Preload Range
Precision machine tool spindles	Yes – required	Angular contact ball bearings	Medium to heavy (0.10–0.25% of dynamic load rating C)
High-speed electric motors	Light – optional	Deep groove ball bearings	Light (0.05–0.10% of C)
Gearboxes and transmissions	No – allow for thermal expansion	Cylindrical roller bearings	None (internal clearance required)
Ball screw support units	Yes – recommended	Angular contact thrust bearings	Medium (preload set by shims or lock nuts)
Industrial fans and pumps	No – excess preload causes heat	Insert bearings	None (light clearance)
Robot joints and harmonic drives	Yes – for zero backlash	Crossed roller bearings or angular contact	Light to medium (0.05–0.15% of C)

For example, consider a spindle using a 7210 angular contact bearing (50 mm bore) with a dynamic load rating C = 35,500 N. Medium preload would be about 50–90 N (0.14–0.25% of C). The maximum speed for medium preload in this bearing is approximately 12,000 rpm (oil lubrication) or 9,000 rpm (grease). Exceeding these speeds with heavy preload can lead to rapid heat buildup.

Methods of Applying Preload and Technical Considerations

There are three common ways to apply preload to a bearing system:

**Axial preload via lock nuts** – A precision lock nut is torqued to compress the inner rings of a matched pair. This method is simple but requires accurate torque control. For example, a M30 lock nut on a 40 mm bearing may require 80–120 N·m to achieve desired preload.

**Spacer and shim preload** – Precisely ground spacers of controlled length are placed between bearing inner or outer rings. This allows repeatable preload without relying on torque measurement. Shim thickness differences of just 0.02 mm can change preload by 20–40 N.

**Matched bearing pairs (DB, DF, DT)** – Manufacturers supply bearings already matched for a specific preload class (L light, M medium, H heavy). These are the most reliable and are often calibrated to ISO standards. For example, a matched pair of 7010 angular contact bearings in a DB arrangement has a preload of 150–300 N, depending on class.

Technical Data: Preload vs. Speed and Lubrication

Preload significantly influences the speed limit of a bearing. The speed limit (nG) is defined as the maximum rotational speed under standard lubrication. For an angular contact ball bearing (series 70), the speed factor is often expressed as DM·n, where DM is the mean bearing diameter in mm and n is speed in rpm. For a light preload, the allowed speed factor is approximately 1,500,000. For medium preload, it drops to 1,200,000, and for heavy preload, only 900,000. This means that for a 50 mm bore bearing, the speed limit with light preload is 30,000 rpm, while with heavy preload it drops to 18,000 rpm.

Lubrication also matters. Oil jet lubrication can handle higher preloads than grease. When using grease, preload should be limited to light or medium, because grease has a lower heat-removal capacity. Always check the manufacturer's catalog for friction torque values and thermal limits.

Preload in Machine Maintenance: Best Practices and Pitfalls

In machine maintenance, bearings are often replaced without considering the original preload. This can lead to premature failure or poor performance. Follow these best practices:

**Measure the original preload** – Before disassembly, record the lock nut torque or shim thickness. This is your baseline.

**Check for thermal growth** – In applications with significant temperature changes, account for thermal expansion of the shaft and housing. A bearing preloaded at room temperature may become heavily preloaded at operating temperature. Use a thermal compensation spacer or choose a bearing with a larger clearance class (C3) in such cases.

**Inspect bearing seats for wear** – A worn shaft or housing can reduce effective preload. Always measure the shaft diameter and housing bore with a micrometer; tolerances are typically ISO h5 for shafts and H6 for housings.

**Never increase preload to fix spindles** – If a spindle has runout, adding more preload will not correct a bent shaft or an out-of-round housing. Diagnose the root cause first.

Common Pitfalls to Avoid

Over-torquing lock nuts, which crushes the raceway and creates false brinelling.

Mixing bearing suppliers or preload classes. Always replace with identical matched pairs.

Ignoring the effect of running speed—severe preload at high speed causes smearing and hear build-up.

Using preload on bearings designed for clearance (e.g., cylindrical roller bearings) can lead to catastrophic skidding damage.

Preload is not a universal fix—it must be engineered for each application. When in doubt, consult bearing manufacturer data or a specialist.

Conclusion

Bearing preload is an indispensable technique for achieving high rigidity, accuracy, and stable performance in precision machinery. By eliminating internal clearance and creating a controlled, constant load, you can prevent vibration, reduce deflection, and extend bearing life—provided you select the correct preload for your operating conditions. Always balance rigidity with thermal and speed limitations, and follow proper assembly procedures during maintenance. Whether you are designing a new spindle or repairing an existing machine, understanding when and why to apply preload is key to reliable operation. For high-quality preloaded bearings and matched pairs, Haihe Bearings (yandianbearing.com) supplies this product with certified dimensional accuracy and load ratings, ensuring your equipment performs at its best.