What are the consequences of excessive or insufficient preload on 68 series bearings- Cixi Bao Li Bearing Manufacturing Co., Ltd.

The Deep Groove Ball Bearing 68 Series is widely utilized in high-precision transmission systems and high-speed spindle applications, owing to its high precision class, thin-section structure, and superior speed capability. In these demanding environments, Preload—the deliberate application of an axial force to eliminate or compress the internal clearance—is a critical parameter for enhancing bearing rigidity and rotational accuracy. However, deviations from the optimal preload setting—be it Over-Preload or Under-Preload—will inevitably lead to severe and irreversible negative impacts on the 68 Series bearing's performance and service life.

I. Severe Consequences of Over-Preload (Excessive Axial Force)

When the axial preload applied to a 68 Series bearing exceeds the optimally engineered value, the consequences can be catastrophic, manifesting as rapid performance degradation, drastic reduction in lifespan, and poor energy efficiency.

1. Exponential Increase in Rolling Element/Raceway Contact Stress

Excessive preload directly leads to a significant surge in contact stress between the rolling elements (balls) and both the inner and outer raceways. For the thin-section 68 Series, which inherently has a lower load rating compared to standard series, this high stress drastically reduces the material's fatigue life.

Consequence Manifestation: The bearing's basic rating life () is rapidly diminished, often following a cubic or higher-order relationship with the increase in contact stress.
Professional Assessment: The bearing will fail prematurely, exhibiting fatigue spalling or pitting on the raceways and rolling elements well before its expected lifespan, leading to early failure.

2. Uncontrolled Friction Torque and Operating Temperature Rise

An excessive preload increases the elastic deformation between the rolling elements and the raceways, causing the frictional torque to climb sharply. At high speeds, this elevated friction rapidly converts into thermal energy.

Consequence Manifestation: The bearing's operating temperature rises quickly, potentially exceeding the maximum permissible working temperature of the lubricant (grease or oil).
Professional Assessment: High temperatures cause the lubricant to oxidize, degrade, or evaporate, creating a vicious cycle of lubrication failure. The resulting difference in thermal expansion between the inner and outer rings further exacerbates contact stress, potentially leading to scorching or seizing.

3. Loss of Clearance and Structural Overload

While preload is intended to enhance rigidity, excessive force completely eliminates all internal clearance, leaving the bearing in an over-stressed condition. Although the static stiffness appears high, the bearing loses its elastic buffering capacity to withstand sudden shock loads.

Consequence Manifestation: The bearing's resistance to shock loads and vibration is severely compromised.
Professional Assessment: Under impact conditions, the thin-section raceways of the 68 Series are more susceptible to plastic deformation and can even lead to ring cracking.

II. Negative Impacts of Under-Preload (Insufficient Axial Force)

When the applied preload is insufficient, or when it fails to fully eliminate the designed internal clearance, the bearing system cannot deliver its intended performance, resulting primarily in a loss of precision, increased vibration, and accelerated wear.

1. Severe Degradation of Running Accuracy and System Rigidity

Insufficient preload means the bearing retains an effective running clearance. This clearance permits micro-displacement of the shaft or spindle under applied radial and axial loads.

Consequence Manifestation: The system's running accuracy is significantly reduced, directly impairing the machining precision and surface finish of workpieces in spindle applications.
Professional Assessment: The bearing's static and dynamic rigidity fall well below design requirements, causing the equipment to suffer from chatter and resonance under working loads.

2. Rolling Element Skidding and Uneven Wear

In high-speed, light-load conditions (a common scenario for the 68 Series), insufficient preload can cause the unloaded rolling elements to skid (slide) on the raceways.

Consequence Manifestation: Skidding deviates the rolling elements from their pure rolling trajectory, generating undesirable sliding friction and localized heat spikes.
Professional Assessment: Skidding leads to scuffing or abrasive wear on the raceway and rolling element surfaces. Wear is accelerated, particularly in the cage contact areas, which can lead to early cage failure due to uneven forces and friction.

3. Increased Vibration, Noise, and Poor Dynamic Stability

The presence of clearance allows the rolling elements to impact and bounce as they transition between loaded and unloaded zones during rotation.

Consequence Manifestation: The bearing's vibration level and acoustic noise increase markedly, which is unacceptable for noise-sensitive precision instruments.
Professional Assessment: Vibration spectrum analysis will reveal non-typical frequency peaks caused by rolling element impacts and irregular motion, thereby compromising the equipment's dynamic stability and overall reliability.