Beyond Vibration Levels: How RONDS Detected Hidden Bearing Creep Before Gearbox Failure

In continuous production industries, the sudden failure of critical equipment often results in significant production losses.

Actually, most major failures do not occur without symptoms. Before complete equipment failure, the internal mechanical structure typically releases a wealth of early signals.

Identifying these hidden risks from complex vibration signals and translating diagnostic insights into actionable maintenance recommendations are at the core of predictive maintenance.

Recently, RONDS diagnostic team, through online condition monitoring, high-resolution vibration analysis, and professional diagnostic techniques, successfully identified a bearing deteriorating trend in a customer's extruder gearbox, and guided the on-site maintenance planning, thereby preventing a potential serious equipment failure.

Case Background: A Critical Production Asset with Hidden Failure Risk

The equipment is a core production asset — an extruder gearbox.

The gearbox features a double-layer internal configuration. The upper layer consists of Shaft 1 (input shaft), which meshes with Shaft 2 on the lower layer. Shafts 2 and 3 on the lower layer are synchronous shafts, working together through synchronizing gears to deliver the output. The currently engaged high‑speed gear ratio is 2.23.

RONDS has deployed an online vibration monitoring system on‑site, providing 24/7 monitoring services to continuously monitor the critical bearing positions.

A total of 11 sensors are installed on the gearbox, with the layout as shown below:

Fault Evolution Process: From Early Degradation to Bearing Crack Failure

Leveraging high‑resolution long‑waveform data and envelope demodulation techniques, RONDS diagnostic engineer divided the entire fault evolution into three distinct phases, achieving full-tracking and precise alert from incipient damage to impending failure.

Phase 1: Early Wear and Incipient Creep Emergence (Dec 2025 – Feb 2, 2026)

After monitoring began in December 2025, the high-frequency acceleration RMS vibration trend at the Shaft 1 position remained stable throughout, with no indication of exceeding any thresholds — relying solely on regular threshold-based alarming, no anomaly would have been triggered at this stage.

However, through detailed analysis of high-frequency acceleration waveforms and envelope spectra, RONDS diagnostic team identified several hidden degradation indicators.

(1) Periodic impact signatures appeared in the time waveform

High-frequency acceleration waveform analysis revealed periodic impact features.

The impact interval was close to the rotational frequency of Shaft 1, indicating abnormal mechanical excitation occurring inside the gearbox.

(2) Early bearing raceway damage was identified through envelope spectrum analysis

Envelope demodulation of the acceleration spectrum revealed the Shaft 1 bearing characteristic frequencies, including:

•  Ball Pass Frequency Inner Race (BPFI)

•  Ball Pass Frequency Outer Race (BPFO)

•  Their corresponding harmonics

These characteristic components indicated that early-stage pitting damage had already developed on both the inner and outer raceways of the Shaft 1 bearing.

(3) Hidden abnormalities were identified at the Shaft 3 output end

Meanwhile, two significant abnormal features were detected at the Shaft 3 output end measuring point.

① Synchronizing gear wear indicated by GMF sidebands

The low-frequency acceleration waveform showed periodic impact responses corresponding to the rotational frequency of Shaft 3.

At the same time, the second-stage gear mesh frequency (GMF) between Shafts 2 and 3 exhibited multiple sidebands spaced at the Shaft 3 rotational frequency.

This pattern indicated that wear had already developed in the synchronizing gear system.

② Inner ring creep identified through noise-floor envelope demodulation

After applying envelope demodulation to the noise floor of the long-waveform acceleration spectrum, a frequency component at 7.397 Hz and its harmonics were detected.

This frequency was slightly lower than the theoretical rotational frequency of Shaft 3 (7.44 Hz).

Diagnostic Interpretation

This subtle frequency deviation became the key indicator for identifying inner ring creep at the Shaft 3 bearing.

Under normal operating conditions, the bearing inner ring maintains an interference fit with the shaft, ensuring that the inner ring rotates synchronously with the shaft.

However, when the interference fit deteriorates due to insufficient fit tightness or mechanical loading conditions, micro-sliding may occur between the bearing inner ring and the shaft.

This phenomenon is known as bearing inner ring creep or inner ring slip.

Once creep begins:

•  Relative sliding develops between the bearing inner ring and the shaft;

•  Frictional heat accumulates at the contact interface;

•  The interference fit further deteriorates, accelerating the progression of damage.

In this case, the detected frequency component slightly below the Shaft 3 rotational frequency represented the actual rotational behavior of the bearing inner ring, indicating that creep had already occurred between the inner ring and the shaft.

On February 1, 2026, RONDS monitoring system triggered a related alarm.

After detailed diagnostic analysis, the diagnostic engineer issued an alert to the site:

•  The Shaft 1 bearing showed early-stage raceway pitting on both the inner and outer raceways, with the Shaft 1 output end bearing showing relatively more severe damage and a clear deterioration trend.

•  The Shaft 3 bearing showed signs of potential inner ring creep, requiring continued monitoring and further assessment.

The analysis established the initial condition of the Shaft 3 bearing and provided the foundation for tracking its subsequent deterioration process.

Phase 2: Creep Progression and Initiation of Inner Ring Cracking (Feb 3, 2026 – Jun 18, 2026)

After the equipment was restarted on February 3, 2026, the vibration trend at the Shaft 3 output end bearing location gradually began to increase.

During this stage, abnormal impact responses continued to appear in the acceleration time waveform. However, the impacts did not yet show a clear repetitive pattern.

Through envelope demodulation analysis of the acceleration spectrum noise floor, the relative rotational frequency associated with the Shaft 3 bearing decreased further to 7.34 Hz.

Compared with the 7.397 Hz observed during Phase 1, this further reduction indicated that the inner ring creep condition was progressively worsening.

Fault Mechanism Analysis

The synchronizing gears installed in this gearbox are helical gears.

During operation, helical gears continuously generate axial forces, which act on Shafts 2 and 3 and transfer additional axial loading to the Shaft 3 bearing.

Under these conditions, the bearing inner ring experiences continuous axial stress.

At the same time, the relative sliding between the inner ring and the shaft generates persistent frictional heat, gradually reducing the integrity of the interference fit.

As the creep condition continued, repeated mechanical stress and thermal effects caused fatigue accumulation at stress concentration areas on the raceway, eventually initiating micro-cracks on the bearing inner ring.

The fault then entered a hidden deterioration stage, where internal damage continued to develop while regular vibration indicators remained relatively limited.

Phase 3: Inner Ring Through-Thickness Crack and Imminent Failure Risk (Jun 18, 2026 – Jun 30, 2026)

From June 18 onward, the vibration trend at the Shaft 3 output end bearing location increased significantly.

Sharp periodic impacts appeared in the acceleration time waveform at intervals corresponding to the Shaft 3 rotational frequency.

This impact pattern is a typical indication of a sharp fracture edge on the bearing raceway surface, where rolling elements generate strong impacts when passing over the damaged area.

The signal characteristics indicated that the inner ring crack had progressed into a through-thickness fracture.

At this stage, the envelope spectrum clearly showed the Shaft 3 bearing BPFI component and its harmonics.

Meanwhile, rich sidebands spaced at the Shaft 3 rotational frequency were also observed, confirming that the inner ring fault characteristics had become fully developed.

Meanwhile, the relative rotational frequency of the Shaft 3 bearing inner ring continued to decrease, reaching approximately 7.22 Hz.

This further confirmed that the inner ring creep condition was continuing to deteriorate.

On June 19, 2026, RONDS diagnostic team issued an escalated alert to the site:

•  The Shaft 3 bearing inner ring had developed a through-thickness crack and was approaching a severe failure condition;

•  The Shaft 1 output-end bearing showed signs of rolling element spalling and cage degradation, creating a potential risk of sudden failure.

Timely Shutdown and Inspection: Preventing a Potential Major Failure

The customer placed high importance on the alert and arranged a planned shutdown for inspection and maintenance.

The inspection results conducted on June 30, 2026, showed strong consistency with the remote diagnostic findings from RONDS:

(1) The self-aligning bearing installed at the Shaft 3 output end showed clear evidence of inner ring creep, together with a through-thickness crack on the inner ring raceway.

(2) The Shaft 1 output end bearing showed indentation wear on the inner ring raceway, together with rolling element wear.

The early identification of the degradation trend and accurate fault diagnosis enabled the customer to prepare spare parts, coordinate maintenance resources, and execute a planned intervention.

As a result, the customer avoided the risk of uncontrolled bearing failure, such as severe seizure or secondary gearbox damage, and successfully transformed a potential emergency shutdown into a controlled maintenance activity.

Core Technical Methodology: Quantitative Identification of Bearing Creep

A common question from maintenance teams is:

Why did the vibration level remain normal even when the bearing was already approaching failure?

The answer lies in understanding the characteristics of bearing creep.

For regular vibration monitoring, overall amplitude indicators such as RMS values are often effective for detecting severe damage. However, in the early and intermediate stages of bearing creep, the overall vibration amplitude may remain relatively stable.

The key indicator of creep is not simply vibration amplitude, but the subtle change in frequency behavior.

A frequency component slightly lower than the shaft rotational frequency can provide direct evidence of relative sliding caused by loss of interference fit. Furthermore, the continuous decrease of this frequency component can be used to quantify the deterioration process.

RONDS Diagnostic Methodology:

(1) High-resolution long-waveform analysis combined with noise-floor envelope demodulation

Regular spectrum analysis may not have sufficient resolution to capture extremely weak fault signatures hidden within noise floor.

By applying high-resolution long-waveform collection and noise-floor envelope demodulation, RONDS was able to:

•  Extract weak impact characteristics from the noise floor;

•  Identify subtle frequency components associated with bearing inner ring creep;

•  Capture the small rotational frequency difference between the bearing inner ring and the shaft.

This enabled the identification of bearing creep before regular vibration amplitude indicators showed obvious abnormalities.

(2) Quantifying creep deterioration through relative frequency deviation

The severity of bearing creep is directly related to the degree of relative sliding between the bearing inner ring and the shaft.

As the interference fit deteriorates:

•  The slip between the inner ring and shaft increases;

•  The effective rotational frequency of the bearing inner ring gradually decreases;

•  The deviation from the shaft rotational frequency becomes larger.

By continuously tracking this frequency shift, the progression of creep can be quantitatively evaluated and the remaining safe operating window can be estimated.

Based on the gearbox operating parameters and vibration analysis results, the theoretical rotational frequency of Shaft 3 was calculated as 7.44 Hz.

However, in the high-resolution long-waveform acceleration envelope spectrum, noise-floor envelope demodulation revealed a frequency component of 7.25 Hz.

This frequency was slightly lower than the Shaft 3 rotational frequency, and the BPFI sidebands were also spaced at approximately this frequency interval.

Theoretically, when a Shaft 3 bearing develops an inner race fault, the envelope spectrum should show:

•  The bearing BPFI and its harmonics;

•  Sidebands spaced at the shaft rotational frequency;

•  The shaft rotational frequency component itself.

This raises an important diagnostic question:

Why did a frequency component slightly lower than the Shaft 3 rotational frequency appear in the spectrum?

When another long-waveform data point was analyzed, the corresponding envelope frequency was found to be 7.227 Hz.

This represented a further decrease from the previously observed 7.25 Hz.

Since the operating speed of Shaft 3 remained unchanged, the frequency variation could not be attributed to speed fluctuation.

A comparative analysis showed that the inner ring fault sidebands were consistently spaced at a frequency slightly lower than the actual Shaft 3 rotational frequency.

This difference became the key evidence for identifying bearing inner ring creep:

When creep occurs, the bearing inner ring no longer rotates perfectly synchronously with the shaft due to loss of interference fit.

The slightly lower frequency observed in the envelope spectrum represents the actual rotational frequency of the slipping bearing inner ring.

Therefore, the frequency difference between the shaft rotational frequency and the measured inner ring rotational frequency can be used as a quantitative indicator of creep severity.

During the subsequent degradation stage, this relative rotational frequency continued to decrease:

From 7.3 Hz down to 7.104 Hz

This continuous downward trend demonstrated that the bearing creep condition was progressively worsening.

Without timely maintenance intervention, the bearing could have eventually progressed toward severe inner ring slip, bearing seizure, and potential secondary gearbox damage.

Through early detection and accurate diagnosis, RONDS enabled the customer to intervene within a planned maintenance window and successfully avoided a major unplanned equipment failure.

Key Takeaways for Industrial Maintenance Teams

Equipment rarely fails suddenly.

Before a fault occurs, the mechanical system has already released early signals. The value of predictive maintenance lies not just in detecting anomalies, but in understanding the fault mechanism and helping customers take the right maintenance action at the right time.

Normal vibration levels do not necessarily mean equipment is healthy. For hidden faults such as bearing creep and early‑stage pitting, combined analysis of waveforms, spectra, and envelope spectra is required to capture the deterioration trend developing inside the equipment.

Bearing faults usually follow a progression from early damage to severe failure. The key is to identify the risk early enough — transforming reactive emergency repairs into proactive, planned maintenance.

Make equipment status transparent. Make maintenance decisions more intelligent.