Bearing Failure: Which Comes First – Heat, Vibration, or Noise?

When engineers conduct routine equipment checks, abnormal bearing vibration, temperature, or noise often triggers an inspection. While these anomalies don't always point to a bearing fault itself (they could be symptoms of other issues), if a bearing is failing, it will exhibit these signs. In practice, when a bearing develops a fault, its vibration, temperature, and noise don't necessarily go awry simultaneously. Field personnel often detect just one or two abnormalities before initiating an inspection. Sometimes, even the sequence in which these symptoms appear can offer valuable clues for fault diagnosis. So, when a bearing fails, what's the actual sequence of heat generation, vibration, and noise? Let's explore this.

The Genesis of Bearing Faults: From Internal Damage

First, let's consider bearing failure from the perspective of failure modes. Standard bearing failure analysis categorizes failures into types such as fatigue, wear, corrosion, electrical corrosion, plastic deformation, and fracture. Regardless of the type, all these modes manifest as some form of damage to the bearing's rolling elements or raceways. Therefore, let's look at the process of bearing vibration, noise, and temperature changes from the perspective of internal bearing damage.

When there's an issue within the bearing's rolling elements or raceways, the first thing that changes is the morphology of the rolling contact surfaces. To make this easier to understand, let's use bearing fatigue as an example.

In the early stages of bearing fatigue, the changes occur within the bearing steel itself. At this point, the bearing steel surface shows no visible alterations, and the bearing's rolling process won't exhibit any outward changes. Consequently, there will be no noticeable shifts in vibration, noise, or bearing heat (temperature).

However, when the first spall appears on the bearing surface, a pit forms, and detached particles may also settle elsewhere on the raceway. When a rolling element passes over this pit or over the spalled particles, stress concentration first occurs within the rolling contact area. This can potentially puncture the lubricant film, leading to secondary damage and possibly localized heat generation. As the rolling element passes over the pit or particles, it no longer rolls smoothly; this irregular motion causes a change in the rolling element's vibration. When this induced vibration falls within the audible range, it's then perceived as noise.

From this analysis, using bearing fatigue as an example, it's clear that when the surface morphology of the bearing raceway and rolling elements changes, the bearing's vibration, temperature, and noise fundamentally emerge simultaneously at the source. So, why is it that during field inspections, we sometimes only detect one or two of these, or notice them in a sequence? This is all related to the propagation and perception of bearing vibration, temperature, and noise.

Why Perception Differs: Propagation and Detection Limits

Let's continue with our fatigue example. If there's a small fatigue pit, the amplitude of the vibration caused when a rolling element passes over it might not differ significantly from the normal operational vibration. From a signal analysis perspective, the signal-to-noise ratio (SNR) is very low at this stage. Even with highly sensitive sensors, the collected signal can be difficult to separate and identify as a fault. At this point, it's hard to distinguish between fault-induced vibration and normal vibration. Only when the fault vibration amplitude becomes sufficiently large can it be reliably identified. If field personnel are relying on their own senses, a small spall inside the bearing might not even be perceptible as a change in vibration.

Since vibration is the excitation source for noise, if the noise triggered by vibration is drowned out by other ambient noise, it also becomes undetectable, whether by sensors or the human ear.

In the same process, changes in the bearing's internal lubrication status and friction conditions will lead to alterations in its heat generation. The temperature change caused by bearing heat is influenced not only by the heat source (heat generated at the fault point) but also by the system's thermal capacity (specific heat and mass). In the very early stages of bearing failure, the heat generated by just a few small points is often insufficient to cause a noticeable temperature fluctuation across the entire system. At this stage, it's also difficult to detect. Only when the generated heat continuously increases (due to the worsening of the failure, of course) will it begin to impact the overall temperature.

From this analysis, we can see that vibration, noise, and heat are essentially three manifestations of an internal bearing fault. Theoretically, they are almost simultaneously excited at the fault location. However, when they are perceived, it's often influenced by their propagation path and the detection methods used.

Decoding Common Field Observations

So, what about those common field observations where temperature is high but vibration is low, or there's noise but vibration and temperature are normal, or high vibration with normal temperature? Let's do a simple analysis:

1. High Temperature, Low Vibration: If the temperature is high, it means there's significant friction and heat generated at the contact points during rolling. If the surface morphology of the friction area has changed (like our fatigue pit example), it should be accompanied by increased vibration (though detection might be delayed due to perception issues). If there's no significant increase in vibration, it suggests that the surface changes might be minimal in the initial stages. This indicates that the lubricant film is poor, leading to increased direct metal-to-metal friction, but the metal surfaces haven't sustained extensive damage yet. In this scenario, an experienced engineer might suspect a lubrication problem rather than a severe bearing defect.

2. Noise, but Normal Vibration and Temperature: Normal vibration and temperature suggest that the overall internal state of the bearing is stable. However, the noise in this case is often at an abnormal frequency. An increase in abnormal frequency noise doesn't necessarily mean a significant increase in overall vibration amplitude. At this point, we can still roughly conclude that the bearing raceway hasn't sustained severe damage (at least initially; damage may develop over time). By analyzing the noise frequency and the abnormal frequencies in the vibration spectrum, you can usually pinpoint the source of the noise.

3. High Vibration, Normal Temperature: Here, "high vibration" usually refers to an increase in vibration amplitude. When the total normal vibration value of a bearing increases, it doesn't necessarily mean lubrication damage. If lubrication is normal, the heat generated by the bearing due to increased vibration load might not be substantial. In such cases, the observed temperature change might be minimal. However, regardless of the temperature, it's crucial to investigate the source of the abnormal vibration.

Of course, the sequential appearance of bearing vibration, temperature, and noise can vary greatly depending on specific field conditions. However, grasping the fundamental mechanisms provides valuable clues. These clues aren't strict laws, nor is there a fixed rule of who comes first. A person's analysis of phenomena and their understanding of underlying principles are the foundation for successful analysis and accurate conclusions. This is what we call "experience."

Bearing temperature, vibration, and noise are frequently used parameters to characterize bearing condition in engineering practice. Abnormal vibration and temperature are two critical indicators of bearing failure. When a bearing fails, field engineers often observe either high temperature (abnormal temperature), increased vibration (abnormal vibration), or sometimes abnormal temperature with normal vibration, or vice versa.