Bearing electrical erosion is a frequently discussed issue, but most of the current literature presents it only in terms of why bearings experience electrical erosion, the characteristic features of bearing electrical erosion, and how to deal with it.
In fact, previous articles on this platform have already provided a detailed introduction to these aspects. This article introduces bearing electrical erosion from several other perspectives.
- What is the process of bearing overcurrent?
- What is the damage process of bearing electrical erosion?
- Can insulating bearings completely solve the problem of bearing electrical erosion?
- How many brushes should be used and where should they be placed?
- What voltage level can cause bearing electrical erosion?
The Process of Bearing Overcurrent?
Other articles have introduced the causes of bearing overcurrent, including issues with motor electromagnetic design, grounding, inverter switching components, mold voltage, and more. This won’t be repeated here.
Regardless of the cause, bearing overcurrent is caused by potential differences in different parts of the bearing and the existence of a current path.
Typical bearings are made of steel, and the inner ring, outer ring, and rolling elements are all metallic conductors, constituting the conditions for a current path. Therefore, when there is a potential difference between these parts of the bearing, the conditions for overcurrent are met, and current will flow through the bearing.
If the bearing’s internals were all stable conductors, such overcurrent wouldn’t seem to significantly impact the bearing. The problem is that there are other factors within the bearing.
First, the issue of the bearing’s lubricant. Generally, the conductivity of the bearing’s lubricant can’t compare with that of metal materials, making it a relative insulator. The oil film between the bearing raceway and the rolling elements acts like a capacitor structure.
When the potential difference between the capacitor plates (the raceway and rolling elements) reaches a certain level, the lubricant film will break down, leading to bearing overcurrent.
If the above breakdown is instantaneous, it forms an instantaneous current. If it happens continuously, it forms a continuous current.
It’s easy to see that when a potential difference exists between the inner and outer rings of a bearing, overcurrent occurs between the outer ring’s raceway and rolling elements and between the inner ring’s raceway and rolling elements.
Wherever a point current forms, it appears on the motor end cover as a non-uniform surface current, which is concentrated at the few contact points between the rolling elements and the raceway.
Considering the dynamic process, there is relative motion between the rolling elements and the raceway. When the bearing carries a load and rotates, the thickness of the oil film inside the bearing is a dynamic process, causing fluctuations in the circuit’s capacitance.
Changes in the number of contact points between the rolling elements and the raceway also affect the current density of the overcurrent.
What Is the Process of Bearing Electrical Erosion Damage?
It’s clear from the above description of bearing current conditions that the transient current punctures the lubrication film, resulting in bearing current.
First, consider the situation of a single puncture. When the bearing lubrication film is punctured, there is an initial occurrence of localized high temperatures (even sparks), which first burn the lubricant (carbonizing the lubricant).
If this instantaneous high temperature doesn’t reach a level that affects the metal, the bearing condition manifested is poor lubrication due to the carbonization of the grease.
In fact, typically, such puncture temperatures are quite high (some data indicate, puncture temperatures can reach up to 4000℃). Clearly, such temperatures will affect the metal material of the bearing.
The metal can melt under such high temperatures, forming tiny electrical erosion pits on the originally machined rolling elements and racing surfaces.
Under a microscope, the condition distribution around the edges and center of the pits (caused by carbon content changes) can be observed. For further details, interested professional readers can inquire separately.
The above explanation pertains to a single case of lubrication film puncture. In actual working conditions, single punctures are almost rare (occasionally occurred in DC motors years ago).
When multiple punctures accumulate, the surface of the bearing’s rolling elements and raceways turns gray, and common scrubbing patterns form on the raceway surfaces. (The formation of scrubbing patterns is a deeper topic and won’t be discussed in-depth here).
Can Insulated Bearings Solve Electrical Erosion Issues?
Before answering this question, it’s essential to clarify the role of insulated bearings in situations of bearing current.
Insulated bearings typically have an insulating coating on both the outer and inner rings. They’re called insulated bearings because the coating has excellent insulating properties.
Currently, mainstream manufacturers provide insulated bearings that are usually marked with an impedance value greater than a specific number under a certain voltage (for example, greater than 50 megaohms under 1000 volts DC).
When using insulated bearings, the bearing steel itself is a conductor, the bearing chamber and shaft are conductors, but the insulating coating in between is an insulator.
As such, the insulating coating of the insulated bearing in the circuit between the bearing and the housing acts like a capacitor.
The nature of a capacitor dictates that it blocks DC but allows AC to pass through, a phenomenon often referred to as “blocking DC, not AC.”
This is why bearing manufacturers mark the insulating ability of insulated bearings with the impedance not lower than a certain value under XX DC voltage.
For ordinary motors, bearing current is usually not DC but a combination of AC and DC. It’s not hard to see that common insulated bearings can effectively block the DC part of the bearing current but will not impede the AC component.
Therefore, an objective assessment of the impact of current insulated bearings on electrical erosion issues should be: Current insulated bearings can block the DC part of the bearing current but cannot block the AC part. They offer some relief to bearing current issues, but they do not entirely solve the problem.
The insulating coating on the aforementioned insulated bearings acts as a capacitive load in the bearing current loop. If an inductive coating could be found, it seems that the problem of bearing electrical erosion could be solved.
Indeed, some researchers started studying this topic years ago. Unfortunately, the optimal solution for an inductive coating has not been found yet (considering factors such as the range of current frequencies). As of now, I have not seen any mature solutions.
How Many Brushes to Place and Where to Place Them?
In the resolution of bearing electrical erosion issues, the approach of using insulating bearings works by “blocking.” Another method is “draining,” specifically by arranging brushes to release the potential difference between the inner and outer rings of the bearing, allowing the current to flow through carbonization, bypassing the bearing.
The selection of brushes involves many factors such as contact resistance and wear, but we won’t delve into those here. Let’s discuss the arrangement of brushes: Where should the brushes be placed, and how many are needed?
As mentioned earlier, the current between the bearing chamber and the shaft is a surface current on the end cap. The current density of this surface current is related to the cause of the potential difference. To facilitate discussion, let’s first discuss the arrangement of bypass brushes under uniform surface current conditions.
We know that when the bearing is under load, the load-bearing area of the bearing should be about 30% of the raceway section (roughly 120°-150°) under normal conditions. Within this area, contact occurs between the bearing rollers and raceway, carrying the load. This is known as the load zone, with other areas referred to as non-load zones.
In the non-load zones, the rollers and raceway do not contact each other, so they do not form a current path (or the path resistance is much higher than the contact area). Hence, the bearing’s overcurrent should be concentrated in the bearing’s load zone.
From actual cases of bearing overcurrent, we’ve seen that the scorched area of the bearing overcurrent in the inner-turning horizontal motor coincides with the contact trajectory of the load zone, concentrating in the load zone.
Such analysis helps us determine that the placement of bypass brushes should be close to the load zone of the bearing.
There are a few finer details that we won’t expand on here, such as the influence of load and oil film on overcurrent.
As for the number of bypass brushes for the bearing, while a higher number of brushes results in lower current density per brush and the brush distribution aligns closely with the current distribution, in reality, it’s influenced by other factors and doesn’t need to be excessive. Usually, 1-2 brushes can meet the requirements.
What Voltage Can Cause Electrical Erosion in Bearings?
From the process mechanism of bearing current, the two important influencing factors are the potential difference between the inner and outer rings of the bearing and the impedance in the entire path.
The potential difference between the inner and outer rings of the bearing is a potential difference of alternating and constant components. For the constant part, the resistance in the path (insulation bearings, insulation coating, insulation end cap) can often provide good isolation.
For example, for insulated bearings, 50 megaohms under 1000VDC can ensure the impedance is not compromised. This can be understood as a withstand voltage of 1000VDC.
It is not difficult to see that in general motor and mechanical designs, it is unlikely that there is such a large DC potential difference between the bearing chamber and the shaft of the equipment, so this withstand voltage level is sufficient for DC.
For the AC part in the voltage, the current is hindered by the impedance. Impedance is related to frequency, inductance, and capacitance values. From the bearing perspective, the inner and outer rings of the bearings, the rolling element, and the lubricating film constitute the main body of the capacitive and inductive reactance.
Moreover, the overcurrent path flows from the outer ring load zone – load zone rolling element – load zone inner ring.
When the bearing is operating, the electrical characteristics of the rolling element contacts in the bearing current path are changing. This includes the change in the number of contacting rolling elements, the change in contact area caused by load changes of the rolling element, changes in lubricant film thickness, and differences in electrical properties of different lubricants, among other factors.
On the other hand, the frequency of the AC component of the voltage of the inner and outer rings of the bearing also directly affects the size of the impedance in the circuit.
The third factor causing electrical erosion of bearings is temperature changes caused by overcurrent. If the discharge of overcurrent only causes the conduction of current, and the heat is not enough to affect the metal, electrical erosion will not occur. For example, slight overcurrent will cause carbonization of the lubricant, and the final failure mode of the bearing is poor lubrication, not changes in the raceway surface.
From the above analysis, it is not difficult to see that the many factors causing electrical erosion of bearings are changing and difficult to determine. Therefore, it is almost impossible to give a definite voltage value that causes electrical erosion of bearings.
In the literature I have seen, several values such as 500mV and 350mV have been suggested by some technicians.
However, based on the understanding of the overcurrent mechanism, these values are more likely to be empirical data that some technicians have explored for a certain type of motor under certain conditions, and their applicability to other occasions is still not convincing to me. (Please note that the discussion of the AC and DC parts in the literature that proposed these values may also need further improvement.)
How to Detect Electrical Erosion in a Bearing?
One can get a basic understanding of how electrical overcurrent affects bearings from the first article in this series. In fact, the process of bearing overcurrent may be related to the extent of the overcurrent (or the potential difference between the inner and outer rings of the bearing), which directly determines the degree of electrical erosion damage to the bearing.
Firstly, in the case of a slight overcurrent, the heat emitted from the electrical discharge will affect the grease, leading to carbonization or minor combustion. This significantly impacts the lubrication performance, and the carbonized grease becomes a contaminant within the bearing.
At this stage, one can observe a change in the bearing grease. This is typically manifested as a change in color or viscosity. Oil analysis would reveal carbonization.
Additionally, such poor lubrication can affect the raceway and rolling elements of the bearing during operation. If the overcurrent intensity is insufficient to impact the bearing steel, then the main surface features of the rolling elements and raceway will present a state of poor lubrication.
If the bearing overcurrent further intensifies, the discharge temperature will affect the bearing steel, leading to actual electrical erosion of the bearing.
During electrical erosion of the bearing, tiny electrical erosion pits form on the surface of the bearing steel. However, when the overcurrent is extremely slight, the surface does not exhibit the classic “fluting” pattern.
The surface appearance at this time is as shown, and a visual inspection often confuses it with bearing load markings.
A fundamental criterion for determining whether a bearing has experienced electrical erosion is to examine the microscopic image of the raceway surface, as shown below:
Microscopic images clearly reveal the pits caused by electrical erosion. The appearance of these pits largely confirms the presence of bearing electrical erosion.
If the overcurrent of the bearing continues to increase, or progresses further, the aforementioned condition will also develop further, eventually leading to the commonly seen “rubbing striations”. There are too many such images, so they will not be included in this article.
In the latest bearing failure standards, electrical erosion of bearings is divided into instances of excessive instantaneous current and bearing leakage current. This represents a change from previous standards. The electrical erosion of bearings introduced earlier is often seen in cases of bearing leakage current.
Instances of excessive instantaneous current have been rare in recent years, and these failures are often related to external factors such as static electricity. This manifests as a larger, and even visually identifiable, pit from electrical erosion, as shown in the figure below.