Introduction to electrically insulated bearings
First, the purpose of electrically insulating bearings
The so-called electrically insulated bearings are also insulated bearings, and electrically insulated bearings include all rolling bearings that can prevent the passage of electric current. Bearings with ceramic coating, both inner and outer rings, are called insulated bearings. The ceramic coating prevents the passage of electric current and has the ability to insulate.
The rolling elements of hybrid bearings are made of ceramic and therefore also have insulating properties. It is made of rolling elements to prevent the passage of electric current.
Second, the selection of bearing insulation
In general, it is very difficult to completely eliminate the potential difference inside the bearing. However, if we can stop or significantly reduce the current flow through the bearing, we can prevent galvanic corrosion of the bearing. A wide range of insulated bearings are currently designed for this purpose. Depending on the type of voltage generated, the insulation method of the bearing is selected.
1. The induced voltage generated along the shaft
2. The voltage between the shaft and the bearing seat
If the voltage occurs between the shaft and the housing, the current flows through each bearing in the same direction. This is mainly due to the common-mode voltage brought by the frequency converter. In this case, the bearings at both ends of the motor should be insulated, and the decisive factor in the choice of insulation is the temporal characteristics of the current and voltage. In the case of DC voltage or low-frequency AC voltage, the insulation effect depends on the pure resistance value of the insulation layer; In the case of high-frequency AC voltages (commonly found in devices using frequency converters), it depends on the capacitive reactance of the insulation.
3. Typical situation of bearing damage caused by overcurrent
1. Traces on raceways and rolling elements
Regardless of whether the bearing is powered by direct current or alternating current (frequency below MHz), we can always find the same form of failure inside the bearing.
2. Electro-erosion groove marks
The so-called electro-erosion groove refers to the continuous periodic groove on the raceway surface in the direction of operation. Most of these phenomena are caused by the current passing through the bearing.
Fourth, under the microscope to check the damaged structure of the overcurrent bearing
It is only by scanning electron microscopy (SEM) that it is evident that almost all damaged surfaces are densely covered with pits and μm solder joints.
Fifth, the process of bearing damage
These pits and solder joints are caused by current discharges between tiny points of contact on the surface of the raceway and rolling elements. In the fully fluid lubricated state, the electric current breaks through the weak point of the oil film, and the energy generated by the electric spark melts the surface of the adjacent metal in an instant.
In the mixed friction state (metal-to-metal contact), adjacent surfaces also fuse, but are quickly separated as the bearing moves. In both cases, the material detaches from the metal surface and immediately solidifies into a solder joint. There are also some solder joints that are mixed with lubricant and others that are deposited on the raceway surface. As the bearing continues to move, these solder joints and pits are also rolled and smoothed. Under the action of a continuous electric current, the process of melting and solidification is repeated several times on a very thin surface layer of the adjacent surface.
6. The influence of current on lubricants
Electric currents can also have a negative impact on the lubricant. The base oils and additives are oxidized and cracked. This change can be clearly seen in the infrared spectrogram. Premature aging and the accumulation of ferrous metal particles can lead to deterioration of lubricant performance and can also lead to overheating of bearings.
Post time: Feb-24-2025
