26 March 2015, 00:00
Electrically insulated rolling bearings feature electrical insulation that is built into the bearing, providing reliable protection against current passage and electrical corrosion. Typical applications are electric motors, generators and other electrical machines. The Austrian manufacturer NKE Austria GmbH offers electrically insulated rolling bearings in several versions. In this article, Klaus Grissenberger, Application Engineer with NKE, explains the causes of passage of electrical current through rolling bearings, damage symptoms and countermeasures, properties and production processes of electrically insulated bearings.
A difference in electrical potential between the outer and inner ring of a rolling bearing can lead to a damaging electric current that permanently damages the bearing’s raceways and impairs its running characteristics. A current flow results in electrical discharge in the contact zone between the rolling elements and the inner or outer ring. This, in turn, causes local melting of the surfaces. The result: pitting, material transfer and local microstructural damage through thermal stress. At least a very thin layer of the eroded surface is re-hardened, making it extremely hard and prone to cracking. This process is called electrical corrosion and often affects bearings in electromechanical applications, such as generators and electric motors, but also machines that are powered by electric motors, such as pumps and gearboxes. In addition, the electric discharge renders the lubricant useless. The base oil and additives contained in the lubricant oxidise, resulting in the characteristic black colour of the lubricant. Premature ageing permanently impairs the lubricant’s ability to form a separating layer between the metal surfaces. The bearing’s damaged functional surfaces and the loss of lubrication action result in a rapid loss of the bearing’s functionality.
Possible causes of undesirable passage of electrical current through rolling bearings
The main reasons for current discharge are known. Asymmetries in the magnetic flux of an electrical machine give rise to a low-frequency voltage between the shaft and the housing, which results in a flow of current through the rolling bearings. Such a passage of current can also result from the use of unshielded asymmetrical electric cables if the earth connection of the machine is ineffective. Another cause is the use of frequency converters: The operating principle of many frequency converters is based on pulse width modulation (PWM) and generates high-frequency common mode voltage, which also results in a passage of current through the rolling bearings. Lastly, electrostatic charging of the shaft and housing followed by discharge through the rolling bearings is also a possible cause.
Damage symptoms and possible countermeasures
Typical signs of electrical corrosion include grey, tarnished tracks in the raceways and on the rolling element surfaces. Melt craters (Figs. 1a and 1b) or fluting is also discernible, mainly on the raceway surfaces. Damage due to current discharge usually manifests itself in increased running noise.
In order to prevent damage of this kind, it is advisable to insulate the bearing seat in the housing or on the shaft. However, this entails additional design measures of the surrounding parts. A simple, economical solution in this case is to use electrically insulated rolling bearings from NKE. Since the key dimensions and performance data of the insulated bearings is identical with those of the corresponding uninsulated models, no changes to the overall design are required.
Electrically insulated bearings – properties and application
Electrically insulated bearings from NKE, designated with the suffix SQ77, are provided with an oxide ceramic insulation layer. An overview of the various design variants of the insulation layer is given in the table. Electrically insulated rolling bearings from NKE offer exactly the same outer dimensions and technical characteristics as the corresponding uninsulated types.
The most important advantages of these bearings are higher operating reliability, which is assured by optimal protection against electrical corrosion. Electrically insulated bearings are less expensive than, for example, applying insulation to housings or shafts. They are interchangeable with conventional bearings because they have the same key dimensions and technical characteristics. They also offer a reduced risk of damage and thus also longer operating life than conventional bearings when used in electrical machines. When electrically insulated bearings are handled properly, there is no risk of damage to the coating.
The main range of electrically insulated bearings consists of cylindrical roller bearings and deep groove ball bearings, but all other bearing types can also be electrically isolated. Application areas include traction motors of railway vehicles, electric motors and generators, particularly in conjunction with fast-switching frequency converters.
Production process and operating principle of electrically insulated rolling bearings
For electrically insulated bearings from NKE, the insulating layer is applied to the outer ring (SQ77 or SQ77C – Fig. 2a) or the inner ring (SQ77E or SQ77H – Fig. 2b) by plasma-spraying, a thin-film technology. During plasma- spraying, an electric arc is created between two electrodes with a suitable gas supply. The plasma jet serves as a carrier medium for applying the aluminium oxide powder (Al2O3) either to the outer or inner ring at high velocity. In order to obtain optimum protection the oxide layer also covers the side faces of the sprayed rings. In the next process step, the layer is then sealed to prevent any penetration by moisture.
The physical effect of the insulation layer depends on the frequency of the voltage that causes the harmful currents in the bearings. In the case of DC voltage, the insulated bearing has Ohmic resistance. The higher this resistance is, the lower the electrical current. The resistance value of the insulated bearings is greater than 50 MΩ, thus limiting the electrical current to a level that cannot cause damage to the bearing.
In the case of AC voltage the capacitive nature of the insulated bearing is advantageous. The bearing then behaves in approximately the same way as a parallel circuit consisting of a resistor and a capacitor with a frequency-dependent resistance, called impedance. The impedance determines the magnitude of the alternating current that flows through the bearing for given values of voltage and frequency. Here too, the impedance should be as high as possible in order to reduce the current to a level that is incapable of damaging the bearing.
To achieve high impedance values, the resistance of the insulating layer must be high and its capacitance must be low. This can be achieved by making the insulating layer as thick as possible and by reducing the overall insulation surface area. When transferred to the bearings, this means that this layer should preferably be applied to the bore of the inner ring. However, the coating is usually applied to the outer diameter for reasons of cost and due to the constraints of the manufacturing process. In most cases, this still results in more than adequate protection against damage from electrical corrosion. Another important property of the coating is its dielectric strength. Depending on their version, the bearings from NKE have a dielectric strength of at least 1000 V or 2000 V.
Overview of the design variants of electrically insulated NKE rolling bearings:
Source: NKE AUSTRIA GmbH; TPR International