Electrically insulated rolling bearings
29 March 2015
Electrically insulated bearings from NKE, designated with the suffix SQ77, are provided with an oxide ceramic insulation layer. These bearings offer exactly the same outer dimensions and technical characteristics as the corresponding non-insulated types.
The insulating aluminium oxide layer is applied to the outer ring (SQ77 or SQ77C) or the inner ring (SQ77E or SQ77H) by plasma-spraying. 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 50MO, 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 (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 1,000V or 2,000V.
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