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Rolling bearings in electric motors, generators or associated equipment are at risk from the passage of electric current, which can damage the surfaces of rolling elements and raceways in the bearing and degrade the grease rapidly. The risk of damage, so called electric erosion, increases greatly if a frequency converter controls the electrical machinery, which is becoming increasingly common. An additional risk for high frequency bearing currents occurs in the application due to the inherent stray capacitances within the electrical machinery.
To protect the bearings against electric current passage, SKF has developed INSOCOAT� bearings – electrically insulated rolling bearings (fig 1). An INSOCOAT bearing is a very economical solution compared with other insulation methods to protect the bearing. By integrating the electrical insulation function into the bearing, SKF has been able to increase reliability and machine uptime by virtually eliminating electric erosion problems.
INSOCOAT bearings have a nominal 100 �m thick layer of aluminium oxide on the exterior surfaces of the outer or inner ring, that can withstand voltages up to 1 000 V DC. The SKF plasma spray coating technique provides an extremely coherent coating of uniform thickness, which is further treated to make it insensitive to moisture and humidity.
SKF INSOCOAT bearings are robust and should be handled in the same way as normal non-insulated bearings.
SKF INSOCOAT bearings are available from stock as
in the most frequently used sizes and variants. The performance data as well as the dimensional and running accuracy of the INSOCOAT bearings are identical to those of standard non-insulated bearings.
The SKF standard range includes bearings either with coated outer ring or coated inner ring of open design. Deep groove ball bearings with Z shields or with RS1 contact seals can also be supplied by SKF. Before deciding upon design and ordering, please consult the SKF application engineering service.
Bearings with electrically insulating coating on the external surfaces of the outer ring are the most common INSOCOAT bearings. They are identified by the suffix VL0241.
For applications where smaller bearings are needed than listed in the product table, SKF recommends the use of hybrid deep groove ball bearings.
Bearings with electrically insulating coating on the external surfaces of the inner ring (fig 2) provide enhanced protection against electric erosion due to the increased impedance because of the smaller coated surface area. They are identified by the suffix VL2071.
If the standard range of INSOCOAT deep groove ball bearings and cylindrical roller bearings is inadequate please contact the SKF application engineering service for information about the complete manufacturing programme of INSOCOAT bearings. Other types and sizes of INSOCOAT bearings that are not listed in the standard range and INSOCOAT bearings with an aluminium-oxide layer up to 300 �m thickness on the bearing outer ring are available on request.
Dimensions The boundary dimensions of INSOCOAT deep groove ball bearings as well as cylindrical roller bearings are in accordance with ISO 15:1998.
Tolerances The INSOCOAT bearings are produced to Normal tolerances. Some deep groove ball bearings are also available with higher accuracy to tolerance class P5. The values of the tolerances conform to ISO 492:2002.
The aluminium-oxide layer applied either to the external surfaces of the outer ring or the inner ring does not influence the accuracy.
Internal clearance INSOCOAT deep groove ball bearings and cylindrical roller bearings are manufactured as standard with radial internal clearance shown within the bearing designation. The availability of bearings with clearance other than standard should be checked before ordering.
The clearance limits are
for deep groove ball bearings
for cylindrical roller bearings
The values are valid before mounting under zero measuring load.
Cages
Depending on the bearing type and size, INSOCOAT bearings are fitted as standard with one of the following cages
For more detailed information on these cages please refer to the sections “Single row deep groove ball bearings – Cages” and “Single row cylindrical roller bearings – Cages”.
Minimum load To achieve satisfactory operation, INSOCOAT rolling bearings, like the non-insulated standard bearings, must always be subjected to a given minimum load. The recommendations for calculating the requisite minimum loads are identical to those of the standard non-insulated bearings and can be found in these sections for
Axial load carrying capacity The axial load carrying capacity of INSOCOAT bearings is identical to that of the standard non-insulated bearings. Recommendations can be found in these sections for
Recommendations for calculating the equivalent dynamic and static bearing loads of INSOCOAT bearings are identical to those of the corresponding standard bearings. Recommendations can be found in these sections for
“Equivalent dynamic bearing loads”
“Equivalent static bearing loads”
“Equivalent dynamic bearing loads”
“Equivalent static bearing loads”
Electrical properties The INSOCOAT layer provides effective protection against AC and DC currents. The minimum ohmic resistance is 50 MO at 1 000 V DC. Tests at SKF have shown that electrical breakdown of the insulating layer occurs above 3 000 V DC.
Design of associated components For insulation reasons it is recommended that for
Mounting and maintenance During mounting INSOCOAT bearings should be handled in the same way as standard bearings. Proper lubrication is important to utilize fully the service life of INSOCOAT bearings. The best method is frequent regreasing.
Additional information For additional information concerning INSOCOAT bearings, please contact the SKF application engineering service.
