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A Guide to Quiet Electrically Insulated Bearings
Silent insulated bearings refer to the general term for bearings that can block the passage of current, have insulation properties, and can rotate quietly. Its insulation performance is usually guaranteed by applying a layer of insulating material on the outer ring or inner ring of the bearing through a special process, or its rolling elements are made of ceramics. The names and codes of insulated bearings adopt the international bearing codes. The outer ring insulated bearing code is vl0241, and the inner ring insulated bearing code is vl2071.
The outer surface of the inner ring or outer ring is coated with an insulating aluminum oxide layer, and a high-quality finish is obtained by applying a plasma spraying process. The coating has high hardness, wear resistance, and good thermal conductivity and insulation properties. The coating has strong adhesion to the substrate and good insulation properties, which can avoid electrical corrosion of the bearing by the induced current, prevent the current from damaging the grease, rolling elements and raceways, and increase the service life of the bearing. In the silent insulated bearing, there is a 100μm-300μm thick coating on the surface of the outer ring or inner ring, which can withstand voltages up to 1000V DC-3000V DC.
A special spraying process forms a coating with uniform thickness and strong adhesion, which can be further treated to prevent moisture and humidity. Silent insulated bearings are cost-effective and reliable than other insulation methods such as shaft or housing insulation. The dimensions and basic technical characteristics of insulated bearings are the same as those of non-insulated bearings and are 100% interchangeable. Suitable for motors and generators, especially variable frequency motors.
Another type of silent insulated bearing is ceramic rolling element bearings. It uses ceramic rolling elements (balls or rollers) made of Si3N4 material; it has excellent insulation properties, its capacitance is basically 40pf, and its insulation performance is better than that of coated bearings. At high temperatures, its DC impedance can also reach the G ohm range, with good insulation properties; ceramic rolling elements have excellent wear resistance and low lubrication requirements. It is particularly suitable for high-speed, low-friction, and low-temperature operation; it operates well under dry friction conditions; for small rolling bearings, ceramic rolling element insulated bearings have good economic performance.
Characteristics of electrically insulating bearings
The main categories of electrically insulated bearing products are: electrically insulated deep groove ball bearings, electrically insulated angular contact ball bearings, electrically insulated cylindrical roller bearings, hybrids of electrically insulated inner or outer ring insulated bearings and electrically insulated ceramic rolling elements. bearings. The characteristics of electrically insulating bearings are:
1. Prevent electrical corrosion
2. High resistance (minimum resistance is 200M, can withstand DC voltage up to 3000V)
3. Good electrical performance (using unique sealant to prevent the penetration of condensation water)
By optimizing the design of the contact state between the rolling elements and the raceway, smaller surface roughness and higher geometric accuracy, the electrically insulated bearing has a remarkable silent effect when running at high speed.
Silent insulated bearing cage
1. Cage for deep groove ball bearings
Stamped cage, riveted, steel ball guided (no suffix)
Machine-cut brass cage, riveted, steel ball guided (suffix M)
2. Cage for insulated cylindrical roller bearings
Glass fiber reinforced PA66 cage, window type;
Rolling element guided (suffix P) Machine-cut brass cage, window type;
Rolling element guided (suffix M) Machine-cut brass cage, window type, inner or outer ring guided (model suffix ML).
Working temperature
The allowable temperature of insulating bearings may be limited by the following factors:
1. The dimensional stability of the bearing rings and rolling elements can reach at least 150°C.
2. The working temperature of steel or brass cages is the same as that of the bearing rings and rolling elements; the maximum working temperature of PA66 cages is 120°C.
3. Lubricant.
Silent insulated bearing design
J20B-breakdown voltage 500VDC, dry working environment, coating thickness 100 microns.
J20A-breakdown voltage 1000VDC, dry working environment, coating thickness>300 microns, preferably used when the outer diameter is greater than 500mm.
J20AB-breakdown voltage 1000VDC, dry and wet working environment, coating thickness 100 microns.
J20AA (AB)-breakdown voltage 3000VDC, dry and wet working environment, coating thickness 200 microns.
J20C-breakdown voltage 3000VDC, dry and humid working environment, coating thickness 200 microns.
Note: The models shown in the size table are standard insulation designs. When the default suffix does not meet the customer’s operating conditions, please clearly indicate the selected insulation bearing suffix when placing an order. For applications requiring higher pressure, Aubearing can also provide bearings with an outer ring with an insulation layer thickness of than 300 mm.
Bearing Model number
1. Single row deep groove ball bearing
6208-J20AA (AB)—6236-J20AA (AB)
6308-J20AA (AB)—6336-J20AA (AB)
2. Single row cylindrical roller bearing
NU208-J20AA (AB)—NU236-J20AA (AB)
NJ208-J20AA (AB)—NJ236-J20AA (AB)
NU2208-J20AA (AB)—NU2236-J20AA (AB)
NU308-J20AA (AB)—NU33 6-J20AA（AB）
NJ308-J20AA（AB）—NJ336-J20AA（AB）
NU2308-J20AA（AB）—NU2336-J20AA（AB）
NU1008-J20AA（AB）—NU1036-J20AA（AB）
Application of insulated bearings in motors
Silent electrically insulated bearings are widely used, mainly covering traction motors and wheels of rail vehicles, AC and DC motors of power transmission systems, wind power generation and related equipment, gearboxes, reducers, engineering machinery and other applications that require insulation requirements for bearings. It is particularly noteworthy that 80% of silent insulated bearings are used in motors.
During the operation of the motor, any imbalance in the stator and rotor magnetic circuits or the phase current around the shaft will generate magnetic flux of the rotating system. When the shaft rotates, these magnetic fluxes will generate a potential difference at both ends of the shaft, which is called shaft voltage. The shaft voltage can excite the circulating current in the loop (closed circuit) formed by the shaft and the housing through the bearings at both ends. This current is called shaft current. In addition, there is a large amount of residual magnetism in the rotor core. For wound rotor motors, if two or windings are short-circuited with the rotor core or the shaft, shaft voltage and shaft current will also be generated. The magnitude of the bearing current is related to factors such as the motor structure, motor power, drive voltage amplitude, pulse rise time, and cable length. The greater the motor power, the higher the drive voltage, the steeper the drive voltage rise edge, the shorter the cable, and the greater the carrying current.
To avoid the shaft current from burning the bearing, effective measures should be taken to isolate the shaft current. For large motors with independent bearing seats at both ends, an isolation gasket made of insulating material can be placed between the bearing seat and the metal base. For ordinary motors with bearings and housings as one, an insulating bearing is generally used at one end (often arranged at the non-main shaft extension end). For occasions with high requirements, insulating bearings are installed at both ends.
Selection suggestions
The key factor in selecting insulation is the characteristics of current and voltage. If it is a DC voltage or a low-frequency AC voltage, the insulation effect depends on the pure resistance value of the insulation layer; if it is a high-frequency AC voltage, such as in equipment using a frequency converter, at this time, it depends on the capacitive reactance value of the insulation layer.