Current-Insulated Bearings Prevent Damage Caused by Electrical Current

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Current-Insulated Bearings Prevent Damage Caused by Electrical Current

Technical Product Information

Damage Caused by Electrical Current and Its Consequences · Current-Insulated Bearings as a Preventive Measure

Damage Caused by Electrical Current and Its Consequences

Rolling bearings used in:

• wheelsets and traction motors (rail vehicles)

• DC and AC motors (drivetrains)

• generators (wind power)

can be exposed to electrical current. In a worst-case scenario, this can damage raceways and rolling elements, which, in turn, causes the motor or generator to fail prematurely and without warning. On top of the extra expenses incurred for repairs, this also means additional costs caused by machine downtime and the resulting production losses.

A much more economical solution is to provide for the use of current–insulated bearings during the planning stage. This reduces maintenance and repair costs, and increases machine availability – all of which means greater value for the customer.

In most cases, it is sufficient to interrupt the electric circuit between the housing and shaft, in order to mount current-insulated bearings at one or both bearing locations, depending on the application.

Generally, current-insulated bearings (ceramic-coated or hybrid) exhibit significantly higher resistance to electrical current than standard bearings.

Current-Insulated Bearings as a Preventive Measure

As a rule, it is very difficult to eliminate the causes of bearing voltages that are induced by the motor. Nevertheless, it is possible to avoid damage to the bearing, if the flow of current can either be prevented or at least significantly reduced. Many types of current-insulated rolling bearings are available today for this very purpose. Which components need to be insulated depends on the type of voltage involved:

Induced Voltage along the Shaft

This produces a circular current, which is closed via bearing 1, the housing, and bearing 2. Such shaft voltages are often the result of an asymmetrical distribution of the magnetic flux within the motor. This is especially evident in motors with only a few pairs of poles. If this is the case, it is sufficient to disrupt the flow of current by insulating one of the two bearings.

Voltage between the Shaft and the Housing

In this case, electrical currents flow through each of the two bearings in the same direction. The most likely cause is the converters’ common-mode voltage. This type of a situation might require insulating both bearings. Which type of electrical insulation is to be used depends on the time response of the given voltages. With DC voltage and low-frequency AC voltage, the choice depends on

Current-Insulated Bearings as a Preventive Measure ·

Typical Damage Caused by Electrical Current Passing

through the Bearing

the ohmic resistance of the bearing. With higher-frequency AC voltage (often encountered in converters!), it depends on the capacitive reactance of the bearing. Basically, a current-insulated bearing acts like a resistor and capacitor connected in parallel. To ensure good insulation, the ohmic resistance should be as high as possible, and the capacitance should be as low as possible.

Typical Damage Caused by Electrical Current Passing through the Bearing

Regardless of whether a bearing was exposed to direct current or alternating current (up to frequencies in the MHz range), the resulting changes to the surface are always the same: Uniformly dull, gray marks in the raceways (Fig. 1) and on the rolling elements. These marks are not very specific and can also be caused by other factors (e. g. lubricating oil containing abrasives).

“Fluting” refers to the washboard patterns that develop along the raceway surface in the direction of rotation (Fig. 2). This type of damage usual indicates that electrical current has passed through the bearing.

An examination under a scanning electron microscope (SEM) reveals that the damage shown in figures 1 and 2 is characterized by densely-packed craters (caused by localized melting) and welding beads with micrometer-sized diameters covering the raceways. This definitively proves that electrical current has passed through the bearing.

These craters and welding beads are the result of electrical discharges between the microscopic peaks that are always found in raceways and on rolling-element surfaces. When a spark penetrates a fully-developed lubricating film at a bottleneck, it causes the adjacent surfaces to momentarily melt. In the mixed-friction range (metal-to-metal contact), the affected surfaces are temporarily fused together, then immediately broken apart again by the rotation of the bearing. In both cases, material also separates from the surfaces, where it immediately solidifies to form welding beads. Some of these beads get mixed in with the lubricant, the rest are deposited on the metal surfaces. Craters and welding beads can be flattened and smoothed as the rolling elements continue to pass over them. If there is a continuous flow of current, the (thin) surface layers, over time, repeat this melting and solidifying process over and over again.

Most bearing failures, however, are caused by fluting (Fig. 2). These washboard patterns in raceways and on rollers form as a result of the combined effects of a continuous flow of electrical current and the vibrational

Typical Damage Caused by Electrical Current Passing through the Bearing · Ceramic-Coated Bearings

properties of the bearing components. Each time the rolling element comes into contact with a sufficiently-large crater, it becomes radially displaced; the extent of the element’s displacement depends on the bearing’s internal geometry and speed, as well as on the loads acting on the bearing. As the rolling element swings back, the thickness of the lubricating film is eroded, resulting in more sparkovers in this area – a self-sustaining process has been triggered. After a while, the entire circumference of the ring’s raceway can become covered with fluting damage. This causes more pronounced bearing vibrations, finally leading to bearing failure.

Calculated current density – i.e., the effective amperage divided by the total area of contact between the rolling elements and the bearing’s inner ring and outer ring (which is dependent on the type of bearing and on the operating conditions) – has proven itself in the field as a reliable criterion for assessing the level of danger posed by electrical current. When current densities are less than approx. 0,1 Aeff/mm2, there is no risk of fluting, according to our present level of understanding. Densities at or above 1 Aeff/mm2, however, are likely to cause this type of damage.

Effect of Current on Lubricant

Electrical current also negatively affects the lubricant,

whose base oil and additives oxidize and develop cracks.

(This is clearly evident under the infrared spectrum.)

The lubricating properties are compromised by premature

aging as well as by an increased concentration of iron

particles, which can cause the bearing to overheat.

Ceramic-Coated Bearings

4: Ceramic-Coated Deep Groove Ball Bearings

Features and Benefits of Coated FAG Bearings

• Oxide ceramic coatings (J20...) provide a high level of insulation. Plasma spraying is used to apply these coatings to the bearing surfaces (Fig. 5).

• Thanks to a special sealant, the J20AA coating retains its insulating properties even in a damp environment. The resulting oxide ceramic coating is very hard, wear resistant, and a good thermal conductor.

• The external dimensions of the current-insulated rolling bearings are in accordance with DIN 616 (ISO 15). This means that they are interchangeable with standard bearings.

• For special applications, such as those with a rotating outer ring, we recommend using an inner ring coated with J20C.

• Starting with the 62-series and up, coated deep-groove ball bearings are available in both open and sealed versions (with lip seals on one or both sides). This enables the user to also benefit from the advantages offered by for-life lubrication.

The Coating Process · Electric Resistance

The Coating Process

The plasma spraying process involves generating an arc between two electrodes to ionize a noble gas that is issued from the plasma torch. The resulting plasma jet is used to carry the injected aluminum oxide powder. This powder is melted by the heat and sprayed at high speed onto the outer or inner ring. When applied in this manner, the oxide layer adheres extremely well to the base material. It is then sealed and ground to size.

Electric Resistance

The coatings are subjected to a 100% quality inspection and guarantee a dielectric strength of at least 1000 VDC (J20AA, J20C) or 500 VDC (J20B), respectively.

Below this voltage, the insulating layer permits only extremely low levels of current to flow through the bearing. It offers resistance to DC currents and to AC currents:

DC resistance

At room temperature, the layer typically has a DC resistance of 1–10 GOhm, depending on the bearing size. As the temperature increases, the DC resistance decreases exponentially, typically by approx. 40–50 % per 10 K. But even at operating temperatures of 60 °C or even 80 °C, the insulating layer still has a resistance of several MOhm. According to Ohm’s law (I = U/R), this means that voltages of up to 1000 V only produce currents that are significantly below 1 mA, which are not critical for bearings.

AC resistance

The insulated unit acts like a capacitor (C) which can accumulate charges. When exposed to an AC voltage, this causes an alternating current to flow through the contact area between the rolling element and raceway. In the case of a harmonic time dependence with angular frequency ., the rms values for current and voltage are calculated using the formula

I = U · . · C.

Analogous to Ohm’s law, Z = 1/.C is the capacitive reactance of the bearing. A bearing with an oxide ceramic coating typically has a capacitance of 2–20 nF, depend­ing on the bearing size. So, at a frequency of 50 Hz, it has a capacitive reactance of 0,15–1,5 MOhm, which is significantly lower than its DC resistance. At higher frequencies, this value decreases even further. Still, in most cases it will be signifi-cantly higher than the resistance of a non-insulated bearing, which, at voltages higher than 1 V, is very low (1 Ohm and less).

Types of Coatings · Range of Sizes

J20B / J20A / J20AA J20C

J20B J20A *) J20AA J20C

Disruptive Voltage 500 VDC 1000 VDC 1000 VDC 1000 VDC

Environment dry dry dry/damp dry/damp

Coating Thickness <100 µm >200 µm W200 µm W200 µm

Applicable 70…1400 mm 70…1400 mm 70…500 mm 70…340 mm

Dimensions outside diameter outside diameter outside diameter inner ring bore

*) Preferably used for bearings with an outside diameter of at least 500 mm.

The surfaces of the rings to be coated must be cylindrical; they must not be interrupted by lubricating holes or grooves.

Bearing Designs with a Ceramic Coating:

-2RSR -2Z

only with

J20C-coating

If desired, other bearing designs can be also be coated (upon consulting with the appropriate technical department).

Recommended FAG rolling bearings with a ceramic coating are listed on pages 6–8.

Ordering examples:

6220-2RSR-J20AA-C3 Deep groove ball bearing with a coated outer ring, with seals on both sides and

radial clearance C3.

NU214-E-M1-F1-J20B-C4 Cylindrical roller bearings with a coated outer ring and radial clearance C4.

6330-J20C Deep groove ball bearings with a coated inner ring

Ball Bearings with a Ceramic Coating

rr

D d

d1

D1

Designation Mass Dimensions Load Rating Fatigue Limiting Reference

Limit Load Speed Speed

dyn. stat.

m d D B r D1 D2 d1 Cr C0r Cur nG nB

min W W

FAG kg mm kN kN kN min–1 min–1

6212-M-J20B-C4 0,98 60 110 22 1,5 95,6 76,1 52 36 2,24 14 000 6 800

6213-J20B-C4 1 65 120 23 1,5 103,1 82,3 60 41,5 2,55 13 000 6 300 6313-M-J20AA-C5 2,55 65 140 33 2,1 117,5 88,6 93 60 3,95 11 000 6 400

6214-2RSR-J20AA-C3 1,11 70 125 24 1,5 110,7 86,8 62 44 2,9 12 000 6 100

6215-M-J2B-C4 6215-M-P6-J20AA-R85-105 6315-M-J20AA-C3 1,42 1,42 3,74 75 75 75 130 130 160 25 25 37 1,5 1,5 2,1 112,8 112,8 133,2 92,5 92,5 101,8 65,5 65,5 114 49 49 76,5 3,35 3,35 4,65 11 000 11 000 9 500 5 900 5 900 5 800

6016-M-J20AA 6216-J20AA-C3 6316-J20AA-C3 6316-M-J20B-C4 F-808916.6316-J20AA 0,997 1,46 3,75 4,44 3,69 80 80 80 80 80 125 140 170 170 170 22 26 39 39 39 1,1 2 2,1 2,1 2,1 111 121,3 141,8 141,8 141,8 94 98,8 108,6 108,6 108,6 47,5 72 122 122 122 40 54 86,5 86,5 86,5 2,34 3,45 5,2 5,2 5,2 12 000 11 000 9 000 9 000 9 000 6 500 5 500 5 500 5 500 5 500

6317-M-J20AA-C3 5,05 85 180 41 3 151,6 114,4 132 96,5 5,8 8 000 5 300

6218-J20AA-C3 2,21 90 160 30 2 139,4 112,3 96,5 72 4,2 9 000 5 100 6318-M-J20AA-C3 6,14 90 190 43 3 157,1 123,8 134 102 5,8 8 000 5 100

6319-M-J20AA-C4 7,05 95 200 45 3 166,9 129,1 146 114 6,4 7 500 4 950

6220-J20C-C3 3,3 100 180 34 2,1 154,8 124,7 122 93 5,4 8 000 4 800

6220-M-J20AA-R95-120 3,9 100 180 34 2,1 154,8 124,7 122 93 5,4 8 000 4 800

6320-M-J20AA-C3 8,64 100 215 47 3 179 138,6 163 134 7,4 7 000 4 650

16021-M-J20AA-C5 1,42 105 160 18 1 141,2 124,2 54 54 2,39 9 500 3 950

6322-M-J20AA-C3 11,7 110 240 50 3 197,4 153,4 190 166 8,6 6 300 4 150

6324-M-J20AA-C3 15 120 260 55 3 214,9 165,1 212 190 9 6 000 3 850

6326-M-J20AA-C3 18,3 130 280 58 4 231,2 178,9 228 216 9,8 5 600 3 500

6230-J20AA 10,3 150 270 45 3 229,1 191,6 176 170 7,8 5 600 3 350

6336-M-J20AA-C4 43 180 380 75 4 317 245,2 355 405 16,3 3 800 2 440

Cylindrical Roller Bearings with a Ceramic Coating

Designation Mass Dimensions Load Rating Fatigue Limiting Reference

Limit Load Speed Speed

dyn. stat.

m d D B r r1 s 1) F D1 d1 Cr C0r Cur nG nB

min min W W

FAG kg mm kN kN kN min–1 min–1

NJ312-E-M1-F1-J20B-C4 2,14 60 130 31 2,1 2,1 1,8 77 109,6 84,4 177 157 26,5 5 000 5 300

NU214-E-M1-F1-J20B-C4 1,29 70 125 24 1,5 1,5 1,6 83,5 109,4 140 137 19 5 300 4 750

NU314-E-M1-F1-J20B-C4 3,16 70 150 35 2,1 2,1 1,7 89 126,8 242 222 30 4 500 4 550

NU215-E-TVP2-J20AA-C3 1,27 75 130 25 1,5 1,5 1,2 88,5 114,4 154 156 21,7 5 300 4 500

NU215-E-M1-F1-J20B-C4 1,41 75 130 25 1,5 1,5 1,2 88,5 114,4 154 156 21,7 5 300 4 500

NU216-E-M1-F1-J20B-C4 1,71 80 140 26 2 2 1,3 95,3 122,9 165 167 22,6 4 800 4 250

NJ316-E-M1-F1-J20B-C4 4,48 80 170 39 2,1 2,1 0,7 101 143,9 110,4 300 275 46 3 800 4 150

NU218-E-TVP2-J20AA-C3 2,36 90 160 30 2 2 1,5 107 139,7 215 217 28,5 4 300 3 950

NUP218-E-TVP2-J20AA-C3 2,46 90 160 30 2 2 – 107 139,7 114,3 215 217 35 4 300 3 950

NJ219-E-TVP2-J20AA 2,94 95 170 32 2,1 2,1 – 112,5 148,6 120,5 260 265 41,5 3 800 3 700

NU219-E-M1-F1-J20B-C4 3,25 95 170 32 2,1 2,1 0,7 112,5 148,6 260 265 34 3 800 3 700

NU220-E-TVP2-J20AA-C3 3,49 100 180 34 2,1 2,1 1,5 119 156,9 295 305 38,5 3 800 3 500

NU320-E-M1-F1-J20AA-C4 8,77 100 215 47 3 3 1,2 127,5 182 450 425 53 3200 3400

NU224-E-TVP2-J20AA-C3 5,8 120 215 40 2,1 2,1 1,4 143,5 187,8 390 415 52 3 200 3 100

F-809035.NU228-E-J20AA 9,39 140 250 42 3 3 2 169 216,7 460 510 59 4800 2600

1) axial displacement

Tapered Roller Bearings with a Ceramic Coating

Designation Mass Dimensions Load Rating Fatigue Limiting

Limit Speed

dyn. stat. Load

m d D T/2B *) r1, 2 r3, 4 d1 Cr C0r Cur nG

min min W

FAG kg mm kN kN kN min–1

F-803477.TR1-J20B 2,64 89,945 146,975 40 3,6 1,5 119 232 355 50 4 800

F-804565.30220-A-J20B 3,7 100 180 37 3 2,5 135 231 290 32 4300

F-803478.TR1-J20B 9,4 117,475 212,725 63,5 8,1 3,3 162,2 490 720 93 3 000

F-803889.32224-A-J20B 9,15 120 215 61,5 3 2,5 175,2 445 650 84 3 000

F-809028.TR1-J20B 10,2 130 225 67 4 3 174,6 480 710 94 2 800

F-804550.01.TR2S-J20B 1) 6,8 140 190 99 2 1,5 150,1 365 780 74 3 400

Z-577634.01.TR2S-J20B 1) 13,5 140 210 130 2,5 2 175,8 585 1 180 75 2 800

Z-580065.30228-A-J20B 8,6 140 250 45,75 4 3 187 390 520 60 2 600

K36990-36920-J20B 3,18 177,8 227,012 30,162 1,5 1,5 203,8 186 400 30 2 600

F-809055.TR1-J20AA 2) 9,34 198,298 282,575 46,038 3,6 3,3 249 345 750 87 2 200

Z-566566.TR1-J20AA 9,28 199,949 282,575 46,038 3,6 3,3 249 345 750 87 2 200

F-807411.TR1-J20B 8,23 240 320 42 3 3 278 380 670 73 2000

F-809146.TR1-J20AA 18,6 240 336,55 65,088 6,4 3,3 284 640 1 250 137 1 800

F-808428.TR1-J20AA 17,2 240,5 336,55 65,088 6,4 3,3 284 640 1 250 137 1 800

F-808428.TR1-J20B 17,2 240,5 336,55 65,088 6,4 3,3 284 640 1 250 137 1 800

Z-547733.TR1-J20AA 22,3 254 358,775 71,438 3,3 1,5 302,8 720 1 370 148 1 700

Z-547733.02.TR1-J20AA 22,3 254 358,775 71,438 3,3 1,5 302,8 720 1 370 148 1 700

*)

Overall width of matched tapered roller bearings.

1)

Tapered roller-bearing matched in an O-arrangement (spacer between inner and outer rings).

2)

Flange at the outer ring.

Hybrid Bearings

As an alternative to coated rolling bearings, FAG offers hybrid bearings that

have ceramic rolling elements and rings made from rolling-bearing steel.

Hybrid bearings have the suffix HC.

The rolling elements are absolutely wear-free and provide the requisite

current insulation.

In addition to ball bearings (Fig. 6), we also offer hybrid versions of our

cylindrical roller bearings (Fig. 7).

Features and Benefits of Hybrid Bearings

• Greatest resistance to passage of current. Even at higher temperatures, DC-resistance is in the GOhm range. Hybrid bearings typically have a capacitance of about 40 pF, which is lower than for ceramic-coated bearings by a factor of 100.

• Higher speeds with less friction, which translates into lower operating temperatures

• Better dry-running properties

Hybrid bearings have a longer grease life than traditional “lubricated for life” bearings (see TI WL 43-1210).

For small rolling bearing sizes, hybrid designs are more cost-effective than ceramic-coated bearings.

Ordering examples:

HC6214-M-P6-C3 Deep groove ball bearing with ceramic balls; machined

brass cage; increased accuracy P6 and bearing clearance C3.

HCN1020-K-M1-SP Cylindrical roller bearing with ceramic rollers; tapered

bore; machined brass cage; increased accuracy SP.

Our field-service engineers will be happy to assist you in selecting the

most suitable and cost-effective designs for your applications.

Properties Unit Ceramic Steel (silicon nitride Si3N4) (100Cr6)

7: Cylindrical Roller Bearing with Ceramic Rolling Elements

Resistivity O · mm2/m 1017 10–1

Density g/cm3 3,2 7,8

Thermal Expansion

Coefficient 10–6/K 3,2 11,5

Modulus of Elasticity MPa 315 000 210 000

Poisson’s Ratio – 0,26 0,3

Hardness HV10 1 600…800 700…150

Hybrid Bearings - Table

Designation Mass Dimensions Load Rating Fatigue Limiting

Limit Load Speed

dyn. stat.

m d D B r D1 D2 d1 Cr C0r Cur nG

min W W W

FAG kg mm kN kN kN min–1

HC6002-2Z 0,031 15 32 9 0,3 28,4 20,5 4 150 2 000 102 30 000

HC6003 0,038 17 35 10 0,3 29,5 22,7 6 000 3 250 157 21 000

HC6212-C4 0,694 60 110 22 1,5 95,6 76,1 40 500 31 000 1 590 14 000

HC6014 0,614 70 110 20 1,1 9,3 82,8 29 000 25 500 1 850 10 000

HC6214-M 1,23 70 125 24 1,5 108 87,1 48 000 39 000 2 050 12 000

Designation Mass Dimensions Load Rating Fatigue Limiting

Limit Load Speed

dyn. stat.

m d D B r1 s 1) E Cr C0r Cur nG

min

FAG kg mm kN kN kN min–1

HCN1006-K-M1-SP 0,115 30 55 13 0,6 1,9 48,5 16 000 17 000 2 330 36 000

HCN1007-K-M1-SP 0,149 35 62 14 0,6 2 55 19 000 20 400 2 700 28 000

HCN1008-K-M1-SP 0,182 40 68 15 0,6 2,1 61 23 600 27 000 3 700 28 000

1) axial displacement

Mounting Examples

1. Three-phase motor

Deep groove ball bearing with a J20AA coating

Technical data: A current-insulated deep groove ball bearing 6316-J20AA-C3 is

Three-phase motor, converter-fed installed at the ventilation end, and a deep groove ball bearing

Power 375 kW 6320-C3 is installed at the drive end. Both bearings are lubricated

Design four-pole with grease. A relubrication device is provided.

2. Axle box roller bearing

Tapered roller bearings with a J20B coating

Idler mounting in Combino low-floor articulated tramcar, one-meter gauge / Freiburg (Germany)

Technical data: Tapered roller bearings (O arrangement):

Mounting Examples

3. Traction motor bearing mounting in an electric tramcar

Deep groove ball bearing and cylindrical roller bearing (both with J20AA coating)

500 kW three-phase motor

A deep groove ball bearing 6316-J20AA-C3 is installed at the ventilation end, and a

cylindrical roller bearing NU320-E-M1-F1-J20AA-C4 is installed at the drive end of the

rotor shaft.

Both bearings are lubricated with grease and protected from dirt and environmental

influences by labyrinth seals.

A relubrication facility was provided.

Traction motor

Ventilation end

Drive end

97/03/07 Printed in Germany by Druckhaus WEPPERT GmbH

Schaeffler KG

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