# The Influence of Lubricant Conductivity on Bearing Currents in the Case of Rolling Bearing Greases

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## Abstract

**:**

## 1. Introduction

^{th}German Tribology Conference (GfT), and extending that—the investigation of rolling bearings subjected to test voltages is presented, where the focus is on the discharge currents, also called “electric discharge machining” currents (EDM currents), and their relation to the electrical properties of the lubricants (especially to the conductivity). For this purpose, lubricants with different rheological and electrical properties were experimentally tested in order to understand their influence on the bearing currents. In addition, this paper presents an evaluating scheme for the bearing currents and two methods for the investigation of the relationship between the lubricating film and the occurrence of the EDM-breakdowns.

## 2. Basics

#### 2.1. Lubricant as Electrical Isolator

#### 2.2. Electric Discharge Machining (EDM) Breakdowns and EDM Currents

## 3. Investigated Greases

## 4. Test Bench and the Measurement Process

## 5. Results

#### 5.1. Results of the Preliminary Investigations

#### 5.2. Results of Phase 1

^{−1}, a load of 200 N and at the steady-state temperature of the bearings.

^{−1}rotational speed, 200 N axial load). The capacitive behaviour of the lubricant film can be measured also with isolating greases, and this film may be broken through due to electric charges.

#### 5.3. Results of Phase 2

- Capacitive, EDM and resistive currents occur with all of the investigated greases.
- With conductive grease, ohmic currents already occur at a lower applied voltage and the EDM-domain can be shortened.

#### 5.4. Results of Phase 3 and Phase 4

- In the case of low applied voltage amplitude (5 V), the influence of conductive lubricants is low. The mean and maximum EDM voltage of conductive lubricants is lower compared to insulating lubricants, but the differences are not significant.
- When a high test voltage (20 V) is applied, the capacitive currents are prevented in the temperature domain investigated using conductive lubricants. Predominantly ohmic currents flow.

## 6. Summary and Outlook

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Relationship between the operating condition, the corresponding lubrication condition, and its electrical properties in rolling bearing.

**Figure 3.**(

**a**) Evaluating scheme of the bearing voltages and currents; (

**b**) investigation method 1 of the bearing currents; (

**c**) investigation method 2 of the bearing currents.

**Figure 6.**Measured specific conductivity of the test greases at switching frequency of 20 Hz, temperature from 30 to 100 °C and at a pressure of 1 bar in a cylinder capacitor.

**Figure 7.**Measurement results of (

**a**) the bearing impedance magnitude, and (

**b**) the impedance phase angle after 16 h preconditioning at steady-state temperature, a rotational speed of 1000 min

^{−1}, and a load of 200 N.

**Figure 8.**Measurement results of phase 2 with voltage variation (1–60 V). Determination of the capacitive, electric discharge machining (EDM) and resistive current ranges at steady-state temperature, a rotational speed of 1000 min

^{−1}, an axial load of 200 N and an inverter switching frequency of 10 kHz (* conductive greases).

**Figure 9.**The average EDM breakdown number per second as a function of bearing temperature at a speed of 1000 min

^{−1}, a load of 200 N and an inverter switching frequency of 10 kHz from phase 3 and 4 of the GESA investigations; (

**a**) measurement results at an applied voltage of 5 V; (

**b**) measurement results at an applied voltage of 20 V.

**Figure 10.**The maximum and mean EDM breakdown voltage occurring at an applied test voltage amplitude for all investigated lubricants; (

**a**) results at an applied test voltage of 5 V; (

**b**) results at an applied test voltage of 20 V (* conductive greases).

Test Grease | Temperature Range (°C) | Base Oil Viscosity at 40 °C (mm²/s) | Base Oil Viscosity at 100 °C (mm²/s) | Type of Base Oil | Type of Thickener | Electric Conductivity |
---|---|---|---|---|---|---|

F1 | −40 to 200 | 130 | 20 | Perfluorpolyether, Ester oil | Polytetra-fluorethylen, Polyurea | Not specified |

F2 | −30 to 160 | 165 | 18 | Mineral oil, Synth. Hydrocarbon | Polyurea | Not specified |

F3 | −45 to 180 | 72 | 9.5 | Ester oil | Polyurea | Not specified |

F4 | −50 to 260 | 190 | 34 | Perfluorpolyether | Polytetra-fluorethylen, Polyurea | Not specified |

F5 | −40 to 180 | 90 | 9 | Polyalphaolefin/Ester oil | Polyurea | Yes |

F6 | −35 to 140 | 82 | 12.5 | Mineral oil, Synth. Hydrocarbon | Lithium | Yes |

Phases | Rotational Speed (min^{−1}) | Axial Load (N) | Applied Voltage (V) | Switching Frequency (kHz) | Bearing Temp. (°C) | Bearing Type |
---|---|---|---|---|---|---|

Phase 1 | 1000 | 240 | - | - | Steady-state | 51208 |

Phase 2 | 200 | 1–60 | 10 | Steady-state | ||

Phase 3 | 200 | 5 | 10 | 10–100 | ||

Phase 4 | 200 | 20 | 10 | 10–100 |

Test Greases | F1 | F2 | F3 | F4 | F5 * | F6 * |
---|---|---|---|---|---|---|

Steady-state temperature (°C) | 40 | 42 | 41 | 55 | 40 | 38 |

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Gonda, A.; Capan, R.; Bechev, D.; Sauer, B. The Influence of Lubricant Conductivity on Bearing Currents in the Case of Rolling Bearing Greases. *Lubricants* **2019**, *7*, 108.
https://doi.org/10.3390/lubricants7120108

**AMA Style**

Gonda A, Capan R, Bechev D, Sauer B. The Influence of Lubricant Conductivity on Bearing Currents in the Case of Rolling Bearing Greases. *Lubricants*. 2019; 7(12):108.
https://doi.org/10.3390/lubricants7120108

**Chicago/Turabian Style**

Gonda, Attila, Resat Capan, Dani Bechev, and Bernd Sauer. 2019. "The Influence of Lubricant Conductivity on Bearing Currents in the Case of Rolling Bearing Greases" *Lubricants* 7, no. 12: 108.
https://doi.org/10.3390/lubricants7120108