# Failure Analysis of Wind Turbine Planetary Gear

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## Abstract

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## 1. Introduction

## 2. Development Trends in Wind Turbine Construction Elements

- Reduction of the turbine structure/weight;
- New drivetrain to eliminate or reduce the size of gearboxes;
- Turbine load analysis and mitigation, considering the dynamic coupling between translational (surge, sway, and heave), rotational (roll, pitch, and yaw) loading and turbine motions, as well as the dynamic characteristics of mooring lines for floating systems;
- Turbine and rotor designs to increase the efficiency and reliability and reduce weight;
- New generators and power electronics to increase the efficiency and reliability;
- Improving wind farm performance, considering interactions between wind turbines and also wind farms (e.g., improved wake models);
- Advanced maintenance strategies, remote monitoring, diagnostics, prognosis and health-monitoring systems, for improving reliability, reducing turbine down time, and operation and maintenance costs;
- Economic modelling and optimization of the overall wind farm system.

## 3. Operational Problems of Selected Wind Turbine Modules (Mechanisms Causing Damage to Toothed Wheels in Gearbox)

#### 3.1. The Forces Acting on the Gear Wheels of the Planetary Gear

_{main}and F

_{PLC-A}are non-zero. To determine whether these moments contribute to bearing failure, it would be necessary to determine whether and by how much these moments increase the bearing loads. However, the authors assumed that for specific gearboxes installed in wind turbines, for which the failure-free life should be between 20 (for onshore turbines) and 30 years (for offshore turbines), there is no need to consider limit states for the loads involved. Therefore, in order to analyze the damage, the classical model of contact stress distribution was adopted.

#### 3.2. The Distribution of Forces Acting in the Mating Gear Wheels System

_{max}as σ

_{H}, we obtain the formula for calculating stresses on the surface of the contacting teeth (including the coefficient of oil viscosity, surface roughness and the peripheral speed)

_{E}—material coefficient accounting for the properties of the mating wheels materials (where: v—Poisson number, E—Young modulus) expressed by the formula

_{B}and Z

_{D}for α ≤ 2-coefficients of one-pair pressure point of a tooth allow converting contact stresses at the pitch point to the maximum stress in the internal point B of the one-pair teeth of the pinion or internal point D of the one-pair pressure of the wheel tooth. If Z

_{B}> 1 or Z

_{D}>1, it is recommended that the coefficient be determined for the wheels if u < 1.5. If > 1.5, then generally M

_{2}< 1.0 (2.5) and Z

_{D}= 1.

_{B}and Z

_{D}are calculated from the formulae:

_{α}-transverse contact ratio

_{B}= 1, if M

_{1}≤ 1 Z

_{D}= 1, if M

_{2}≤ 1

_{B}= M

_{1}, if M

_{1}> 1 Z

_{D}= M

_{2}, if M

_{2}> 1

_{H}—the coefficient of the contact zone depends on the geometry of the surface of the contacting teeth. It takes into account the impact of the gear tooth sides curvature at the pitch point on contact stresses and conversion of the peripheral force on the tooth reference cylinder to the force normal to the tooth surface on the pitch cylinder and equals

#### 3.3. Fatigue Wear Mechanism in Gear Wheels

- Creation of microgaps due to fatigue of the material and propagation of cracks;
- Splitting microgaps by pressed-in lubricant during the rolling contact of teeth (growth and propagation of cracks is due to the oil wedge effect)—Figure 8;
- Breaking off material particles from the surface layer (oil breaks off particles of metal which weakened or lost cohesion with the native material)—Figure 9.

## 4. Conclusions

#### Highlights

- Observations of actual gear damage allow us to conclude that there is no single rule determining the probability of gear wear rate;
- The authors indicate that it is necessary to identify the sources and analyze the gases emitted in the gear wheels region;
- Oxygen depolarization may occur on the surface of mating gears in a wind turbine, which is the cause of corrosion “foci”;
- If we assume that the first symptoms of gear wear appear under the influence of environmental and corrosive factors, and that they reveal themselves particularly early in the presence of current flow (corrosion cell), it is likely that the time of their occurrence depends on the frequency and value of stray currents in the gearbox;
- The occurrence of stray currents does not directly lead to electrical failures, but can have a significant impact on accelerating the wear process of wind turbine gearboxes—they are the cause of fatigue corrosion.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

PLC-A and PLC-B | the radial forces of the main bearing |

PL-A and PL-B | the radial forces of the planetary bearings |

σ | stress |

ρ | radiuses of the curvature |

α_{w} | angle of the contour |

ε_{α} | transverse contact ratio |

F_{n} | peripheral force |

Z_{E} | material coefficient accounting for the properties of the mating wheels materials |

v | Poisson number |

E | Young modulus |

Z_{B} and Z_{D} | coefficients of one-pair tooth point pressure at the inner point B of the one-pair pinion tooth pressure or the inner point D of the one-pair wheel tooth pressure, respectively |

Z_{H} | the coefficient of the contact zone |

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**Figure 1.**View of the main wind turbine modules (with permission of ifm.com) [15].

**Figure 2.**The planetary gear of the wind turbine (1—bearings of the high-speed section; 2—medium speed stage bearings; 3—planetary gear bearings; 4—gearbox teeth) (with permission of Olympus) [19].

**Figure 4.**The distribution of contact stresses on the surface of the mating wheels (C—pressure point, σ—stress, ρ—radiuses of the curvature, α

_{w}—angle of the contour, F

_{n}—peripheral force).

**Figure 6.**The effect of planetary gear teeth wear in wind turbines in the form of spots caused by gases released inside the nacelle.

**Figure 8.**Spalling wear of gear wheels of the wind turbine planetary gear due to the cyclical impact of contact stresses.

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**MDPI and ACS Style**

Bejger, A.; Frank, E.; Bartoszko, P. Failure Analysis of Wind Turbine Planetary Gear. *Energies* **2021**, *14*, 6768.
https://doi.org/10.3390/en14206768

**AMA Style**

Bejger A, Frank E, Bartoszko P. Failure Analysis of Wind Turbine Planetary Gear. *Energies*. 2021; 14(20):6768.
https://doi.org/10.3390/en14206768

**Chicago/Turabian Style**

Bejger, Artur, Ewelina Frank, and Przemysław Bartoszko. 2021. "Failure Analysis of Wind Turbine Planetary Gear" *Energies* 14, no. 20: 6768.
https://doi.org/10.3390/en14206768