Stress tes ng, par cularly at the tooth root of wind turbine gearbox gears, is crucial for ensuring

their reliability and

lifespan. This involves evalua ng the stresses induced by the opera onal loads and assessing their impact on fa gue
resistance. Specific techniques include measuring residual stresses, which can significantly affect fa gue life, and using
strain gauges to monitor load intensity distribu on. Finite element analysis and experimental methods, like tooth root
bending fa gue tests (TRBF), are also employed to simulate and analyze stress condi ons.

Here's a more detailed breakdown:

1. Residual Stress Measurement:

 Importance:

Residual stress, especially compressive stress at the tooth root, can significantly improve fa gue life by counterac ng the
tensile stresses induced by the load.

 Methods:

 Contour Method: Measures deforma on changes a er cu ng a sec on of the gear tooth, indica ng
residual stress.

 X-ray Diffrac on (XRD): Analyzes the diffrac on pa erns of X-rays to determine strain and stress levels.

2. Load Intensity Distribu on Measurement:

 Importance:

Understanding how load is distributed across the tooth flank is crucial for iden fying poten al stress concentra ons and
areas of high loading.

 Method:

Strain gauges are installed at the tooth root to measure strain distribu on and then processed to determine load
intensity distribu on.

3. Stress Simula on and Analysis:

 Finite Element Analysis (FEA):

Used to model gear geometry, material proper es, and loading condi ons to predict stress distribu on and iden fy
areas of high stress concentra on.

 Tooth Root Bending Fa gue Tests (TRBF):

These tests simulate the fa gue loading condi ons on a single gear tooth to determine its bending strength.

4. Factors Influencing Stress:

 Tooth Geometry:

Pressure angle, profile shi , and addendum/dedendum length can significantly impact tooth root stress, as discussed in a
ResearchGate study.

 Material Proper es:

Hardness, case depth, and residual stress profiles all play a role in determining a gear's fa gue performance, as explained
in a ResearchGate study.

 External Load:
Higher external loads increase the risk of fa gue failure, as shown in another ResearchGate study.

 Gear Misalignment:

Misalignment can lead to uneven load distribu on and concentrated stresses, increasing the risk of tooth failure.

5. Types of Fa gue Failure:

 Tooth Interior Fa gue Fracture (TIFF): Cracks ini a ng within the tooth core, o en at the junc on of the
hardened layer and core.

 Tooth Flank Fracture (TFF): Fractures occurring on the tooth flank, o en due to contact fa gue.

By using these stress tes ng techniques and considering the various factors that influence stress, engineers can design
and select gears that are more reliable and less prone to fa gue failure in wind turbine gearboxes.

Stress tes ng of wind turbine gearbox tooth roots using strain gauges involves placing strain gauges on the tooth root to
measure the strain induced by the mesh force. These measurements help determine the mesh load factor, which is
crucial for gear ra ng and sizing. The strain gauge data, when processed, provides insights into load distribu on and can
be used to validate finite element simula ons.

Here's a more detailed explana on:

1. Purpose of Stress Tes ng:

 Wind turbine gearboxes experience high stresses, par cularly in the tooth roots, due to the constant mesh forces
between gears.

 Stress tes ng helps engineers understand how loads are distributed within the gearbox and iden fy poten al
areas of weakness or stress concentra on.

 This informa on is essen al for op mizing gearbox design, improving reliability, and ensuring long-term
performance.

2. Strain Gauge Placement:

 Tooth Root:

Strain gauges are typically placed on the tooth root, where the highest stresses are expected.

 Specific Loca ons:

The loca on of strain gauges is carefully chosen to capture the maximum strain and avoid areas with large stress
gradients, according to a study on experimental evalua on.

 Mul ple Gauges:

Mul ple strain gauges are o en used to capture the strain distribu on along the tooth root and obtain an average value
of load sharing.

3. Data Acquisi on and Analysis:

 Strain Measurement:

The strain gauges measure the deforma on (strain) of the tooth root under load.

 Load Intensity Distribu on:

The measured strain data is processed to determine the load intensity distribu on on the tooth flank.

 Mesh Load Factor:

The mesh load factor (Kγ) is calculated from the strain measurements and provides a measure of how loads are shared
between planet gears.

4. Benefits of Using Strain Gauges:

 Accurate Load Sharing:

Strain gauges provide a direct and accurate measure of load sharing between gears, which is crucial for gearbox design.

 Valida on of Simula ons:

Strain gauge measurements can be used to validate finite element simula ons (FE) and ensure their accuracy.

 Early Failure Detec on:

Monitoring strain changes in real- me can help detect early signs of damage or wear, allowing for proac ve maintenance
Experimental Evalua on of Wind Turbine Gearbox Structural Models Using Fiber Op c Strain Sensors

Reducing the cost of energy (CoE) has become one of the main research drivers in Wind Energy (Ref. 1). As a result, wind
turbines have experienced a significant increase in rotor diameter. This can be understood considering the equa on for
the generated power (P):

(1)

where

ρ is the air density;

A is the area swept by the rotor;
v is the wind speed;
Cp is the power coefficient;

Power is propor onal to the swept area and grows with the square of the rotor diameter.

Geared drivetrains dominate land-based or onshore wind energy. It is es mated that 75 percent of turbines have a
gearbox (Ref. 1). The leading OEMs, such as Vestas, Siemens Gamesa Renewable Energy (SGRE), General Electric (GE),
and Acciona-Nordex all use geared drivetrains in their onshore turbines. The latest SGRE onshore pla orm 5.X has a
power ra ng of 6.x MW and rotor diameters of 155 m and 170 m. In offshore sites, the share of direct drive turbines is
more significant, but Vestas and Ming Yang also use geared drivetrains and target rotor diameters as large as 236 m with
expected rated powers of 15 MW. The design requirements for wind turbine gearboxes are given by standards IEC 61400-
4 (Ref. 2) and AGMA 6006 (Ref. 3). Torque is the main sizing factor in gearboxes. Assuming a limita on to maintain the p
speed of the blade’s constant, the rota onal speed decreases linearly with the rotor diameter. Therefore, input rotor
torque (T) grows with the cubic exponen al of rotor diameter.

The significant increase in rotor diameters has pushed gearbox manufacturers to introduce mul ple technological
innova ons to boost the torque density of current designs. Torque densi es of 200 Nmkg−1 are now available thanks to,
for example, new gearbox architectures with more planetary stages and planets per stage, new materials, improved
manufacturing tolerances, and addi onal surface finishing techniques. To achieve compact and lightweight drivetrains, a
trend has emerged toward increasing the mechanical integra on of the main bearing, gearbox, and generator (Ref. 4).
Overall, these light designs increase stress on gearbox components, and accurate structural models are needed to
maintain or even increase gearbox reliability. Structural models based on the finite element method (FEM) with a high
level of complexity to capture all interac ons between gearbox components are widely used for this purpose. These
models must be validated through experimental evalua on to achieve the desired degree of confidence. Once validated,
FEM structural models provide a suitable pla orm to op mize gearbox components to increase torque density further.

The main objec ve of this study is to perform an experimental evalua on of the structural model of a five-planet first
planetary stage from a modern 6MW wind turbine gearbox. The FEM structural model comprises the rotor side housing,
also known as the torque arm housing, the first stage ring gear and the transi on housing between the first and second
ring gear. Strain measurements on the outer surface of the ring gear obtained in a full load back-to-back test bench have
been used to validate the structural model. Op cal fiber strain sensors have been used because they offer a higher
signal-to-noise ra o, are immune to electromagne c interference, and allow a more straigh orward installa on because
mul ple strain sensors can be accommodated in a single fiber.

This study has been conducted using a Siemens Gamesa Renewable Energy (SGRE) gearbox manufactured by Gamesa
Energy Transmission (GET) shown in Figure 1. The gearbox is a 3-stage gearbox; the first and second are epicyclic
planetary stages, and the third is a parallel stage. A drawing of the sha s and gears in this gearbox is shown in Figure 2.
The structural housings of the gearbox have been omi ed for clarity. The first planetary stage has five planets, and the
second stage has three. The rated power of the gearbox is 6.x MW and weighs approximately 44,000 kg. The first stage of
this gearbox has been chosen for this study because it is the most cri cal one due to the high torque that it supports.