ABSTRACT：Non-torque loads induced by the wind turbine rotor overhang weight and aerodynamic forces can greatly affect drivetrain loads and responses. If not addressed properly, these loads can result in a decrease in gearbox component life. This work uses analytical modeling, computational modeling and experimental approaches to evaluate two distinct drivetrain designs that minimize the effects of non-torque loads on gearbox reliability: a modified three-point suspension drivetrain studied by the National Renewable Energy Laboratory (NREL) Gearbox Reliability Collaborative (GRC) and the Pure Torque® drive- train developed by Alstom. In the original GRC drivetrain, the unequal planetary load distribution and sharing were present and they can lead to gear tooth pitting and reduce the lives of the planet bearings. The NREL GRC team modified the orig- inal design of its drivetrain by changing the rolling element bearings in the planetary gear stage. In this modified design, gearbox bearings in the planetary gear stage are anticipated to transmit non-torque loads directly to the gearbox housing rather than the gears. Alstom’s Pure Torque drivetrain has a hub support configuration that transmits non-torque loads di- rectly into the tower rather than through the gearbox as in other design approaches. An analytical model of Alstom’s Pure Torque drivetrain provides insight into the relationships among turbine component weights, aerodynamic forces and the resulting drivetrain loads. In Alstom’s Pure Torque drivetrain, main shaft bending loads are orders of magnitude lower than the rated torque and hardly affected by wind speed, gusts or turbine operations. Copyright © 2014 John Wiley & Sons, Ltd.

KEYWORDS wind turbine; drivetrain; Pure Torque; three-point suspension; non-torque   loads

1. INTRODUCTION

On average, gearboxes in wind turbine drivetrains have not been achieving their expected design life.1,3 Premature gearbox failures have a significant impact on the cost of wind farm operations.2 A large number of damaged gearboxes require re- pairs or complete overhauls during the service.2

Geared drivetrains, the most prevalent design for land-based wind turbines, consist of a main shaft, main bearing(s), gearbox, generator coupling and generator. Different rotor supports and bearing configurations are used across various manufacturers, which can be grouped into four categories: (i) three-point suspension (Figure 1), (ii) two-main-bearing sus- pension (Figure 2(a)), (iii) integrated drivetrain (Figure 2(b)) and (iv) hub support drivetrain with a flexible coupling, such as Alstom’s Pure Torque drivetrain (Figure 3). In the three-point suspension, the rear main bearing of the two main shaft bearings is integrated into the gearbox at the planetary stage. The two-main-bearing suspension uses two separate main bearings that ideally carry all the non-torque loads from the rotor and transmit them into the tower through the bedplate. Because the four-point design is an overconstrained system, it might be sensitive to drivetrain structural deflections. The integrated drivetrain has the main bearings integrated into the gearbox. The non-torque loads are transmitted through the gearbox housing. The Pure Torque drivetrain uses a set of flexible couplings to connect the rotor with the main shaft (also called torque shaft). The use of the flexible couplings, such as those applied in the Pure Torque drivetrain, eliminates most

non-torque loads from the main shaft and gearbox. Among all these drivetrain configurations, the three-point suspension drivetrain—historically, the most widely used configuration—is highly sensitive to non-torque loads.

Nearly all horizontal-axis wind turbine drivetrains carry various combinations of torque and non-torque loads. Non- torque loads consist of primarily the turbine rotor overhang weight and aerodynamic loads. The three-point suspension drivetrain studied by the National Renewable Energy Laboratory (NREL) Gearbox Reliability Collaborative (GRC) trans- mits significant main-shaft bending loads into the gearbox.4,5 The bending moments on the main shaft measured in the field tests were up to 64% of rated torque, and similar loads were replicated in the  dynamometer.5