Increasing the Reliability of Wind Turbine Gearbox

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The gearbox's reputation for a high failure rate is linked to the extreme engineering challenge that gearbox technology faces in wind applications, and the difficulty in properly assessing the loads – and in particular the non-torsional loads that pass through the gearbox – and how these affect bearings and gears. Some manufacturers have chosen to move to direct drive to reduce the number of moving parts in the wind turbine more exposed to wear. But this has led to wind turbine specific generator designs that are usually more expensive and often come together with a long-term maintenance contract with the Original Equipment Manufacturer (OEM), which does not necessarily meet the operations and maintenance (O&M) concept of flexibility expected by customers.

Of course, much has been done in the last decade to design and manufacture gearboxes ensuring a high quality, often with associated with over-engineering and increased cost. Many efforts are also being put in to performing proper monitoring and maintenance to detect and prevent any avoidable damage. These efforts have limited gearbox breakdowns in infancy, and sometimes allowed some maintenance activities to be initiated earlier than before, but they have not helped resolving a key cause of the problem: the rotor support concept, and how it distributes loads among the wind turbine structure and the gearbox.

Failure in Conventional Rotor Support Concepts

Traditional rotor support concepts typically feature either one or two bearings as shown below.

Gearbox connection

In a one bearing configuration, shown in the left hand image, the rotor shaft is supported by one main bearing and by the proper gearbox that is attached by two torque arms to the bedplate. Generally, the single main bearing does not absorb bending moments which result from the blades acting on the rotor shaft and, as a consequence, the planet carrier bearings transmit loads to the gearbox housing that are absorbed by the torque arms. Using this design principle, a gearbox absorbs additional loads introduced by the rotor shaft bending moment and also, to a lesser extent, those due to deflections of the bedplate and main bearing.

The single bearing concept is basically a three point suspension for the hub, one point is the front bearing and the other two are the gearbox torque arm supports. All forces produced by the wind on the rotor are going through the gearbox to the structure, and therefore the gearbox itself becomes part of this structure. Conventional gearbox design techniques used in other industries have simply been proven insufficient to deliver designs that can bear such highly variable loads in all directions over 20 years of operation.

In the two-bearing configuration B, shown above right, the rotor shaft is supported by two main bearings. With this arrangement the residual bending loads transmitted by the rotor shaft to the gearbox depend essentially on the stiffness of the double main bearing configuration and on bedplate stiffness.

Multistage gearbox

Except in a few cases of major turbine concept issues or gearbox defects, modern wind turbines gearboxes usually do not fail in the first few years of operation. Turbines in the 1.5–3 MW class have been built on the experience of smaller machines where gearbox failure was a chronic issue, and wind turbine and gearbox designs have been improved, allowing gearboxes to work properly in the first years of operation. However, inspections after 3–5 years performed on gearboxes of these large wind turbines usually show that major gearbox overhauls or replacements will be required in the next few years.

With one or two gearbox replacements expected over the 20-year lifetime of the turbine, even more in very windy sites, many customers are required by their lenders to include risk provision for extra material breakdown in the gearbox in their project business plan. 

Improving Reliability with Novel Support Concept

As previously mentioned, the main problem of conventional rotor support structures is that the gearbox is performing structural and mechanical functions at the same time, which makes it challenging to simulate loads properly at the design stage. This is especially critical in a component as complex as a gearbox, which is basically designed to withstand mechanical loads. This challenge is illustrated by the recent debate in the US about whether gearbox failures are due to the gearbox ability to withstand the specified loads, or to the fact that real loads experienced by the gearbox are higher than those specified by the wind turbine manufacturers.

An efficient way to solve this problem is to use a rotor support concept that separates structural behaviour from mechanical behaviour  This allows designers to simplify the way the loads are transmitted in the drive train, and therefore specify the drive train components with figures that are much closer to the real loads.

Wind turbine parts

As shown in above, the rotor, supported directly by a cast frame on two main bearings, is not supported by the gearbox, which is fully separated from the supporting structure. The two bearings divert weight and other loads to the main frame.

The key feature of this arrangement is that torque transmission is performed independently of rotor support. The shaft and gearbox are thus protected from potentially damaging bending loads. The concept decouples bedplate deflexion from the main shaft by means of a front elastic coupling that allows a certain degree of misalignment required in the system. The gearbox is allowed to pivot freely when the bedplate deflects. This ensures that only pure torque is going into the gearbox, allowing higher gearbox reliability without overdesign of the gearbox or unnecessary preventive maintenance costs.

Deflection loads

The figure above: Deflection loads (red arrows) are transmitted directly to the tower whereas only torque (dark green arrows) is transmitted through the shaft to the gearbox

A cast frame goes entirely through the hub to support it and drive all deflection loads (red arrows) to the tower, as it shown above. The shaft, connected to the hub at the front of the turbine, transmits pure torque to the gearbox.

Technical Validation

The technical benefits of this rotor support concept have been exhaustively validated in the field by measuring strains and displacements at several points in the structure and drive train. This experimental information has been used to complete and correlate the global virtual design models based on Finite Element Method-ANSYS and Multibody-SAMCEF design tools.

The figure below shows the most relevant results of this technical analysis in which, in addition to the Alstom Pure Torque validation, the global behaviour of the entire rotor support and the drive train is compared with a standard rotor support concept when a bending load is applied to the hub-rotor. For this comparison a standard rotor support concept with two main bearings has been used. When considering the standard rotor support concept, results clearly indicate the development of strain/stress all along the drive train, mainly in the bearing-shaft contact corners, but also affecting internal parts of the gearbox. In comparison, using same nominal bending moment and colour scale, the Alstom Pure Torque concept distributes strain/stress in the structural parts, isolating the drive train from bending moments.

Ansys result

The above figure: FEM comparison between a standard two bearing rotor support concept (top) and Alstom Pure Torque rotor support and drive train (bottom) when a bending load is applied to the hub-rotor.

Equivalent results have been obtained using multibody numerical analysis.

Results indicate a clear reduction of the radial bearing load for any relevant number of cycles when Alstom Pure Torque is considered compared with the standard configuration.