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Windtech in-depth: GE takes gearbox support to 2.2MW

WORLDWIDE: GE announced a new 2.2MW IEC class II turbine model with a 107-metre rotor diameter in May, with an unexpected drivetrain choice.

GE’s 2.X… The 2.X series turbines, now up to 3.2MW
GE’s 2.X… The 2.X series turbines, now up to 3.2MW

The 2.2-107 is an expansion of GE's "Brilliant" turbine platform, and the US manufacturer calls it an evolution of the 1.5MW series. Its drivetrain system, with a three-point gearbox support, is a proven design and economical, making it popular with many leading wind turbine suppliers.

Until this launch, GE had two distinct product platforms: the 1.X (up to 2MW) and 2.X (from 2MW). The 1.X portfolio originates from the former Tacke 1.5MW turbine of 1996, subsequently expanded to include the 1.6MW, 1.7MW and 1.85MW sister/successor models.

According to a GE spokesperson, the company's product development strategy is now focused on migrating to new 1-2MW and 2-3MW turbine class portfolios.

Rotor bearing

GE has already produced more than 20,000 1.X series turbines with non-integrated drivetrains featuring three-point gearbox support, while other manufacturers offer larger turbines with this type of drivetrain, to at least 3.4MW. GE's 2.5MW-plus turbines use a rather different solution with a bearing housing for two bearings and a main shaft incorporated in the cast main carrier, and a gearbox attached to the main shaft rear.

Conceptually, a three-point gearbox support incorporates a single rotor bearing connected to a cast or steel main chassis in front, and a main shaft rigidly attached to the gearbox input shaft. The gearbox has two torque side supports, which provide one rigid and two semi-rigid connections with the main chassis.

A single rotor bearing system has fewer components and is said to be cheaper to manufacturer than a two-bearing solution. However, improper design could cause harmful rotor bending moments entering into the gearbox, increasing the risk of premature damage and gearbox failure.

Another well-known disadvantage of three-point gearbox support is that when the gearbox exchange is necessary, the entire rotor might need to be removed, incurring considerable extra time and costs. Nowadays turbine manufacturers increasingly fit a main shaft-clamping device to alleviate this issue and enable a (cost) effective gearbox exchange. Specialised component suppliers also offer such devices as retrofit product for older turbines.

GE's initial 2.X (2.3-2.7MW) series, introduced in 2003, incorporated a new mechanical drivetrain solution, also included in the later 2.5-100/103, 2.75-120 and 3.2-103 models. This features a compact main shaft with a double-row tapered roller bearing facing the rotor and a cylindrical roller bearing located behind it.

Again, similar to the 2.X, gearbox exchange is enabled by disconnecting the complete unit and moving it back to be lifted out by crane, leaving the rotor and main shaft assembly intact and functional.

The main shaft assembly rotates in a cast chassis with precision-machined concentric bearing bores. The gearbox is attached to the main shaft rear side, which GE says the benefits are the transfer of "pure" rotor torque into the gearbox and a full absorbance of rotor-induced axial forces, as there is no free play in the unit.

The 2.5-100 features a favourable head mass (nacelle + rotor) of 122-124 tonnes, but mass figures for the latest successor models are not available.

Of all 2.X versions combined, around 1,300 units were operational by the end of 2013.

Two-bearing alternatives

The gearbox exchange issue was the main reason for Senvion (formerly Repower) developing a two-bearing solution for its 5MW 5M offshore turbine introduced in 2004.

The main shaft is supported by two rotor bearings and the gearbox with side supports attached to the main shaft rear end. To exchange the gearbox, the complete unit is disconnected and moved back to be lifted out by crane, leaving the rotor and main shaft assembly intact and functional. Another claimed but debated benefit of non-integrated geared drivetrains with a two-bearing solution is that rotor-bending moments are fully prevented from entering the gearbox.

Siemens applies a functionally comparable two-bearing solution in its 3.6MW (2004) and latest 4MW geared offshore turbines. In addition, Vestas uses the two-bearing solution up to 2MW, Gamesa now up to 2.5MW, and Acciona up to 3MW, and Senvion up to 6.2MW (6.2M152).

Besides GE's fully integrated main shaft plus bearing assembly used for the 2.5-3.2MW models, two separate bearing housing-units can also be individually bolted to a main chassis, or alternatively as bearings incorporated into a single cast housing.

Medium-speed

The single-cast housing offers structural benefits and nearly eliminates misalignment risks when all main components are joined with flange connections. It is increasing popular in the current medium-speed onshore/offshore range, including the Gamesa G128-5.0MW and the latest Samsung S11-7.0MW and Vestas V164-8.0MW giants.

So, while geared turbine suppliers can choose from a wide variety of main shaft and bearing support design solutions, each with merits and drawbacks, a single ultimate solution has yet to be found.

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