Cylindrical roller bearings reduce weight in wind turbines

Paul Boughton
Ralf Petersen explains how cylindrical roller bearings have improved reliability and wear life in new wind turbine gearboxes.

Finite element method (FEM) analysis was undertaken to produce compact cylindrical roller bearings that save weight, and absorb uneven load distribution, delivering optimised life in a patented new multi-stage planetary/helical unit that extends the development of wind energy plants into the multi-megawatt area, ie up to 5MW and above.

An essential advantage of the new differential gearbox is its slim outer diameter, with only a slightly increased total length compared to existing - but lower capacity - MW gearboxes. Consequently, although offering increased plant power, the new unit delivers a 15 per cent weight reduction compared with today's gearbox concepts. The component dimensions also play a major role in this reduction. The dimensions of the internally and externally toothed gears, the roller bearing and the planetary carriers of the differential gearbox are similar to those of the gearboxes from 1.5 to 2MW class. This offers major advantages for manufacturers of wind energy plant, as the majority of components used in the new gearbox are already produced in large numbers and with consistently high quality.

In the differential gearboxes, unlike conventional generator gearboxes of the multi-megawatt class, four or more planetary gears do not revolve around the sun wheel in the input stage. Instead, they have two input stages, A1 and A2, each fitted with three planetary gears, offering the advantages of a static determination paired with freely adjustable sun pinions. Torque reduction is undertaken in the two planetary stages, using NSK's EM series CRB in the planetary gear wheels. Importantly for standardisation and cost savings, these bearings correspond closely to the original sizes of bearings used in smaller megawatt class gearboxes. The use of paired EM series cylinder roller bearings, with one-piece solid brass cages, has proven to be the optimum bearing type for this application.

The bearings are able to also absorb uneven load distribution through load point displacement in the planet meshing. The planet carrier A2 is fixed as a housing part and therefore requires no bearing. The bearing of the hollow wheel B of the planet stage B operates in tapered roller bearings to enable high rigidity and axial precise control.

EM series bearings are also used in the helical gear stage of the differential gearbox. The bearing of the helical gear stage is realised in a classical fixed-free-end bearing arrangement. The free-end bearing is designed using EM series CRB, and the fixed bearing through tapered roller bearing pairs with axial bearing clearance matched with the application. This bearing arrangement is already tried and tested in gears with megawatt class gearboxes and is used in the differential unit with only minor modifications.

In determining the best bearing arrangement for the new wind turbine gearbox, NSK employed a wide selection of analysis and calculation tools, including analysis of component deformations - component loads and deformation as well as resulting bearing loads and deformations; analysis of internal friction ratios - kinematic consideration of rolling elements, tilting and crowning behaviour of the rolling elements, and thickness of lubrication film, heat generation and dynamic friction moment; analysis of stress in the roller bearing, rolling contact stress and internal stress distribution; wear parameters, the pv-value (the product from contact stress and circumferential speed; this value gives a statement on possible friction and wear states).

The EM series cylindrical roller bearings meet modern industry demands for improved levels of load capacity, reliability and operating life. They provide higher performance, substitute for two-piece brass cage designs - and also competitive one-piece designs - in applications where increased performance demands mean that the latter bearings can offer either the life, or the levels of reliability required.

The one-piece cage design of the cylindrical roller bearings enables a greater number of larger rollers to be integrated, resulting in up to 30 per cent higher load capacity than conventional rivetted brass cage bearings of similar size. The optimised one-piece cage design also ensures higher rigidity, lower temperature running, quieter operation and longer operating life.

The key to the increased complement of larger rollers of the EM Series bearings - and their commensurate higher load capacity - is the improved strength of the unique one-piece brass cage design. In tests the cage of the EM Series has proved to be four to five times stronger than the two-piece designs of major competitors and two times stronger than competitive one-piece designs. For the user, in many applications this means that bearings can be downsized to reduce costs without affecting machine performance or bearing life.

Enter 69 or at www.engineerlive.com/ede

Ralf Petersen is with NSK Deutschland GmbH, Ratingen, Germany, www.eu.nsk.com

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