By John Krouse, LMS International

Jari Toikkanen, manager of the research and test group at Moventas, a tier-one wind turbine gearbox supplier, has seen the number of noise and vibration tests ordered by wind turbine original equipment manufacturers (OEM) quadruple in the last five years, with many projects requiring same-day turnaround.

“Tests are done primarily to improve product reliability and meet strict demands from regulatory agencies such as the AGMA (American Gear Manufacturers Association) and European ISO standards,” said Toikkanen.

Wind turbine gearbox undergoing vibration analysis. Photo, Moventas.Click here to enlarge image

Particular attention is focused on studying vibrations of the wind turbine’s gearbox, which uses a combination of planetary and helical gearing to step-up rotor speed 100-fold for driving the electrical generator. Another major component of interest is the torque arm that connects the gearbox to the turbine framework.

Engineers perform extensive modal impact testing to ensure that resonances of these components do not match the excitation frequencies of the surrounding structure or gear mesh frequencies, thus exciting potentially damaging vibrations in the framework, rotor blades, drive shafts and the tower. Generally, the goal is to avoid the modal frequency range of 80 to 250 Hz for the torque arm and 400 to 800 Hz in the rest of the housing structure. When resonances are identified within or near these ranges, engineers shift the modal frequencies by modifying the geometry of the gearbox components and torque arm, typically optimizing stiffness properties by changing part thicknesses and shapes.

Toikkanen said the process is complicated by variable gearing frequencies that excite gearbox and torque arm vibration modes at different rotor blade speeds. This can range from an input rotation of a few revolutions per minute for a light breeze to a maximum of 10 times that for gale-force winds.

Moventas uses LMS Test.Lab software with an LMS SCADAS Mobile data-acquisition system with eight channels and an integrated suite of tools for modal analysis. LMS Test.Lab also allows for on-line monitoring.

Visualization may be particularly helpful to the engineers. With the animated mode shapes displayed together on the same screen—and with plots such as frequency response functions (FRFs) showing vibration amplitude versus frequency at key locations on the gearbox—engineers can see how the gearbox housing bends and twists at various frequencies. They can then identify which bearings are transmitting vibrations and determine critical gear-mesh harmonics.

Moventas worked with LMS Engineering Services to measure gearbox rotational vibration on the low-speed input and high-speed output shafts for a wind turbine manufacturer. Signals from accelerometers mounted directly on the low-speed shaft were fed into LMS Test Lab for analysis. Signals for the high-speed shaft were obtained from a laser vibrometer system measuring rotational velocity. A series of operating response color maps accurately identified rotational vibration and related resonances for both shafts, data needed by the wind turbine manufacturer to simulate the dynamic performance of the entire drivetrain.