Upfront: Words on wind by Romax

Gearboxes do not need to be the Achilles heel of wind, says Richard Smith, a principal engineer at Romax Technology.

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With the growth of wind-power generation, and in particular for installations offshore, the industry needs to put more thought into design for serviceability to remain a viable and credible energy source.

I've climbed many turbines of different manufacturers and power ratings - offshore and onshore - and, from what I have seen, there are three fundamental aspects of the design of gearboxes that, if better thought through, could seriously improve the profits of a project over the lifetime of the turbine:

- They should be easy to build, using simple manufacturing with good repeatability, and assembled using simple tooling and processes;

- They should be simple to maintain, inspect and service and repaired with relative ease; and

- They should use proven and reliable technology - not designs based purely on theoretical models.

This is standard practice for every other established industry - rail, marine, automotive, aerospace, military, conventional power generation - and they all save money, time and effort in doing so.

Easy to build

Double-row tapered roller bearings are used extensively in wind-turbine gearboxes. They are good at locating shafts and gears and, providing they are correctly located themselves, can operate to their expected longevity.

However, this correct location does require some thought, especially as every bearing is subject to manufacturing tolerances. In general, the wind industry uses a system of either specifically machined spacers, or a selection of shims combined to meet the required thickness, to ensure the bearings are correctly seated.

But this process needs both detailed and accurate measurement and machining. This is challenging to achieve in a factory when the gearbox is new, and downright difficult when it is in service. As a result of this, we continue to see high failure rates here.

Automotive gearboxes use such bearings in a similar way, needing equally accurate location. However, almost every vehicle gearbox in existence uses a much simpler, infinitely adjustable, one-size-fits-all solution that can also be adjusted in service as the bearing wears. These adjustable bearing retainers only require simple tooling to allow the bearing to operate to its expected design life.

A fundamental part of maintaining a high-asset drivetrain is the ability to inspect it. Wind-turbine gearboxes regularly contain planetary stages within their drivetrains that are difficult to inspect because of their complex architecture. So why do we continue to add to this difficulty by placing inspection points in totally irrelevant locations? Why do we make it so difficult to inspect some of these components that only the most skilled can conduct the exercise to a meaningful level? Instead, we should move the inspection points to a location that makes it difficult to inspect incorrectly. Or incorporate dedicated pipework that leads the inspector's endoscope directly to the chosen site of inspection.

One might argue that this adds to the manufacturing cost, but when you compare this to the cost of a failure, which may include boat hire, diesel, direct labour, lost revenue and wear and tear, then this additional cost immediately becomes an operational benefit. "Easy to inspect" has been applied to every civil and military gas turbine in service. They have dedicated and targeted inspection ports that enable rapid and repeatable inspections with huge cost and time reductions.

Simple to maintain and repair

Every drivetrain should be relatively easy to repair and designed to enable as much up-tower repair as practical. Due to their high operating speed, and consequential reduced fatigue life, high-speed shaft bearings are a regular point of failure in wind-turbine gearboxes, and these bearings are accepted as a maintenance item. So why do we continue to see high-speed shaft assemblies that cannot be changed in situ and either need removal and exchange of the whole drivetrain or removal of failed components within the gearbox casing?

We can compare this scenario to another type of automotive gearbox. Motorsport gearboxes often require either unscheduled repairs or ratio changes in a matter of minutes or even seconds. This is achieved not by complexity of design or costly manufacturing, but simply through intelligent design.

These issues are what identify wind-turbine gearboxes as the industry's Achilles heel. Yet they are completely avoidable. Intelligent and experienced design of these items will reduce manufacturing and operating costs and dramatically increase both lifespan and output.

Richard Smith is a principal engineer at Romax Technology.

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