Gearboxes do not need to be the Achilles heel of wind
With the growth of wind power generation and in particular for installations offshore, the industry needs to put more thought into design for serviceability in order to remain 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:
a) they should be easy to build, using simple manufacturing with good repeatability, and assembled using simple tooling and processes,
b) They should be simple to maintain, inspect and service and repaired with relative ease, and
c) 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, off highway, automotive, civil aerospace, military, conventional power generation, and they all save money, time and effort in doing so.
Easy to build
Double row taper roller bearings are used extensively within wind turbine gearboxes. Such bearings are very good at locating shafts and gears, and providing they are correctly located themselves, they can operate to their expected longevity.
This correct location does require some thought however, especially as every bearing is subject to manufacturing tolerances. In general, the wind industry uses a system of either shims or specifically machined spacers to ensure the bearings are correctly seated.
But this process needs both detailed and accurate measurement and machining, which 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 of these bearings.
Automotive gearboxes have a similar use of such bearings, and a similar requirement for 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.
Easy to inspect
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 can be difficult to inspect due to 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? Why, for example, do we not move the inspection points to a location that means it is difficult to inspect incorrectly? Or incorporate dedicated pipework that leads the inspector’s endoscope directly to the chosen site of inspection?
A counter argument may be additional manufacturing cost, however when you compare these costs to the cost of a failure which may include boat hire, diesel, direct labour, lost revenue and wear & tear, then these additional cost immediately becomes an operational benefits.‘Easy to inspect’ has been applied to every civil and military gas turbine in service. These all have dedicated and targeted inspection ports that enable rapid and repeatable inspections with huge reductions in cost and time.
Simple to maintain & repair
Every drivetrain should be relatively easy to repair and designed to facilitate 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 within wind turbine gearboxes. This is not a revelation, these bearings are accepted as a maintenance item. So why, as an industry, do we continue to see high speed shaft assemblies that cannot be changed in situ, and that either require the removal and exchange of the whole drive train 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. This is achieved not by complexity of design or costly manufacturing it is achieved 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 costs, reduce operating costs and dramatically increase both life and output.