When is 20 year gearbox life not 20 years?

It is well known that many wind turbine gearboxes have a design life of 20 years. However, it is also well known that a great number of gearboxes don’t last for 20 years and fail prematurely… so why the discrepancy?

The answer lies in the way that gear and bearing lives are defined. The life of a gearbox component is stochastic, not deterministic - this means that we can’t predict the exact date on which a component will fail, but we can estimate the probability that it will last for a given duration.

The life of a bearing is generally defined as the ‘L10’ life – the duration after which 10% of the bearings will fail. If L10 for one bearing is 20 years, there is a 10% chance that this bearing will fail in less than 20 years. This is interesting because it forces us to think about a lifetime in terms of probabilities – something that reliability engineers have been doing for years. You may have also heard engineers talking about ‘Weibull curves’ – these are simply a type of distribution that works very well to describe failure rates in mechanical components due to fatigue and ageing. Weibull curves are a powerful tool and are used for reliability engineering in many industries.

A Weibull distribution describes failure rates well for a single bearing experiencing ageing/fatigue failure

 

Now it starts to get interesting: what happens if we combine the L10 life for every bearing in a drivetrain to calculate a ‘system level’ life? A typical wind turbine drivetrain has 20-25 bearings, including the main bearing(s), gearbox and generator bearings. A very simple calculation for a drivetrain with 25 bearings, all with an L10 design life of 20 years, indicates that the probability of one or more bearings failing within 20 years is 93%. So, by this logic, almost all gearboxes in a wind farm are likely to fail within 20 years. This seems shocking but actually isn’t far from reality – many gearboxes have been changed or gone through an uptower repair of some kind, such as a new high-speed shaft or a new set of generator bearings before the 20-year life is up.

Now let’s look at another example: how many gearboxes will fail within 7 years? In our experience, lots of gearboxes are replaced or repaired within this time. Running the simple calculation again indicates that the probability of one or more bearings failing within 7 years is 37%. So more than a third of gearboxes might suffer some sort of bearing failure – again, this doesn’t seem too far from reality.

A typical wind turbine contains 20-25 bearings which all must be considered in a system-level reliability calculation

These results come from a simplified calculation and are intended to show the overall trends, but they show some startling findings. In reality, this approach can over-estimate the failure rate because bearings in the gearbox will actually have a design life of more than 20 years – safety factors are applied during gearbox design, and other design rules are followed that are not covered by the bearing standards. In practice, some bearings have a design life well in excess of 20 years because their size is dictated by other factors such as stiffness or safety factors during extreme load cases. On the other hand, the calculation can under-estimate the gearbox failure rate because many non-fatigue failure modes are not considered and gear failures are not considered in the examples above.

And this is why the term “design life” can be misleading, and one reason why many gearboxes in the field are failing in less than 20 years. One solution is to employ better reliability engineering methods throughout the life of the turbine, and we expect to see the industry talking a lot about this topic over the next few years. Romax InSight will certainly be leading the way – we currently use very sophisticated versions of these methods to analyse and forecast failure rates for owners and investors. Using design standards and simulations alongside a huge amount of operational data and historical failure rates, we are able to provide very accurate predictions of drivetrain failures.

 

A RomaxWind gearbox model can be used to calculate bearing lives based on proven design standards. These lives are inputs for the system reliability calculation

 


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