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Changing a Gear: Gearbox Lubrication and Its Impact on Wind Turbine Lifespan

The lifespan of an average wind turbine is around 23 years, but turbines are under pressure to last longer, and work more productively.

Technological advances in wind turbine designs and the increasing search for locations with higher wind speeds - including offshore options - mean that turbine manufacturers must constantly keep pace, and the correct maintenance of turbine gearboxes is central to achieving this.

Finding the right supplier

Different drivetrain components have different requirements, and the right choice of lubricant is crucial to the smooth running of gearboxes and therefore the overall performance of turbines. Indeed, investing in the right lubrication could help to save a typical operator managing 50 wind turbines up to US$250,000 year-on-year.

To identify the right lubricant, users need to initially understand why gearboxes fail. The reputation of gearbox turbines has been somewhat challenged over recent years because of reportedly high failure rates. The target lifespan of 20 years has apparently not been reached before repair or overhaul has been required. That said, this isn't always the case - a major turbine manufacturer has reported that the mean age of their turbines is 23 years, with excellent gearbox reliability within that. In any event, at the moment gearbox turbines provide the majority of systems being commissioned and built today.

Gearbox failure (and how to avoid it)

There are three main reasons why many gearboxes fail - namely micropitting, bearing failure and foaming. All are serious, long-term problems but each can be relieved with lubrication which will extend lifespan and save the cost of replacing or repairing equipment. In order to avoid these problems, it's important to first understand why they occur.

Micropitting

Micropitting is surface fatigue, which can result in micro-cracking and the formation of minute micropits which can sometimes give a metal surface a frosted or grey appearance. In some instances micropitting can cause whole gear teeth to break off.

Although micropitting accurately describes the appearance and mechanism of the problem, it is sometimes also referred to as fatigue scoring, flecking, frosting, glazing, grey staining, microspalling, peeling and superficial spalling.

This phenomenon occurs under mixed-film elastohydrodynamic lubrication (EHL), where oil film thickness is of the same order as surface roughness average, and where load is borne by surface asperities and lubricant. In addition to contact stress due to normal loading, sliding between gear teeth causes tractional forces that subject asperities to shear stresses.

Micropitting is complex, unpredictable and difficult to control - despite extensive research on the problem. That said, there are ways to help prevent it happening in the first place. Engineers should maximise lambda (which essentially involves using a thicker film to coat the gear teeth and prevent them from touching), optimise gear geometry, optimise metallurgy, optimise lubricant properties and protect surfaces during running in.

Bearing failure

Bearings are among the most important components but are also often very fine and can damage easily. In particular the bearings which support the shaft that holds gear teeth in place have very fine tolerances, and can be damaged by even small particles.

The potential reasons for bearing damage are numerous. For example, working beyond the original design specification - speed, load and temperature could all change due to the changing requirements of a site. Also, careless handling and seals that are too tight can cause insufficient bearing clearance while inadequate or unsuitable lubrication can also cause failure.

Damage can be split into two categories - in the first instance primary damage occurs, signs of primary damage include wear, indentations, smearing, surface distress, corrosion and electric current damage .

Primary damage can then lead to more serious, secondary damage including flaking and cracks which can ultimately cause equipment failure. Even at the primary damage stage, bearings may have to be scrapped because they are causing excessive internal clearance, vibration and noise. Most failed bearings show signs of both primary and secondary damage. Another issue that can shorten bearing life is the impact of wear particles from the gear box - such as those emitted from micropitting.

A final potentially damaging problem is foaming, where the high speeds and loads that are consistent with wind turbines can cause severe churning, pushing air into the oil, meaning the lubricant doesn't pump or circulate, reducing its effectiveness. Additionally, foam can cause a potential safety hazard if it spills on to the floor.

Future gazing

So lubrication is essential to eliminating many of the problems of gearbox maintenance, and therefore turbine maintenance generally and while using an inferior lubricant can achieve short-term savings but can cause risky long-term effects. It has been estimated that the potential annual savings of switching to gear oil could be up to $5000 per wind turbine operated. This is when taking into account associated reductions in the typical number of oil changes, and the time and cost of changing parts, including labour costs. If you include potential lost revenue due to turbine downtime and the cost of fully replacing a gear box, the savings could be considerably higher.

The right lubrication can help to enhance performance, lifetime and productivity, as well as reduce downtime - all of this can help to deliver commercial benefits and competitive advantage. With increasing pressure on turbine machinery to work harder and last longer, lubrication is more important than ever.

Daryl Luke is global product manager at Castrol Industrial.

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