Gears - Revolving and Evolving
Gear, gears, drivetrain, gearbox
A gear is normally but not exclusively a rotating machine part with teeth or cogs. When two gears mesh in a rotating machine they impart motion from one to the other, the motion will change direction. Rotation speed and torque can remain the same, increase or decrease depending on the gears relationship to each other. When two gears mesh in this way we have a transmission or a drivetrain. Put a number of these gears into a housing, with an input shaft and an output shaft and what we have is generally known as gearbox.
There are many types of gears and they are used in infinite number of applications. Without even knowing or thinking about it we use gears in our everyday lives and we have done so for hundreds of years. Wrist watches are probably closest to home for most of us, food mixers or the gearbox in our car. There is not a day goes buy in our modern lives where we are not directly or indirectly affected by gears. So, for a technology that’s been around for so long why all the song and dance when it comes to the wind turbine.
Wind mills have been around for thousands of years. The Greeks as far back in the first century is one of the earliest known instance of using a wind-driven wheel to power a machine. I can’t guarantee there were gears involved but changing rotation speed, direction and torque through pulley wheels and belts is a similar principle. The gear tooth and mesh replace the belt but the mechanical advantage gained from different sized wheels or gears is the same basic principle. If you ever get a chance to visit a more recent working wind mill used for grinding grain it is well worth the trip. In these you can see on a massive, open and expanded scale how motion is imparted from the sales through the main shaft and a series of wooden or iron gears depending on the vintage of the machine, to drive the grinding stones.
This is just an expanded version of what takes place in a wind turbine gearbox. The gearbox is the heart of the turbine it converts the slow rotation and the high torque on the rotor side of the gearbox through the transmission in the gearbox to high speed low torque at the generator side.
If you have difficulty understanding torque, visit one of the gear manufacturers stands at a trade show. If they have a demonstration gear on display, try to rotate the input shaft by hand. Once you have exhausted yourself and suffered a hernia, move around to the output shaft and try the same. This you will find incredibly easy. You are still moving the same components, weighing exactly the same but due to the low torque at the output shaft and the mechanical advantage of the drive train you are able to do this easily.
We can now see clearly why a gearbox is used in the wind turbine’s drivetrain and what it does. Let’s take a look at how it has developed in the very short time that our industry has existed as a real producer of energy.
In the very early part of the 1980s, wind turbines went into what could be really called serial production for the first time to be used as commercial generators of energy. At the time the industry was an infant and in many ways still is when compared to others. Many gearboxes were little more than adaptions of gearboxes used in industrial or agricultural applications. These gearboxes were relatively simple affairs with two or three stage spur gear drives between input and output. These worked but were not without their problems, size, weight, noise and reliability being among these. As our industry has progressed, turbine size increased and regulations have become more stringent, the development of the gears and the gearbox has needed to keep pace and adapt.
The first major change in kW class gears was the move towards using a planetary gear arrangement coupled to a double helical stage. The net effect of this was to reduce weight and increase torque density. A good example of this was the reduction in weight of a straightforward helical gear used in a V44 660kW turbine from around 5.5 tonnes to 4.4 tonnes, a massive weight reduction of 20%.
Torque density is the relationship between the weight of the gear and the torque it is capable of handling. As gear designs and materials become more efficient the torque density figure rises. So we can see that in the relatively short time that our industry has existed turbine size and the gearboxes within these turbines have had to cope with the size increase while in real terms reducing in size them-selves. Gearboxes now exist as single planetary with two helical stages, double planetary with one helical stage and even three planetary stages. Modern gearboxes are now complex and highly efficient drive train systems and there reliability in recent years has significantly improved.
A double planetary stage gear with a single helical stage
Reliability has always been a stone thrown at the gearbox and some of the stone throwing was certainly justified in the early days, however as the industry has grown, learned the lessons from other industries and developed itself, reliability is an ever improving area. Better maintenance regimes, condition monitoring/management, better oils and oil care have all played their part. Significant development by gearbox manufacturers, more powerful design software modelling, better manufacturing technology, improved materials and understanding of the turbine operating environment have all been crucial in this area. A prime example of where these factors have combined along with the prevalence of the planetary gear arrangement is the adoption of the integrated bearing within the planet wheel.
Bearing failure in planet wheels has been a major cause of gearbox failure in the past and still exists today where non-integrated bearings or badly designed integrated bearings are used. These failures are almost impossible to correct in the nacelle and are always costly when it comes to factory service.
The problem manifests itself in the same way irrespective of the turbine/gearbox size. The root cause of the problem was identified by Moventas over a decade ago and is a lecture in itself however the solution that is proven to work is to use a planet wheel with the bearing outer race form ground into the planet wheel. This is generally referred to as an integrated planet wheel bearing. Moventas has been using this in serial production on all gears at 850kW and above and has pioneered the upgrade to existing gear designs to integrated planet wheel bearing during workshop service.
The previous examples are just a few of the many changes made to wind turbine gearboxes over the last thirty years. The development never stops and at Moventas we strive for ever increasing quality and with that quality ever increasing reliability. In a world where wind is one of the only energy generation forms where the cost of generation is reducing, we aim to make a difference. By being better we can reduce the cost of generation further.
The writer, David Moss, is a Service Sales Manager with Moventas. This article was published in the May/June 2014 issue of the Wind Energy Network magazine.