A look at ring gears inside a windmill with Sandvik Coromant.

Wind turbine gearboxes may not appear that complicated at first glance, but every wind turbine gearbox incorporates at least two ring gears. However, in reality, ring gears are just as complex and challenging to machine as any other wind turbine component.

Ring gear turning, drilling, and gear teeth milling operations each pose their own specific challenges and can involve lengthy cycle times. Advancements in cutting tools designed specifically for each one of these operations can help significantly, but only if they are applied in all three operations. When applied across the board, today’s cutting tool technologies can potentially reduce overall ring gear machining cycle times by as much as 25% to 30%.

Transforming Wind into Energy

Wind turbine blades rotate at 10rpm to 30rpm, but the speed entering the turbine’s generator needs to be one hundred times higher. One way that is accomplished is through ring gears within the turbine’s gearbox.

Ring gears embrace the gearbox’s planetary gears, allowing them to transform low incoming speed to high outgoing speed. For every planetary gear there needs to be a ring gear.

Wind turbine ring gears can be big, ranging in size from 4ft in diameter to almost 6ft in diameter and from 6″ to 12″ thick. They typically start as steel forgings with hardness ratings in the 35Rc to 45Rc range, and involve turning, drilling, gear teeth milling, and grinding operations.

Wind Turbine Ring Gear

Ring Gear Machining

The first operation in ring gear machining is rough turning the gear’s overall shape – ID, OD, and face surfaces. Raw ring gears start as steel forgings, and the tough abrasive skin left by that process puts enormous strain on cutting tools. Because of a ring gear’s forged hardness, this first machining operation is similar to hard turning.

Turning tools typically used for rough machining are standard diamond or triangle-shaped turning inserts. However, today’s square-shaped inserts offer increased stability, toughness, and strength for cutting through forging layers at more aggressive feedrates than those possible with other turning inserts. Square turning inserts, including those with the Sandvik Coromant GC4225 grade, usually have 15° lead angles that spread chips out across the insert’s cutting edge. This generates thinner, lighter chips, which do not damage the cutting edge as they curl up and over the insert. Plus, this action helps increase tool life, while the generation of thinner chips allows for higher material removal rates, faster feeds, and shorter roughing cycles.

Sandvik Coromant’s GC4225

The GC4225 grade features a medium temperature CVD coating and a post treatment. The post-treatment is a mechanical process done after the initial coating is applied, and when the insert has cooled. Similar to shot blasting, it will relieve cracks and stresses that resulted from the cooling process.

Once forged surfaces are machined off, shops will turn ring gears to a finished size – leaving grind stock for later generation of the internal gear teeth. For the finishing operation, inserts with wiper geometries are used.

Using the GC4225 grade also gives shops the option to dry machine ring gears. These types of inserts are actually designed for it, but many shops will still run coolant.

Ring gear turning and possibly drilling operations are usually done on big vertical turning lathes (VTL), since these machines can handle the sizes and weight of the components. Although without live tooling capabilities on its VTL, a shop must then transfer ring gears to other machines for drilling.

The holes in ring gears are for mounting other gears, usually ones with external teeth, to the ring gears. Drilling ring gears can be tough, mainly because of the high length-to-diameter-ratio drills that are required. Most often, shops drill ring gears using regular fluted, solid drills, such as solid cobalt ones. The trend, however, has been steadily heading toward insert-type drills.

CoroDrill 805

Insert-type drills, such as Sandvik Coromant’s CoroDrill 805, are the latest tool technology for drilling ring gear holes. These types of drills allow shops to drill to depths up to 12 times the tool’s diameter. Also, the drills are more economical to use because they provide multiple cutting edges for indexing, which eliminates the downtime and costs associated with either re-sharpening or replacing fluted solid drills when they wear.

Most shops producing wind turbine ring gears will also machine other turbine components requiring gear teeth. These include pinion gears, slewing rings, and others that are done on expensive dedicated gear milling machines. Any inconsistency in the gear milling operation makes the final gear grinding operation more complicated and time consuming, placing high demands on the gear machine tooling.

Machining Internal Gear Teeth

Machining the internal gear teeth of ring gears involves removing a lot of material and has long been done using solid high-speed steel cutting tools. They are imparted with ground forms that can be re-sharpened, once or twice, before all critical dimensions are lost.

CoroMill 170

Instead of solid gear cutting tools, many shops have switched to indexable insert gear cutting tooling technology such as the CoroMill 170, which is a disc cutter for internal gear teeth, using high performance inserts that can both rough and finish. Specifically for large gears, the cutter generates gear wheel profiles in accordance with DIN 867 and allowance in accordance with DIN 3972-4.

Once ring gear teeth are roughed and finished machined, they are ground to size. Ring gears are then heat-treated to a hardness of 60Rc.

Regardless of which components a shop is producing for wind turbines, productivity is key. The demand is growing rapidly, which puts pressure on higher machine utilization. However, there is also a need for advanced machining to get the highest output, and today’s tooling technology plays a critical role.

Article originally published in Today’s Energy Solutions 2.2011.

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