Anatomy of a Wind Turbine – Part II

This article is a continuation of “Anatomy of a wind turbine.”

Work Large

Gears for utility-scale wind turbine applications are very large in diameter—up to 70 inches—and have face widths up to 56 in.

Obviously, the equipment we must use to manufacture components as large as these must also be very large and heavy-duty. Our gear grinding, shaping, and hobbing machines are the largest available.

Special workholding and fixturing must be able to bear the heavy weight of the large gears and reduce the vibration and movement of the gear blank during hobbing and roughing. We must carefully and uniformly torque the clamping fasteners during these operations to prevent workpiece movement and gear blank distortion during the roughing operation.

The gear blank must have accurate mounting and indicating surfaces to help control the pitch line runout to critical features (bearing journals, splines, and so forth) and to minimize lead error. Lead error is the deviation of the actual advance of the gear tooth profile from the set value. This can interfere with good contact in the gear assembly—a real problem atop a 400-foot wind turbine! Crowning is one technique we use to overcome this effect.

Achieve Exacting Material Composition

In addition, the gears require very exacting material composition and heat-treat processing of the carburized steel. The gear must be designed and manufactured for low rolling resistance and long life because of the high costs of maintenance, repair, and associated downtime of the gearbox assemblies once they have been erected. We have to carefully document and control every step in the manufacturing of these gears to achieve the high accuracy and reliability needed for operation in extremely high-wind environments.

Minimize Distortion, Cracking

We machine the gears out of carburized steel. The heat treatments and stress-relief operations associated with that material must be exacting to minimize part distortion and expansion, as well as to achieve the required metallurgical properties. Often we have to preheat the forging or barstock.

To minimize gear distortion, we insert the gear vertically into a quench tank during the hardening phase. The dwell time in the quench tank is critical to performance and material stability. Even the slightest variations in the cooling rates can cause distortion that might be undetected in normal inspection.

Heat treatment itself can cause microcracking that must be ground out, so we have to allow enough stock to achieve final case depth. When you heat it up at 1,600, 1,700 degrees F and drop it into the quench tank, it goes from 1,600 degrees to 300 degrees just like that. If you don’t do everything right during that transformation, it’ll distort. So after, we grind all the faces, and then gear-grind.

Some of these very fussy components that have to be just right, we don’t compromise. We use magnetic particle inspection and grinding burns using nital etching to detect cracks.To properly distribute the load on the gear teeth, we sometimes off-center crown-grind the tooth geometry.

Modify Tooling

Rigid, heavy-duty hobbing machines using roughing hobs or gear milling (gashing) cutters are required for the coarse-pitch gears. We use coarse-pitch diamond dressing rolls and special grinding wheel abrasives to machine the large, high-accuracy gear grinders to produce efficient, accurate results and to prevent grinding burns and cracks.

The cutting fluids must have the proper viscosity, the right amount of extreme-pressure additives, and must be directed to the exact location of the workpiece and cutting tool interface to optimize results. We routinely sample and adjust the fluids.

Build for Durability

Using the correct bearing clearances and preloads is critical to long life and proper gearbox operating temperature.

Gearboxes must last a long time. They are 200 feet up in the air, and they are very, very heavy. If you have to take the gearbox out, it’ll cost quarter of a million, half a million dollars.

We are not a typical gear manufacturer. All of them can make gears. Some of them can assemble gearboxes. But they do not design them. If they do not design them, they miss finer points, like a bearing preload, a very critical application. And when you do a bearing in a gearbox assembly, you set the looseness of the bearing. If you set the preload properly, there has to be a little bit of leeway in the alignment. And also when the gears have a longer shaft, they deflect differently than a shorter shaft. So the gear is not in the middle. You have to make an adjustment for those things.

When you build things like spindles—20,000 rpm, 30,000 rpm spindles, that speed—the design of the bearing, the selection, the mounting, the accuracy of the bearing, … is phenomenal. If you don’t do it right, it’ll be shot in two hours.

We rely on sophisticated measuring techniques with bearing inspection gauges to help ensure these results. The type and method of lubrication and proper sealing affect the gearbox’s performance. Computerized analysis and testing of gearbox performance is crucial to ensuring durability.

N.K. Chinnusamy is president of Excel Gear Inc., 11865 Main St. Roscoe, IL 61073, 815-623-3414, [email protected], www.excelgear.com. Chinnusamy is a member of the Great Lakes Wind Network.