A WT 2000 replacement gearbox heads to testing.

The next generation of drivetrains will be large and operate in harsher environments (offshore) than those previous. They will have to be more reliable, easier to service, repair, monitor, and include effective diagnostics and prognostics. These Gen2 units will have to carry greater torque and rotor-bending moments, and show drivetrain dynamics with lower frequencies, for more durable machinery. Operational costs and financial risk must also come down.

The need for improved reliability has driven several efforts in the industry, including:

• A major revision to ISO 61400-4, “Design Requirements for Wind Turbine Gearboxes”

• The long running NREL Gearbox Reliability Collaborative (GRC)

• Stricter wind gearbox requirements in the 2010 edition of the Germanischer Lloyd “Guideline for Certification of Wind Turbines”, and

• Current revision efforts to AGMA 6006-A03, “Design and Specification for Wind Turbine Gearboxes”

The analysis is of a RomaxWind model of an NREL GRC drivetrain with a 750-kW gearbox. RomaxWind was released as a wind turbine specific version of the RomaxDesigner; software used by the automotive OEM’s for geared transmission design and refinement.

Engineering firms have been developing and improving design methods and software since the beginning of the wind industry. Manufacturers have been improving their product by implementing a range of tests such as full load end-of-line testing, off-axis load and dynamic development testing, 100% nital etching, stricter incoming material specifications and inspections, and improved techniques for the heat treatment of large components. Turbine designers have a better understanding of the loads and dynamics in the machinery.

Rotating machinery specialists such as Romax Technology have been involved in these activities along with the development of the next generation of wind-turbine gearboxes. To date the company has developed 14 multi-MW models ranging from 1.5 to 5.5 MW, which are used by 10 different turbine manufacturers, most of which are installing turbines in the Far East. “We won’t design a wind gearbox for a manufacturer unless they go through certification either by GL, DNV, TUV or an equivalent,” says Romax Director of renewables Andy Poon. “This along with our own due diligence on the prototype development and manufacturing helps ensure high-quality manufacturing for our products.”

The company is working on improving the installation and serviceability of the high-speed stage of the wind turbine gearbox. This stage in any wind-turbine gearbox is most susceptible to wear and premature failure, given the high number of cycles and high inertia of roller bearings in larger gearboxes. The company is also developing a diagnostics capability for damage and wear in the gearbox, with condition monitoring equipment and specialized software. If wear in the high-speed stage is detected early enough, then an up-tower replaceable cartridge can be easily fitted by a technician. The high-speed cartridge is easy to assemble. The technician simply removes the whole unit and replaces the high-speed-stage bearings and pinion in the new cartridge. Bearing preload is preset in the factory to ensure good performance. Double helical gears are used in several models on the parallel stages in combination with the high-speed cartridge. This negates the requirement of the axial constraint to resist the load due to gear helix. Then the high-speed bearings need only react to radial loads. It eliminates the problem of double row, taper-roller bearings being unloaded in reverse torque situations.

An easily changed high speed cartridge is designed for a proposed 5-MW gearbox. A recent condition-monitoring product, Romax IDS, can detect wear early enough to allow, if necessary, a planned and orderly replacement.

Another recent significant development is a new way of adapting the high-speed bearing to many common wind turbine gearboxes in service. The concept of this new design was developed after evaluating many offshore machines. Most bearings are manufactured with high quality material, to high tolerances and surface finishes, and then fitted with no real idea of the actual preload,” says comapny Test Team Leader Richard Smith. “Industry falls down on the last 10% of the job. A new approach to retrofitting the high-speed stage takes aim at setting accurate bearing preload, which results in longer bearing life than the previous practice.”

Another feature of the recent gearbox is a split-torque design with rotating ring gears and a dry sump. This removes a common problem of debris accumulating in the 6 o’clock position of the stationary ring gear and the early failure of this gearing. Gravity feeds oil into a sump below the gearbox, so the gear stages need retain no oil. Churning is minimized so less aerated oil returns to the cooling system and drag losses drop.

In addition, the company has an advantage because it has been a long-time developer of commercial engineering software for designing gearboxes as well as being a large wind turbine gearbox design firm. Gears and bearings are designed with microgeometry, and clearance and preload settings after accounting for whole system structural deflections, non-torque loading at the rotor, and the effect of thermal expansions and off-nominal tolerances.

Romax software shows strong validation with extensive gearbox measurement data provided by the NREL GRC, and the software is certified by Germanischer Lloyd for gear contact analysis and rating. The analysis allows for improving function, performance, weight, and cost of gear trains.

The planet carrier provides an example of lessons learned. Generally, manufacturers use cast-steel carriers to meet strength requirements. However the steel has high scrap rate, can be a difficult to work with, and has stringent requirements for nondestructive testing.

Careful analysis has led the company to a suitable lower cost and lower weight (sometime by 1,000 lbs) cast-iron planet carrier. “We worked with GL to approve GJS 400 spherodal graphite cast iron for the planet carrier,” says Design Team Leader Dave Saysell. “By careful analysis and rearrangement of the structure, we can achieve equivalent deflections, strength, and fatigue life with this lower-cost material. Critical attention is paid to strength near the planet-pin fit and the carrier wind-up.” Environmental considerations are important because the material is more suitable for warmer climates. WPE

By: Ashley Crowther, VP Engineering, U.S. Wind Technology Center, Romax Technology Inc., www.romaxtech.com

Windpower Engineering & Development

The FusionDrive combines a gearbox and PM generators into one unit.

Wind gear manufacturer Moventas and generator manufacturer The Switch, have announced the delivery of the first commercial order for FusionDrive, a gearbox and generator combination. The first delivery is going to Germany-based DeWind.
“We think FusionDrive is the next big thing for the wind industry. It includes the benefits of a hybrid drive, but Moventas and The Switch have taken the integration even further,” says Dr. Sungkon Han, Managing Director of DeWind Europe.

The developers say FusionDrive is the answer to the challenge of turbines needing to be bigger in size and power, while the race is on to lower the cost of energy. The developers say FusionDrive is the smallest and lightest combination of gearbox and generator. Studies have confirmed that it is a technically optimized solution to limit rotational speed. More important to turbine manufacturer and energy provider, the unit provides the highest energy yield and the best serviceability in the market, say the two companies.

The unit requires minimal maintenance, say the companies. The gear and generator can be split, and all components are changeable in the nacelle enabling best serviceability in the market.
Moventas

Moventas.com

Windpower Engineering & Development

President of Gearbox Express Bruce Neumiller talks with Chris Schoenher from the Wisconsin department of administration.

A new company, Gearbox Express (GBX), along with state and local public officials, recently celebrated the official grand opening of its new 43,000 ft² facility in Mukwonago, Wisconsin. The company says it is the only one in North America focused on providing independent, down-tower, wind-gearbox remanufacturing services. They are expected to eventually employee 100 people.

While GBX understands the overall tone of the wind energy industry is considered flat, it is confident that by helping develop the aftermarket infrastructure, they’ll impact job growth and the economy.

“There are more than 26,000 active turbines in the United States and it’s a fact that the gearboxes will need replacing during their lifetime,” said company CEO Bruce Neumiller. “Wind farm owners want their investments to keep running so there’s a tremendous opportunity for us to help protect and manage their assets.”

The three wind-industry vets that started GBX succeeded in attracting millions of dollars of investment capital along with a $3.4 million low interest, revolving loan from the Wisconsin State Energy Program.

A new gearbox is going in. Gearbox Exchange says it will keep an inventory of remanufactured gearboxes in its GBXchange program to allow for immediate swap-outs when they’re being removed from the turbine.

“The people at GBX have developed a business model that helps address a critical economic issue by bringing wind turbines back on-line much faster,” said Chris Schoehnerr, Deputy Secretary, Department of Administration for the state of Wisconsin. “A key element of Wisconsin’s past and future success is having driven entrepreneurs with technical manufacturing experience. That creates profitable new businesses and brings jobs to our state.”

The company says its climate-controlled facility offers a highly-flexible, technologically-advanced test stand that “rivals any in the world,” according to Neumiller. GBX technicians remanufacture the gearboxes to the latest revision level and then conduct a rigorous load test on its regenerative test stand. The test stand is a critical component of its business model and was the largest focus for investment. The company says it will keeps an inventory of remanufactured gearboxes in its GBXchange program to allow for immediate swap-outs when they’re being removed from the turbine.

“We are truly gearbox guys,” said Neumiller. “Our careful industry analysis work shows a clear need for an advanced company providing dedicated gearing, bearing, and gearbox expertise. And there is a critical need for one that uses the best equipment and parts.”

Gearbox Express
gearboxexpress.com

Windpower Engineering & Development

The show was hosted by a London-based company. I find there’s just something about the British accent that adds an extra air of pleasantery during the registration and moderation. A representative from Make Consulting started things off by touching on a hot topic: the extension of the PTC. The company says it expects the credit to be renewed, probably for another year, after the November elections. This is an opinion I also heard at the ABB event earlier this week. I have to say it makes sense, as it’s a shame clean energy has become an area of political division when what it really needs is bipartisan support. But I suppose, in this political climate, we should be thankful just to have it renewed in the nick of time.

Make Consulting also noted that many of the turbines online today are becoming outdated. Even 5-years-old models are starting to be considered old-fashioned and could benefit from upgrades or repair—though it’s hard to know how much value doing so holds for availability, and financing isn’t easy.

All about predictive maintenance
Stuart Cameron from Romax Technology discussed a point I’ve heard often lately in the wind industry. He encourages the shift from reactive to predictive maintenance. Out of reactive, preventive, and predictive maintenance, predictive offers the best balance of failure and maintenance costs. Cameron says predictive maintenance offers up to 50% cost reduction over reactive maintenance. This is especially helpful with turbine drivetrains and gearboxes because the industry is seeing an increasing failure rate in these components. Moventas agrees, stressing condition monitoring and understanding what went wrong the to help prevent problems. The representatives also noted that performing maintenance tasks together as much as possible is important to saving time and money.

A bit on blades
The same is true for blade maintenance. Getting up-tower is time consuming and costly, so it makes sense to performance as much maintenance as possible while you’re up there. Wind Energy Solutions (WES) gave a great presentation on blade predicaments and encourages predictive maintenance as well. The company representative recommends starting a database on blades at the end of the warranty and deciding when to do maintenance ahead of time (probably during the non-productive season). He also suggests keeping to a periodic maintenance schedule. For blades, things to check for include rotor and aerodynamic imbalance, which are often due to installing blades at incorrect angles. He says the biggest aging issue with wind blades is leading-edge erosion. The problem can be fixed little by little with protective tapes or coatings.

WES says scheduled maintenance makes common sense because it increases availability, reduces large repairs by focusing on fixing smaller ones first, reduces crane and mobilization cost, and increases aerodynamic efficiency. However, due to lack of data to support cost savings, the WES representative says he has a hard time convincing operators that this is the way to go. The second biggest problem is lightning damage, which can be rectified through lightning-system testing. There are also problems with icing, but without any effective solutions. “If I had the solution, I’d be making some big money,” as he puts it.

There were many other informative sessions including some on improving gearbox reliability, collecting data to improve availability and more. I’d say the conference was definitely worthwhile, and so was my trip to Dallas. But now it’s back to Cleveland for me. After a week away, I’ve learned a lot. But I think I’ve had enough of hotel rooms for awhile.

Windpower Engineering & Development

ZF Services combines the global customer service activities as well as ZF Wind Power as an OEM gearbox supplier (above). With more than 76 locations in 36 countries, including 650 partners, ZF Services has a full-coverage service global network.

ZF Friedrichshafen AG has expanded its wind power services by becoming an authorized repair center for all legacy models of Hansen Transmissions in wind turbines. To support OEM and owner-operator turbine service across North America, the repair facility, in Vernon Hills, Ill., has received approval to expand wind services, including load testing, material handling, and manpower. In June, 2011 the State of Illinois awarded ZF Services $1.6 million to support the expansion.

“The Hansen line is a great opportunity to better serve customers with more flexibility,” said Scott Gardiner, Director of Wind Services NA, ZF. “With the addition of Hansen, ZF can now serve all major global markets in North America, Europe, and Asia with a product range that covers the entire megawatt spectrum.”

ZF completed the acquisition of all remaining shares of Hansen Transmissions International in November 2011. In 2010, the Group achieved a sales figure of €12.9 billion with about 70,000 employees. To continue success, the company annually invests about 5% of its sales (2010: € 646 million) in R&D

ZF Services
www.zf.com

Windpower Engineering & Development

A wind-turbine gearbox manufacturer has expanded its field-service portfolio to include the inspection and repair of the entire helical side of the gearbox up-tower. Moventas General Manager Steve Casey says its entire process can be performed in just three days, and the company (www.moventas.com) has successfully completed over 20 of these repairs to date.

Moventas says its on-site repair services can replace failing gears and bearings in about three days, and without need for a large boom crane.

“By using a mobile service unit and a small hydraulic crane, we eliminate the costs associated with mobilizing a large boom and secondary cranes normally required to bring a complete gearbox down from the nacelle,” he says. Shipping costs to and from the workshop are also eliminated.

A repair crew from Moventas has removed a gearbox top cover. Standard procedure says the crew is to inspect bearing and gear surfaces along with all interface components and connections.

A Moventas crew drives a self-contained mobile workshop to the wind-turbine site that contains all tools, components, and software required to repair the entire helical side of a gearbox. Each component, including the high-speed pinion and intermediate and low-speed gear assemblies, is lowered into the workshop where it is cleaned, inspected for damage and wear, and replaced as needed. In lieu of workshop-dynamometer testing, all up-tower helical repairs include condition monitoring using the company’s Condition Management System (CMaS). Casey says using CMaS ensures the same quality of repair as those performed in the company workshops. WPE

Windpower Engineering & Development

Further Summit topics include:

• O&M markets and trends for 2012 and beyond

• End of warranty options (EOW)

• Retrofit technology and repowering

• Advanced monitoring technologies

• Life extension of critical components

• Major part failure, upgrading and repair

• Grid integration and curtailment

Global gear manufacturer David Brown Ltd. will present a paper in the Summit’s “Solutions for asset-life cost containment” session, as well as participate in the interactive panel discussion focused on improving gearbox reliability. The company will discuss how best to use the latest industry experience, proven technology, and O&M strategies to maximize power generation and reduce costs in wind farms.
A team from the gear manufacturer will address how leveraging global experience, such as that from mission critical industries, goes beyond current equipment specifications to drive the wind-turbine-gear industry. The discussion will focus on David Brown’s operations model as a case study.
“Our goal at the Summit,” says Tom Marek, David Brown’s North American Business Unit Leader Wind Energy, “is to talk frankly about gearing issues in the wind industry and then how our global engineering, service, and production network can address the needs of wind-turbine owners. It is important we address the industry’s gearing issues to move forward to lower life costs, and ultimately the cost of energy for the industry’s operators.”

This follows the recent announcement of the company’s acquisition of the Canadian industrial gear manufacturer Unigear Industries Inc, as part of its North American growth strategy, as well as its recent signing of a multi-million dollar contract with Samsung Heavy Industries to design, develop, and manufacture an innovative 7 MW offshore wind gearbox.

Companies confirmed include E.ON Climate & Renewables, AES Wind Generation, Iberdrola Renewables, Duke Energy, EDP Renewables, Infigen Energy, Edison Mission, TransAlta, Wind Capital Group, enXco, GE, Acciona, NERC, Praxis, Hytorc, Foundation Engineering, Complete Wind and many more.

Wind Energy Update
http://www.windenergyupdate.com/operations-maintenance-usa

David Brown
www.davidbrown.com

Windpower Engineering & Development

Unigear Industries in Montreal is capable of large custom gears.

David Brown, a manufacturer of industrial gearing and support services, says it has acquired Montreal based industrial gear manufacturer Unigear Industries Inc. Unigear is a privately held business known for its high quality gears. With its flexibility, innovation, quality, and performance it has an excellent reputation for customer responsiveness and a high standard of customer service in the North American gearing market.

David Brown (DB) already has a presence in key markets in North America, such as mining, and oil and gas, as well as through its premier double-enveloping worm gearing business Cone Drive, based in Traverse City, Michigan. The acquisition of Unigear and its industrial gear manufacturing and service capability is a key part of DB’s global and North American growth strategy. The business will be brought under the DB umbrella and will trade under the name: Unigear – a David Brown Company. The Unigear team, including President Ron Mehra and VP Peter Zurcher, will remain with the company and play key leadership roles going forward.

The parent company says it has a vision for the future and a defined growth strategy developed around expanding in key global markets including mining, oil and gas, conventional power, rail, and renewable energy such as solar, wind, and hydro. The acquisition of Unigear provides DB with wider access to many of these strategic industry segments coupled with local capability to manufacture and service industrial gear products for the North American market.

DB says expanding its aftermarket service offering is integral to its strategy to provide customers with locally employed specialist teams delivering world-class service. As well as becoming a North American manufacturing center of excellence, the company will also establish a service center at its Montreal facility to support customers in Canada. Additionally, the business is planning further expansion through the opening of a service center in the mining region of Kentucky, with near term plans for the establishment of additional service centers in strategic locations across North America.

David Brown
www.Davidbrown.com

Windpower Engineering & Development

By: N.K. Chinnusamy, President of Excel Gear Inc.

Gearboxes designed to acceptable and proven industrial standards can fail in wind turbine applications for reasons not fully understood. One reason could be the automated stopping and re-starting of the generator, which results in torque that far exceeds the levels of maximum rated power.

Wind-turbine applications require developing experimental and analytical tools along with instrumentation to fully understand the machine’s operational characteristics. Recent generations of wind turbines use condition-monitoring systems capable of measuring input and transient peak torques, along with the radial and axial movement of bearings and shafts to detect the onset of in-service failures before they occur.

Generating gear loads 
Wind turbine gearboxes connect a relatively low-speed rotor to a high-speed generator. Synchronous generators must run at synchronous speed, while induction generators run slightly above that (e.g. 105% of synchronous speed but that depends on load). For 60-Hz power, synchronous speed is found by dividing 3,600 by half the number of electrical poles in the generator. Synchronous speeds can be 3,600, 1,800, 1,200 rpm and so on. Rotor speed depends upon its diameter, typically 10 to 20 rpm. This combination requires a large speed-up ratio. Gear boxes for 60-cycle power with large speed-up ratios are complex in design and require epicyclic (planetary) gear sets. Planetary gear boxes have multiple meshes, speeds, and a complex power flow.

A varying wind speed and turbulence place loads on the input shaft in addition to the torque that all prime movers exert. The blades are long compared to the length of input shaft. In addition, they operate in the boundary layer so wind speed is higher at the top of the rotor sweep compared to the bottom. And then each blade tip generates a vortex which increases turbulence for downwind units.

The design of the rotor, hub, and main shaft is of utmost importance for the stability of the rotor. Varying wind speeds and turbulence produce moments on the end of the input shaft which cause shaft deflection and loads on bearings. If these effects can be isolated from the gearbox input shaft, then they do not impact the gearbox design. When the effects are not isolated, care must be taken so gear meshes are not misaligned.

Load equalization in planetary gears must be addressed for long gear life. The planet carrier must accommodate the centering force of the planet gears. Mesh frequencies must be set to prevent exciting resonant frequencies.

To make matters worse, wind-turbine gear boxes must work in extreme cold or hot temperatures. This requirement makes it difficult to find an ideal lubricant. Lubrication of bearings and gears is somewhat difficult in epicyclic gear boxes and care must be taken to assure that all bearings and gears are adequately lubricated under all temperature conditions. Cleanliness and the proper amount of lubrication are critical for the life of the components. During run-in on a test rig, cleanliness of lubrication must be checked periodically and proper lubricant flow must be established at low-speed running. Only then should final tests be run at full load and speed.

What went wrong? The broken gear comes from a three-stage industrial planetary gear box. The gear, made of 4320 carburized and hardened steel, failed at about 150 hours. Defective heat treatment was first suspected, but visual inspection revealed gear tooth surfaces like new and no evidence of defects, indentation, or overload. The gear runs on bronze bushing and its bore finish was rough with small cracks. But how deep? Metallurgical evaluation confirmed that gear tooth hardness and case depth were to spec but hardness in the bore was low. The failure was attributed to insufficient lubrication–the bronze bushing was almost friction welded to the gear. The cracks were deep, almost extending to the root. Essentially, the gear was falling to pieces.

Most gear failures will be the result of fatigue, which can be either surface or bending fatigue. However, tooth breakage can occur due to misalignment or deflection under load. Gears may fail due to defective material or heat treatment, cracks in hardening, gear grinding, or any combination of these. Pitting or scoring may also cause gear failure. Premature bearing failure accelerates the gear failure even when gears are correctly designed and manufactured.

Because gearboxes are located in nacelles 60 to 100 m off the ground, maintenance and repairs are difficult and expensive. Even routine maintenance is time-consuming. This means there is a high premium on long life and reliability.

The growth of the wind-turbine industry is creating technological developments focused on the manufacturing larger, more precise, and optimized gearing. The need for better performance, quieter operation, and higher efficiency means the primary goal is to limit losses and control other factors that reduce efficiency.

Because of the number of units that will be manufactured and installed, there is need for longer life and higher efficiency. One way to achieve longer life and higher efficiency is to develop new technology and computer tools that optimize gear geometry. Examples of technological development are improved finish with REM technology and electro-polishing. These technologies, which are used in automobile racing, are having significant, positive impacts in gearing.

In building a wind-turbine gearbox, it is also important to establish the correct bearing clearances, preloads, and proper gearbox operating temperatures critical to long life. Sophisticated measuring techniques with bearing-inspection gauges are necessary to ensure these results. Verification of gearbox performance through computerized analysis and testing is a crucial step to ensuring long life. The critical factor here, as with all similar power transmission applications, is that the gears are properly designed and manufactured. Other mechanical components that make up the assembly, along with the gearing, must be applied and designed so the overall system performance does not suffer shortcomings that could affect the unit’s performance and life.

Most gear failures will be the result of fatigue, a condition dependent on the force on the tooth and the number of tooth-load cycles. If a tooth has more than one load cycle per revolution, this must be taken into account when calculating life. Although life is usually stated in hours, it is the number of load cycles that is important. For example, if a pinion drives two gears, its life will be one half that of the pinion driving one gear (assuming the same torque on both gears).

Preliminary planetary design
Recent software can assist with gearbox design and analyses. One such program, Excel-lent, includes a table of commonly used gear materials with values from AGMA standards. If a special material is used, it can be added to the table. Also, if a higher-grade material is used, material values can be changed to reflect the new material.

Planetary gearboxes have multiple meshes and speeds, and complex power flows. This requires determining the number of stress cycles and torque for each gear. Planetary design can be evaluated using, for example, calculations from (gear researchers) Merritt chapter 11 or Buckingham page 129. The software mentioned calculates life based on a mesh of a single pair of gears. The software converts between life and stress cycles with:

C_s = 60Lω
where C_s = Stress cycles; L = life, hours; ω and ω = speed, rpm.

The Excel Gear staff performs a static backlash and compliance test. In it, the output shaft is locked and bi-directional torque is applied to the input shaft. An encoder measure rotation of the input shaft. Software plots rotation vs. torque, a hysteresis curve that allows determining torsional stiffness and backlash.

If a sun gear has three planets, the load input to the mesh calculation is 1/3 the total load. The actual life is also one-third the calculated life. For example, if the total power to a simple three-planet stage is 90-kW, the mesh power is about 30 kW. If the calculation shows a 300,000-hour life for the sun gear, the true sun gear life is 100,000 hr.

The reduction in load on the teeth in the mesh has a larger effect than the increased number of gear tooth load cycles. Consequently, the result of having three planets is much longer life.

For example, consider a simple planetary system consisting of a fixed internal gear with 72 teeth and an input shaft carrying three planet gears with 27 teeth driving a sun gear with 18 teeth. Following the calculations in Buckingham’s text, we get values in the table below.

The equivalent speed for calculating life is obtained by dividing the pitch-line velocity by the gear pitch circumference. The life obtained is for the given input speed. Using these values will give the life for an input speed of 100 rpm.

Notice that the power in the two planet gear meshes (PA and PS in the table) are less than the transmitted power, PT. Power in planetary meshes can be lower, as shown here, or much higher. When the power is higher, it referred to as circulating power.

A planetary box will ordinarily have three planets. In this case the table would be reflected in Value that change for a gearbox.

The input screen for gear design software Excel-ent shows a few values to consider.

With the speeds and loads determined, we proceed to the gear design. The planet gears are reverse loaded on every cycle. This means that 70% of the bending-fatigue strength should be used in the calculations. The easiest way to do this is with a new material. For example, if we plan to use 4140, add a new material with a name such as “4140 70% Bending”. Only the bending-fatigue value is changed for reverse loaded gears. All other values remain the same. Inputs to the design program would then be as they appear in the input screen.

The design program uses approximate methods so there is only one material specification for both gears. Note that the safety factor selected is “Wind Turbine and Critical Apps”. This uses a safety factor of 1.56 in calculation instead of the standard 1.0.

Data is then transferred to the analysis section, also part of the design software. The analysis program allows selecting different materials for the two gears. Note that the special material titled “4140 70% Bending” has been selected for the planet gear. Also, the Profile shifts have been set to zero. Profile shifts may be used in planetary gearboxes but they must be picked considering all three gears – sun, planet, and ring. For preliminary designs, using a zero value is best.

The analysis screen for Excel-ent tells a little about the proposed gear design.

Examining the data in the analysis section reveals a power capacity slightly low for sun-gear surface fatigue. This can be changed by increasing the face width to 90-mm or by changing the material to one with a higher surface-fatigue value. Ease in making such changes allows for checking many combinations.

Designing an epicyclic gear train, especially a compound one with an overall ratio of about 80:1, is typically a complex task. Many kinematically correct solutions will not work because of excess circulating power. Using software such as Excel-lent for load and life calculations can take a large amount of time off the task because non-feasible solutions are eliminated early.

When a designer selects three planets, care must be taken so that the sun and internal gears are coaxial. The three planets have a strong centering effect. If the sun and internal gears are not coaxial, a large load may be transferred to the bearings. In that case, both the bearings and the gears may fail prematurely.

A third software section provides design and manufacturing data. The software mentioned, for example, includes an option for calculating correction factors for balancing beam strength or specific sliding, a requirement for wind-turbine gear boxes.

Gearbox design is a complex, iterative process. The three programs in the Excel-lent software package can greatly reduce the time needed to process repetitive calculations. WPE

Windpower Engineering & Development

Wind turbine gearbox -Winergy

The design of a wind turbine gearbox is challenging due to the loading and environmental conditions in which the gearbox must operate. Torque from the rotor generates power, but the turbine rotor also applies large moments and forces to the wind-turbine drivetrain. It is important to ensure that the drivetrain effectively isolates the gearbox, or to ensure that the gearbox is designed to support these loads, otherwise internal gearbox components can become severely misaligned. This can lead to stress concentrations and failures.

Wind-turbine drivetrains undergo severe transient loading during start-ups, shut-downs, emergency stops, and during grid connections. Load cases that result in torque reversals may be particularly damaging to bearings, as rollers may be skidding during the sudden relocation of the loaded zone. Seals and lubrication systems must work reliably over a wide temperature variation to prevent the ingress of dirt and moisture, and perform effectively at all rotational speeds in the gearbox.

Gear and bearing fatigue standards by AGMA and ISO are used for design, these only capture a subset of the potential failure modes of the components. For instance, the ISO 6336 gear standard provides an established method for calculating resistance to subsurface contact failure and for tooth root breakage. The standards are doing their job, but these are not the most common failure modes observed in windturbine gearboxes. More common causes of failure are manufacturing errors such as grind temper or material inclusions, surface related problems, such as scuffing or micropitting, and fretting problems from small vibratory motions, such as may occur when a machine is parked. Scuffing is adhesive wear and subsequent detachment and transfer of particles from one or both of the meshing teeth (ref ISO13989-1). It can happen quickly and is generally considered to be associated with an absence or breakdown of the lubricant film under high loads (ISO 13989-2). Micropitting is a surface fatigue resulting from generation or numerous surface cracks, and is associated with insufficient film thickness (ISO 15144-1). Film thickness is affected by sliding speed, load, temperature, surface roughness, and chemical composition of the lubricant.

Many wind-turbine gearboxes have also suffered from fundamental design issues such as ineffective interference fits that result in unintended motion and wear, ineffectiveness of internal lubrication paths and problems with sealing. Improving the resistance of future gearbox designs to all these issues is a key for the future cost of energy generated by wind turbines.

Windpower Engineering & Development