The test machine measures friction levels for a combination of seal material, shaft surface, and lubricant.

Nacelles become oily places when wind seals don’t work well. To find better seals, our companies developed a way to work in close cooperation with users to design tailor-made solutions that improve gearbox performance and life-seal predictions. Here’s what we learned.

A little background
Radial shaft seals are used in industrial gearboxes to prevent leakage and keep out contamination. The gearbox lubricant is usually selected according to the requirements of the gears and bearings, and not necessarily the seals. An equally important criterion for selecting the right radial shaft seal considers the influence of the lubricant on the wind turbine seal material along with load, speed, temperature, and life.

Gear oils are classified according to DIN standard 51 517/03 and, therefore, comply with considerable protection requirements against fretting and rolling bearing wear. Elastomer compatibility under static conditions is another consideration. However, practical experience has shown this is not enough to fulfill the multiple industry requirements regarding seal reliability and life under assumed operational conditions.

Industrial gear oils 
These normally consist of a mineral or synthetic-base oil. The base oil is 85 to 98% of the lubricant, complemented by additives. In Germany, 70 to 80% of gears are lubricated with mineral oil, while the remaining 20 to 30% use synthetic gear oils. Most wind-turbine gearboxes are filled with a synthetic lubricant.

Mineral oils have been the traditional basis for gear oils for decades. Additives normally dissolve well in these base oils. Elastomer (seal) compatibility is predominantly influenced by the additives selected.

Polyalphaolefins (PAO) are the most commonly used base oils for synthetic gear oils. They show better viscosity-temperature behavior, good low-temperature characteristics, and help reduce friction. Unlike mineral oils, elastomer compatibility with polyalphaolefin oils is influenced by the combination of base oils and additives.

Polyglycols (PAG) are another important group of synthetic gear oils. They have the same benefits of polyalphaolefins and like mineral oils, elastomer compatibility comes from the additives.

Synthetic esters offer the same advantages as polyglycols and polyalphaolefins. However, unlike mineral oils, elastomer compatibility is influenced by the different ester oil types and the additives.

Shaft seal materials 
Seals are usually made of rubber-like materials called elastomers. These long-chain molecules combine with additives to provide specific and technical material properties. Sealing materials based on butadiene-acrylonitrile and fluorinated rubbers are, with few exceptions, the proven and preferred wind turbine sealing materials used in industrial-gear applications.

Butadiene-acrylonitrile rubber (NBR) shows good swelling resistance in hydrocarbons along with high resistance to hot water, and inorganic acids and bases. However, when exposed to benzene, chlorinated hydrocarbons, esters, polar solvents, and polyglycol-ether brake liquids, considerable swelling occurs.

Fluorinated rubber (FKM) shows high temperature resistance, high chemical stability and good swelling resistance in hydrocarbons. Like NBR materials, FKM materials also tend to swell considerably when exposed to polar solvents and flame-resistant hydraulic fluids.

Interactions in the tribological system
The function of a radial shaft seal primarily depends on its geometry and the elastomer material. Radial shaft seals work like a small pump transporting liquids, gases, and dirt particles around it through the sealing edge. This effect is also used to pump small leaked amounts back into the sealed area. This intentional unmeasurable leakage is required for reliable lubrication and function of the radial shaft seal. It also has a considerable influence on its service life.

Leak-free sealing of the rotating shaft is ensured by the pressure of the sealing lip on the shaft. This force generates an asymmetric pressure distribution on the shaft dependent on the design of the seal-lip angle and tension-spring space. The pressure together with the elastic material properties cause the pump effect.
Considerable heat generates under the shaft seal’s sealing edge due to friction generated by the pressing force, the rotating speed of the shaft, and other factors,

Permanent interaction between the sealing material and lubricant is due to strong shearing forces combined with atmospheric oxygen. Therefore, it is critical to optimize the combination of shaft seal, lubricant, and shaft surface finish to ensure trouble-free operation.

Elastomer compatibility 
The chemical and physical resistance of the sealing material to the proposed lubricant must be considered along with the sealing material and operational temperature range. The behavior of rubber-like materials toward liquids is tested to DIN ISO 1817 in the specific medium or standard test liquids.

To determine static elastomer compatibility, S2 standard-test pieces and discs are punched from a 2-mm thick test sheet and stored in the test medium. For better correspondence between static and dynamic test results and practical conditions, Freudenberg increased the test duration for mineral oils from 168 to 1,008 h. After immersion in the test medium, test pieces are inspected with regard to changes in hardness, tensile strength, and ultimate elongation according to DIN 53504. Volume changes are determined according to DIN ISO 1817. Standard static compatibility tests were traditionally used for lubricant approvals, but it is now more representative to conduct dynamic seal testing as well. Dynamic oil compatibility tests following DIN 3761 and reducing friction deserve further discussion.

The illustration provides a closer look at the pump action or area of the seal. The tan arrows indicate lubricant circulation.

Flender AG developed the testing program. Dynamic seal testing is undertaken on DIN-standard test benches. The radial shaft seals are analyzed by measuring relevant functional parameters and by visual inspection. For approval, the variation compared to the original measurements is decisive. Visual inspection results are based on experimental values.
The friction between seal and shaft is an important indicator of the expected wind turbine seal life. It should be kept as low as possible because the friction heat generated by the sealing system influences gear efficiency and lubricant temperature. Temperature increases of only 10°C reduce the life of the radial shaft seal and gear oil by half.

The cooperative project, called Lube&Seal, focuses on determining the frictional moment, or power loss, for all wind seal and lubricant combinations as a function of speed, temperature, lubricant additives, and viscosity.

Initial findings on lubricant additives prove that different additives in the same base oil considerably influence the frictional moment and temperature in the shaft seal’s sealing zone. The example shows that the frictional moment and temperature are considerably higher with the lubricant polyglycol (PAG) 1 than with PAG 2 over the entire speed range, regardless of the oil sump temperature.

The cross section shows a typical elastomeric radial-shaft seal at work. The spring (circular detail) pulls it tight against the shaft.

Energy and CO2 emission reduction 
Basic research projects have shown that the right combination of radial shaft seal (material and shape) and lubricant can considerably reduce friction. A few rough calculations deliver the following figures, assuming a typical industrial gearbox with three seals and running for 5,000 hrs p.a. Then the power loss using:

-Standard radial shaft seals is about 90 W
-Optimized standard radial shaft seals is about 60 W

The potential energy-saving for an entire installation means a remarkable potential reduction of CO2 emissions. A gear manufacturer making one million gearboxes per year could reduce industrial CO2 emissions by about 25 tons simply through changing oil and seals (1 kW corresponds to average carbon dioxide emissions of about 500 g).
Industry requirements regarding tightness and reliability of radial shaft seals are ever-increasing. Elastomer compatibility of a lubricant, besides speed and temperature, is of particular importance to meet these increased requirements. Experience shows that testing only static elastomer compatibility is insufficient. Dynamic seal testing is indispensable because only these tests allow reliable conclusions on the long-term behavior of the shaft and seal tribosystem under lubricant exposure.

With an ideal combination of radial shaft seal, shaft, and lubricant, wind turbine seal life can increase three-fold and friction at the sealing lip can be reduced by up to 30%. As a consequence of this work, it’s also possible to considerably reduce CO2 emissions. However, the complex interaction between lubricant and sealing material must be thoroughly investigated to realize these reductions. WPE

By:
Erich Prem, Product Development Engineer Leadcenter Simmeringe, Freudenberg Simmerringe, www.Freudenberg-ds.com
Hermann Seibert, Head of Application Engineering, Klüber Lubrication Munchen KG, www.kluber.com

Windpower Engineering & Development

Clifford Spring Co. developed an energy absorbing spring for the oil and gas industry that is non corrosive, chemical resistant, with high and low temperature resistance. The company selected a Vitrex Peek to develop the spring.

Victrex Peek (polyaryletherketone) polymers is said to be a good choice when replacing metal wind-turbine components. The high performance and light-weight material allows up to a 70% weight reduction. This can lower stress on the components and reduce energy needed to power the turbine. The tribological characteristics of this thermoplastic compared to metals also helps reduce downtime due to the inherent ability to perform with or without lubrication.

The polymers are useful in wind turbine applications such as bearing separators, connectors, braking systems, and pitches and yaw drive components. The material can be applied to wind equipment to create products that are more corrosion and high temperature resistant, light-weight, durable and stable. Most notably, wind turbine products benefit from PEEK’s strength and wear properties. Victrex polymers also prevent galvanic corrosion, and are an exceptional insulator for generators, motors or transformers. The polymers help extend equipment life, reduce product failures, improve thermal performance and material strength.

Victrex Corp.
Victrex.com

Windpower Engineering & Development

Seals, indicated in black, are used on a variety of bearings in a nacelle. Improving seal function improves the reliability and efficiency of the turbine. -Simrit

• Size – shaft, housing bore, and available seal width
• Temperatures – continuous and maximum
• Application parameters – equipment, sealing surface misalignment to housing bore, dynamic shaft run-out
• Media – type and level of lubricant
• Pressures – continuous and maximum
• Shaft surface speed – continuous and maximum. From these, select either lip seals or isolators.

Lip seals are typically retained in a housing bore by a rubber-to-metal, or a metal-to-metal press fit requiring considerable installation force. Press fits can let metal shavings enter a bore housing, leading to contaminated lube and premature bearing failure. Also, nicks, burrs, or scratches on a shaft surface can damage a seal lip and produce a leak. A mounting tool prevents damage, such as lip roll-over.

By comparison, isolators are easy to install. Isolator seals facilitate the installation and maintenance of sealing systems. They usually have O-rings on their inner and outer diameters to seal on the shaft and against the bore. To prevent damaging O-rings, the sealing surfaces of the shaft and bore must be cleaned prior to installation. O-rings are not dynamic sealing elements so they are not subject to wear. Once equipment is cleaned and inspected, an isolator can usually be installed by hand pressure alone.

Leak detectors (sensors) on some seals measure leakage. (Read more in 1:09 Sensors) Other sealing tasks in a nacelle include:

At the hub where blades pitch: These junctions must be weather tight yet allow rotation. Several designs in a range of materials are well suited for the application. For instance, a form-pressed continuous ring works well on the large dimensions. Its good performance is due in part to a rust-proof tension spring which pulls the seal against a rim. The seal comes in 200 to 1,700-mm diameters. Profile rings for large seal areas are batch vulcanized for advantages over glued rings.

Profile rings come in standard materials and others, and are said to ensure long working lives, low torques, high resistance, and ensure against press-out. They are well suited for sealing large-diameter bearings on pitch and yaw mechanisms. Main shafts: An on-site joined seal concept is said to replace a typical turbine main-shaft seal in less than 30 min and without major disassembly. On-site joined seals claim to offer the same integrity, life, and performance as the seal fitted during manufacture by an OEM.

Slewing bearings: One company has added two more formulations to its line of nitrile butadiene rubber (NBR) sealing materials for grease and ozone resistance, low-compression set, and low-temperature capability. Both materials were developed for long-term reliability and superior resistance to environmental and chemical concerns.

The company says the recent NBR materials offers excellent cold flexibility, good compression set, and demonstrates superior aging in several greases, including Kluber, Fuchs, and Mobil. Recent premium materials are said to provide superior compression set and are optimized for use in Shell Rhodina BBZ greases.

Offshore wind foundations: These provide one of the more unusual sealing tasks for wind power. These are built with structures such as monopiles and tripods. A monopile (single leg) provides an example. After the pile is driven into the sea bed, its top extends up to about 16-ft., but below the water surface. A transition piece, about 80-ft high, is lowered over the top of the pile and will clear the water by some 40 ft. An inflatable grout seal, much like an inner tube, is then fitted in the space between the pile and transition piece. This seal inflates with a few psi to close the substantial gap between monopole and extension. Grout is then pumped into the gap above the grout seal to produce a strong joint. A floating crane then installs the turbine.

Windpower Engineering & Development

Seals, indicated in black, are used on a variety of bearings in a nacelle. Improving seal function improves the reliability and efficiency of the turbine.

In July 2008, the Energy Efficiency and Renewable Energy Department of the U.S. Department of Energy published a document 20% Wind Energy by 2030 that included wind-energy volume projections. The projections were based on many factors and assumptions, including justification of capital, improvement of wind-turbine efficiencies, lower annual maintenance costs, and reliable component life.

Increasing component reliability to achieve 20% wind energy will require advancements in component design, materials, and testing methods to validate wind-turbine components in the harshest environments. No component should be overlooked in the quest to increase energy captured by wind turbines. For example, if the seal for a blade pitch bearing—which many consider a commodity—is unreliable, designed improperly, or made with poor material, it will leak grease and eventually prevent the blades from pitching. The whole turbine will under-perform. Hence, small components can have a big impact when not properly designed or tested.

Slewing-bearing seals should also keep grease in and detrimental environmental elements (dirt, salt, water, sand) out of the bearing’s working elements. This sounds simple, but many factors contribute to the proper function and reliability of the seal. As OEMs build larger wind turbines and expand into new regions of the world, some factors will change to influence life expectancy and reliability of slewing-bearing seals. Thus, seal suppliers require close cooperation with those bearing companies, grease suppliers, and M&O groups to ensure the proper evaluation and evolution of these components.

Realizing that a single component cannot drive the 20% energy goal alone, it is vital to recognize and appreciate how each turbine

The seal cross section illustrates the complexity to which Simrit can design. Bearings in each turbine model can differ from each other and would require different seal cross sections. Seals for any diameter bearing are extruded, cut to length, and bonded.

component plays an important role in making the turbine industry successful. As a result, many companies continue to invest in new materials and designs. Two recent material examples are Ventoguard 453 and 454, formulated for ozone resistance and reduced compression set, as well as newly improved seal geometries that keep grease where it belongs, increases seal life, and improves turbine service.

In addition to design and material innovations, new technologies are being developed to address industry-related challenges. One example is a way to reduce seal friction that can influence the motor size needed to pitch a turbine blade. Larger wind turbines require large slewing bearings (over a meter in diameter) hence large diameter seals. Reducing the friction between bearing and seal helps decrease pitch-motor size. Smaller motors mean a lower-cost machine to rotate the blade and—to some extent— less tower weight.

Growing the wind industry market to 20% in North America will require continuous collaboration and experimentation to increase the reliability of wind-turbine components. Improved material properties and seal geometry of slewing-bearing seals is just one way to increase turbine life and reduce overall project costs. WPE

By: Steve Koch, Special Sealing Products Division of Simrit, Freudenberg-NOK Sealing Technologies

Windpower Engineering & Development

Extensive testing showed Turcon M12 resistant to most all media, including a broad range of lubricants, and has outstanding wear resistance and friction characteristics.

Turcon M12, a PTFE-based material launched in 2011 by Trelleborg Sealing Solutions, has received a 2011 Innovation Award from Flow Control Magazine. According to the company, the PTFE-based sealing material has unrivaled performance in key hydraulic sealing characteristics such as friction, wear, and high-pressure operation. Testing shows Turcon M12 resistant to most media, including a broad range of lubricants, and has outstanding wear resistance and friction characteristics. The cost-effective material also provides an extended seal life, as well as a wide operating window in temperature, pressure, and velocity.

 “Turcon M12 has exceeded even our expectations,” said Trelleborg Sealing Solutions Product Manager Nancy Getz. “On most every parameter it is better than, sometimes significantly, than previously recommended compounds or was at least equal to them.”

Trelleborg Sealing Solutions
www.trelleborg.com

Windpower Engineering & Development

A cross-sectional size of 25 mm x 32 mm so the UltraWind P1 Seal is intended to easily retrofit standard elastomer seals.

The Timken UltraWind P1 Seal is a significant wind-turbine bearing seal with a polyurethane design that provides increased resistance to abrasion for longer wind-turbine service life and more reliable performance than most other commonly used sealing materials. “Seals have an integral role in maximizing wind-turbine uptime and productivity because they prevent lubrication leakage and bearing contamination,” said Hans Landin, director of Process Industries original equipment and wind energy at Timken. “However, over time the cumulative impact of abrasive forces caused by varying loads and speeds, as well as extreme temperature fluctuations, rain, snow, debris, and lubrication challenges, can significantly reduce seal performance in wind-turbines. The new UltraWind P1 Seal addresses the problem with the latest polyurethane technology.”

In addition to its durable polyurethane base, the Timken UltraWind P1 Seal contains a variety of other features, including:

• A flexible sealing lip that handles misalignment or run-out in the application of the bearing. The lip’s special profile also helps minimize heat generation and cone wear while helping to accommodate bearing deflections.
• A corrosion-resistant, stainless steel garter spring that helps prevent rust.
• A machined design for a broader, more diverse range of applications, plus ease of installation via stress minimization. This design also allows for multiple positions at the cone lip OD contact; and

The Timken Company
www.timken.com

Windpower Engineering & Development

The company even held a party in anticipation of this momentous occasion. What started out as a simple gathering soon became a source of company pride and success. Over 400 employees, partners, clients, family members and special guests from around the world gathered to commemorate this achievement.

“It took Magni 37 years to achieve this milestone,” says The Magni Group’s Founder-Chairman, Dave Berry. “Now, with Magni’s rapid growth, the company is on course to produce one million gallons every year.”

The event featured lavish food, raffle prizes, and a number of surprises, including the high-energy Eastern Michigan University cheerleaders who burst through Magni Industries’ closed side doors to get the party started. On-stage, the Berry family clapped along with the crowd as the choreographed dance squad performed to the songs, “Celebration” and “The Final Countdown.”

As everyone anxiously awaited the 10 millionth gallon of coating product coming off the on-site production line, they counted down in true Times Square fashion. Three, two, one…and confetti filled the tent, after everyone squeezed off rounds from their six-round, celebratory popper pistols.

To put the icing on the cake, Detroit City Councilwoman Saunteel Jenkins, Oakland County Deputy Executive Phil Bertolini, and 13th District Michigan Congressman Hansen Clarke stopped by to pay tribute to Magni’s accomplishment. Speaking of cake, everyone also enjoyed a piece of the special four-tier, building-shaped dessert.

Once the confetti settled, Magni Group’s President Tim Berry unveiled another off-the-agenda surprise when he presented Dave Berry, company owner, (and dad), with a giant 3.5’x1’, accordion-folded birthday card, signed by every Magni employee from the 20 Magni facilities worldwide. It took five months for the card to travel across six continents to collect 380 signatures. The card was printed with “Happy Birthday” in seven languages—English, French, German, Japanese, Korean, Portuguese, and Chinese.

Other surprises included Ted Berry, Magni’s Executive VP and singer-guitarist of Magni’s house band, giving his fellow musicians new logo-imprinted stools, stickers, and coasters.

“Only Magni people and our cherished clients can bring such profound energies to an event and make it unforgettable”, says Jen Hall, Magni Marketing. “It’s just magical to feel the enthusiastic spirit only hard-working, good-hearted partiers can bring.  And it’s a relief to finally have all the special secrets revealed!”

With all of the excitement, believe it or not, no one even noticed the rain.

The Magni Group Inc. www.themagnigroup.com

Windpower Engineering & Development

“The windpower industry is growing and evolving at a rapid pace,” says Reddy Tudi, sales director, renewable energy, Simrit. “To meet the needs of this developing industry, Simrit engineers are continually driving the development of materials and seals that will extend the performance of windpower applications.”

Ventoguard 453 and Ventoguard 454 are the next-generation of the company’s line of NBR materials. Primarily used in slewing bearing profile seals in wind turbines, both materials were developed for long-term reliability and superior resistance to environmental and chemical concerns.

Additionally, each material was designed to address a specific windpower-related challenge with the ultimate goal of helping prolong the maintenance cycle of the sealing application. The company says Ventoguard 453 is a premium material that offers excellent cold flexibility, very good compression set and demonstrates superior aging in several greases, including Kluber, Fuchs and Mobil. Simrit’s Ventoguard 454, also a new premium material, provides superior compression set and is optimized for use in Shell Rhodina BBZ greases.

Simrit www.simrit.com

Windpower Engineering & Development

Application data required to select a seal includes:

•Size – shaft, housing bore, available seal width

•Temperature – continuous and maximum

•Application parameters – equipment, sealing surface    misalignment to housing bore, dynamic shaft run-out

•Media – type and level of lubricant

•Pressure – continuous and maximum

•Shaft surface speed – continuous and maximum.

From these, select either lip seals or isolators.

Lip seals are typically retained in a housing bore by a rubber-to-metal or metal-to-metal press fit, requiring considerable installation force. Press fits can let metal shavings enter a bore housing, leading to contaminated lube and premature bearing failure. Also, nicks, burrs, or scratches on a shaft surface can damage a seal lip and produce a leak. A mounting tool prevents damage, such as lip roll-over.

By comparison, isolators are easy to install. Isolator seals facilitate the installation and maintenance of sealing systems. They usually have O-rings on their inner and outer diameters to seal on the shaft and against the bore respectively. To prevent damage to O-rings, the sealing surfaces of the shaft and bore must be cleaned prior to installation. O-rings are not dynamic sealing elements so they are not subject to wear. Once the equipment is cleaned and inspected, the isolator can usually be installed by hand pressure alone.

There are many other sealing tasks in a nacelle. Hydraulic equipment, of course, needs them. Leak detectors (sensors) on some seals measure leakage. Onboard electronics then provide some analysis and can send results to a computer or telephone. This allows remotely monitoring a seal and scheduling an exchange when necessary in a normal maintenance interval.

German DIN 3760 standards describe function and lifespan for such seals. The sensor-seal combination is available in many different dimensions. The seals protect motors and machines in original equipment and provide options for maintenance personnel. Designs in special materials are available especially for wind turbines.

Where blades meet hubs also call for a seal. These junctions must all be weather tight yet allow rotation. Several designs in a range of materials are well suited for these applications. For instance, a form-pressed continuous ring also works well on the large dimensions encountered in wind turbines. Its good performance is due in part to a rust-proof tension spring which presses permanently against the seal rim. The seal comes in 200 to 1,700-mm diameters.

Profile rings for large seal areas are batch vulcanized for advantages over glued rings. Profile rings come in standard materials and others. The rings are said to ensure long working lives, low torques, high resistance, and security against press out. They are well suited for sealing pivoting large diameter bearings found on pitch and yaw mechanisms.

One of the more unusual sealing tasks on turbines deals with offshore foundations. Wind-power stations there are built using structures such as monopiles and tripods. How the turbine is installed depends on soil properties, water depth, and contractor experience. A monopile (single leg) provides an application example. After the pile is driven into the sea bed, its top will extend up to about 16-ft below the water surface. A transition piece, about 80-ft high, is lowered over the top of the pile and will clear the water by some 40 ft. The space between the pile and transition piece is sealed by an inflatable grout seal, much like an inner tube. This seal inflates with a few psi to close the substantial gap between monopole and extension. Grout is then pumped into the gap above the grout seal to produce a strong joint. A floating crane then installs the tower to the extension and the turbine.

Windpower Engineering & Development

Trelleborg Sealing Solutions
www.tss.trelleborg.com/us

Windpower Engineering & Development