New ideas in wind turbine main bearing and gear seals

Bernd Koch, CEO, Merkel Heavy Duty Hydraulics Div., Freudenberg-NOK Sealing Technologies

Bernd Koch

Bernd Koch

Efforts to maximize the potential of wind as a reliable power source have led to significant investments in onshore and offshore-wind turbine technology. One predominant industry trend to emerge is to increase the size of the wind turbines, thereby maximizing the output per unit.

Merkel_Radiamatic_R55_incl_functional_comparison (2)

Merkel Radiamatic R55 raidal shaft seal includes a deflector lip. The inset box illustrates how the R55 improves over conventional designs when it must work on shafts up to 1.6 mm smaller or larger than nominal.

This trend puts pressure on engineers who are trying to cope with the challenges of harsh environmental conditions and structural requirements due to more weight, increased heights, stronger vibrations, ongoing maintenance, along with high expectations regarding function and the lifecycle of parts. To control costs, wind-turbine parts must meet a lifetime expectation of 20 years. Because of this expectation, reliable sealing technology is one key element for higher performance and longevity.

Sealing a wind turbine raises numerous challenges such as contact surface (roughness and hardness), conditions of use (vibrations and shaft eccentricities), environmental conditions (extreme heat or cold, UV rays, moisture), internal media (oil and grease), and more. Seal suppliers must recognize and understand these challenges to identify and develop the proper seal designs for current and future applications and their environments.

Because of the limited testing available for wind turbines due to increasing size, sealing experts know that ensuring optimum growth in the wind power market requires advances in component design, materials, and testing methods.

The four main bearing seals examined here show the new thinking in their design that aims to keep bearings and gears working for their expected 20-year life.

Seals for the main bearing

The Merkel Radimatic R35 comes with an optional bonding set. This allows bonding the seal to most any large diameter shaft.

The Merkel Radimatic R35 comes with an optional bonding set. This allows bonding the seal to most any large diameter shaft.

The most crucial seal in a wind turbine is the main bearing seal. Its task is to keep the lubricant in place and protect the bearing against harsh environments.

The Radiamatic R 55, a new radial shaft seal successfully addresses these challenges. The seal includes a deflector lip that protects wind-turbine systems against environmental influences such as dust, water, and salt air, and it eliminates need for a second seal – a common design element used in existing wind systems.

The seal reduces installation space, eliminates the expense associated with the second seal, makes no functional compromises, and can even offset radial play or shaft eccentricity without neglecting its sealing and deflecting functions.

In comparison tests, the R 55 demonstrates exemplary behavior in shaft contact and in the prevention of leakage and wear. The deflecting and sealing lips always stay in contact with the shaft to keep lubricant in and contaminants out. In addition, the sealing lip maintains linear contact in the event of positive shaft deflection, thus contributing to a long service life.

Another seal for the main bearing

Merkel Enviromatic EA provides a deflector with a precise sealing lip. The inset box shows that standard V-ring seals work with greater stresses on shafts larger or small than recommended.

Merkel Enviromatic EA provides a deflector with a precise sealing lip. The inset box shows that standard V-ring seals work with greater stresses on shafts larger or small than recommended.

Another field-proven radial shaft seal for wind turbines is made of the material HNBR (Hydrogenated Nitrile Butadiene Rubber). It provides a constant line force on the sealing lip to securely handle all types of oil or grease-lubricated bearings. A special feature, the so-called rubber lock (also available for the type R 55), allows easy on-site seal replacement on site without dismantling surrounding components. The seal is cut at the rubber lock and precisely rejoined by the Merkel RK 15 bonding set after placing it around the shaft. This seal provides wind system manufacturers with significant advantages.

The R 35 is often used in combination with a standard V-ring or complemented by a deflector, such as the one described next.

Deflector for the main bearing and gearbox

Another solution for protecting a main bearing is the Enviromatic EA, a deflector seal with an axially acting sealing lip that forms a sealing system in combination with standard grease or oil-lubricated radial shaft seals. This seal, also installed at the gearbox, is typically used in combination with a labyrinth seal.

The Enviromatic EA’s deflector protects the main seal and consequently the roller bearings as well as the gearbox against dust, scale, carbon, particles, spray water, ozone, salt air, and more.

Merkel Radimatic RCD works with labyrinth seals.

Merkel Radimatic RCD works with labyrinth seals.

The deflector – as well as all radial shaft seals – are made of HNBR, called Ventoguard 467, a material that addresses industry challenges such as ozone resistance, while providing a critical added benefit of extending the life of the system. The deflector’s special geometry allows for uniform contact pressure of the sealing lip, even when there is a large axial clearance.

The Enviromatic EA’s high wiping action ensures a tight seal against environmental factors. It provides high wear resistance and is available for shaft diameters from 200 to about 4,000 mm. Freudenberg-NOK uses a proprietary technology to vulcanize endless elastomer seals that are larger than 10-m diameter on a 2.5 m press. Also, the precise bonding technology found in the Radiamatic types R 35 and R 55 seals offer a time-saving replacement of various seals solutions on site, when needed.

Touchless main gearbox protection

The main gearbox may be the heart of a wind turbine. It is the link between rotor and generator, when the turbine is not a direct-drive design. A polytetrafluoroethylene (PTFE) labyrinth seal called Radiamatic RCD is an efficient design that offers reliable sealing performance with little to no wear, friction, or self-heating.

The seal’s advantages include a consistently reliable sealing function, service life that lasts several decades, and cost savings that result from shaft surfaces and housings that are inexpensive to manufacture.

This two-part seal features a rotating inner ring and a stationary outer ring. The inner ring is open and is kept in place on the shaft by springs. The outer ring sits securely in the housing bore. At the entrance, the seal prevents oil from splashing on the inner labyrinth by the centrifugal effects of the rotating inner part.

The main components of the seal are made of PTFE, which means that they are ozone resistant, slow to age, and chemically resistant to additive oils – all of which are relevant for wind turbine gears. This, along with a non-contact operation in wind turbine gears, means the service life of these seals is expected to last several decades. WPE

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