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Bearings for small wind turbines

02 August 2012

The large wind turbines that have become a familiar sight across the UK landscape over the last few years are now set to be joined by smaller generating units, both grid-connected and stand-alone, as power generation becomes more localised, especially in rural areas.

Small wind turbines are usually defined as having a capacity of up to 50kW, with a typical blade diameter of at least 2m. The requirements of small wind turbines are identical to their larger cousins: namely, high efficiency, high reliability, low noise and minimum maintenance. Noise reduction is a particular focus in small wind turbine design, especially when units are located adjacent to residential or commercial properties; turbine shaft bearings can have a significant impact in this area.

NSK has been involved in the design and specification of bearings for small wind turbines from their inception, and the experience and expertise it has gained is now helping designers and manufacturers to achieve optimum output and reliability, both on existing designs and on new developments that seek to achieve even higher levels of efficiency and reliability.

Irrespective of whether a turbine is large or small, the main shaft bearings that transmit rotary motion from the blades to the generator are among the most critical components of the entire turbine design. Small wind turbines generally employ either deep groove ball bearings or spherical roller bearings as main shaft bearings, the choice of bearing type depending on the design and rating of the turbine. If the loading permits, deep groove ball bearings are a reliable and cost effective option.

NSK Self Lube housed bearing units are widely used across the small wind turbine sector, because of their ability to reduce bearing (component) complexity and mounting costs. Ready assembled to bolt into place, Self-Lube units are easy to mount and able to accommodate an initial misalignment of up to two degrees. They are fitted with seals and are factory charged with a suitable amount of lubricant, which can be replenished via a grease nipple provided on the bearing housing.

In addition to its installation advantages, a key operational benefit of Self- Lube units is its ‘super-finished’ bearing raceways, which ensure quieter running and extended lifetime under boundary lubricated conditions – a specific advantage in small wind turbine applications that have low noise requirements, relatively low running speeds and long maintenance intervals.

For larger turbines, typically greater than 20kW, spherical roller bearings in split plummer blocks tend to be used to support the main shaft. In common with Self-Lube, split plummer blocks (such as NSK’s SNN range) provide easy installation and maintenance. Equipped with spherical roller bearings, they are capable of supporting high axial and radial loads as well as misalignment, at the same time delivering reliability and long operating life.

Also offering specific operating advantages in small wind turbine applications is NSK's HPS (High Performance Standard) range of spherical roller bearings. Due to their internal design, all spherical roller bearings have a sliding motion between the rollers and raceways, known as differential slip. This slippage is most noticeable at low speeds, resulting in wear on the raceways and rollers.

NSK's HPS spherical roller bearings feature a special outer ring raceway profile that minimises slippage, prolonging bearing life under conditions of relatively low speed and marginal lubrication, which are typical of many small wind turbine applications.

Sealing and lubrication
Selection of type aside, bearing sealing and lubrication are other important factors to take into account. While the running torque of an open bearing is usually very low, the addition of integrated contact seals on ball bearings, or rubbing seals in plummer block housings, can greatly increase friction and torque levels, lowering the efficiency of the turbine. This low torque requirement needs to be balanced against the need for contaminant exclusion and grease retention.

NSK’s best compromise is the use of non-contact rubber seals on deep groove ball bearings, the sealing lips of which form a tight labyrinth via a specially machined groove on the bearing inner ring, while avoiding full contact. For plummer blocks, felt seals and rubber lip seals have relatively high running torque; and after lengthy exposure to the weather, noise (squeaking) and wear can be issues with these seal types. For optimum efficiency and low noise, as far as plummer blocks are concerned, NSK recommends the use of non-contact labyrinth seals.

Selecting the right type of grease, its quantity and the lubrication interval will have a direct effect on bearing life. The main selection criteria for small wind turbine lubricating greases are long lifetime, low torque and noise, low temperature capability and, in the case of blade pitch bearings, the ability to withstand high loads and small amplitude oscillation.

NSK's HLT (high/low temperature) mounted bearing units, which form part of the Self-Lube range, are factory charged with a low noise and low temperature grease, allowing the bearings to operate successfully at temperatures as low as –40oC. For sealed ball bearings, NSK's in-house developed greases such as NS7 and EA3 give excellent low torque and low noise performance over a wide temperature range.

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