Bearing technology critical for small wind turbine performance

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 standalone, 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 requirements are a particular area of focus in small wind turbine design, especially regarding sites that are located adjacent to residential or commercial properties. With constant blade and main shaft rotation, bearings are a critical factor in achieving reduced noise across these conurbations. NSK has been involved in the design and specification of bearings for small wind turbines from their inception, and the resulting experience and expertise gained is helping designers and manufacturers to achieve optimum outputs and reliability from their small wind turbines, 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 the most critical bearing component in the 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 provide a reliable and cost-effective bearing 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 initial misalignment of up to 2 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 their installation advantages, a key operational benefit of NSK's Self-Lube units is their superfinished bearing raceways. Superfinished raceways result in 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.

Support for high axial and radial loads

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 bearings, split plummer blocks (NSK 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 high levels of reliability and 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. However, the design of NSK's HPS spherical roller bearings has a special outer ring raceway profile that minimises roller slippage, resulting in longer operating life, particularly under conditions of relatively low speed and marginal lubrication, which occur in many small wind turbine applications.

Equally important as bearing selection, in ensuring optimised wind turbine performance, is bearing sealing and lubrication. 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 has found that the best compromise between these two requirements is the use of non-contact rubber seals (NSK suffix VV) on deep groove ball bearings. The sealing lip of VV seals forms a tight labyrinth, via a specially machined groove on the bearing inner ring, while avoiding full contact. This gives a good level of compromise between high efficiency and good sealing.

For Plummer blocks, felt seals and rubber lip seals have relatively high running torque. In addition, NSK's experience in the small wind turbine industry has shown that when exposed to the elements over time, noise (squeaking) and wear can be issues with these seal types. For optimum efficiency and low noise, NSK recommends the use of non-contact labyrinth seals with Plummer blocks (NSK TS-U).

Low-noise, low-temperature grease

As regards lubrication, selection of the correct grease type, grease quantity and relubrication interval not only has a direct effect on bearing life, it can also be the main factor in determining the wind turbine service interval. 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 oscillations. NSK's HLT (high/low temperature) mounted bearing units, 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 -40degC. 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.

The ability of NSK to provide complete bearing systems in wind turbine projects was evidenced in a recent project in which savings of €295,000 were delivered to a manufacturer of small turbines. The savings were achieved through a programme of material substitution and longer-life lubrication technology.

NSK became involved in the small wind turbine project because the customer was experiencing problems due to the prohibitive cost of bearings. The manufacturer was facing a stark choice: improve the purchasing and manufacturing costs for the project, or it would not continue.

An NSK Engineer visited the customer to review the machine design. This highlighted several areas where unit build cost could be reduced substantially. Following the visit, a supporting design justification report, and bearing and grease life calculations were produced.

These not only underpinned the design cost reductions, but also promised benefits such as simplified assembly procedures, and improvements in overall bearing performance, reliability, and bearing life.

The customer acted upon NSK's recommendations; substituting NSK premium brand single-row radial ball bearings, where before there had been a selection of mixed-brand products, including single-row radial ball bearings and angular-contact bearings. The NSK bearings are lubricated by NSK's long-life grease which, it is claimed, delivers a grease life of over five times that of the mixed-brand bearing package. This improvement in grease life has enabled major reductions in maintenance to be achieved, helping to keep the small turbine in service for substantially longer. The quantification of this improvement, in conjunction with the additional improvements gained from the NSK package, has been calculated by the customer to exceed €295,000.

To find out more about NSK's range of bearings for wind turbines, visit the website at www.nskeurope.com.