Bearing manufacturer SKF has introduced new high-capacity cylindrical roller bearings (HCCRB) offering increased load carrying capacity and longer life in wind turbines.
The new bearings are targeted at all the classical cylindrical bearing positions in wind turbine gearboxes where non-separate mounting is acceptable – mainly in planetary wheels.
The unique feature of this HCCRB is that load carrying capacity has been increased substantially while maintaining the boundary and internal dimensions of standard cylindrical roller bearings. This means that the new design combines the load carrying capacity of a full complement bearing with the benefits of a caged bearing. As well as higher carrying capacity the new designs offer increased life. Calculations show that in one particular application the SKF Explorer version of the HCCRB will increase bearing rating life by 35% when compared to the standard full complement version, and 43% when compared to a standard caged version.
According to the ISO 281 standard there are two ways to increase the load carrying capacity while maintaining standardised boundary dimensions:
* Increase the dimensions of the rollers and maintain the same number of rollers.
* Increase the number of rollers and maintain the roller dimensions.
This method presents other technical problems because increasing roller dimensions reduces the inner and outer ring thicknesses and also the width of the side flanges. This results in reduced ring stiffness and flange strength which in turn increases the risk of reduced bearing life due to increased wear, fretting corrosion and ring creep or even ring fracture.
Increase roller count
This method offers theoretical improvements. The design that allows the maximum number of rollers is the full complement design. With this design, the maximum number of rollers is placed between the rings leaving no space for a cage and this type of bearing is available from a number of bearing suppliers. Such bearings have limitations because the rollers are always in direct contact with each other, causing sliding and increased friction and heat generation. Under certain circumstances, such as higher speeds, this can lead to wear and premature bearing failure.
Bearings with cages (the vast majority of bearings produced worldwide), do not have this problem because the rollers sit in cage pockets that prevent contact. However, the addition of the cage takes up space which reduces the maximum number of rollers possible.
New cage design
SKF achieved the HCCRB’s unique feature by a completely new window type cage design that resulted in two versions:
* An outer ring shoulder guided cage (code JA).
* An inner ring shoulder guided cage (code JB).
With these cages an additional one or two rollers per row can be accommodated. Even more rollers can be fitted in customised bearings. The rollers deliver the additional carrying capacity, while the cage increases bearing life and overall performance compared to a full complement version.
The new cage designs were tested in prototype HCCRB bearings for more than one year in many different tests to fully evaluate their capabilities and compare them with bearings fitted with standard cages and full complement bearings. All tests showed no limitations of the new bearing designs compared to standard designs. In fact the new cage designs provide the following additional benefits:
* Improved oil flow by decreased cross section of the cage resulting in reduced heat generation.
* Lower weight resulting in reduced inertia.
* Reduced slip in low load conditions providing reduced risk of smearing.
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