What to know when purchasing Duplex Bearing Sets
By Cory Mittleider on 3/19/2026
When doing wind turbine gearbox repairs, either in the shop or the field, there are many applications that use two bearings next to one another. Often, for improved performance, bearing manufacturers supply these as a “Duplex Set”, so let’s explore what that means.
What does duplexing mean?
A duplex bearing means that there are 2 individual bearings included together as a matched set within one part number being offered. Since there are several different bearing types used in wind turbine gearboxes this can look different for different positions and can sometimes have different nomenclature, but in most cases duplexed bearings will include a “D” in the nomenclature.
Why duplex bearings?
The benefit of duplex bearings is the additional tolerance control applied as the matched set vs buying single bearings. Since the two bearings in a duplex set are matched at the factory they will offer improved performance and load sharing between them vs buying singles and putting them together.
Example 1: Cylindrical Roller Bearings (CRB) used in planetary section
One of the most popular planet bearings is an NNCF type CRB. This bearing type has 2 rows of rollers but a single inner and outer ring as shown here.
NNCF type cylindrical roller bearing
A common bearing type used in planet wheels of wind turbine gearboxes.
Since these are very commonly used as 2pc side by side in the planet wheel they are typically duplexed to take advantage of the tighter tolerances and ensure equal load sharing.
2x NNCF bearings installed in planet wheel
The gif below shows what it means to purchase a duplex set of these NNCF bearings to be used in the planet section of a gearbox. This illustrates how even though 6 bearings were technically removed from the planet section, you only need to purchase 3 duplex sets to replace them.
Purchase of duplex sets for planet section repair of wind turbine gearbox
Example 2: Tapered Roller Bearing (TRB) used in HSIS-GS position
Many gearboxes use a pair of TRB’s on the gen side position of the HSS & HSIS shaft and repairs to this section of the gearbox are common to do uptower. When looking at a drawing you will see 2 distinct bearings or during disassembly you will find 2 separate bearings, but just like above, often these bearings are often offered as a duplex set, and because of the design of TRB’s this set often (but not always) includes a spacer as seen in this picture.
Tapered bearing set including 2 individual bearings and a spacer between the outer rings
The gif below shows what it means to purchase a duplex set of TRB’s, what you may see in the part number, and how they will appear when reinstalled in the gearbox.
Note: the gif shows a duplex set of 2 different tapered bearings that have the same ID and OD, but different width. The same breakdown shown also applies when the set is 2 of the same size bearing.
Purchase a duplex set of tapered bearings for HSS or HSIS wind turbine gearbox replacement
When opening a crate containing a duplex set you should be aware of what is inside. The gif below shows one of the more complex types which is a duplex set including 2 different TRB sizes to demonstrate the packaging and labeling method but the same concept would apply to other duplex bearing types as well. Note: this gif shows NSK’s methods specifically, other manufacturers may use slightly different methods.
Contents and labeling of duplex tapered bearing set with spacer
The labels on all parts will have the same truncated part number from the crate. Often with a set of different TRB’s this truncated part number only shows one of the two bearing sizes, but don’t worry, as you read further on this label you will see indication if the part is from bearing A or bearing B as well as which component is inside (inner ring vs outer ring).