Combined Needle Roller / Angular Contact Ball Bearings

Needle roller / angular contact ball bearings combine a radial needle roller bearing with a radial angular contact ball bearing to produce a low cross section bearing that can operate at high speeds and accommodate heavy radial and light axial loads. The radial loads are accommodated exclusively by the needle roller bearing.

SKF supplies these bearings in the following two series:

• NKIA series (fig), accommodates axial loads in one direction
• NKIB series (fig), accommodates axial loads in both directions

Note Bearings of both series are separable. In other words, the inner ring can be mounted separately from the outer ring, roller and cage assemblies. However, the bearings must be kept together as supplied � e.g. the outer ring, roller and cage assembly from one bearing are not interchangeable with the roller and cage assembly from another seemingly identical bearing.

Combined needle roller / angular contact ball bearings, NKIA series

SKF needle roller / angular contact ball bearings in the NKIA 59 series (fig) can accommodate axial loads acting in one direction and can therefore locate the shaft in one direction.

For short shafts, where changes in length as a result of thermal expansion are relatively minor, two bearings in the NKIA 59 series can be used in a back-to-back (mirrored) arrangement (fig).

Combined needle roller / angular contact ball bearings

Combined needle roller / angular contact ball bearings, NKIB series

Needle roller / angular contact ball bearings in the NKIB 59 series (fig) are able to locate a shaft in both directions with an axial play between 0,08 and 0,25 mm. To facilitate mounting, the bearing has a two-piece inner ring. When mounting the inner ring, it is important that the two pieces are mounted immediately adjacent to each other without any gap between them.

Dimensions The boundary dimensions of bearings in the NKIA 59 series are in accordance with Dimension Series 59 of ISO 15:1998.

With the exception of the inner ring width, which is extended on one side, the boundary dimensions of bearings in the NKIB 59 series are in accordance with Dimension Series 59 of ISO 15:1998.

Tolerances SKF supplies needle roller / angular contact ball bearings to Normal tolerances in accordance with ISO 492:2002 as standard.

The following tolerances in the NKIB 59 series are exceptions:

• The bore diameter of the narrow part of the inner ring is slightly larger.
• The width tolerances of the complete inner ring is 0/�0,3 mm for all sizes.

Internal clearance SKF supplies needle roller / angular contact ball bearings with Normal radial internal clearance as standard.

The values for radial internal clearance are listed in the table and are in accordance with ISO 5753-1:2009. They are valid for unmounted bearings under zero measuring load.

Misalignment SKF needle rollers are relieved slightly toward their ends. The modified line contact achieved between the raceways and the needle rollers provides favourable load distribution in the bearing and minimizes damaging edge stresses. It also enables needle roller / angular contact ball bearings to accommodate a minimum amount of misalignment. The detrimental effect of misalignment increases with increasing bearing width and load. Needle roller / angular contact ball bearings are sensitive to misalignment. If misalignment can not be avoided, contact the SKF application engineering service.

Cages The radial needle roller bearing is fitted as standard with a machined steel (fig) or a sheet steel cage (fig). The balls of the angular contact ball bearing are retained in an injection moulded cage made of glass fibre reinforced polyamide 66 (fig).

Note Limited by the polyamide 66 cage, needle roller / angular contact ball bearings should not be operated at temperatures above 120 �C.

The lubricants generally used for rolling bearings do not have a detrimental effect on cage properties. Exceptions are a few synthetic oils and greases with a synthetic oil base and lubricants containing a high proportion of EP additives when used at high temperatures.

For detailed information about the temperature resistance and the applicability of cages, refer to the section Cage materials.

Shaft and housing tolerances For needle roller / angular contact ball bearings SKF recommends machining the shaft seat to a k5 tolerance class, and the housing bore seat to an M6 tolerance class.

Tighter fits than those achieved with these tolerance classes should be avoided as they could have a negative influence on bearing performance.

In addition to their interference fit on the shaft and in the housing, the bearing rings should be located axially by a shaft shoulder or snap ring.

Lubrication

To facilitate efficient lubrication, the needle roller bearing has an annular groove with one lubrication hole in the outer ring. Depending on the application, needle roller / angular contact ball bearings can be grease or oil lubricated. In the case of grease lubrication, the needle roller bearing and ball bearing should be filled with the same lubricant. If grease lubrication is going to be used, the bearing should be lubricated before mounting.

Relubrication intervals for needle roller and angular contact ball bearing must be calculated separately. The lower of the two intervals should be used.

For additional information about lubrication, refer to the section Lubrication.

Minimum load To achieve satisfactory operation, needle roller / angular contact ball bearings, like all ball and roller bearings, must always be subjected to a requisite minimum load acting in both the radial and axial directions. Applying a requisite minimum load is especially important for bearings that operate at high speeds or are subjected to high accelerations or rapid changes in the direction of load. Under these conditions, not applying the correct minimum load can result in damaging sliding movement between the rollers and raceways caused by the inertial forces of the rolling elements and cages, and friction in the lubricant.

The requisite minimum radial load to be applied to the radial needle roller bearing can be estimated using

Frm = 0,02 C

The requisite minimum axial load for the angular contact ball bearing can be obtained using

where

Frm = minimum radial load [kN]

Fam = minimum axial load [kN]

C = basic dynamic load rating [kN]

C0 = basic static load rating [kN]

dm = mean bearing diameter

= 0,5 (d + D) [mm]

n = rotational speed [r/min]

When starting up at low temperatures or when the lubricant is highly viscous, an even greater load may be required. However, the weight of the components supported by the bearing, together with the external forces, generally exceeds the requisite minimum load. If this is not the case, the needle roller / angular contact ball bearing must be subjected to an additional radial and/or axial load.

Equivalent dynamic bearing load

The equivalent dynamic bearing load is equal to the calculated bearing load for the radial as well as the axial part of the combined bearing.

Therefore, for the radial needle roller bearing

P = Fr

and for the angular contact ball bearing

P = Fa

whereas Fa must not exceed 0,25 Fr

where

P = equivalent dynamic bearing load [kN]

Fr = actual radial load [kN]

Fa = actual axial load [kN]

Equivalent static bearing load

The equivalent static bearing load is equal to the calculated bearing load for the radial as well as the axial part of the combined bearing.

Therefore, for the radial needle roller bearing

P0 = Fr

and for the angular contact ball bearing

P0 = Fa

whereas Famust not exceed 0,25 Fr

where

P0 = equivalent static bearing load [kN]

Fr = actual radial load [kN]

Fa = actual axial load [kN]

Load carrying capacity and life It is necessary to calculate the life of the radial needle roller bearing and the angular contact ball bearing separately for combined needle roller / angular contact ball bearings.

Combined needle roller bearings | Combined needle roller / angular contact ball bearings | Combined needle roller / thrust ball bearings | Combined needle roller / cylindrical roller thrust bearings