Needle roller / cylindrical roller thrust bearings
SKF needle roller / cylindrical roller thrust bearings in the NKXR series (fig. 1
) combine a radial needle roller bearing and a cylindrical roller thrust bearing. The cylindrical roller and cage thrust assembly is identical to the 811 series. These combined bearings are supplied without an inner ring. In applications where the shaft cannot be hardened and ground, bearings can be combined with an inner ring (fig. 2
). Appropriate inner rings are listed in the product table
and must be ordered separately.
Needle roller / cylindrical roller thrust bearings in the NKXR series are separable. Both the cylindrical roller and cage thrust assembly and shaft washer can be mounted separately from the rest of the bearing. These bearings, without the designation suffix Z, should be oil lubricated whenever possible, as oil facilitates an adequate supply of lubricant to the bearing.
Needle roller / cylindrical roller thrust bearings with the designation suffix Z (fig. 3
) have a stamped steel cover without lubrication holes. The steel cover extends over the shaft washer of the cylindrical roller thrust bearing. It is firmly attached to the housing washer that is integral to the needle roller bearing outer ring. The cover makes these bearings non-separable.
Needle roller / cylindrical roller thrust bearings can accommodate axial loads in one direction and can, therefore, locate the shaft in one direction only. For short shafts and where changes in length as a result of thermal expansion are relatively minor, two bearings can be mounted back-to-back (fig. 4
). For these types of arrangements, SKF recommends elastically preloading the roller thrust bearings with cup springs (Belleville washers). This elastic preload helps to prevent the rollers from skidding if one of the thrust bearings becomes unloaded. Preload also improves the performance of the bearings while reducing noise levels.
Depending on their design, series and size, SKF needle roller / cylindrical roller thrust bearings are fitted with one of the cages shown in table 1.
The lubricants generally used for rolling bearings do not have a detrimental effect on cage properties. However, some synthetic oils and greases with a synthetic oil base and lubricants containing EP additives, when used at high temperatures, can have a detrimental effect on polyamide cages. For additional information about the suitability of cages, refer to Cages and Cage materials.
SKF supplies certain needle roller / cylindrical roller thrust bearings capped with a stamped steel cover over the thrust part of the bearing (fig. 3). The cover, which has no lubrication holes, forms a gap-type seal to retain the grease in the bearing. These bearings are identified by the designation suffix Z.
Greases for capped bearings
The thrust part of combined needle roller bearings with the designation suffix Z is supplied greased as standard. They are filled with high-quality grease under clean conditions.
The relatively large quantity of grease in the bearings means they can be operated for long periods before relubrication is required. SKF recommends SKF LGWA 2 grease, if relubrication is required.
The technical specifications for the initial grease fill and the grease for relubrication are listed in table 2.
Depending on the design and size, SKF supplies needle roller bearings with different features to facilitate efficient lubrication and relubrication.
The needle roller bearing of all combined bearings has an annular groove with one lubrication hole in the outer ring.