Needle Non Thrust Roller Bearings in two series:
NX series  with a full comple­ment thrust ball bearing
NKX series  with a thrust ball bearing with a cage
These combined bearings:
consist of a radial needle roller bearing and a thrust ball bearing
are supplied without an inner ring
can be combined with an inner ring , where the shaft cannot be hardened and ground
can accommodate axial loads in one direction and, therefore, locate the shaft in one direction only can be mounted back-to-back, for short shafts and where changes in length due to thermal expansion are relatively minor
For these types of arrangements, SKF recommends preloading the thrust ball bearings with Belleville washers (cup springs). This preload helps to prevent the balls from skidding if one of the thrust bearings becomes unloaded. Preload also improves performance of the thrust ball bearings while reducing noise levels.