Needle roller / thrust ball bearings
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 (fig. 3), where the shaft cannot be hardened and ground (→ product table, must be ordered separately)
- can accommodate axial loads in one direction and, therefore, locate the shaft in one direction only
can be mounted back-to-back (fig. 4), 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.
- consist of a radial needle roller bearing and a full complement thrust ball bearing (fig. 5)
- are suitable for applications where there are moderate radial loads and lighter, single direction axial loads
- have low cross-sectional height, which enables shaft centrelines to be positioned close together like for multi-spindle drills
can be mounted with a snap ring, or against a shoulder in the housing bore, for axial support
The snap ring groove in the outer ring provides a cost-effective and axially space-saving solution (fig. 6, → product table).
- are most often oil lubricated and therefore supplied without grease
have a stamped steel cover that
- extends over the shaft washer of the full complement thrust ball bearing
- is firmly attached to the radial needle roller bearing
- makes these bearings non-separable
- has lubrication holes as standard
has no lubrication holes for bearings with designation suffix Z (fig. 7)
These bearings can be grease lubricated.
- consist of a radial needle roller bearing and a thrust ball bearing with a ball and cage thrust assembly identical to the 511 series (fig. 8)
- permit relatively high-speed operation
- are located axially in one direction by the outer ring flange
- can be mounted separately from both the ball and cage assembly and shaft washer
- should be oil lubricated, as there is no cover that retains the grease in the bearing
- have no stamped steel cover, as standard
have a stamped steel cover for bearings with designation Z (fig. 9) that
- has no lubrication holes
- extends over the shaft washer of the thrust ball bearing
- is firmly attached to the housing washer that is integral to the needle roller bearing outer ring
- makes these bearings non-separable
SKF supplies certain needle roller / thrust ball bearings capped with a stamped steel cover over the thrust part of the bearing (fig. 7, fig. 9, designation suffix Z). The cover, which has no lubrication holes, forms a gap-type seal to retain the grease in the bearing.
When capped bearings must operate under certain conditions, such as very high speeds or high temperatures, some grease may leak. For bearing arrangements where this would be detrimental, appropriate actions could be taken.
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 (table 1) under clean conditions.
The relatively large quantity of grease in the bearings means they can be operated for long periods before relubrication is required (→ Perform calculations). SKF recommends SKF LGWA 2 grease (table 1), if relubrication is required.
The needle roller bearing used in combined bearings has an annular groove with one lubrication hole in the outer ring.
Needle roller / full complement thrust ball bearings in the NX series without the designation suffix Z have a cover with lubrication holes over the thrust part of the bearing (fig. 6). Most often they are oil lubricated and, therefore, SKF supplies these bearings without grease.