Tandem Thrust Bearing’s Selection
Three pieces of information are necessary when selecting a suitable tandem bearing:
– the maximum possible bearing OD
– the desired calculated rating life under operating load and speed
– the maximum screw pressure.
The maximum possible bearing OD results from the intermeshing of two screws in the case of twin screw extruders. In the process, the crest diameter of one screw contacts the root diameter of the other screw. The resulting tight center distance (Figure 2) and the output shaft near the tandem bearing limit the radial design space. The length of a tandem bearing is normally not a crucial factor in bearing selection. For this limited envelope, tandem bearings are available in the 31.5 to 900 mm OD range.
The required calculated rating life under operating load and speed is generally between 20,000 and 40,000 hours for extruder gearboxes. Smaller gearboxes such as those used in extruders for laboratory applications have shorter life requirements. Here, a value of 5,000 hours is sufficient.
Another factor to consider in bearing selection is the maximum screw pressure. Due to the deformation of the components and the resulting stresses, a maximum permissible axial load Fa max may not be exceeded. The value for Fa max, the dynamic load rating C as well as the static load rating C0 can usually be found in the tandem bearing delivery drawing.
The value Fa max refers to the permissible strain on the components (rings, washers, etc.). For this reason, the maximum screw pressure should be known. On the other hand, the dynamic and static load ratings refer to the entire rolling element set. Since the value for Fa max is always lower than C0, the static load rating C0 is not relevant for the selection of a tandem bearing. INA can provide gearbox manufacturers with a tandem bearing data sheet so that all relevant data can be recorded precisely. This allows the optimum bearing solution to be recommended.