1. Fits and clearances for high-speed bearings: Since high-speed bearings must meet the requirements for both high precision and high-temperature operation, the following two points must be considered regarding fits and clearances:
(1) Changes in dimensions and hardness as the temperature rises from ambient to high levels;
(2) Changes in the force system and shape caused by centrifugal forces at high speeds.
In short, maintaining bearing precision and operational performance through the selection of fits and clearances under high-speed, high-temperature conditions is a challenging task.
To ensure minimal raceway deformation after installation, the interference amount for an interference fit (e.g., for a KOYO 7319AC angular contact ball bearing) cannot be excessive. However, centrifugal forces at high speeds and thermal expansion at high temperatures can either counteract the normal pressure at the mating surfaces or cause the fit to loosen; therefore, the interference amount must be calculated carefully, taking these two factors into account. An interference level that is effective at ambient temperature and normal speed may prove ineffective for high-speed bearings.
If the resulting conflict is too great (a situation typically encountered only at ultra-high speeds), a dual lubrication approach-combining under-race lubrication and hydrostatic lubrication-may be adopted; this strategy has the potential to push the bearing's dmn value beyond the 3-million mark.
Thermal expansion at high temperatures can either counteract the normal pressure at the mating surfaces or cause the fit to loosen; therefore, the interference amount must be calculated carefully, taking these two factors into account. An interference level that is effective at ambient temperature and normal speed may prove ineffective for high-speed bearings.
When determining clearance for high-speed bearings, one must consider not only the factors mentioned above but also the impact of the shaft's thermal elongation on clearance. The goal is to achieve optimal clearance during operation (i.e., at the operating temperature), with this clearance established when the centers of the ball grooves in the inner and outer rings are precisely aligned. Since high-speed bearings aim to minimize relative sliding and internal friction, it is best to avoid adjusting clearance by axially offsetting the inner and outer rings (as might be done with SKF 71900C/DB angular contact ball bearings). When considering bearing interference fits and internal clearances, it is essential to account for the material's tendency to soften and deform at high temperatures, as well as the potential for permanent deformation resulting from repeated thermal cycling between ambient and high temperatures.
2. Requirements for Associated Components
High-speed bearing applications require the rotating system to undergo precision dynamic balancing. The bearing seating surfaces on the shaft and in the housing must meet dimensional and geometric accuracy standards that exceed typical requirements-specifically regarding coaxiality and the perpendicularity of abutment shoulders relative to the housing bore or shaft journal. High-speed operation and elevated operating temperatures must be taken into account when addressing these factors.
Shaft support systems require both high rigidity and minimal mass. To resolve this conflict, measures such as reducing surface roughness and enhancing surface strength can be employed to increase support rigidity, while hollow shafts may be utilized to reduce system mass.
