Discuss the 'Shim End' vs. 'Plain End' configuration for wave springs in high-speed bearing preload applications.

In high-speed bearing applications (e.g., turbochargers), the interface between the wave spring and the bearing race is critical. 'Plain ends' terminate at a wave peak, which can create localized 'point' loading, potentially leading to race distortion or uneven wear. 'Shim ends' (or flat ends) are integrated into the final 360 degrees of the spring, providing a flat parallel surface. This ensures that the spring force is distributed evenly over the entire circumference of the bearing race ($360^∘$ contact). This uniform preload is essential for preventing ball skidding and maintaining the stiffness of the spindle assembly at high RPMs, though it does slightly increase the spring's solid height.