Bearings
Angular contact, or ball bearings are commonly used in the industry. The ball bearings when seated in the bearing seat, provides a known tolerance for the ID. These bearing offer high accuracy and high speed, as well as the taper roller bearing, yet less contact surface. The taper roller bearings provide more contact surface than other bearings, which allows for more rigidity. The taper roller bearing also permits greater side-load and have increased longevity in dirty environments.
Bearings fail for a variety of reasons. Bearings wash out from seal failure, as the seals fail coolant gets into the tool washing the grease out of the bearings. A bearing can get small pieces of material or debris into the rollers and it will cause a rough spot or notch in the bearing. Misalignment of bearings could be from install or from crashes that are not hard enough to damage external parts. Bearings will fail when overheating occurs from running the tool too fast or long.
Ultimately preventive maintenance can eliminate most of these issues. The bearings and seals are almost always going to need to be replaced before the larger items are damaged. When washout occurs the tool will sound rough and when spun by hand will be notably different. This is a great time to send it in for repair because it will likely only require bearings and seals and can be turned around with relative speed. When a tool continues to be run after the point of washout it will create issues like pitting on gears, or the seal surface groove on the spindle, which cost more on the repair and sometimes takes longer due to the need for special parts.