Definition
A condition in a tapered roller bearing in which the small end of the rollers contacts the inside of the cone (inner race) shoulder, rather than rolling smoothly along the tapered race surface. This indicates improper bearing seating, wear, or incorrect installation, and it causes localized loading, heat, and accelerated bearing failure.
Plain English
When the tiny ends of the rollers in a wheel bearing start dragging against the inside lip of the bearing instead of rolling freely along the surface they were designed to roll on. It is a sign the bearing is not sitting correctly and is on its way to failing.
Context Anchor
Seen in aircraft maintenance discussions about installing or inspecting propellers that use tapered centering cones.
Derivation
Cone refers to the inner race of a tapered roller bearing, which is shaped like a truncated cone. Bottoming is a mechanical term meaning a part has reached the limit of its travel and is contacting a surface it should not. Together, the rollers have moved far enough inward that they are riding on the bottom shoulder of the cone instead of the proper rolling surface.
Why Pilots Care
Avoids catastrophic blade strikes that can destroy the rotor system or airframe.
Analogy
It is like tightening a cap that stops against a hidden shoulder before it actually presses down on what it is supposed to hold. It may feel tight, but the part underneath is not secure.
Intuition Check
Do not read bottoming as a normal seating action. In this term, bottoming is a fault: the cone has stopped too soon and has not properly secured the propeller.
Example Sentence 1
The mechanic rejected the main wheel bearing during the annual inspection because signs of cone bottoming were visible on the inner race.
Example Sentence 2
Low rotor RPM in autorotation can quickly lead to cone bottoming if not corrected.