This machined section of a rotating shaft is specifically finished to interface with a plain bearing or the inner race of a rolling-element bearing. It requires precise dimensional tolerances, high surface finish, and often localized hardening to resist wear. In fluid film bearings, it rides on a microscopic wedge of pressurized oil rather than touching the bearing material directly.
We need to polish the main journal because those deep scratches will tear up the new babbitt bearing in an hour.
Related terms
Bearing
A machine element that constrains relative motion to a desired direction and reduces friction between moving parts.
Journal box
A protective housing that encloses a rotating shaft end and its bearing to retain lubricant and block debris.
Bushing
A cylindrical sleeve used as a plain bearing to reduce friction and wear inside a hole.
Radial shaft seal
A flexible ring that presses against a rotating shaft to retain lubricant and exclude dirt.