journal

A journal is the cylindrical or conical surface of a rotating shaft that makes direct contact with a bearing. It is the working surface, typically polished to a smooth finish, that transmits the load from the shaft to the bearing and allows controlled rotation. The journal diameter, roundness tolerance, and surface finish directly determine how well the bearing will perform and how long it will last.

Journals are usually smaller in diameter than the main body of the shaft and are formed either as an integral part of the shaft or as a separate component pressed or shrunk onto it. Common journal diameters range from a few millimeters in precision instruments to several hundred millimeters in heavy industrial machinery. The surface is typically finished to a roughness of Ra 0.2 to 0.8 micrometers for hydrodynamic bearings and Ra 0.05 to 0.4 micrometers for rolling-element bearings, since even small imperfections can cause bearing wear or failure.

The load capacity and speed rating of a shaft assembly depend heavily on journal design. A journal running in a hydrodynamic (sliding) bearing generates an oil film that separates the shaft from the bearing surface; the journal diameter, clearance, and oil viscosity together determine film thickness and load-carrying ability. Misalignment, deflection under load, or eccentricity of the journal can break this film and cause metal-to-metal contact, leading to rapid wear, scoring, and seizure.

Failure and maintenance

Journal damage is one of the most common bearing failures in rotating machinery. Scoring (parallel grooves along the journal surface) and spalling (material loss in discrete patches) result from inadequate lubrication, contamination, or overload. Wear causes the journal diameter to shrink; if clearance becomes too small, friction increases, temperature rises, and bearing life drops sharply. Periodic measurement of journal diameter using calipers or micrometers is standard condition-monitoring practice in facilities running critical equipment.

The term 'journal' derives from the Old French word for a daily account or record, which in English came to mean any small or regular thing, and in bearing terminology has meant the rotating part itself since the 17th century. In modern usage, 'journal bearing' refers to any sliding-surface bearing, whether it acts on a journal or on the exterior of a rotating member, though 'journal' alone always means the shaft surface itself.