In what kind of a properly lubricated bearing is there no metal-to-metal contact while the shaft is rotating?

In a properly lubricated journal bearing, there is a formation of a lubricating film between the shaft (journal) and the bearing surface. This film is crucial because it creates a hydrodynamic effect that raises the shaft above the bearing surface, preventing any metal-to-metal contact as the shaft rotates. The lubricant fills the space between the surfaces, ensuring that the moving parts glide over each other smoothly without direct contact, which significantly reduces friction and wear.

While other types of bearings, such as needle, ball, and roller bearings, also function to support loads and minimize friction, they operate on the principle of rolling contact rather than maintaining a continuous lubricating film like in a journal bearing. This rolling action may still allow for some minimal wear and direct contact under certain conditions, particularly if lubrication fails or is inadequate. Thus, the unique mechanism of journal bearings relies on a full hydrodynamic film to fully prevent metal-to-metal contact during operation.