Historically speaking, fluid film bearings were the first type of rotary bearing to appear simply because they are more simple than rolling-contact bearings. Early sliding bearings were almost certainly first made from wood after the invention of the wheel in prehistoric times, and these were lubricated with grease derived from animal fat. Metal wheel journal bearings followed. When the industrial revolution began in the eighteenth century, machine rotational speeds reached a point where lubricated metal journal bearings became hydrodynamically isolated so that there was no metal-to-metal contact. The initial discovery was certainly by accident, but soon an empirical methodology was developed followed by the science of tribology related to journal bearings.
A Journal Bearing Has Hydrodynamic Lubricating Layer.
When a journal bearing begins rotating, there is very little lubricant between the hole and shaft at the contact point, H0, and rubbing occurs. Therefore much friction needs to be overcome when starting a hydrodynamic journal bearing. When the bearing has reached sufficient speed, the lubricant begins to wedge into the contact area and hydrodynamic lift is attained.
Various ways have been devised for keeping a journal bearing lubricated. The lubricant often cools and cleans the bearing in addition to its lubrication function. For instance, in a siphon wick lubrication arrangement, as illustrated below, gravity provides the pressure head needed to keep the lubricant flowing to the bearing surface.
Siphon Wick Journal Bearing Lubrication
Another method of delivering lubricant can be done by a capillary wick, as illustrated below. The necessary pressure head is developed through capillary action within the fibers of a textile wick, as with the phenomenon of a candle wick.
Capillary Wick Lubrication Method
For applications that require significantly decrease the amount of starting friction, a hydraulic lift can be introduced.
Hydraulic Lift Lubricated Journal Bearing
Hydraulic lift journal bearings have been successfully applied to slow-moving, heavy rotating masses such as those found in steel rolling and paper mills.