Journal Bearings: Lubrication Modes
Bearings are devices used to transmit a load from one surface to another when a relative motion exists between the surfaces. This is most easily achieved by either some form of sliding action or rolling motion. Based on this fundamental difference, bearings are typically divided into two classes, nominally rolling element bearings or sliding surface bearings.
Sliding bearings can be further categorized by the direction of load support that they provide. Radial support is achieved using journal bearings (bushings), which typically support radial loads of varying magnitude and orientation on a shaft. Axial loads are typically reacted by thrust washers which often comprise of a simple flat washer seated against a thrust collar on the shaft.
One of the critical elements which need consideration when designing sliding bearing is the type of lubrication regime under which the sliding bearing has to operate. Three potential lubrication and corresponding wear regimes exist, namely; hydrodynamic lubrication, mixed-film lubrication, and boundary lubrication.
If a lubricant, in the form of a fluid or grease, is introduced between sliding surfaces, the bearing can be designed in a manner that allows a pressure to be developed by the action of the rotating components so that the load is transmitted by fluid pressure rather than metal to metal contact between the surfaces. Bearings designed in this manner are referred to as ‘full-film’ or ‘hydrodynamic’. Surface wear in this lubrication regime is extremely low and in some cases non-existent. Several critical variables which influence the formation of such hydrodynamic films include the geometry, tolerances, motion of the bearing surfaces, the nature and consistency of the fluid employed, and the supply of the fluid/lubricant.
It is almost inevitable however that contact between two bearing surfaces will occur at some point during operation, such as startup or high load conditions, however, some bearings, especially in slow speed applications, are designed to operate under these conditions. Contact of large asperities (surface irregularities) in the presence of a lubricant is referred to as boundary lubrication. Surface wear under this lubrication mode can be relatively severe and is typically dependent on the nature and material of the opposing surfaces. Boundary lubrication exits in liquid (or grease) lubricated bearings when there is no load carrying film and the lubricant serves mainly to keep friction fairly low. Cast bronze bearings are used extensively for boundary lubricated applications as certain bronze materials, including high-leaded tin bronze, offer additional lubrication through solid particles contained within the bulk material. Mixed film lubrication occurs where there is intermittent contact between opposing surfaces in the presence of a lubricant, where the transmitted load is carried partially by fluid pressure and partially by asperity contact. Wear under this lubrication regime can be light to moderate and is heavily dependent on the type of lubricant used and the bearing material selected.
The design of sliding bearings requires the full consideration of several areas which are key to the successful operation of the bearing including, bearing geometry, bearing kinematics (motion), type of lubricant/dispersion method, and bearing material selection. These seemingly interrelated factors need careful consideration but these complexities should not preclude their use as well designed bearings can provide good performance with low operational and maintenance costs.
Published in Technical Tips by Origen Engineering Solutions on 1 April 2019