Hydrodynamics Effect on Efficiency 2

[Phil_H]

This may be rudimentary but how does the efficiency of a bearing material change if you were to go from a metal to a coated metal material if there already was a thin film boundary layer between the bearing and the wear surface?

 

[MikeHalloran]

ISTR that plain bearings are sometimes plated with soft metal,
for (I'm guessing) two reasons:
To provide a sacrificial layer to wear off during break-in, trapping any abrasives that may be present,
and to provide a visual contrast so you can see how badly the engine has been bent on disassembly.



Mike Halloran
Pembroke Pines, FL, USA

 

[romke]

whenever the shaft contacts the bearing more friction and wear will occur and thus some efficiency will be lost. most engine bearings are coated with a thin coating that can more or less temporarily act as kind of solid lubricant and thus more or less prevent more severe damage to the bearing. that very thin coating is not meant to embed dirt - it's thickness is insufficient, dirt and wear particles may be embedded in the actual bearing metal which usually has a steel backing to support it. most coatings are based on a lead/indium alloy, for applications where maximum reliability is required (eg ships engines in navy vessels) sometimes a coating based on a silver alloy is used capable of withstanding the load for a somewhat longer amount of time.

experiments have shown that modern nano types of coating may be able to still better prevent damage due to temprarily lack of sufficient lubricant, however producing that kind of bearings on a industrial scale still poses lots of problems. unfortunately, also that type of coatings is not yet sufficiently stable - the coatings can deteriorate quite rapidly in actual service.

 

[Phil_H]

okay so from the sounds of it, solid-solid contact is not ideal therefore it's best to have some lubrication that is dirt/grime free on top of the mated surface. When not sufficiently covered or there are design concerns therefore, metals or polymers might be used as a sacrificial layer where needed for visual inspection. This could have nothing to do with efficiency. The efficiency will be driven more by the fluid film losses and not the solid material wear surfaces. Feel free to correct me where wrong, thank you for the help.

Also, nano-tech is extremely difficult to manufacturer, I worked in a pilot scale lab trying to deposit MoS2 and BN filled materials at 1-100nm thicknesses. Wasn't scalable and extremely difficult to have a full layer with no crystalline breaks. Fun project though and very challenging.

 

[tbuelna]

Ideally, your journal bearing should be designed for hydrodynamic contact conditions under normal operation. The only time it should operate under boundary contact conditions are brief periods during start/stop. As others noted, the surface of fluid film bearings typically have a thin coating of material that is soft enough to prevent abrasive wear of the mating journal surface during brief periods of operation under boundary contact conditions. However, this coating is normally thin enough that even if it gets locally worn away, it won't have any significant effect on the bearing's fluid film efficiency/performance.

The materials used for journal bearings are selected based on requirements like surface fatigue, metallurgical compatibility, heat transfer efficiency, CTE matching, adhesion resistance, etc. A well designed/manufactured/assembled/maintained journal bearing should not experience any significant "wear". And its service life will normally be limited by surface fatigue capability of the bearing backing material, which is steel for most automotive engine bearing applications.