How Do Plain Journal Bearings Work Within A Motorcycle?

Kevin Cameron explains why plain journal bearings are generally not the cause of an engine’s demise.
Plain journal bearings are at the heart of nearly all modern engines. A one-piece forged crankshaft must have bearings made in halves for the bearings to be assembled over the crank. The nature of those bearings is determined by the fact that when people tried to have rotating parts supported by a film of oil rather than by balls or rollers they had to deal with the problem of oil contamination by wear particles or dirt.
Engines for years were sand cast with a rough surface finish. You guessed it: There might be sand in that finish. What happens when a piece of grit gets between the crankshaft journal, which is highly polished and truly cylindrical, and the bearing? If there is no place for that piece of grit to go, it cuts the daylights out of both of them and might cause the bearing to seize. So it was learned very early that the bearing had to be made out of material that was softer than the journal or another rotating part.
In the early days, bearings were poured. Liquid Babbitt metal was poured into the bearing in a thin layer and then finished after it was cast in place to make a cylindrical seat for the journal. But because that was a very slow way to build things and incurred a lot of costs, the idea of having the soft bearing metal on a steel backing ready to simply drop into place had a certain appeal.
A process was developed in Indianapolis and the manufacture of this type of bearing was licensed by an outfit in England that made them for aircraft engines. If a piece of grit got between the hard journal on the crankshaft and the bearing that supported it, the grit was simply pounded into the soft material and ceased to do harm.
To supply these bearings with a steady supply of oil, a wedge must be formed. Viscosity-the internal resistance of the oil-causes the oil to be swept into the open end of that wedge and dragged toward the loaded zone, which is where the journal is closest to the bearing surface. In an engine that is working hard, the distance of closest approach between the journal and the bearing surface is measured in microns, which are millionths of a meter.
In a dragster, the pressure in the loaded zone can be as high as 12,000 psi. Oil pumps typically produce 40 to 60 psi so it’s clear that oil-pump pressure cannot support the load. The combination of oil viscosity and the rotation of the journal supports the load so a stable condition is reached in which the bearing is floating on a film of oil. Because the clearance in a bearing is extremely small and the oil film is present, such a bearing has great damping properties.
Plain bearings are now a mature technology. When an engine is finally worn out and goes to the scrapyard, crankshaft bearings are generally not the cause of its demise. Plain bearings are very durable as long as they get oil that is clean and of an appropriate temperature because oil is constantly keeping the two parts separate.
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[@daveg4495]

Cycle World is lucky to have Kevin Cameron, I've been reading is advice for an embarrassing number of years & have never been steered wrong.

[@duyhuynh9646]

I've learned so much from Mr. Cameron's explanation about plain bearings.

[@EeezyNoow]

Thanks Kevin. In 1965 I was studying for a BSc degree in Mechanical Engineering at Leeds University (UK) and we were shown a diagram of the oil pressure distribution across a plain (shell) journal bearing - which is like a sine curve (maximum at the centre, zero at the 2 edges). Any scratch or imperfection in the shell bearing liner results in almost a double sine curve ie. a lower max overall load bearing capacity. Moral: - warm your engine before opening throttle too wide and regular oil and filter changes.

[@JackdeDuCoeur]

I ALWAYS learn something important from Mr. Cameron. I hope he continues to write and make videos like this forever.

[@shreedevi2005]

Another awesome vid. Thanks so much for this series with Kevin Cameron.

[@bluetick1528]

Kevin Cameron has a particular genius for explaining mechanical things. I first read his column in Cycle Magazine almost 30 years ago. (Yes, Cycle, full stop.) Please keep the videos coming. Subscribed.

[@monteiro5306]

Please, more videos. Greetings from Brazil, Kevin and Cycle World.👍👍

[@samcooke2742]

This guy makes me glad to be a Cycle World subscriber. The mag is great, but these videos are beautiful. A nice, practical, accurate description of the fluid mechanics of plain bearings in this one. Seeing H1 parts in action brings a tear to my eye, even if the bike was the widowmaker of a generation.

[@jamesf2656]

Massively underrated video

[@runalongnowhoney]

I fondly recall reading Mr. Cameron's explanation of plain bearings years and years ago in his TDC column of Cycle World; it was a milestone moment of comprehesion in my mechanical engineering journey.

[@Psycrow11]

Kevin you are the "Man" and one of the biggest reasons Im a CycleWorld Subscriber!

[@fixpaana]

Fantastic explanation. Watching these videos makes me want to meet Kevin! The sheer knowledge and the ease with which he shares the same is really something that inspires respect. Great work Cycle World!

[@handsfree1000]

Thank you Kevin (from the UK) very interesting. The history of every day things that we take for granted.

[@robertrishel3685]

Fantastic! Keep them coming.

[@chriswilliams1096]

Brilliant explanation that covers all the bases but does not require a PhD to understand. Thanks.

[@davidciesielski8251]

thank you

[@matthewkirsch327]

Thank you.

[@cpuuk]

Awesome! I'm learning :-)

[@shoominati23]

My CB350 has roller bearings.. SO do the GS1000's . Apparently you can put silly amounts of boost through those things like the drag racers do, but they don't last as long.

[@blipco5]

I knew this but could hear it over and over. By the way, the background music is perfect, way better than that euro disco dance stuff that from what I've heard on the internet is played at gay nightclubs.

[@exnihilo415]

Hate to break it to you but gay nightclubs have higher standards of EDM excellence than the sad royalty free loops that accompany many videos on the internet.

[@blipco5]

ex415...I'll have to take your word for it.