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I assume that ball bearings are very tough balls of steel that, with proper lubrication, cut friction and remain operative for a useful amount of time.

What I don't get are rod and main bearings, which seem soft and fragile, even with lubrication.

Why and how do they function?

The rod and main bearings are a sliding type of bearing that rely, for the most part, on hydrodynamic lubrication. That is, the journal literally floats in a thin film of lubricant.

However, the bearing material must also be able to withstand intermittent direct contact with the journal under the recipricating load of the rotating mass.

No, they do not contact. If they do, you are either using the wrong oil, don't have enough oil pressure or the clearances are wrong.

The reason they are soft, is absorb fine particulates instead of scoring the journal.

Quote:

What about startup (and shutdown)?

The viscosity of the oil prevents that. At least with modern oils it does.

I always turn my engines over several times, until some oil pressure starts to register on the gauge, after they have been sitting for more than a week. I do this to avoid a dry engine start-up which causes excessive bearing and cylinder wear.

(o{}o)

I crank up my 1922 Model T ford which has no pressure for lubrication, and in the 50 years I have been driving it I have never adjusted a main or rod bearing.

I believe all jet turbine engines are a sleeve type bearing spinning at thousands of RPM's with oil pressure.

So technially speaking there is no friction in a ball bearing as the hard balls just roll between the races.

Jim :)

Amazing.

It can't possibly work. But it does....