Big end bearings (Three bearing engine)

[Colin Reed]

Hi Andrew.
The friction is created by the oil going through the pump, oil gallerys and a thin layer between bearing and crank, cam followers and camshaft, cam gears. Hence one of the reasons for Oil coolers.

Colin
Nz

[Bob Culver]

According to various books the oil film is just a few .0001 inch thick, spot pressure in a modern may be 20 tons square inch, down and up, and temp 30 C deg higher than sump. The heat is generated by the severe shearing and the power absorbed. Nearly all is carried away by the oil. (I have encountered cars with cigarette papers behind the shells and which have operated OK) You can easily work out the sliding velocity, high on revving moderns with large dia bearings.
A lot of engine heat is picked up by the oil, esp from underside of piston, and head under ohv gear. I gather many motorbikes rely heavily on a flood of oil for engine cooling.

[Chris KC]

Oil flow through the big end bearing only accounts for about 50% of the cooling, the rest is conducted away through the shaft (subject to decent contact as mentioned by Colin, and I would add good surface finish is important, not only snug fit). On engines with intermediate plain main bearings oil flow through those takes most of the remaining heat out of the shaft.

[Ritchie Wilson]

It seems genuinely miraculous that Bob's few tenths of a thou oil film separates the bearing surface from the journal sufficiently to practically eliminate bearing wear. 
This film is maintained by the rotation of the shaft as long as there is a sufficient supply of oil. 
In the old, less precise, days the bearing was made slightly under size and then "run in" by being used. The bearing wears until it is the optimum dimension. After that wear is, again, practically eliminated.
It all seems rather unlikely.
The alloy used in plain bearings was invented in 1839 by Mr Babbitt. I imagine a lot of luck was involved in his coming up with the mixture - still used today.
The mechanical gods must have been smiling!

[Bob Culver]

Conditions in bearings do seem miraculous. In horizontal Jowett engines when run for modest mileage on clean oil, the only evidence of rubbing contact is often at the little loaded bottom of the main bearings where no film on initial movement.
With the much higher psi loading films are very thin in modern cars and hence filters and additive oils more important.