Re: Truck bearings: Solid vs. RB

Anthony Thompson <thompson@...>

Dave Evans wrote:
If we wish to be precise, then the term solid bearing is incorrect, and in fact does not appear in engineering mechanics texts.
The bearings we are discussing are "journal" bearings, and they are of the Hydrodynamic sub-type.
Nope. These bearings are journal bearings because they bear on the axle journals. The journal is the part of the axle that the bearings ride on. It does not matter whether they are roller bearings or solid (or plain, if you prefer) bearings, they are all journal bearings.

Richard is correct that once the steady state film thickness was established (at some unknown, to me, speed) the friction increased with speed - but only linearly, so the total friction per unit distance traveled was nearly constant (no journal bearing fuel penalty for running faster - but there was a fuel penalty for aerodynamic drag at higher speeds.)
Tests showed that at speeds like 30 or 40 mph, the difference in rolling resistance between plain and roller bearings was negligible. The big difference, and as Dave say, a very important difference for steam power, was at starting speeds.

Note that friction equals heat generation, so the high speed risk was that the heat generated per unit time increased (but not per unit distance traveled), and it could reach a point where the heat could not be rejected fast enough to the atmosphere, and the oil would overheat, potentially resulting in loss of the bearing film - and... Hot box)
This is probably true, but my impression from reading and from talking to railroaders of that era is that hotboxes occurred because the waste would get snagged or bunched and get into the bearing surface, and presto, much reduced lubrication. This could and did happen at quite modest speeds. Whether really high-speed trains ever had hot boxes from speed alone, I don't know, but if so, one would expect a LOT of the cars in such a train to have the problem.

Tony Thompson Editor, Signature Press, Berkeley, CA
2906 Forest Ave., Berkeley, CA 94705
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