Re: Truck bearings: Solid vs. RB
Gentlemen,toggle quoted messageShow quoted text
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.
The term "anti-friction" bearing is applied to roller bearings and other types of bearings where the main relative motion is rolling instead of sliding.
I can understand why marketing types might then label Journal bearings "friction" bearings, even if mechanical engineers never did.
Journal bearings use some form of lubrication between two sliding surfaces. Hydrodynamic journal bearings draw lubricating oil into the bearing gap using fluid mechanics. The lubricant is not under pressure (as it is in auto engines, which use Hydrostatic journal bearings.)
From my engineering book:
"The film pressure is created by the moving surface itself pulling the lubricant into a wedge shaped zone at a velocity sufficiently high to create the pressure necessary to separate the surfaces against the load on the bearing."
This is why the friction of hydrodynamic journal bearings can be so high when they first begin to turn. They may first start with direct metal-to-metal contact but even with a very thin oil film the drag can be much higher than "normal". Since journal bearing friction is a function of film thickness - the thinner the film, the higher the friction (mathematically this friction is a function of one over the film thickness). As speed increases the bearing is designed to induct more oil to create a thicker film.
This is much different than roller bearings, and helps to explain why, during the steam era, if the train got under a few mph and the locomotive lacked the pull to accelerate, the train could just "grind" (figuratively, not literally) to a halt. And it explains why steam era engineers didn't walk out trains at very low speeds - better to keep the moving cars above the speed that could lead to a sure stall as more cars were placed in motion (and hence all of that slack action - a good reason for those buffered draft gear and underframes).
So for steam era journal bearings, until the speed increased to a point where the design film thickness was established, friction could actually drop as speed increased. I do not know the "crossover" speed where drag began to increase as speed increased. Conversely, this crossover speed was also the speed where drag INCREASED as speed dropped. Perhaps it is in an old railroad engineering book.
Makes one wonder if steam may have lasted a little longer if roller bearings had been more widely applied - they are much easier to start and keep rolling at low speeds, which was often a steam locomotive's greatest challenge, and helps explain why steam locomotives pulling modern trains under heavy loads seem to do so well - probably better than when they were built.
This also explains why journal bearing cars appear to come to a sudden stop in the last phase of coasting - they actually do stop quickly (think about the implications for a journal bearing era hump yard.) As they slow, the friction will increase significantly once the film begins to thin (and the slower speed results in an even thinner film - so the friction rapidly builds as the car comes to a stop). I bet hump rider's intuitively understood this, even if they did not know the physics.
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.) 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)
And talk about Journal Bearing "hot boxes" - an older colleague at work once told me stories of how when he was a kid, in Portage, PA (on the PRR main), the kids use to take "those oily rags" out of the journal boxes in the winter time to help start camp fires when they were outside playing ice hockey. The innocence of youth....
Nothing is ever fool proof, because fools are so ingenious ;-)
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