Re: Truck bearings: Solid vs. RB


John Hagen <sprinthag@...>
 

Very enlightening Dave and thanks for posting.



It would seem to me that the speed at which the oil layer would establish
itself would depend on how much weight was riding of the bearings. With the
weight of a loaded car versus that of an empty I think it would take a
higher speed to drag that oil completely around the bearing surface. And the
temperature would also effect it. In sub zero temps that oil is not going to
be easy to drag into that "squish" area.



I never really thought about all this but basically what has to happen the
lubricant must actually lift the weight of the car up off the bearing
surface. That fact that it even happens without an oil pump is somewhat
amazing to me. Here is a lesson for drivers. Even though your vehicles
engine has an oil pressures pump it takes a bit of time. Measured in
seconds, to get that pressure to the bearings. The colder the longer. That
is why all manufacturers recommend letting you engine idle for a bit before
moving off and not revving it even when not in gear.



If you have a relatively small car that can be easily pushed try pushing it
when it is fully warmed up. Then try it on a warm morning but before
driving. Lastly, wait for COLD weather and try pushing that now virtually
immobile block of icy metal. This will demonstrate just how resistant to
movement oil can become in frigid temps. Twenty-five years in the automotive
service business and I know the answers. It's a great thing to tell a
customer when he/she complains that the car gets poorer fuel mileage in cold
weather.



John Hagen



From: STMFC@... [mailto:STMFC@...] On Behalf Of Dave
Evans
Sent: Thursday, April 26, 2012 17:25
To: STMFC@...
Subject: [STMFC] Re: Truck bearings: Solid vs. RB





Gentlemen,

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....

Dave Evans

Nothing is ever fool proof, because fools are so ingenious ;-)

--- In STMFC@... <mailto:STMFC%40yahoogroups.com> , tyesac@...
wrote:

Richard, Pierre,

Right you are! If anybody thinks that solid bearings are obsolete
technology, think again. Anything driven by a crankshaft has solid bearings;
from a formula 1 race car to diesel locomotive. Properly lubricated, they're
a low resistance bearing that's able to tolerate heavy loads. A key issue
for the changeover for the railroads was that roller bearings have less
finicky lubrication requirements, typically only the roller cage requires
greasing at initial installation. Having large percentages of the freight
car fleet that doesn't require constant vigilance for oiling helped tip the
scales for the more expensive roller bearings.

Now if we could only get some model RR manufactures to drop the "friction
bearing" term.

Tom Casey


Thanks Richard, for that clarification. I've never been comfortable with
the "friction bearing" phrase myself and I've always wondered what the
correct term should be. In part because a solid bearing is closer to a
bushing than a bearing in my world of mechanical creations.
Pierre Oliver

--- In STMFC@... <mailto:STMFC%40yahoogroups.com> , Richard
Hendrickson <rhendrickson@> wrote:

Several times lately I have noticed members of this group using the
term "friction bearings" to differentiate solid bearings from roller
bearings, despite objections on previous occasions from myself, Tony
Thompson, and others. The correct terminology for solid truck
bearings is "solid." "Plain bearings" is an acceptable alternative.
"Friction bearings" is wrong and misleading. All bearings have
friction, including roller bearings. "Friction bearing" was invented
as an advertising ploy by roller bearing manufacturers to imply,
incorrectly, that roller bearings were frictionless. In fact, though
roller bearings have much less starting resistance than solid
bearings, they have considerable friction which - as with other
bearings - increases as load and speed increase. The term "friction
bearings" was never adopted by railroad mechanical engineers, who
knew better; it does not appear in the dictionary section of any
edition of the Car Builders' Cyclopedia. So please, guys, don't
perpetuate the mistake of calling solid bearings "friction" bearings.

Richard Hendrickson





-----Original Message-----
From: Pierre <pierre.oliver@...>
To: STMFC <STMFC@... <mailto:STMFC%40yahoogroups.com> >
Sent: Wed, Apr 25, 2012 7:49 pm
Subject: [STMFC] Re: Truck bearings: Solid vs. RB




Thanks Richard, for that clarification. I've never been comfortable with
the "friction bearing" phrase myself and I've always wondered what the
correct term should be. In part because a solid bearing is closer to a
bushing than a bearing in my world of mechanical creations.
Pierre Oliver

--- In STMFC@... <mailto:STMFC%40yahoogroups.com> , Richard
Hendrickson <rhendrickson@> wrote:

Several times lately I have noticed members of this group using the
term "friction bearings" to differentiate solid bearings from roller
bearings, despite objections on previous occasions from myself, Tony
Thompson, and others. The correct terminology for solid truck
bearings is "solid." "Plain bearings" is an acceptable alternative.
"Friction bearings" is wrong and misleading. All bearings have
friction, including roller bearings. "Friction bearing" was invented
as an advertising ploy by roller bearing manufacturers to imply,
incorrectly, that roller bearings were frictionless. In fact, though
roller bearings have much less starting resistance than solid
bearings, they have considerable friction which - as with other
bearings - increases as load and speed increase. The term "friction
bearings" was never adopted by railroad mechanical engineers, who
knew better; it does not appear in the dictionary section of any
edition of the Car Builders' Cyclopedia. So please, guys, don't
perpetuate the mistake of calling solid bearings "friction" bearings.

Richard Hendrickson




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