From: iain@ee.ryerson.ca (Iain Campbell)
Subject: Re: Oil Bearings (was Re: Grand Canyon Railway)
Organization: Ryerson Polytechnical Institute, Toronto
Date: Tue, 22 Dec 1992 15:25:13 GMT

In article <1992Dec21.223512.12593@mel.dit.csiro.au> ajw@squid.mel.dit.CSIRO.AU (Andrew Waugh) writes:
>In article <BzDIIv.A1L@hplabs.hpl.hp.com> hintzman@hpljah.hpl.hp.com writes:
>>Now to the question:  Why do friction bearings limit the speed of the
>>train?  Related question:  is it true that most railroads will not allow
>>friction bearing stuff to pass over their rails anymore?  What does
>>this mean for tourist railroads and excursions?  What the heck is a
>>friction bearing, anyway (I can probably guess...)
>
>'Friction bearing' is a tautology. All bearings have friction. What

The correct technical term for the bearings being decscribed here as
"friction bearings" or "oil bearings" is a journal bearing.  Such a
bearing consists of a turned shaft (the "journal") which rotates 
inside a closely fitted housing( the "bearing").  

This housing may completely encircle
the journal, in which case it is called a full bearing, or, it
may only support a portion of the circumference of the journal.
In railroad usage, as far as I know, the bearings used were half
bearings, in which only the upper half of the journal was supported.
(as a matter of interest, the most common example of a full journal
bearing can be found several times in every automobile engine in 
the world, ie, the crankshaft, connecting rod (both big and little
end) and camshaft bearings in 99% of all auto engines are full 
journal bearings). 

The bearing is normally either made of, or lined
with, a material which is softer than the journal material.  This
is so that the bearing can conform as closely as possible to the 
shape of the journal.  This is necessary because of the principle
which makes these bearings work (and they do work very well ... keep
in mind the note about auto engines ... there may in fact be more 
journal bearings in the world than just about any other type of
bearing).  The principle is known as "hydrodynamic lubrication".
In short, as the journal rotates, whatever lubricant is supplied,
be it oil or grease, is drawn around with the journal into the space
between the journal and the bearing.  If the journal is turning fast 
enough, this has the effect of generating a pressure in the lubricant.
This pressure actually supports the journal in the bearing, hence
the load placed on the bearing is carried by a thin film of pressurized
lubricant.

It is important that the space between the journal and the bearing, 
called the bearing clearance, be kept as small as possible, as 
larger clearances allow lubricant to leak out of the bearing faster,
causing a drop in the lubricant pressure.  Owners of older autos
will see this as a drop in normal operating engine oil pressure as
the engine ages ... it is NOT all the fault of wear in the oil pump,
but mostly wear in the crank and con rod bearings, making it difficult
for the oil pump to maintain pressure.  (Those familiar with performance
cars will know that high performance oil pumps are not high pressure
pumps, but high volume pumps ...)

As another post has noted, brass was the preferred bearing material in 
railroad usage.  Also, it was my understanding, and I am interested to
know what other netters know about this, that grease, not liquid oil,
was the preferred lunricant for car journal bearings.

Also, as I have mentioned above, journal bearings are quite suited to
high speed usage, and I am puzzled to know where anyone would get a
30 mph restriction on journal bearings.  ALL cars up into the 50's
were equipped with journal bearings, and they sure weren't all restricted
to 30 mph !!!

If there is enough interest, I'll talk about why journal bearings have 
been replaced by roller bearings in another post.

If anyone is interested in fiurther reading, I recommend "Design of 
Machine Elements", author VM Faires.  It was my 3rd year machine
design text and is an excellent, if overpriced reference ...


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