Re: Truck bearings: Solid vs. RB


Sorry Tony,

The Mechanical Engineer's definition of "journal" is that portion of the shaft that rotates within a bearing, AND the relative motion between the two is sliding. There is NO sliding motion in a roller bearing, so a roller bearing does not have a "journal".

Some in the railroad industry may have retained the term "journal" to refer to the entire assembly, or the portion of the truck that is fastened to the outer portion of the bearing, or even to the portion of the shaft that the roller bearing is pressed onto (a legacy use, but incorrect), but roller bearings are not journal bearings.

Today, roller bearings are often included with the axle assembly - it is not something installed in the field. The tapered rollers roll between a "cone", typically press fit onto the axle, and the cup, which is the outer, stationary assembly that is mechanically fixed to the truck. Roller bearings are actually pairs of tapered roller bearings at each truck side frame (So two pairs per axle) so they can also react the thrust loads that result from curves (rated at about about 25% of the radial load). It is the need to react thrust loads, without creating excessive surface loads on the rollers, that contributes to the fine tolerances required for railroad roller bearings, and hence why they are an integral assembly that is pressed onto a portion of the axle with a very high precision outer diameter. Perhaps some still call this region of the shaft a journal, but technically that is incorrect.

Nowhere in Timken's railroad literature does the term "journal bearing" appear. There is, in one of their maintenance guides, two instances where they interchange the word axle and journal, but ONLY when they discuss machining the axle to the proper diameter. But as soon as they discuss installing the bearing, it is installed on the axle, not a journal. The only other appearance of journal refers to publications.

So roller bearing and journal bearing are the proper engineering terms to distinguish between the two bearing types.

The term "Plain" bearing does not appear in any of my engineering books.

Dave Evans
Lehigh University

--- In STMFC@..., Anthony Thompson <thompson@...> wrote:

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

Tony Thompson Editor, Signature Press, Berkeley, CA
2906 Forest Ave., Berkeley, CA 94705
(510) 540-6538; fax, (510) 540-1937; e-mail, thompson@...
Publishers of books on railroad history

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