I am looking for explanations and reasoning behind the design of coupling rod bearings. Our Swedish 1307 has bearings with full babbit surfaces. Our smaller German built HHB 4 has bearings with two large babbit pads poured in each bearing half, but appears to have Bronze as part of the working surface. How were these design choices made?

Many standards and practices were based on the supervising engineers preferences in the design stage, some were later altered based on either the changing of the guard in the engineering offices, available materials to use for repairs, shop foreman's preferences, technological evolution, or just about any set of circumstances you can imagine. The more managed the railroad, the greater likelihood a standard would me maintained, the less structured took a more ad hoc approach just to keep trains rolling, industrial and shortlines....... what do we have we can patch this up with NOW? Porta wrote a paper on wrist pin design for coupling rods which may be of interest.

Should we retain the coupling rod bearings as they are, with partial babbit, or should we bore the halves out and pour full babbit bearings?

How have they been doing as they are set up now? Changing what doesn't work makes sense, changing what does not so much........ if they have run cool and given good life why change them?

That is my view. The engine is a new rebuild - last run in 1962. There are some others with bolder ideas.

Steven,

When you use the term "coupling rods", I assume you are referring to what we here in the States would call side rods, that connect and drive steam locomotive driving wheels.

If so, I have often wondered the same thing regarding plain bearing practice in some applications. Not to say Babbitt pockets were nonexistent in this country, but my observation has generally been that this design in side rod (and also driving box crown brass) bearings is a foreign-to-U.S.A. practice.

Being lead-based, Babbitt bearing metal is of course very soft. As such, I understand that a desirable trait of Babbitt bearing metal was its ability to "embed"; that is, absorb a reasonable amount of foreign matter that may enter the bearing, and retain this matter so that it does not keep circulating throughout the bearing surface and therefore potentially create a scored and overheated bearing. So, my best initial guess is that Babbitt pockets were designed in some bearing applications to take advantage of this desirable embedding characteristic. I have also read that the soft nature of Babbitt metal lends itself to conforming more quickly and accurately to lengthwise irregularities on a journal surface. However, I struggle to grasp the advantage here when considering a bronze bearing that has a few separate Babbitt pockets designed in, yet retains a substantial "land" of bronze between the pockets.

In this country, standard practice was typically to use a solid bronze bearing - the only additional feature being varying styles of lubrication grooves or cavities. This might make for a bearing that is less forgiving to the introduction of foreign matter, but I could see how it would also make a bearing that is lower in first cost. The exception to this was/is in car-type bearing shells, and my guess as to why they were/are Babbitt lined is to allow them some freedom of exact fit with the journal diameter of the axle, and allow them to more amenable to wearing in with a journal diameter that was less than a perfect match. My understanding is that car bearing practice was a little different 80 years ago than it is today, with replacement bearings being machined to a few different standard diameters. (This made for more efficient manufacture and inventory when plain bearings by the tens of thousands were in service.) The car inspector would select whichever was closest (but likely not exact) to a proper fit with the axle journal, throw it in the box, and go.

If there is some other more scientific or technical reason behind Babbitt bearing pockets being designed into plain bronze bearings, I'd be very interested to hear that.

-Erich Armpriester
Strasburg Rail Road Mechanical Services

Thanks Erich - do you or anyone else have some handy web links to drawings?

Your description is correct - I am describing the side rod bearings with the pockets of babbit.

Out of sight, out of mind - I never thought about this until our "young guns" said, "hey, lets bore all this stuff out and just pour a full surface babbit."

I am not really sold on that whole "absorbing foreign matter" stuff. We experience endless stress because the babbit cracks with the slightest mishandling or misallignment. On another locomotive, we often run with significant cracks and flaking in the bearing. We have one drive pin that is known to be not in alignment.

I really think the sales pitch is periodic renewals, a la Burt Lancaster, by just melting it out and repouring.

I think if the pockets are poured just a little thick, they take a lot of the thrust. But if the babbit fails, the engine can still roll on the bronze.

My son has pictures and I asked to post. MAYBE IF HE READS THIS HE WILL POST THEM.

As Erich noted, babbit lined rod bearings are unusual practice in the USA. I suspect that it may have been used more in applications with oil lubrication. With the more common use of hard grease lubrication here, solid bronze bushings are more or less standard and hold up quite well. Solid bronze bushings will work with oil lubrication as well if the oil supply is constantly maintained. As for knuckle pin bushings, I have seen them made of steel or bronze, depending on railroad preference, and both work well.

Another variation to a pressed-in bronze bushing is a floating bushing. The bushing is a push fit into the rod and is able to rotate. Per the drawings, the USATC Consolidations originally came out with pressed in bronze bushings. Apparently at some point the Alaska Railroad converted them to floating bushings as that's what was there when we received it. Probably a preference of the Master Mechanic at the time.

The main bearings were originally bronze but have been reworked and lined with Babbitt per drawings that we got showing SP practice.

Stuart didn't have time, so I am posting the side rod bearing photos from HHB 4.

Thanks for posting the photos. I've never seen a babbit pocket before.

Eric

Wondering if perhaps the original design was for oil lubricated pins and waste was where the babbit is now? Probably overthinking but I cant seem to come up with a rationale for the current design. Just happy it seems to work well in service.

We have a couple of German two foot gauge flatcars and the journal brasses for them look just like those rod brasses. They have those babbitt pockets and I have not been able to figure out why. I am thinking this may be something of a European practice.

.

Greetings:

Kelly is correct: the crown brasses on Chinese steam locomotives have several pockets (about 1-1/2x8 inches) cast into them, which according to the blueprints, are filled with babbitt which extends above the bronze crown brass about 1/8+ inch. The crown brasses are lubricated with common journal oil via foam rubber pads about 4x10 inches which are set into rectangular sheet metal forms which are welded to the oil cellar to hold the pads in place.

ALSO, the crown brasses are pressure lubricated by the mechanical lubricator for the machinery of the locomotive. These bearings run at very low temperatures compared to our last remaining locomotive which uses grease to lubricate the driving axles. Valley Railroad No. 3025's driving axles run at about 80~100 degrees Fahrenheit compared to our No. 40 which runs at 110~140+ degrees Fahrenheit.

I'd say that the percentage of bearing surface which is babbitted and lubricated with oil on the Chinese steam locomotives is a well thought out design. Should the babbitt wear down, the bronze bearing is still there. I would note that most large Southern Pacific locomotive driving axles were/are lubricated with oil, as are, I believe, at least some of the locomotives at Strasburg Rail Road.

Be well, stay safe,
J.David