weight of a 4-8-4's connecting rods???

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weight of a 4-8-4's connecting rods???
Posted by Compressor man on Monday, November 3, 2008 6:33 PM

I was just watching a train show on the television, it focused on the 4-8-4 and there were several "chase shots" with the locomotive running at a high rate of speed. I could not help but wonder how much those huge connecting rods weighed. I mean, that is one BIG chunk of iron flying around and around at a frightening speed. Does anyone have some idea what one of these things weighs?



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Posted by dldance on Monday, November 3, 2008 7:46 PM

I don't have an exact weight, but in the book "Superpower:The Making of a Steam Locomotive," by David Weitzman, the billet for the main side rod of a 2-8-4 is noted as weighing about 3,600 lbs.  Now - that billet will lose some weight as it receives final machining but I think that 1.5 tons is a good rough estimate.  And that is just one of the rods.  Of course that is balanced by the rod on the other side and the counterweights - but it is still a lot of moving mass.


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Posted by Gunns on Monday, November 3, 2008 8:40 PM

In 1945 the 2926 had it's wartime (original) side rods replaced with high strength "ultra light weight" connecting rods and roller bearings. We haven't had the rods off yet but the metal feels different and sounds different when you top on it. I do know that everything was rebalanced in the upgrade.




ps I changed my sig photo to one taken of the 2926 rods. taken from the rear, note the profile.

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Posted by Compressor man on Thursday, November 6, 2008 5:38 PM

Hey thanks for the replies guys. The guess of about 3000 lbs was pretty close to what I thought they must have weighed. Its just hard to believe that bearings can stand up to the tremendous forces that they must have experienced when that loco was really moving, but obviously they did stand up to it!!!


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Posted by timz on Thursday, November 6, 2008 5:54 PM

I'm guessing 3000 lb is too high. Steel's density is about 7.8, isn't it? If so, 3000 lb would be 10650 cubic inches-- e.g. cross section 8.9 inches square and 135 inches long. Don't think they averaged that thick.

As I recall, the articles in Rwy Age and Rwy Mech Engr didn't usually give the rod weights, just the reciprocating weights-- and don't ask me how to convert a main rod's "reciprocating" weight to its total weight.

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Posted by feltonhill on Thursday, November 6, 2008 7:31 PM
Somewhere in the distant recesses of my memory, which is sometimes out to recess a lot lately, I recall reading that a PRR I1's main rod weighed about 1100 lbs. Dimensionally, this rod would be a lot longer and heavier than most any 4-8-4 that I can think of. I believe this may have been in a Trains mag sometime around 1955-56 re: the final I1 service of out Northumberland on ore trains. MOre on the subject, I'm currently in Roanoke, and may be able to find what a Class J's mainrod weighed. Be back tomorrow night if I find anything
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Posted by Modelcar on Saturday, November 8, 2008 11:03 AM

Of course that is balanced by the rod on the other side and the counterweights - but it is still a lot of moving mass.


As for balancing on each axle and using the quote above....of the rod being balanced by the "rod on the other side"....Wouldn't that alone just balance {that axle}, statically but certainly not dynamically. 

Perhaps each side was balanced as best possible between the rod / journal-bearing weight and the counterweight.


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Posted by dredmann on Monday, November 10, 2008 1:38 PM

Some basic observations:

(1) Usually a billet out of which something is machined is much heavier than the ultimate item. If the billet weights 3600 pounds, the finished rod is probably more in the range of roughly 1000 - 1500 lbs.

(2) I think that the usage "main side rod" is incorrect or at least misleading. As I understand, the main rods are the two* long rods that run from the pistons to the crank pins on the primarily-driven wheel; the side rods run from the crank pins on the primarily-driven wheel to pins on the other wheels. Suppose you have a locomotive with eight drivers; you might have the main rods going from the pistons to, say, the third driver (from the front) on each side, and then side rods running from the third driver to the fourth driver, the third driver to the second driver, and the second driver to the first driver.

(3) The term "connecting rod" is to me somewhat less precise; I think most people use "main rod" or "side rod" when being specific.

* Obviously on some locomotives you'd have four, or even six, but two was by far the most common.


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Posted by Piper106a on Wednesday, November 12, 2008 5:46 PM

Don't know about a 4-8-4, but in the 1941 Locomotive Cyclopedia (1971 Kalmbach reprint) page 730, a Timken advertisment compares their light weight rods to conventional rods for a 4-6-4.  Some weights; 

Conventional main rod - 1003 pounds, Timken lightweight main rod - 529 pounds.

Side rods; conventional - 922 pounds, Timken light weight side rods - 638 pounds.

Those are the weights for each side of the engine.


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