Side Rods

[Keith Hayes]

I have a nice photo of Espee 4449 as my screen saver. I noticed the other day that the main rod is attached to the second driver, and this got me to wondering why? Why not #3?

A quick study of NG lokies indicates that six couples engines (2-6-0s and 4-6-0s) always have the rod connected to the middle wheelset.

Engines with 4 axles vary between the second and third wheel. C-16s and the earlier B-4s have rods connected to the second axle. But 71-73 and the Brooks engines connect to the 3rd axle. Same is true for C-17s and 18s, but not the 19s. C-21s, 25s and the Ks all have the rod on the third wheel.

Yet the big 4-8-4s go back to the second wheel.

Why?

Did any classes have a mix?

[Mike Trent]

Hi, Keith.

As steam locomotives evolved through the late 1800's, the science of physics advanced, and improvements relative to greater tractive effort and efficiency took root in the steam locomotive accordingly.

By 1897, it was accepted that one of the ways to increase both of these areas was to use the 3rd driver on a 2-8-0. This allowed, because of the longer length to the crankpin, a higher level of efficiency.  1897 was the year the B-4-E class was built. The following year, the B-4-F class engines were built by Brooks for the C&N ry, and they also employed use of a third axle main driver.

There are different ways to calculate tractive effort on a steam locomotive, but length of the main rods is not generally considered. Rather, boiler pressure, bore and stroke of the cylinders, and driver diameter are the typical factors.

On D&RGW narrow gauge 2-8-0's, it is interesting to consider the C-18 and C-19 classes. The C-18's were rated in the conventional way at 18,000 lbs TE, hence C-18. The C-19's, which included three engines which were leased to the C&S in 1936, were rated in the conventional way at 19,000 lbs TE. The D&RGW was phasing out the C-19's because it was found that the C-18's, which had the 3rd axle as the main driver, pulled their tonnage better than the slightly heavier C-19's. That is why C-19's were available for lease. On the C&S, engine crews constantly complained that the C-19's couldn't pull tonnage nearly as well as the B-4-E class, which was also rated at 19,000 lbs TE, despite having added half flanges to the blind drivers. The C-17's were classed the way they were because they were likely a bit smaller. The C-18's may have been classed in the way they were to differentiate them from the C-19's, as they probably could have been rated at 19,000 lbs.

At the time the B-3-C's had been rebuilt from the old Cooke design, the main rods were not changed in position.

I don't know why the SP Northern you have on your desktop had the main driver at #2, but they must have done it for a particular reason, possibly for a certain area of their system where that worked better. The UP "800's", which I would consider the Gold Standard of Northerns, had the main drivers at #3. The third series of 800's was built in 1944.

 

[cdressel]

Just a comment about the discussion:

I would think that a high-speed locomotive would want the minimum mass for a main rod.  The longer the main rod, the harder to keep the locomotive in balance at different speeds.  In other words, the main rod should be connected to driver set #2, rather than #3.

I can't think of any narrow gauge locomotives that normally operated at the speeds of the modern Northerns.  So, the balancing of the main rod wasn't quite as critical.  The manufacturer could couple it to driver set #3 without too many disadvantages regarding balancing and harmonics.

Just my 2-1/2 cents.

Chuck

[Keith Hayes]

That was my thinking,  Chuck.

Mike, I share your feelings for the FEF-4s, however the main rod is connected to #2, as it is on N&W 611.

Curiously, the Big Boys and Challengers use the #3 driver: that is a lot of steel in motion.

I had not thought about the rod and tractive effort relative to the 18s and 19s. This explains why 315 and 318 survived so long. Tell us more, Mike.

[SteveG]

I don't have a copy here at work to check, but I'm pretty sure that there's mention in the Narrow Gauge Pictorial for the C&S motive power, that the C&S experimented with different main rods on some of the Moguls--I recall a pair of pictures of one of the engines with the main rod connected to different drivers at different times.
I'll check again this evening, if no one beats me to a copy to verify first.

[Chris Walker]

That would be the #37, a Consolidation Steve.  Separate views showing 2nd Driver then 3rd Driver connected.  The Mason Bogies had the 3rd Driver connected, can't say I've seen a Mogul with 3rd Driver connected though.  Awaiting your picture.

[SteveG]

I stand corrected. Thanks!

[Mike Trent]

Hi Keith, I think Chuck nailed it with his comments. Right you are on the FEF's. I've been away far too long!

I don't know much more about the C-18-C-19 comparisons. I believe I gleaned that several years ago through a discussion on the NGDF by guys far more knowledgeable than me. What I was able to add was to confirm what I learned from Doug Schnarbush  about how the C&S enginemen disliked "them" Rio Grande engines.  It might be possible to pull up the discussion by using the search engine on the NGDF.

[linnwm]

The main two issues to deal with are weight and angularity. The shorter the rod, the less the weight but the more angularity which saps power and tends to strain crosshead guides. The longer the rod, the more weight to counterbalance, but much less angularity.