Re: K-27's rebuilds

A lot of compound bashing in this thread!

I submit that “compound” wasn’t the reason behind the initial conversions of the K-27’s nearly so much as “Vauclain” was. As has been pointed out, compound steam locomotives in the form of Mallet articulateds were used right up to the end of steam, and on the Rio Grande, the #3400’s and #3500’s had long and successful careers as compounds and were never converted. On the other hand, Vauclain compounds were converted to simple locomotives wholesale within a very few years of their construction, to the point of being nearly extinct by when, about the 1920’s?

The problem with the Vauclain compound comes from its piston rods being placed out of line to the wrist pins. If you look at an indicator diagram of a Vauclain compound working at long cutoff, the work produced by the two cylinders was nowhere near even, with the low pressure piston producing most of the thrust during the first half of the stroke, and the high pressure piston producing most of the thrust during the second half of the stroke. This would result in constant racking of the crossheads, and I expect that the “high maintenance costs” so often mentioned in histories were specifically piston rods that were constantly breaking where they entered the crossheads.

It speaks to the salesmanship of Samuel Vauclain that first he was able to sell his brand of compounding so widely during the brief period that it was popular, and then able to sell the same railroads thousands of simple cylinder conversion kits for locomotives that were only a few years old.