It seems to me that in the past there was some discussion about how the drivers were “quartered”, so to speak, on three cylinder steam locomotives. Some saying they were set at an even 120 degrees apart, others saying different. Was it ever determined without a doubt how this was actually done?
In as much as it has been said (and heard) that the exhausts are not evenly spaced resulting in a stuttering sound, just how were the three cylinders timing set in relation to each other?
Everything I’ve found indicates that the cranks were set at 120 degrees apart. Someone actually measured a 3-cyl Alton 0-8-0 park engine about 3 years ago and confirmed this. It shouldn’t be too hard to check because there’s an SP 4-10-2 (5021??) and a UP 4-12-2 (9000) surviving. There’s still controversy re: the off-beat exhaust. Some say it’s the angled center cylinder, other say it’s difficult to get the conjugated valve gear set exactly right, or that it slips out of time after a small amount of wear.
The cranks for the outside cylinders were set 120 degrees apart, but the internal crank was not exactly 120 degrees from the others because the inner cylinder was raised above the plane of the outer cylinders. On the Japan National Railways C53 class 3-cylinder 4-6-2, the angle was close to, but not exactly, 118 degrees.
This was intended to create more even torque, but does result in a ‘syncopated’ exhaust.
The Gresley gear on the later UP 4-12-2s was fitted with roller bearings, which alleviated the problem of wear throwing the center cylinder out of timing. Some of the earlier locos of that class had the Gresley gear removed in favor of a third set of Walschaerts valve gear.
It’s interesting to note that the JNR 3-cylinder Pacifics were much shorter lived than the older C51 class 2-cylinder 4-6-2s, some of which soldiered on almost to the end of steam operation in Japan.
Chuck
Thinking in the way of a V8 engine, how would this effect the exhaust?
If you check with Mr Meislahn I think he’ll tell you he “confirmed” (to the degree possible) that the A&S cranks were 126-114-120, as you’d expect with the inside cylinder 6 degrees from horizontal.
Seems a safe bet to me that any engine with conventional Gresley-Holcroft gear (i.e. a short 1-to-1 lever pivoted at the end of a long 2-to-1 lever) is intended to have the pistons reaching front dead center at even 120-degree intervals of driver rotation. So the cranks themselves can’t be 120-120-120 if the center cylinder is inclined.
So why is 3-cyl exhaust uneven? Well, next question is, was it always uneven? Did German and English 3-cyl engines sound like that? I’m guessing not. (Of course, their center cylinders weren’t always inclined-- sometimes they drove the lead axle.)
Timz scores again! I checked the Meislahn drawing (which I forgot I had) and sure enough it’s 126-114-120. Memory ain’t what it used to be. I stand corrected.