07-17-2012, 10:15 PM
Warning: I got on a bit of a monologue below - but I love the topic! (Herc, apologies for derailing your thread)
Hi Sumpter!
Yes, true enough - my amazement is around the part that is not replaced - the part on which every bearing surface is directly or indirectly attached: the engine bed. Older steam had built-up frames in which alignment of anything was never a given and were a source of heavy maintenance. This also connects to my comment about steam locomotives beating themselves up - reciprocating weight (piston, crosshead and a component of the main rod) had to be offset on the main driver. I've read (this is mostly book knowledge with a smattering of info from people with steam experience) that stationary steam locomotives generally had about a 50% rotational overbalance on the flywheel to compensate for the reciprocating forces. This was manageable when the wheel could be held in a fixed position (i.e. bolted to a factory floor). However, applying that much overbalance to a railroad steam locomotive was tremendously destructive to rail and locomotive as the drivers hammered the rail while rotating, leading to broken frames and rail - and sometimes at very low speeds (25 - 35 mph). One argument I've found interesting is that suspension exacerbated the issue as it allowed the wheels that much more travel, and with no damping of the motion - no shock absorbers on these guys!
Rule of thumb was to set overbalance to about 30 - 35%. This was acceptable for rail and roadbed (hammering was reduced), but aggravated the problem of yaw - the locomotive "nosing" back and forth as a result of the now underbalanced reciprocating forces. This was also damaging to rail (knocked it out of gauge left and right) and made it difficult to keep the frame in tram (all axles perpendicular to the long axis of the locomotive). Two and four wheel leading trucks helped dampen nosing - one reason you see four wheel leading trucks on faster locomotives. As a bit of a tangent, one reason that N&W's J could attain high speeds (111mph) with relatively small 70" drivers was a due to a leading truck centering design that had stiff resistance to nosing. General thinking was that there was not enough room on the driver for enough weight to control nosing at high speeds; this design compensated for that. N&W's compromise gave the J excellent acceleration and maintenance of speed with heavy consists up significant grades (due to the small drivers), with the capability of speed, but at the cost of an extremely stiff locomotive that wasn't much good for any other task.
Back on topic - so steam locomotives are rough on themselves, and built-up (bolted together) frames were less and less up to the task of retaining tram, alignment with the cylinders, and I don't know what else. About the time that steam was getter faster and more powerful, steel casting technology was improving, eventually resulting in the ability to cast the entire frame - cylinders included - from pilot to buffer plate. Union Pacific's second (I think) and final order of 4-12-2's had one piece cast frames (and with the additional complexity of three cylinders, to boot!) that were fantastically huge. Flip through the fantastic book "The Union Pacific Type" by William Kratville for some casting and machining photos of these particular engine beds.
Cast steel "engine beds" had become more and more common in the late thirties and early forties, especially on big, fast steam. They were a HUGE improvement over built up frames given the forces that steam locomotives subject themselves to, minimizing frame related maintenance, breakage and tram issues. Secondary items such as wear on rod and axle journals was also reduced since these parts were held in rock-solid alignment compared to built up frames.
Anyone who has the chance to approach a modern steam locomotive closely should take a long look at these cast pieces of (huge) - they are beautiful.
All of the above is why I find the 844 such a marvel. 65 years of pounding, and still running!
Sumpter250 Wrote:All of the "wear points"....those contact points where one item moves against another.... have replaceable bearing surfaces, which are inspected and replaced often.
Hi Sumpter!
Yes, true enough - my amazement is around the part that is not replaced - the part on which every bearing surface is directly or indirectly attached: the engine bed. Older steam had built-up frames in which alignment of anything was never a given and were a source of heavy maintenance. This also connects to my comment about steam locomotives beating themselves up - reciprocating weight (piston, crosshead and a component of the main rod) had to be offset on the main driver. I've read (this is mostly book knowledge with a smattering of info from people with steam experience) that stationary steam locomotives generally had about a 50% rotational overbalance on the flywheel to compensate for the reciprocating forces. This was manageable when the wheel could be held in a fixed position (i.e. bolted to a factory floor). However, applying that much overbalance to a railroad steam locomotive was tremendously destructive to rail and locomotive as the drivers hammered the rail while rotating, leading to broken frames and rail - and sometimes at very low speeds (25 - 35 mph). One argument I've found interesting is that suspension exacerbated the issue as it allowed the wheels that much more travel, and with no damping of the motion - no shock absorbers on these guys!
Rule of thumb was to set overbalance to about 30 - 35%. This was acceptable for rail and roadbed (hammering was reduced), but aggravated the problem of yaw - the locomotive "nosing" back and forth as a result of the now underbalanced reciprocating forces. This was also damaging to rail (knocked it out of gauge left and right) and made it difficult to keep the frame in tram (all axles perpendicular to the long axis of the locomotive). Two and four wheel leading trucks helped dampen nosing - one reason you see four wheel leading trucks on faster locomotives. As a bit of a tangent, one reason that N&W's J could attain high speeds (111mph) with relatively small 70" drivers was a due to a leading truck centering design that had stiff resistance to nosing. General thinking was that there was not enough room on the driver for enough weight to control nosing at high speeds; this design compensated for that. N&W's compromise gave the J excellent acceleration and maintenance of speed with heavy consists up significant grades (due to the small drivers), with the capability of speed, but at the cost of an extremely stiff locomotive that wasn't much good for any other task.
Back on topic - so steam locomotives are rough on themselves, and built-up (bolted together) frames were less and less up to the task of retaining tram, alignment with the cylinders, and I don't know what else. About the time that steam was getter faster and more powerful, steel casting technology was improving, eventually resulting in the ability to cast the entire frame - cylinders included - from pilot to buffer plate. Union Pacific's second (I think) and final order of 4-12-2's had one piece cast frames (and with the additional complexity of three cylinders, to boot!) that were fantastically huge. Flip through the fantastic book "The Union Pacific Type" by William Kratville for some casting and machining photos of these particular engine beds.
Cast steel "engine beds" had become more and more common in the late thirties and early forties, especially on big, fast steam. They were a HUGE improvement over built up frames given the forces that steam locomotives subject themselves to, minimizing frame related maintenance, breakage and tram issues. Secondary items such as wear on rod and axle journals was also reduced since these parts were held in rock-solid alignment compared to built up frames.
Anyone who has the chance to approach a modern steam locomotive closely should take a long look at these cast pieces of (huge) - they are beautiful.
All of the above is why I find the 844 such a marvel. 65 years of pounding, and still running!
Matt Goodman
Columbus, Ohio
Columbus, Ohio