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[quote]QUOTE: Originally posted by Overmod Interesting that in the late '20s it was recognized that *running* resistance of roller vs. plain bearings was not that different. The reliability and 'sealed maintenance' characteristics of roller bearings imho are more significant than starting antifriction in many cases. I remember (I think) that the limiting lifetime of Timken freight-car bearings was limited by wheel wear, rather than anything about bearing metallurgy or tribology, as early as the 1970s. With respect to steam roller bearings, the situation is a bit complex. The NP engine is, of course, the famous "Four Aces", built for Timken as a demonstrator; a successful as well as good-looking locomotive. Many roads did not use roller bearings in the trailing truck, due to firebox heat and ash problems, if I remember rightly. Rollers on the driving axles could get interesting. N&W, and perhaps other roads, had early versions that were incorporated in the hubs (Louis Newton has a rather good picture of one in his Rails Remembered volume 3). Something like this was my solution to the inside cranks on a Withuhn conjugated duplex locomotive, where the inner race diameter of the 'main' drivers is very large, greater than the crank circle; the hollow axles are of very great diameter, and external bridges between the bearings, outside the throw of the conjugating inside rods, provide the necessary lateral stiffness. Main drivers are not ideally provided with lateral motion, so there is less problem with designing suspension, frame, etc. Later modern steam power used very substantial axlebox structure (IIRC called a 'cannon box' or something similar; I don't have references nearby) to keep the bearings "perfectly" aligned; this lessened the requirement for tight and near-perfect adjustment of axle alignment in the frame (e.g. via Franklin self-adjusting wedges). Timken used tapered rollers, while SKF used a barrel-shaped roller (which had some of the lateral self-aligning and thrust geometry of a ball bearing while preserving line contact, or at least that's the description I was given) There are roller bearings on some axles in the Baldwin 60000 demonstrator (of 1926) preserved at the Franklin Institute in Philadelphia. I remember this because the bearings took a 'set' while the engine sat on display for so many years ("egg-shaped" was the term they used at the time to describe it) and needed to be rebuilt a few years ago. My opinion is that the 'datum' or reference axle on a 4-8-4 should be the main driver axle. Since the locomotive is almost certainly double-acting, it doesn't 'matter' from a thrust standpoint whether the self-adjusting wedges are at the front or rear of the axlebox -- my preference is to put them at the rear because that 'unloads' the wedge a bit more, permitting easier self-alignment action, during acceleration. All the axles need to have the wedges at the same side, with the non-wedge side being precisely machined, built up if worn, etc. -- it is the precision and accuracy of this machining that controls the clearances mentioned above, including the rod clearances. An interesting case involves the geometry of side-rod bearings on lateral-motion axleboxes. There is no particular difficulty with the wheel bearings here, because of the substantial axlebox construction. But the rod bearings -- that's another story! One solution is to provide a spherical bearing (like a ball-and-socket joint) outboard of the actual antifriction roller bearing outer race, lubricating it appropriately. UP 844, I believe, is an excellent 'poster child' for observing exactly how this sort of lateral motion is best implemented... If something here is wrong, I invite correction (e.g., I had not recognized that two Allegheny locomotives had actually survived!) RME [/quote] So, the machined face of the bearing journal block guides in the frame on the opposite side of the wedge blocks is what actually determines CL distance between axels. (whew!) That should mean then that the tapered blocks are used only in maintaining a tight fit between the guides as they wear. What about freight and passenger trucks, do they use tapered rollers?
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