by Allen Hazen
I was rereading the article in the March 2008 "Railfan & Railroad" about MotivePower's MP36bbb (bbb = blahblahblah: there are numerous variants) commuter locomotives. (They have EMD engines, and EMD seems willing to support Wabtec-MotivePower's efforts: a way of selling commuter locomotives that are perhaps half EMD by value without designing their own. So even if I WEREN'T comparing the design to some "classic" EMDs I wouldn't feel too bad about posting to this forum! (Grin!)) The article describes them as having "semi-monocoque" carbodies: the side walls are doing stuctural work; they aren't just "cowl" units. (Good thing: the design is already excessively heavy, and roadswitcher designs need heavy frames, as witnessed by the extra framing Santa Fe had to put on its CF-7.) There are a couple of photos in the article of units under construction, and the framing looked as if it was very different in design from the sort of truss frame on an E or F unit. The classic carbody truss frame had a mixture of vertical and diagonal elements, just like the frame of a truss bridge. (*) The MP36 seems to have mainly verticals: there is a diagonal in the "cell" right behind the cab and another at the extreme rear of the unit, but between these it looks as if the platform and the rail at the roofline are connected only by a "ladder" of verticals. I was taught that triangles were better for forming a rigid structure than rectangles (explaining the presence of diagonal elements in typical truss forms), so I was surprised by this. At a GUESS (note: guess-- this is speculation by a non-professional, on the basis of what was in the article) this means that the side walls of an MP36 are designed to carry a smaller propoertion of thetotal load than those of an E or F. Anybody here know more?
(*) There was a photo-- I think in the F-unit article in "R&R" a few months ago-- showing TA locomotives for the Rock Island under construction. The arrangement of vertical and diagonal elements in their frames was much more simple and regular than that in later F units. At a GUESS, the TA was built with a "first draft" truss design, and the F-unit's more complicated structure was the result of more careful analysis, putting elements of the truss structure in exactly the places needed, given the weight distribution of the unit. (Obviously a locomotive used in multiple unit freight lashups would be exposed to greater stresses than one designed for single-unit use on lightweight passenger trains, so it would make sense to re-do the stress analysis as part of the process of transforming the T into the FT!) ... On the basis of Preston Cook's sketches in (I think) the same F-unit article, it looks as if the FP-7/9 and FL-9 truss frames were designed by plugging additional cells into basic F unit design. Which might not have produced quite the optimal structure, but doubtless simplified the design and construction process: much of the structure of these longer units seems to be in common with the standard F design.
(I am posting a shorter version to the "other builders" forum.)
PS: The December 2008 "R&R" has arrived in Melbourne. If I have any comments after I've had a chance to read part 3 of the E-unit article I will post them in a few days.
(*) There was a photo-- I think in the F-unit article in "R&R" a few months ago-- showing TA locomotives for the Rock Island under construction. The arrangement of vertical and diagonal elements in their frames was much more simple and regular than that in later F units. At a GUESS, the TA was built with a "first draft" truss design, and the F-unit's more complicated structure was the result of more careful analysis, putting elements of the truss structure in exactly the places needed, given the weight distribution of the unit. (Obviously a locomotive used in multiple unit freight lashups would be exposed to greater stresses than one designed for single-unit use on lightweight passenger trains, so it would make sense to re-do the stress analysis as part of the process of transforming the T into the FT!) ... On the basis of Preston Cook's sketches in (I think) the same F-unit article, it looks as if the FP-7/9 and FL-9 truss frames were designed by plugging additional cells into basic F unit design. Which might not have produced quite the optimal structure, but doubtless simplified the design and construction process: much of the structure of these longer units seems to be in common with the standard F design.
(I am posting a shorter version to the "other builders" forum.)
PS: The December 2008 "R&R" has arrived in Melbourne. If I have any comments after I've had a chance to read part 3 of the E-unit article I will post them in a few days.
