• EMD "DD-series" official thread (covers all variations)

  • Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.
Discussion of Electro-Motive locomotive products and technology, past and present. Official web site can be found here: http://www.emdiesels.com/.

Moderator: GOLDEN-ARM

  by Pneudyne
The UP initial order for 25 6600 hp “Centennial” units was announced in the journal “Railway Locomotives and Cars” (RLC) for 1968 December.

An interesting comment in the subject item was: “The new locomotives could be carried on Electro-Motive's four-axle trucks which were used on the 5000-hp models or might be mounted on a span-bolster running gear which was utilized by Alco and GE on their 1963 models.”

That would suggest that UP had a preference for the span-bolster running gear and at least explored the possibility of using it under the Centennial locomotives. I imagine that EMD would have resisted, and anyway, given that deliveries were scheduled to start in 1969 April, there may not have been enough time for such a major engineering change as compared with using the stablished D-D running gear. No EMD model designation was mentioned in the item. Presumably that awaited finality on the running gear choice.

Going back to the DD35 in 1963, although this was no doubt inspired by UP’s idea of a three-unit, 15 000 hp consist for fast freight haulage, the two prototypes were not built to a UP order, so EMD had a free hand in choosing the running gear, opting for D trucks which offered lower weight and a lower centre of gravity as compared with the span-bolster arrangement. On the other hand, Alco and GE both built their initial examples against a UP order, so would have had to meet UP’s requirement that they used recycled GTEL4500 span-bolster equipment. Given that the prototype GTEL4500 was an Alco-GE project, one assumes that neither had any technical problem with that.

EMD also differed in offering a four-unit consist for 15 000 hp, namely GP35+DD35+DD35+GP35. I have seen it said that in part this was because it was thought that a leading D-D unit might be harder on the track, although whether that emanated from EMD itself or a third-party observer is unknown. I imagine that under hard pulling conditions, there might be some mitigation of lateral railhead forces of the trailing units in curves, although just how much is unknown. (And the reverse might be true under dynamic braking.) Be that as it may, any concerns that may have existed about a leading D truck were not inimical to the development of the DD35A soon after the DD35 entered service.

Perhaps by the time the Centennial was planned, the UP had acquired some data to suggest that the span-bolster arrangement was preferable to the D trucks in respect of track wear and tear, hence the “either/or” situation mentioned in the RLC item. But if so, it was evidently not a sufficient gap to disqualify the D-D running gear.

Regarding the EMD D trucks themselves, something I have not been able to ascertain is whether they had provision for lateral movement, controlled or uncontrolled, of some of the axles. Given the 17 ft 1.5 in wheelbase, one might have expected this. The patent mentioned upthread (https://railroad.net/emd-quot-dd-series ... ml#p307774) does not seem to cover this aspect, rather focusing on the benefits as compared with the span-bolster arrangement, and some of the structural aspects. An RLC 1963 June article on the DD35 and GP35 described the former as having “fully flexible” trucks. But this could be interpreted as referring to the combination of the flexicoil secondary springing with the long-travel primary springing, without necessarily implying any lateral flexibility for the individual axles relative to the frame.

By way of a comparison point, the D-trucks under the GN W-1 class motor-generator electric locomotives, with B-D+D-B running gear, had a wheelbase of 16 ft 9 in:. The inner axles of the D trucks were allowed 1.5 inches lateral movement each way under the control of a rubber sandwich spring mounted above the axleboxes. (This might have been similar to the arrangement used on the rigid-wheelbase axles of the steam locomotive “Centipede” tenders.) Whether the Baldwin 2-D+D-2 diesel-electric running gear of the 1940s was similarly equipped is another unknown.

Returning to the RLC 1968 December item on the UP Centennial, it also included this commentary, reflective of what has been said upthread: “A frequent criticism of the two-engine locomotive is the loss of availability inherent with malfunction of either of the prime movers. On the other hand, many of the servicing and maintenance costs involved with locomotive units are on a unit, rather than a horsepower, basis, encouraging the use of the largest possible individual units.”

Thus there was a trade-off whose optimum solution would vary with situation and also over time, dependent upon the state of locomotive technology. In 1968, the UP had five years of experience with operating large twin-engined units, and a lot longer with the EMD E-series passenger units, which it acquired through to the mid-1960s. So one assumes that it had “run the numbers” and had come to the conclusion that on balance, the twin-engined Centennial was justified for its operations at the time. There is evidence that the UP was mindful of the drawbacks of a single powerplant failure. Apparently, its main objection to the Alco C855 was that with its wet-block engines, a power assembly replacement necessitated by say a scuffed liner was a backshop operation that took the locomotive out of service for a day or two. On the other hand, with the EMD and GE dry-block engines, a power assembly change could be made on an urgent basis within a few hours, and without moving the locomotive to a backshop.

Aside from that, I understand that the Centennials were at a disadvantage when locomotive through-running became more commonplace, as the neighbouring roads preferred “standard” locomotives. Thus the optimum trade-off point probably shifted to being against the specialized, twin-engined locomotives.

D-trucks (as distinct from D-assemblies in a rigid wheelbase) were relatively rare. As best I can determine, EMD was alone in building D-D locomotives with independent trucks. The other examples used combination, articulated trucks. The GN W-1, B-D+D-B, has already been mentioned, as has the Baldwin 2-D+D-2 group. Beyond those were the “Little Joe” DC electrics, also 2-D+D-2, and a solitary Russian diesel-electric prototype of 1924 with the 1-C+D+C-1 wheel arrangement, where the centre D section was a swivel-truck, not part of the main frame. (It was one of the very first tribo locomotives.)

  by bogieman
Regarding lateral flexibility, the D trucks used standard to EMD Hyatt cylindrical roller bearings where the journal box is laterally captive between the pedestals of the frame and lateral travel of the axle is allowed by the cylindrical rollers with relatively little lateral resistance until a thrust block is contacted by the end of the axle. The nominal lateral free clearance to the thrust block is set to +/- 3/16" for total axle lateral of 3/8". The thrust block has a rubber pad behind it that allows about 1/8" compression at about 20,000 lbs. lateral force IIRC. The EMD maintenance instructions make no mention of increasing the lateral travel at the middle axles, kind of surprising to me.

The secondary suspension is a flexicoil design with about +/-2" lateral travel.

I didn't get involved in bogie design until the DD35 were in service for 20 years so don't know the design thinking then. I do know after the DDM45's for Brazil which used a meter gauge version of the DD bogie, there was never any thought of building more with that bogie as we recognized wheel wear was problematic; any future 8-axle loco would use a span bolster with 2-axle bogies as we did when I designed the GBB bogie for the SD70ACe-BB built recently in Brazil. I did give the order in 1987 to scrap the pattern equipment for the DD bogie since we weren't going to cast any more.