HT-B Truck

[Engineer Spike]

I have wondered what EMD was trying to accomplish with this design? Why did they go back to the Blomberg design?

[Allen Hazen]

Well, the "HT" in the designation probably stands for "high traction." The HT-B truck was used on 3500 hp BB prototypes at a time when many railroads thought even 3000 hp needed six axles (witness the popularity of the SD40-2 versus SD45-2). So, at a guess, the new truck design offered higher adhesion. It might be interesting to compare it to Alco's Hi-Ad and to the MLW "ZWT" (zero weight transfer) and GE's FB-2 designs. My suspicion is that in each case the point at which tractive effort was transferred to the body of the locomotive was lower-- closer to axle level-- than in earlier designs used by the same builder (I know this is the case with Alco's Hi-Ad; I think GE's FB-2 is similar in principle to the Hi-Ad, substituting a more easily maintained stack of elastomeric and metal plates for the Alco design's coil springs), thereby reducing the weigh transfer that tends to unload the leading axle.

Why wasn't it adopted more widely? Technological advance in locomotives was painfully slow in that era: the main railroads were very conservative in what they would order. Something NEW, something that would involve NEW SPARE PARTS INVENTORY, even if clearly somewhat better than the existing (Blomberg with extra shock absorbers) design, would be passed up. So: maybe the HT-B turned out not to be very much better than the Blomberg, maybe it was more expensive, but I suspect it might have been the market flop it turned out to be even without drawbacks like these. (Compare: the HT-C truck introduced on six-axle Dash-2 designs was supposed to improve adhesion by reducing weight transfer as compared to the older Flexicoil truck. But one major railroad-- Conrail-- insisted on having its SD40-2 and even its initial orders of SD50 built with the old truck.)

(((Question I don't know the answer to: could HT-B and Blomberg trucks be interchanged, or did they require differences in the locomotive underframe? If the latter, there would be an additional motive for conservative motive power authorities in purchasing railroads to stick to the older design.)))

[bogieman]

I posted this about 3 years ago on Trainorders.com:

"The HTB was a great advance in weight shift compared to the GP swing hanger truck - the weight shift within the truck was nearly zero under adhesion whereas the GP is pretty bad. However, the advent of super series wheel slip control pretty much eliminated the need for a zero weight shift truck. That, combined with the increased cost of the truck and the underframe mods it required, caused its early demise. I am confident the ride would have been fixed if there were enough of them out there. In principle, it has softer primary springs then the GP and more deflection in the secondary as well compared to the GP with compression rubber springs, the standard at the time. I suspect the lateral stiffness of the secondary chevrons was the source of the rough lateral ride, but with some tweaking of the chevron angle, that could have been reduced. But the GP with its swing hangers is about as soft laterally as practical, and the +/- 2.25 inches travel to the secondary suspension stops is huge by comparison to other trucks. I'm not sure what profile wheels the GP40X's were delivered with but when used to pull passenger trains it was recommended they change the profile to 1:40, as used on the F40PH, to limit hunting.

Incidentally, in 1984 the only remaining spare HTB frame in LaGrange was used to fabricate the first prototype of a radial truck, which eventually led to the HTCR truck."

The HTB had rubber secondary springs placed on an angle that focused their reaction at rail level, not much different in principle than the ZWT truck. It did require a different underframe as it used a larger centerbearing than the GP swing hanger truck which was mounted at a lower height relative to the rail, about 28" or so above the rail. And the bottom plate had to be higher because of the increased travel primary springs which were the same as the HTC used. Head to head adhesion testing on the GP40X lococ, half of which were built with GP trucks showed no advantage to locomotive adhesion although there was less difference in tractiion motor currents. EMD was also doing a good business at that time with locos built with trade-in trucks so those things conspired to kill the truck.

Dave