Hi Kcsivils,
You are correct about the NG15's leading Bissell truck arrangement, which incorporates the locomotive's pilot truck wheels and the first set of drivers into a single bogie. The loco is essentially a 4-6-2 with the "second pair" of "pilot truck" wheels powered. This was a design, at least similar, to a Krause-Helmholtz bogie. Interestingly, the NG15s were the replacements for the NG10 4-6-2s built by Baldwin.
On an NG15, the "driven axle" is hollow - like a pipe - with the wheels mounted to it. The actual driving axle feeds through the center of this "pipe" and utilizes the usual outside frame driving wheel boxes, shoes, wedges, etc. to mount it to the locomotive's main frame and drive rods. There is sufficient clearance between the two axles to allow room for a mechanism that transmits the motion of the inner axle to the outer axle. The two concentric axles are mounted together by means of a Klein-Lindner housing.
The book "Namib Narrow Gauge", by Moir and Chrittenden, does not say how the motion of the "fixed" axle is transmitted to the "hollow" axle on the NG15's. But Wiener's book "Articulated Locomotives" states that a bogie truck design developed by Krauss-Helmholtz is able to successfully connect a Klein-Lindner equipped driving axle and an unpowered bogie axle into a single bogie truck, so my guess is that the NG15s used this arrangement.
The Klein-Lindnerlocomotives pioneered this axle-within-axle design to reduce the fixed wheel base of its locomotives. That's why the O&K 60cm "Kriegslok" 0-8-0Ts could make such tight turns. They had only a 900mm fixed wheelbase and yet a 3000mm total wheelbase.
As stated previously, the front driving axle is attached to the mainframe of the locomotive via the axles mounted within the Klein-Lindner housing. It is also attached to the "front truck" via a yoke incorporated into the pilot truck frame. The yoke ends in a ball joint (Bissell) which is mounted within a socket that is built into the Klein-Lindner housing under the mid-line of the front axles. This arrangement allows the front driver to be "steered" into the curves by the unpowered bogie wheels. The bogie frame pivot is located under the front cylinder block, like any front truck pivot would be.
Best Regards,
Glenn
Edited 2 time(s). Last edit at 08/01/2014 09:13PM by christensenge.
