Murphy Siding Can anyone explain the need for span bolsters connecting the pairs of trucks?
Most other arrangements will tend to put more weight on the axles that are on a 'high' spot, and 'unload' those in low spots, thereby creating a greater tendency for those axles to slip when under a large tractive effort load.
- Paul North.
Following what Paul said, I offer this about the span bolsters:
The shorter the rigid wheel base of each truck, the sharper the curves that can be negotiated. In order to permit this locomotive to negotiate tight curves, it uses a pair of individual B trucks at each end rather than one D truck having a continuous truck frame and rigid wheel base.
If you study the geometry of using four independent trucks under a locomotive, you can see that you cannot simply attach each truck to the locomotive frame with its own pivoting bolster bearing. Each end of the locomotive can only have one pivotal truck attachment point. So the pair of individual trucks has to attach to that one pivot point on the locomotive frame.
At each end of the locomotive: the span bolster is used to bridge the two independent trucks and then the center of that bridge attaches to the locomotive frame. The span bolster has a pivot bearing at each end on its bottom which rides atop the center pivot bearing of each truck. The span bolster also has a pivot bearing at the center of its top which attaches to and suports the single pivot bearing of the locomotive frame.
So the span bolster pivots on the locomotive frame, and the two trucks pivot on the two ends of the span bolster.
Bucyrus The shorter the rigid wheel base of each truck, the sharper the curves that can be negotiated. In order to permit this locomotive to negotiate tight curves, it uses a pair of individual B trucks at each end rather than one D truck having a continuous truck frame and rigid wheel base.
The Illinois Terminal four truck electric locomotives were capable of negotiating some ridiculously tight curves, seem to recall 50' radius or sharper.
- Erik
Thanks both of you for the explanation. Each pair of B-B trucks has one pivot point to the frame- kind of like on an HO locomotive. The span bolster picots where it meets the truck. But, that pivot is in the up & down direction? As I picture it, the span bolster is connected to both trucks. As such, wouldn't that make the set of B-B trucks the same length on each side, so it acts, and pivots just like a D truck, but able to pivot up and down on uneven track?
Thanks to Chris / CopCarSS for my avatar.
Just to clarify a bit: The trucks do not attach directly to the locomotive frame. They attach to the span bolster, and the span bolster attaches to the locomotive frame.
The span bolster and the two trucks all pivot with the pivot swing in a horizontal plane. That is, their pivot axes, or pivot pins are positioned vertically.
I suppose the span bolsters can accommodate some rocking motion fore and aft, and side-to-side, in addition to their pivoting in the horizontal plane. There is that same type of action of the body relative to the trucks with a normal locomotive or freight car. The pivot bearing is loose enough to accommodate the rocking motion.
Here is a good explanation including a drawing of a military gun with railroad wheels using span bolsters.
http://en.wikipedia.org/wiki/Span_bolster
Here is a heavy haul car with each end riding on four trucks. The four trucks use a main span bolster that rides atop two secondary span bolsters. And those secondary span bolsters ride atop the trucks. The main span bolster has the draft gear and coupler mounted on its end, so it extends further out beyond the two end trucks
http://www.flickr.com/photos/alcomike/3433929307/
Here is a locomotive with the same span bolster arrangement as the TEM14
http://nick86235.smugmug.com/Trains/Away-Fixtures-2007-chasing/17789124_zLpGHg/1359194725_RXm4s76#!i=1359197007&k=LS3p8tT
On the TEM14, what is unique is the large prominent beam outside of the trucks. Typically the span bolster is mostly hidden because its width is confined within the track gage. The beam on the outside of the Russian locomotive appears to be an extension of the span bolster in order to give it support and suspension from the side sill of the locomotive frame. With that feature, they may actually have a more complex center bearing, such as a ball joint, to accommodate the rocking and pitching motion that I mentioned above. If that is the case, this would seem to be a highly advanced locomotive truck system.
From Googling around the web, it would seem that the locomotive in question is a Genset style rebuild of an older TEM-7a locomotive. That model is a Soviet era roadswicher built in large numbers and originally had a 12 cylinder 2,000 HP V-12. The rebuilt unit has double gensets. The core locomotive uses 4 B trucks, all motored, so unless the rebuilding made major changes to the trucks it does have all 8 axles powered.
As an interesting aside, in the early 90's a Texas based commodities broker excepted a number of TEM-7a's as pavement from the Russians for export grain. These were stored for many years at the Port of Houston but no North American buyer could be found. I believe they all went to scrap early this Century:
http://www.trainweb.org/southwestshorts/russian.html
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
I have been inside these guys, they were on the PTRA’s Brown lead stored for a long time, very primitive inside.
The bar on the side of the trucks is an equalizer, the purpose of it is to keep the trucks from rocking up and down in excess…it forces the lead wheels on, say the trailing truck, downward when the lead truck moves up.
On bad track, it will slow the up and down “harmonic rock” effect of the trucks bouncing across a dip, or bouncing on a high spot.
It also slows the side to side sway or harmonic rocking of the body.
Dampers would be a better term I guess.
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