Question about commonwealth tender trucks

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Question about commonwealth tender trucks

Post by Steamin » Mon Nov 25, 2013 8:22 pm

Question about commonwealth tender trucks bolster for frame link arrangement. I'm working on a set of freelance truck castings (no prints, of course) and I cannot figure where the swing link pin holes should be positioned relative to the upper swing link pin holes on the frame.

The holes for the swing links on the frame are fixed at 4-3/8" apart at the top of the truck frame. When I drill the holes for the lower swing links in the bolster, I can make the distance wider than the frame holes (a), the Same distance apart as the frame holes (b), or smaller than the frame hole distance (c).

Each position seem to have some influence on which side of the bolster lifts when it swings side to side, but I can't wrap my head around what moves (the truck?), what is stationary (the bolster?) relative to the frame and track.

?What should it be - A or B or C?

Bolster Question.png
What position should be links be in? A or B or C?

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Re: Question about commonwealth tender trucks

Post by Pipescs » Mon Nov 25, 2013 9:49 pm

There are a lot smarter people out there than me on this subject.

Some one correct me if I screw this up.

All the designs I have looked at for the Rust Bucket I am doing use pins B The distances between the top pins matching the distance between the bottom two pins.
but I can't wrap my head around what moves (the truck?), what is stationary (the bolster?) relative to the frame and track.
The bolster is fixed to the frame of the engine (other than in rotation) so is relative to the frame.

The Frame of the truck is fixed to the track by the wheels.

The swing links allow the truck to stay on the track while the frame/ bolster are allowed to swing out of line in a curve. The swing links providing a centering effect to bring it all back in line as the engine returns to straight track
Charlie Pipes
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Re: Question about commonwealth tender trucks

Post by johnpenn74 » Tue Nov 26, 2013 12:27 am

The key element to remember is that a straight, verticle member has a centering effect on the bolster and therefore the car.
As the bolster swings out and lifts up vertically, horizontal forces will increase which is where we get our centering effect from.

Leaning members, like the ones at A and C, have a tilting effect on the king pin of the lead truck of a locomotive, causing the frame to tilt into the curve. This eventually went away and straight/ tilted links on lead trucks were replaced with heart links which maintain a straight or parallel motion (no leaning effect). Any leaning into a curve is a product of super elevation. Of course, we are talking about lead trucks here.

John Pennington

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