what are radial trucks?

[wisandsouthernkid]

i was reading the forum below and i just remembered that was one of my long lost questions what are radial trucks and what other types are there. thanks

[oltmannd]

Radial is where the axles in the truck are always perpendicular to the rail, or parallel with the curve's radius, if you prefer...

Conventional trucks keep the axles always perpendicular to the truck frame so the don't align exactly with the rail in curves.

[fredswain]

Here's a picture:

http://www.emdiesels.com/lms/en/locomotive/innovations/radial_bogie/index.htm

 

[wisandsouthernkid]

so conventional trucks are like the drivers on steamers they never move?

[trainfan1221]

They turn to negotiate the curves but as one solid unit.  The radial trucks let all wheelsets turn.  I am sure this helps preserve the rails too.

[oltmannd]

They turn to negotiate the curves but as one solid unit.  The radial trucks let all wheelsets turn.  I am sure this helps preserve the rails too.

Right.  Conventional trucks can pivot under the locomotive but the axle alignment is fixed relative to the truck frame.

In a conventional truck, the axles are all always parallel to each other.  In a radial truck that navigating a curve, they won't be.

[Paul Milenkovic]

For inquiring minds who need to know this sort of thing, there are two kinds of radial trucks: self-steer and forced-steer.  The EMD truck is self-steer. 

A cone tapered pair of wheels on a solid axle has a natural tendency to steer into curves, in fact in oversteers and would "hunt" or "nose" back and forth if not properly constrained by the neighboring axles.  The self-steer truck connects the journal boxes with links to the wheels, and the axles are allowed to align to the same curve, essentially "bending around the curve."  The forced-steer design is probably a two-axle truck answer to the Talgo guided axle system.  When the train goes around a curve, the truck side beam or bolster pivots, and that pivoting of the bolster is connected to cranks and links to precisely move the journals relative to the truck frame to get the same radial steer effect.  The forced-steer design is regarded as something for the type of high-speed trains they have in Europe as it has less chance for some kind of self-oscillating hunting getting set up

The reason for the EMD radial truck is that a 3-axle truck can be as sharp-curve friendly as a 2-axle truck, and as you know, EMD is no longer making 2-axle trucked locomotives these days.  The reason for the forced-steer trucks that the Swiss SIG and others make is to suppress hunting at high speeds without the stiffness in the journal boxes that increases wear.