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Wheels:
Truck and Wheel Assembly:
A common design has 4 wheels on 2 axles mounted on an assembly called a
truck(or bogie), with a single truck on each end of a railcar. Each pair of wheels
is pinned to its axle so that they both rotate at the same rpm. The whole thing
is held together by gravity with no bolts or other fasteners. The center pin on
the truck fits into a bearing mounted on the railcar and the truck is free to pivot
left and right around it. Nothing steers it - the wheels simply caster to follow the
tracks. This is a picture of a typical wheel assembly of a train car.
[CREDIT] http://www.railway-technical.com/trains/rolling-stock-index-l/bogies.html
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Wheel geometry:
Each wheel is manufactured with a flange on its inside face. In normal running
the flange doesn't touch the rails. Instead, the truck and wheels remain centered
on the rails due to the conical profile of the wheel surface which tapers towards
the outside of each wheel. Thus the wheel diameter is larger near the inside
(flange side or gauge side) and smaller near the outside (field side). The flange
comes in contact with the rail only in the event of a major left-right shift of the truck.
The conical taper helps to accomplish at least two objectives :
First, it reduces the contact spot of the wheel surface and rail to an area the size
of a fingernail thus reducing the rolling resistance of the wheel... This photo shows
the contact viewed from track level.
"If you want to make a wheel that can carry a vehicle across a prepared surface of some
kind you can choose from several possibilities. The pneumatic tyre on a concrete road represents
one possibility in which the load is spread over a relatively large area. The steel wheel and the
steel rail represent the opposite extreme: the load is concentrated within a tiny contact area so as
to minimise rolling resistance."
(The following link is somewhat technical; the statement concerning contact size
vs.rolling resistance is at the very end.)
https://the-contact-patch.com/book/rail/r2019-rolling-contact
This subject is still a work in progress for me because there are what seem to be
valid references which make no mention of the contact spot in reference to roll-
ing resistance.
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Second, it allows the truck and wheels to self-center continuously on the rails
despite minor flaws and deformities of the track. This self-centering also allows
the truck and wheels to negotiate curves without damage to the wheels or rails.
Self-Centering:
The principle of self-centering relies on the varying diameter of the cone-shaped
wheel surface. Unless each rail is contacting the same spot on each wheel sur-
face, the effective diameters of each "wheel" will be different - with a larger
"wheel" on one side and a smaller one on the other. Because both wheels are rotating
at the same rpm the larger one will cover more distance per rpm causing the truck to turn
slightly toward the smaller wheel. This causes the smaller wheel
to move to a spot having a slightly larger diameter.The opposite effect takes
place on the larger wheel causing it to move to a position having a smaller diameter.
If the wheels overshoot the center postion then the above actions re-
peat in the opposite direction until the wheels are centered. This process is
known as "hunting" and under certain circumstances can be a problem.
afk May 25, 2024