Post by Graeme WallPost by ReclinerPost by Graeme WallPost by M***@dastardlyhq.comOn Wed, 7 Aug 2024 10:01:57 +0100
Post by Graeme WallPost by M***@dastardlyhq.comInteresting episode this week. But why on earth when standing next to the
rail carrying train didn't she ask how it goes around corners?? I assume the
rails flex but can they flex enough for example to get around the kensington
curves and doesn't the strain put a huge wear on the flanges of the wagons
carrying them?
Kind of an obvious question I thought but perhaps not.
You'd better amazed how much rails can flex, I've watched a trainload
coming out of Eastleigh yard via the reverse curves of a crossover to
get onto the main line. Fascinating to see and not as much flange
noise as you'd expect.
One rail flexes easily, but they were loading dozens of them onto each
wagon.
Each one flexes individually.
Presumably they’re carried as a loose bundle, and can move around
individually on curves? So, some of the movement can be accommodated by
the rails shifting around, as well as flexing?
They are not bundled in any way, ie no fastenings to hold them together.
They are only restrained by the vertical stanchions on the wagons.
Here's a US example of 400m long welded rails being carried round a fairly tight bend:
I found this on-line:
Perhaps surprisingly, the vertical axis of a big, heavy, seemingly stiff piece of rail - which is the plane involved in
going around curves - is significantly less strong = weaker = more flexible than the rail's stiffness in the horizontal
axis (which is what carries and distributes the weight of the wheels across the ties). That's because the stiffness of
the rail is a (complicated) function of - put simply - how much metal is farthest away from the center of the rail. For
the horizontal axis, the most metal - the head and the base - is pretty far away from the middle of the rail. For the
vertical axis, most of the metal in the head is pretty close in to the web, and of the base, only the comparatively thin
outer edges are farthest away from the web. I'll try to find some values for either I ("Moment of Inertia") and/ or S
("Section Modulus") for one of the common mainline rail sections to give you a sense of the magnitude of that
difference.
The other surprising aspect is that the relative stiffness of the rail is defined by the fraction I (same I as above) /
L, where L is the length of the stick of rail or string of Continuous Welded Rail ("CWR"). Clearly from this fraction,
for a given rail section as the length gets longer, the stiffness gets lower. What may not be obvious is that for a
typical string of CWR, that length is from 35 to 40 times longer than a conventional 39 ft. stick of rail, so it's
stiffness is only 1/35 to 1/40 as much, or only from around 3 % to 2.5 % as much. That's why the CWR is so flexible -
it can droop into a shape looking like a piece of overcooked spaghetti if it is not placed carefully on the ground. I'll
also see if I can find a good photo of that.
Our old friend Archimedes (the Greek mathematician) who famously said, "Give me a lever long enough and a place to
stand, and I can move the world" would appreciate this next point: Another way to look at this is that the rail is
fixed in the middle (which it is, clamped down with "hold-down" or restraint cars to keep it from moving fore and aft
while in transit, such as during braking). The rest of the cars in the train - and the track it is running on - act
like multiple continuous levers to bend the rail as needed. To use RWM's example, think of a single string of 1,500 ft.
long CWR, with the end car in the rail train being 750 ft. away. How much force do you think that car has to exert on
the side of the rail from that distance away, in order to get the rail to bend the little bit that is needed to go
around a broad radius railroad mainline track curve ? The answer, of course, is not much. And that end car of the CWR
train has lots of helpers along the way in the form of the carrying rollers on each car, each doing its share to help
with the bending around curves. I've never had occasion to push the limits of a CWR train too hard on curves, but I'm
confident that it could negotiate curves as sharp as around 14 degrees (400 ft. radius) - which is a little sharper than
most new industrial spurs these days - if done carefully and at a slow speed. I wouldn't want to try it on a 22 or 23
degree curve, though (250 ft. radius) without special arrangements and equipment, or only a few pieces on the rail train
at a time, etc.
As far as length: Typicals lengths of CWR strings vary. 1/4 mile = 1,320 ft. (33.85 pieces of 39 ft. rails) is common,
as is 1,440 ft. (36.92 pieces of 39 ft.) for some reason, but there's no reason why they can't be a little longer or
shorter as RWM's 1,500 ft., for example. (New rails from the steel mill always include a percentage of "shorts", so the
fractional lengths of full 39 ft. long pieces is not a problem at all. And yes, I know that new rails from the mill now
come in lengths from 78 to 82+ feet to minimize the number of welds - just trying to keep it simple here, folks.)
In my experience I've had occasion to "special order" CWR string lengths to avoid wastage if I didn't need the entire
length - say, to fit between 2 turnouts (switches) - or a little longer to minimize the number of field welds needed to
join the strings together, esp. if there was a tight time schedule for the installation. For example, for one little
project that was 6,630 LF long I ordered and got 4 CWR strings at 1,660 ft. long (each 1/4 the distance) so each rail
only had to have 3 "intermediate welds", plus the closure weld at each end. The guys at ConRail's Lucknow Rail Welding
Plant just north of Harrisburg, PA and at Amtrak's plant in New Haven back then were always willing to add a couple of
roller cars to the rail train to accommodate the longer lengths, and to mark and load the special length strings
separately - including shorter strings. All I really ever had to do was let them know that I had a special need, and
tell the plant superintendent clearly and in writing just what I needed and why. (I didn't hurt to treat them "nicely",
either, I'm sure.)
- Paul North.
https://cs.trains.com/trn/f/111/t/153512.aspx
