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Greg.Procter
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 Posted: Sun Oct 26, 2008 4:38 am Post subject: Re: Big trains question about coupling |
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On Fri, 24 Oct 2008 02:58:08 +1300, Wolf Kirchmeir <wolfkir@sympatico.ca>
wrote:
| Quote: | Roger T. wrote:
I agree. Free slack action is a serious concern to North American
locomotive engineers. Freight cars will typically have at least 1 inch
of
slack per coupler if not more, that's about 2" per car and on a 100 car
freight that's about 200 inches, or 16 feet or 4.9 metres of free slack
Correction. I miss calculated.
If each car has 1" of free slack at each end, then a 100 car train has
199
coupled ends or 199 pairs of couplers coupled together which translates
to
(1" per coupler x 2 couplers coupled together = 2" of slack per pair of
couplers) that's 199 x 2 = 398" or about 33' feet or about 10 metres.
Ok, new figures accepted. But it's still not much compared to the length
of the train. A 100 car train will be 5,000 feet or more. Less than 1%
of train length.
Lots of free slack and that's not counting cushioned underframe slack..
So, what are the "issues with slack"? They vary, and not all of thme are
bad. As I understand it, free slack can be an advantage when starting a
train, as the load is incrementally increased as the slack runs out.
I've read that engineers will actually reverse the engine against the
train before moving forward. Is this common practice?
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In the case of historic British coal trains, that was the norm. Their
trains mostly were pulled by 0-6-0 tender locomotives right up to the
1950s.
That was all that was required to move the maximum length trains
allowable. Of course they could only achieve something like 15mph and
spent most of their time in sidings waiting for 60-100mph passenger trains.
| Quote: |
OTOH, on a line with many short grades, shorter than the train, slack
action (both kinds) can cause problems, which are minimised by running
shorter trains and running them slow. Eg, the Huron central (Sudbury -
Soo, runs through my town) runs trains around 40-50 cars averaging 50ft,
at about 15-20mph. They could run them up to 30-40mph, but the track is
very bad. ;-)
Sprung slack has more complicated effects. A certain amount is
necessary, to dampen horizontal shocks transmitted to the cars and
lading. Too much can be a problem, especially when helpers are used on
hilly lines on which long trains are run. Rough track can set up
oscillations. Not easy to analyse, even by people who study it or work
with it.
FWIW, the slack action in model trains is much more than on the
prototype. I just tested it on my layout, on a 7-car train, it's about 4
scale feet (HO)!
HTH
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Using Opera's revolutionary e-mail client: http://www.opera.com/mail/ |
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Greg.Procter
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| Posted: Sun Oct 26, 2008 4:43 am Post subject: Re: Big trains question about coupling |
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On Fri, 24 Oct 2008 10:06:34 +1300, Erik Olsen <erik.olsen@ishoejby.dk>
wrote:
| Quote: | Bob May wrote:
Back in the steam engine days, the slack was pretty necessary to
start a long train. Remember that a steam engine can pull a train
that it can't easily start while a diesel can start a train that it
can't get up to a speed where it can run without running into heating
problems in the electric motors.
The reason for the difficult start was the use of friction bearings.
With roller bearings it is much easier to start a heavy train.
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You still need to accelerate the great mass of the train!
"Stiction" which is the resistance to movement could be of the order of
3-4 times the rolling friction of plain bearings, but only double for
roller bearings. |
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Wolf Kirchmeir
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| Posted: Sun Oct 26, 2008 6:20 am Post subject: Re: Big trains question about coupling |
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Greg.Procter wrote:
| Quote: | On Thu, 23 Oct 2008 15:34:56 +1300, Wolf Kirchmeir
wolfkir@sympatico.ca> wrote:
[...]
AFAIK, there should be no free slack with screw couplings, but when I
observed switching in Austria a couple years ago, there was definitely
more than an inch of free slack. The buffers parted 20cm or more when
the loco started up the cut of cars. The coupler links were simply
lifted into place (ie, "three link couplings" in effect), to save time
I guess.
You probably witnessed a shunting move in a station area.
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Quite so:
"...I observed switching..."
--
Wolf Kirchmeir |
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Roger T.
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| Posted: Sun Oct 26, 2008 1:24 pm Post subject: Re: Big trains question about coupling |
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"Greg.Procter" <procter@ihug.co.nz> wrote in message
news:op.ujlse4ttt7a1n0@promodel-5a9821...
| Quote: | On Thu, 23 Oct 2008 10:51:48 +1300, Roger T. <rogertra@highspeedplus.com
wrote:
The only difference between Europe and North America is that Europeans,
even
within the same country, mush the operational detriment, have not
standardised on one coupler system.
Huhh???? They standardised in 1849!
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No they haven't. There are all kinds of different couplers that are not
compatable.
| Quote: | Britain's railways aren't a part of "Europe" as such.
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Agreed.
Like Canada isn't part of "America".
--
Cheers
Roger T.
See the GER at: -
http://www.islandnet.com/~rogertra/ |
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Bob May
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| Posted: Mon Oct 27, 2008 4:07 am Post subject: Re: Big trains question about coupling |
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Yes, roller bearings do make the job a fair bit easier than with the older
friction bearings but the problem is still there. I don't know how many
cars you may have in a train and how heavy those cars are when loaded but
back when Krauss-Maffei did the 6 locos for the SP and D&RG railroads, they
could only test one loco at a time and even then couldn't get the full power
out of the loco due to the lack of ability of the train to take the strain.
Over here, the locos ran in 3 unit sets, often with another loco(s) of the
diesel-electric type to move a train on the SP and D&RG railroads.
Stretch that train out and try to start it and there will be a whole lot of
power needed to start it moving. Diesels can often get them rolling but
steam engines definitely couldn't even today with all of the roller bearings
on the axles.
--
Bob May
rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net
Erik Olsen <erik.olsen@ishoejby.dk> wrote in message
news:4900e767$0$90272$14726298@news.sunsite.dk...
| Quote: | Bob May wrote:
Back in the steam engine days, the slack was pretty necessary to
start a long train. Remember that a steam engine can pull a train
that it can't easily start while a diesel can start a train that it
can't get up to a speed where it can run without running into heating
problems in the electric motors.
The reason for the difficult start was the use of friction bearings.
With roller bearings it is much easier to start a heavy train.
--
Venlig hilsen/Best regards
Erik Olsen
http://www.modelbaneteknik.dk/
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Bernhard Agthe
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| Posted: Tue Oct 28, 2008 2:30 pm Post subject: Re: Big trains question about coupling |
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Hi,
Greg.Procter wrote:
| Quote: | The advantage of the side buffers is the elimination of slack, and the
resulting damage to goods.
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Even with side buffers and screw couplers you will have some slack
(although only little). It is still possible to back up against a train
(compressing the buffers) and then start forward (releasing them). You'd
have to tighten the screws quite a bit to prevent that - and then the
buffers would resist the train going around a curve.
So I'd say that with screw couplers and buffers (european style) there
is very little "free" slack, but still a bit of "sprung" slack.
If handled carefully, a bit of slack is no problem, but you wouldn't
want excessive slack in a high-speed-train. It's all a question of
proportion ;-)
| Quote: | The planned Euro centre coupler was to be the ultimate in fine
tolerances so that the inherant slack would be negligable, but of course
that level of manufacturing tolerance costs!!!
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Well, there should have been *very* little "free" slack (you still need
a bit for the locking plates to slide into place), but you still need
the coupler mount to include a buffer. Again, the buffer needs to have
some movement allowed which you could use instead of slack  Compare
the Scharfenberg couplers - they are close-tolerance meant to fit the
form of the opposite one. They even use a sophisticated locking
mechanism which "pulls" them together. But when coupling several
four-car 130-ton multiple units you definitely need some buffer and you
need torsional as well as lateral freedom which makes a very complicated
coupler shaft. You wouldn't want that coupling to be completely rigid ;-)
Have fun... |
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Wolf Kirchmeir
Guest
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| Posted: Tue Oct 28, 2008 5:43 pm Post subject: Re: Big trains question about coupling |
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Bernhard Agthe wrote:
| Quote: | Hi,
Greg.Procter wrote:
The advantage of the side buffers is the elimination of slack, and the
resulting damage to goods.
Even with side buffers and screw couplers you will have some slack
(although only little). It is still possible to back up against a train
(compressing the buffers) and then start forward (releasing them). You'd
have to tighten the screws quite a bit to prevent that - and then the
buffers would resist the train going around a curve.
[...] |
Back in the days when I stood on open end-platforms and watched the
coupler action, I noticed that occasionally the buffers on the outside
of the curve would part. The screw couplings weren't tightened enough, I
guess.
HTH
--
Wolf Kirchmeir |
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Greg.Procter
Guest
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| Posted: Thu Nov 06, 2008 8:11 am Post subject: Re: Big trains question about coupling |
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On Wed, 29 Oct 2008 01:43:46 +1300, Wolf Kirchmeir <wolfkir@sympatico.ca>
wrote:
| Quote: | Bernhard Agthe wrote:
Hi,
Greg.Procter wrote:
The advantage of the side buffers is the elimination of slack, and the
resulting damage to goods.
Even with side buffers and screw couplers you will have some slack
(although only little). It is still possible to back up against a train
(compressing the buffers) and then start forward (releasing them). You'd
have to tighten the screws quite a bit to prevent that - and then the
buffers would resist the train going around a curve.
[...]
Back in the days when I stood on open end-platforms and watched the
coupler action, I noticed that occasionally the buffers on the outside
of the curve would part. The screw couplings weren't tightened enough, I
guess.
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Two points there:
- depending on the design of the buffers, some are/were sprung with a
semi-eliptical spring behind the two buffers which will equalize on
curves. Others have/had self contained coil springs.
- On lines with tight curves and with rolling stock fitted with
self-contained spring buffers the couplers wouldn't be tightened all the
way. If the buffers reach the end of their travel those 4 wheel coaches
could derail. (on the principal that something has to give somewhere)
Regards,
Greg.P. |
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