Re: Helper LocomotivesM.

Msg # 990 of 1000 on RelayNet Trains/models/railways etc
Time: Saturday, 8-11-7, 1:32
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James Robinson  writes:
> "wa-kiki"  wrote:
> > I've seen it in prototype operation and I've seen it on film where a
> > locomotive will be put somewhere in the middle of a train to help move
> > a really heavy load such as a coal unit train. But can someone tell me
> > how this actually works.
> It all depends on how many locomotives are used as helpers, and where
> they are placed in the train.
> In many case, the helpers are controled by data radio from the lead
> locomotive.  When the engineer moves the throttle on the locomotive at
> the front, the radio link causes the same actions on the locomotives in
> the middle.  They can also be run indpendently by the engineer at the
> front if necessary.  In most cases this isn't done.
> There are also manned helpers, where the two engineers coordinate their
> actions using a voice radio. In this case, the engineer in the middle
> typically fully opens the throttle once the train gets moving, and leaves
> it there until the train gets to the top of the hill. Or conversely
> applies the maximum dynamic brake until the train reaches the bottom of
> the grade.
> As far as the coupler slack is concerned, consider that on a conventional
> train, the coupler between the last car of the train and the second to
> last car only has to pull one car.  The next coupler ahead has to pull
> two cars, and so on to the front of the train, where the first coupler
> behind the locomotives has to pull the whole train.
> The problem is that if the train is heavy enough, and the grade steep
> enough, the force needed to pull the train might be higher than the
> strength of the couplers toward the front end of the train, and the
> couplers could break.  Helpers are then used to avoid splitting the train
> up, since two trains use more capacity than a single longer train.
> When helpers are inserted, they can be put in a number of places.  Some
> put them in the middle, some at the end, and some a few cars from the end
> of the train.
> Consider a 100 car train where all the cars weigh the same, and the
> helper locomotives are placed exactly in the middle, behind 50 cars.  If
> the same number of locomotives are on the head end pulling as are in the
> helper set, then effectively you get two identical trains running up the
> hill, which are just coupled together. The force on the coupler
> immediately in front of the helper set would be practically nil.  The
> force on the couplers immediately behind each of the locomotive sets
> would be about 1/2 of the force behind the head-end locomotives if the
> helpers had not been put in the train. Thus the coupler strength is no
> longer an issue.
> Now move the helper set toward the rear of the train, say at the 75
> point.  Since there would be only 25 cars behind the helpers, the force
> on the coupler immediately behind the helpers would be much lower than
> when it was located in the middle of the train.  In fact, it would be
> about 1/2 as much.
> Since the helpers would be working just as hard as before, that means
> they will not only be pulling the cars behind, but will also be pushing
> some of the cars ahead.  Given the amount of tractive effort they
> provide, they will be pushing all the cars up to the mid-point of the
> train, where the helpers had been located in the previous example, and
> the locomotives on the head end will pull the rest.  Like the first
> example, the coupler between the 50th and 51st car will have practically
> no force on it.
> That is the basic principle, and with different locomotive placement and
> number of locomotives it just varies where the neutral point will be in
> the train.

Seems like a good description in an idealized situation.  Since
the rail lines are almost never completely consistent grades,
the neutral point will fluctuate as the grade varies.  I've seen
descriptions of operations where the front units will be braking
while the more rearward units will still be pushing/pulling at
full bore as a long train goes over a hill.  So there is some
art to getting the power balance correct at any moment.

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 * Origin: RelayNet  MoonDog BBS Brooklyn, NY (900:100/50)

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