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Author Topic: reverse loop  (Read 411 times)

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Offline TrevL

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reverse loop
« on: October 11, 2017, 11:00:27 am »



A llittle advice please.
The above Anyrail plan (apologies for the poor quality of my use of it)  is a portion of the track I'm building.
The main loop itself is not a problem, however, the short length of track between the two points looks to me like a reverse loop.  I realise I could wire this using  DPDT switch, and but I would prefer not to stop the locos but drive straight through.  So which is the preferred module I need so I can drive through in either direction please?
Cheers, Trev.


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Offline Delboy

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Re: reverse loop
« Reply #1 on: October 11, 2017, 12:33:36 pm »
Hi Trevor,
As you know, I am by no means an expert but I think you could used any reverse loop module connected to the line between the points. I hope someone will step in and confirm this.
Dennis.
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Offline Nigel Cliffe

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Re: reverse loop
« Reply #2 on: October 11, 2017, 01:15:14 pm »
For any solution, you need to break the track in at least two places.  There are many solutions. 

The simplest is as Delboy describes, but has a problem with a train covering both breaks simultaneously (leads to confusion as to which way is "correct").  If you only use locos which are shorter than the diagonal section, and your stock doesn't have metal wheels then it works.  BUT anything with trailing pickups can (and will) cause it to fail, such as DMUs with lighting in the rear, etc.. .
   
So, a much safer method would be to put in three breaks: at both exit routes (heading to the right) from the upper turnout, and at the lower exit route (heading to the left) from the lower turnout.   Everything within the loop area after these three breaks is fed from a local track feed which has to be switched over to match the required polarity. 


Methods to reverse the polarity:

You could spend money on an electronic auto-reverser. These only work for DCC, so can't be used if you want to use DC.  If doing this, pick a solid state one, and one which has the ability to set the sensing current low because N locos don't draw much current, and this technology relies on a short circuit which isn't nice to the wheels and pickups.   
And check that your choice of auto-reverser is quick enough to work with your DCC systems.  Some DCC systems have very quick shutdown in the event of a short circuit, others are fractionally slower (in all cases, its a fraction of a second).  The very fastest are quicker than a lot of auto-reversers, so the DCC system shuts down ahead of the auto-reverser operating, which makes the auto-reverser useless.

You could use a switch as you describe, its cheap, but a bit clunky.  Works with DCC and DC.

You could use a switch associated with the turnout mechanisms.   This might be the panel switch which throws your turnouts, or it might be a switch associated with the motor or movement of the blades.  If you lack enough terminals on the switch or turnout motor, then one terminal can operate a relay which achieves the change-over.  Thus, when the turnouts are set, the polarity changes over automatically.   If both turnouts for the cross-over work together, you'll only need the polarity switching associated with one of them (I'd pick the left one).    This method also works with DCC and DC. 



- Nigel

Offline Steven B

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Re: reverse loop
« Reply #3 on: October 11, 2017, 01:19:20 pm »
What does the rest of the layout's track plan look like? The loop may not be the best place to feed from the auto-reverser/switch.


Steven B.

Offline TrevL

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Re: reverse loop
« Reply #4 on: October 11, 2017, 04:09:04 pm »
What does the rest of the layout's track plan look like? The loop may not be the best place to feed from the auto-reverser/switch.


Steven B.





It's bacically a double track figure of eight with a viaduct over the loop in question. there will be a shunting yard in the empty space in the middle, but that is not pertinent to my issue. 
Again, apologies for my use of anyrail, it took me over an hour to draw this, and it's not very acurate, but the general design is.
The reverse loop(RL) bit is a little over 3 foot long, so there will be no issue with trains being too long.
 As I would like  the trains to enter this RL from either direction without stopping, it's a question of which module would be best to choose, bearing in mind it would need it to flip polarity twice (once going in and again going out I think correct me if I'm wrong)
It is DCC. I've used the +ve and -ve labels just to signify how the droppers will be fed.
« Last Edit: October 11, 2017, 04:13:34 pm by TrevL »
Cheers, Trev.


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Offline Steven B

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Re: reverse loop
« Reply #5 on: October 11, 2017, 04:55:59 pm »
I've got my crayons out:




The red loop is stand-along and can be fed directly from your DCC controller.

The green and blue sections cause the issue with polarity. It's generally recommended that the auto-reversing section is longer than your longest train. As a result I'd feed the blue track directly from the DCC controller. The green track should be switched via either a auto-reversing (AR) module or micro-switches linked to the points..

You could however, power the green section directly and the blue section via the AR module.

Steven B.
« Last Edit: October 11, 2017, 04:58:39 pm by Steven B »

Offline TrevL

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Re: reverse loop
« Reply #6 on: October 11, 2017, 05:32:38 pm »
Aah, that's where my lack of knowledge kicked in, I assumed I only needed to  reverse the three foot section where it crosses over, but put like that, Ican see how that would work, Thank you.
Now which module should I choose?
Cheers, Trev.


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Offline lil chris

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Re: reverse loop
« Reply #7 on: October 11, 2017, 10:06:34 pm »
    @TrevL                         Hi there Trev I saw your message. I had a double track main line reverse loop's on my old layout, you have to be carefull what you buy some are not compatible with all control systems. You do not say what system you are using, I am using a NCE powercab based system but I have upgraded to a Smart Booster system. Also how are you controlling your points, also are you using live frogs etc. I would buy a Dcc specialties PSXAR auto reverser which is digital but they are a bit expensive but you could also use it to control the points for the reverse section. It works with stall motors Cobalt and Tortoise types.http://www.digitrains.co.uk/ecommerce/search/psx-ar-auto-reverser.aspx.
I have also successfully used a Hex Juicer to control one of the loops, you can use two of the connections for this which leaves the other 4 to control the switching of 4xlive frog points.http://www.digitrains.co.uk/ecommerce/search/hfj003u-hex-frog-juicer-universal.aspx

You can also buy the Dcc Specialties Onguard auto reverser which is a lot cheaper, but this is the one that depends on which control system you use, I actually still have one of those in my box.
Lil Chris
My layout here East Lancashire Lines
http://www.ngaugeforum.co.uk/SMFN/index.php?topic=29492.0

Offline Nigel Cliffe

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Re: reverse loop
« Reply #8 on: October 11, 2017, 10:12:44 pm »
Or,   as alternatives to lil Chris' suggestions....   connect the wiring to switches associated with changing the turnouts.  Its cheaper, and once installed has potentially fewer hassles.

Offline jpendle

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Re: reverse loop
« Reply #9 on: October 11, 2017, 10:56:32 pm »
Hi,

I used two frog juicers to control the polarity on my DCC reverse loop.

When it comes to size, you only need the loop to be as big as your biggest loco, The carriages and wagons on a train don't care about polarity.

The exception is if you are running a Multiple unit that requires more than one decoder, or banking a train, or if you have an MU (such as a Pendolino) with a driving car at each end.

Frog juicers and reversing modules typically need nothing more than DCC power to operate. The required switching is done automatically. Either when your loco enters or leaves the reverse loop.

Regards,

John P

Offline Nigel Cliffe

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Re: reverse loop
« Reply #10 on: October 12, 2017, 07:20:45 pm »
When it comes to size, you only need the loop to be as big as your biggest loco, The carriages and wagons on a train don't care about polarity.

It DOES matter if you use an auto-reverser and have ANY stock with metal wheels, regardless of whether they have pickups. 

Consider a short reversing section, longer than your loco, but shorter than the train.  The loco is straddling the break at the exit (which is fine) and is moving out of the reversing section pulling a train.  A wagon with metal wheels (but no pickups) has a single wheel touching both sides of the track isolator on one rail (momentarily) on the entry to the reversing section whilst the loco is still straddling the break at the exit.  There is now a dead-short through the wagon wheel, the reversing section, the loco pickups and back out the far end.  The auto-reverser will go hay-wire (not sure which way to change) and hopefully the command station will shut down to protect everything.

If the wagon with metal wheels has pickups, the situation is more prolonged, no longer a momentary effect, but one which persists whilst loco and wagon straddle the breaks.   


Which is why there is advice about length of reversing loops being longer than the longest train.


- Nigel

Offline jpendle

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Re: reverse loop
« Reply #11 on: October 12, 2017, 08:54:02 pm »
I stand corrected, Nigel is right. He usually is  :-[

Regards,

John P

Offline Nigel Cliffe

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Re: reverse loop
« Reply #12 on: October 13, 2017, 09:33:28 am »
I've got my crayons out:




The red loop is stand-along and can be fed directly from your DCC controller.

The green and blue sections cause the issue with polarity. It's generally recommended that the auto-reversing section is longer than your longest train. As a result I'd feed the blue track directly from the DCC controller. The green track should be switched via either a auto-reversing (AR) module or micro-switches linked to the points..

You could however, power the green section directly and the blue section via the AR module.


I'd argue that it doesn't matter whether the reversing is applied to the green or blue sections.   They are equivalent.   So, its going to come down to wiring convenience, potential future expansion of the layout, and personal choice. 

A reversing section of adequate length does matter (as Steven says).  But so does the length of the non-reversing section.   If the non-reversing section were shorter than the longest train, then the same short-circuit I've outlined in a post above will apply with the train straddling both ends of the non-reversing section and shorting out. 


Terminus layouts with Y-arrangements may be different, then there are more options as to what gets reversed and what stays as "normal".   


- Nigel


Offline TrevL

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Re: reverse loop
« Reply #13 on: October 13, 2017, 02:29:29 pm »
Thanks everyone for your invaluable input.
Now I've got my head round it, could the middle leg of the blue section (as it is 3 feet long, one length of flexy track and longer than anything I will ever run) be the reverse loop only please?
Cheers, Trev.


Time flys like an arrow, fruit flies like a banana!

Offline Nigel Cliffe

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Re: reverse loop
« Reply #14 on: October 13, 2017, 02:47:00 pm »
Thanks everyone for your invaluable input.
Now I've got my head round it, could the middle leg of the blue section (as it is 3 feet long, one length of flexy track and longer than anything I will ever run) be the reverse loop only please?

Yes, provided it is longer than your longest train. ( three feet in N gauge is only about six or seven coaches, which isn't massively long).     Put the isolators near to the turnouts at each end of the section. 


If using an auto-reverser its simple to wire.   
If using other methods there are solutions, but depends how you want to approach it:  tied to turnout blades, tied to signals, or track sensors to detect train at end of section.   


 

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