Reverse loop problem

[ajwblue]

I have installed a Hornby Reverse Loop Module on a triangle (Wye point?), and in general it works well so I am confident that I have installed it correctly to my Elite controlled layout. However, I have found that some of my larger steam locos ( Mallard, Flying Scotsman & Duke of Gloucester) short out on one of the three points on the triangle. The only "cure" I have found so far is to run the locos at a higher speed and the locos hesitate over the point but manage to keep going without shorting.  I have checked all the connections, track levels and back to backs and all seem fine, as confirmed by the successful operation of other locos. Can anyone explain what might be causing this, and is it something that I will have to live with? Thanks 

[Chrissaf]

Is the section of track that is protected by your RLM longer than your longest train ?

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The whole length of your train (loco plus rolling stock) must cross the entry into the RLM protected track section before the first axle of your train crosses the exit of the RLM protected track section. Is the train causing the issue, longer than your others that do not exhibit this issue.

[ajwblue]

I have just run the loco (Mallard) only through the isolated section protected by the RLM, and when the loco reaches the next set of points on the triangle  the loco shorts at slow speed. The loco does successfully go over two points on the circuit before reaching the first point on the triangle. I believe, therefore, that there must be some problem with my set up of the RLM, but why does it only manifest itself on certain locos?

[2e0dtoeric]

Which type of points are you using - live or dead frog? That may have some bearing on the matter.

[ajwblue]

Thanks for reply. They are all Hornby points with DCC point clips attached.

[Fishmanoz]

Ajw, a diagram of what you have done will certainly help us help you.  Don't forget to show where you have put your IRJs and where your RLM is connected.  

 

That said, I can't see why you think you may have wired it wrongly, the point should be either wholly within the RL isolated section, or wholly outside it, meaning the RLM will not be trying to reverse the current as the point is reversed and any shorting due to some other reason.

[Chrissaf]

I would also add, that if it was a basic RLM wiring fault, then one would expect ALL locos to be affected, not just one. I concur a diagram showing your layout topology for the WYE area, showing insulated rail joiner locations and RLM connection positions would help immensely.

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[96RAF]

I would be checking back to back dimensions on the errant locos especially front bogies.

[ajwblue]

To all who have replied to my post, as a first time user of the Forum can I thank you for your most helpful comments. Having been assured that my wiring of the RLM was OK, I decided to try relaying the track around the points, and hey presto I have managed to cure the problem. It's still a bit sensitive with the larger steam locos, but I will work on improving this. Once again my thanks, without your comments I would have spent many more hours trying to resolve this. Lesson learnt is to make sure the basics of track alignment and back to back dimensions are spot on! 

[Belah]

The whole length of your train (loco plus rolling stock) must cross the entry into the RLM protected track section before the first axle of your train crosses the exit of the RLM protected track section

This puzzles me.  I have three reverse loops installed all of which form part of continuous circuits.  Two of the protected reverse loops are approx one metre long and the third is approx 60 cm. I run 9 coach double header expresses and 36 wagon length trains with loco and banker.  These are all run with the locos  as consists and the front of the train is through the reverse loop before the end of the rolling stock or banker has even entered the reverse loop.  I never have any problems. One loop is controlled by a hornby reverse loop controller and the other two by Hornby power boosters.  This seems to contradict this advice. I am just about to make an alteration which will slightly reduce the length of the shortest loop and am now anxious not to make a change to something that is stable but not in accord with correct practice. Any suggestions would be welcome.

[Chrissaf]

Firstly, do your coaches & wagons have plastic wheels. If they do, then there is no issue. The best practice guideline about the RLM section being longer than your longest train is to prevent the RLM electronics from continually ,flip flopping, as a result of rapid consecutive short circuits being detected as the train wheels (metal ones) pass over both the RLM section input and output at the same time. This will stress the RLM electronics as well as potentially your controller power supply. For this to make sense, it is necessary to fully understand how a RLM functions electronically.

[Belah]

Thanks for quick response.  I will check wheels as suggested. The coaches are all Bachmann as are most of the wagons.  The banker loco obviously has metal wheels and is controlled in a consist.  I did ensure that the reverse loop length was sufficient to contain the two double header locos.

[Belah]

The mineral wagons definitely have metal wheels.

[Fishmanoz]

The advice on isolated section being longer than the longest train is to avoid potential problems as Chris describes.  You will only get actual problems if there is an electrical connection between wheels on the same track.  If these are entering the RLM when any similar arrangement is leaving, it will give rise to the flip-flop polarity change stressing the RLM.  This will mean not only metal wheels but no plastic bushes, metal axles and metal bogies on 8-wheel stock or metal chassis on 4-wheel or more stock. 

 

Given you haven't had a problem to date, you must not have such wheel-to- wheel connections on your stock. And clearly your banker is not entering the RLM until after the main loco has left. 

 

So so for the moment, you are ok and shortening an isolated section, as long as still longer than your double headers, will not be a problem. But if you in future get any all metal rolling stock, then you may get a problem whether your isolated sections are as now or shorter. 

[Belah]

Thank you very much.  I understand the rationale as you have described it and feel happy to continue with the current arrangements.

[Chrissaf]

In Fishy's post he describes the RLM protected section track being bridged to the non protected track via rolling stock with metal chassis (top image representation). It is more likely that the majority of rolling stock chassis will not provide an electrical path between wheels for many reasons. Plastic bushes between wheels and axles, plastic bogies etc. The issue still potentially arises with the rim of the metal wheel itself providing the electrical bridge between the rails (see bottom image).

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How much effect the wheel rim bridging will have on RLM function will depend upon the the speed that the rolling stock is travelling across the insulated rail joiner track connection versus the switching latency of the RLM itself. Put another way, the wheel bridging may occur more rapidly than the switching response time of the RLM circuit. So even if the RLM doesn't actually reach its trigger current level, it is still potentially being stressed. For my (non Hornby) RLMs, switching latency is measured in microseconds not milliseconds. I don't have a switching latency value for the Hornby R8238 RLM.

[Fishmanoz]

Good thinking Chris. The things is, we know individual wheels can't be triggering the RLM as it has been working with a short isolated section already, so will probably continue to work with a shorter one.  Then what continual brief shorts will do is unlikely to be any good.