Fiddling about with couplings

[Richard Hall]

A bit of reading for a quiet Bank Holiday weekend.  When I came back into the hobby about ten years ago I had very little space available, so I set about constructing an N gauge branch terminus in the traditional manner.  Straight away I was into problems with couplings.  I wanted to be able to operate the layout "hands off" and no way could I do that with the standard Rapido coupling. I tried Microtrains knuckles and found that although they work very well with nice big American bogie boxcars they aren't so great with the standard 10' wheelbase British four wheeler, especially in N gauge where the vehicle can yaw some ten degrees either side of the centre line.

 

So I set about designing a coupling that would meet most of the following needs: automatic uncoupling including delayed uncoupling, easy fitment to N gauge models without major modification, easy to assemble, robust, inconspicuous and cheap.  Over six years and seven redesigns I went from the coupling on the left (my first attempt, with hand-filed hook, which somehow survived by hiding in the bottom of my scrapbox) to the one on the right.

 

 

I named it the "Magpie" coupling because it steals design features from just about every tension-lock since the days of Hornby tinplate. The buffing bar and central hook are obviously DG-inspired, but the coupling uses an open ended loop, fixed delay bar rather than a lifting latch, and is designed around a 2mm square plastic shank which can be shaped to fit just about anything. This close-up shows the main features including the delay bar and the drop arm with a few turns of fine iron wire round the end.  

 

 

This version turned out to work very well, even in exhibition conditions.  Not 100% reliable, but certainly better than 90% for first time coupling / uncoupling, and it was very rare for it to fail to work on the second attempt.  Problems were usually caused by couplings being slightly out of spec: drop arm angle, delay bar length and the shape of the lifting loop were all critical. I found that the fairly heavy, large diameter (0.7mm) nickel silver loops very seldom suffered from the dreaded "loop clash", so I could have loops both ends of all my wagons and plain hooks on the locomotives, which is ideal for a terminus - fiddle yard layout and avoids needing to find clearance for a drop arm on the pony truck of something like an Ivatt 2MT.

 

Early on in the development process I ran into problems with the first or second vehicle in a train randomly derailing.  I traced this to the pivot point for the loop being lower (relative to the horizontal) than the drawhook: when put under significant load the loop would try to achieve a straight line pull from hook to pivot, and in doing so would lift the front wheels on a small, light wagon clear of the track. That was the biggest redesign, putting the pivot in line with the hook.  The loop rests on the buffing plate just a few degrees above horizontal.

 

Like many coupling designs, this one requires a small "shuffle" to set the delayed uncoupling function: the two vehicles are drawn a fraction apart over the magnet to flick the loops up above the delay bars, then pushed back together.  Not ideal visually but I could live with it: the problem was that any steel content in the vehicles (including axles) would result in the magnet pulling them back together. As it happens most N gauge wagons have had non-magnetic axles for some years but I still had to get rid of steel ballast weights. However, 2mm Association wagon axles are mild steel...

 

So I had a choice: yet another redesign to incorporate a DG-style lifting latch, or stop being awkward and just use DGs like everyone else.  I ordered DGs, started reading the instructions, noted the geometry (pivot point well below the drawhook), contemplated trying to solder steel wire to a fine brass loop and wondered whether "loop clash" was as big an issue as some people say. Then I looked at the DG etch and thought "there's a lot of meat around those pivot holes, I wonder if I can fold the pivot "ears" up rather than down".  This being the result:

 

 

 

I have attached these to one end of two wagons and so far they seem to work pretty well although in need of minor fettling (compare the drop arm angle on the shock open with the 16 tonner above).  Next stage is to build a few more and see if they work on "Longframlington". I want to try them with steel axles as well.  For now, the main problem I have is with the lifting latch.  The loop cross shaft restricts the latch movement, so the latch has to bend through two angles and the end has to be cut quite short. I am having problems getting the latch to drop down under its own weight. Opening out the slot for the latch even a tiny fraction makes it waggle around too much.  I might try soldering a small brass strip on top of the latch to add a bit of weight.

 

I realise these couplings aren't as delicate and "fine scale" looking as the DGs but the photos make them look more obtrusive than they are, and the one above would look better with a chemically blackened loop. 

 

Richard

 

[Flying Pig]

49 minutes ago, Richard Hall said:

For now, the main problem I have is with the lifting latch.  The loop cross shaft restricts the latch movement, so the latch has to bend through two angles and the end has to be cut quite short. I am having problems getting the latch to drop down under its own weight. Opening out the slot for the latch even a tiny fraction makes it waggle around too much.  I might try soldering a small brass strip on top of the latch to add a bit of weight.

 

Just an idea - could you use a thin springy bit of plastic for the latch?  It could be firmly glued into the body of the coupling, or even (we can dream) moulded integrally with it...

[Richard Hall]

Grrr couplings.  Have spent an unproductive day working on different designs for the lifting latch.  I have something which almost works, but there isn't quite enough metal around the pivot "ears" to be able to drill a couple of 0.4mm holes without breaking through the edge.  I have played about with the standard DG latch but I really don't like it at all and would much prefer a more "engineered" solution. I have one more idea to try for now but it's a bit fiddly.

 

Keeps me busy anyway while I try to sell some unwanted N gauge tat to raise funds for Billingboro's baseboards.  Cost of living crisis and all that: my modelling is going to have to be self-funding for a while.

 

Richard

[Nigelcliffe]

10 hours ago, Richard Hall said:

Grrr couplings.  Have spent an unproductive day working on different designs for the lifting latch.  I have something which almost works, .....

 

I assume you've also looked at the B&B coupling, which is another DG-variation.  That might give you a few more ideas on the latch approach.  

 

- Nigel