Two weeks ago I started telling you about my OO Gauge Fixed Link Couplings, you can find the post here. In this week’s post I’m going to share with you the design and different types I’ll be making available and why.

As the name suggests these are fixed link couplings and can’t be uncoupled.  They’re designed to allow strings of rolling stock to be coupled permanently in sets with the correct-looking couplings.  This is ideal for exhibition layouts or block trains which don’t need to be shunted.

The two basic styles, as pictured below, are Instanter, shown in green and 3 link, shown in red.

The actual style of Instanter or 3 link is purely cosmetic as this simply forms a rigid fixed link between the NEM connection.  Why did I do this instead of leaving the chain links loose?  Well, if the links were loose it would alow the coupling to close up when a locomotive is pushing the rolling stock, but this would mean the loco will be pushing on the buffers. Unlike the real thing this doesn’t work very well on most ready-to-run OO rolling stock and tends to lead to derailment.  Having the chains fused turns the coupling into a drawbar and as it’s centered on the rolling stock it allows long trains to be pushed without any issues. To fuse the chain links I have drawn a bulge making them a bit fatter where the links meet; this causes them to overlap and become one solid piece.

As I said above, making the connection into a solid drawbar gives the advantage of allowing the train to be pushed but what about going around curves?  The NEM sockets do allow a bit of sideways movement but depending on which manufacturer will depend on how much. So I have designed in a flexible section at each end which allows for some more movement.  This can be seen on the Instanter connector below.

As the coupling bends either to the left or right one of the gaps will open slightly.  When the train is being pushed the gaps close equally keeping the force straight.

Below I have two brand new OO Bachmann covered hopper wagons linked with the coupling above.

As the trucks are twisted to the max, where the buffers touch, the NEM socket swivels as well as the flexible section in the coupling.

The different types, apart from Instanter and 3 link, refer to two variations in the NEM sockets.  The first is length, or rather the distance the NEM socket is set back from the front of the rolling stock.  Although the NEM standards specify the size and position of the socket, not all manufacturers have them in the same place. Length is also governed by the tightest radius curve on your layout.  The tighter the radius the further apart the rolling stock needs to be before the buffers lock and cause a derailment.  With some exhibition layouts which only have big radius curves my shortest couplings can be used.

The second is height, some NEM sockets are lower than others.  To solve this I did an in-depth survey of all the main manufactures and determined that the issue could be overcome with three different shapes, which are all shown below.

The top one is simply a straight coupling.  The second is a cranked coupling and the third is a step-up coupling.  Below you can see the same two Bachmann hopper wagons with a straight Instanter fitted.

The Bachmann NEM socket is low and consequently so is the coupling.  By using a step-up coupling it raises the Instanter.

The cranked coupling is designed to be used when connecting rolling stock from different manufactures.  For instance, below are a Hornby and a Bachmann coal wagons.  The Hornby NEM socket is higher than the Bachmann but using a cranked coupling solves the problem.

You many have also noticed the original plastic molded hook is still on the model above the coupling.  My couplings technically are still lower than the real thing but doing it this way means you don’t have to modify your rolling stock if you don’t want to as the new couplings simply plug-in.  I will also be offering an advanced range which raise the couplings to the corrected height but this will require the plastic molded hook to be cut off.

Next week I’ll share with you the full range of my OO Gauge Fixed Link Couplings as well as a way to judge which ones you need.

As promised in last week’s post, this week will be about 3D printed parts.  And these parts are OO Gauge couplings.

Traditionally OO gauge locomotives and rolling stock in the UK have relied on the hook and bar coupling or tension-lock shown below.  These date back to the Hornby Tri-ang era and although they work well they are rather unsightly and are in no way prototypical.

For many years different manufacturers trimmed and improved the design but there was no getting away from that huge hook and bar; otherwise their locomotives and rolling stock would not couple up to everybody elses.  Also there had to be a lot of gap between the couplings to allow trains to navigate very tight curves.  Starter sets and train sets always come with a tight loop of track and all the manufacturers wanted their trains to go around them.

Naturally exhibition layouts didn’t want this type of coupling and the modelers invented all sorts of ways to close couple and accurately represent the real couplings for their era. Many kits are now available but on older models a bit of cutting and drilling is normally required and this can get a bit tedious and expensive.

More recently the majority of manufactures have now standardised their couplings in that they have added a NEM pocket to the underside of their rolling stock.

A NEM pocket or socket, as pictured below, is a standard pocket which complies to NEM 362.  This stipulates the size and position of the socket.  The intention is that all rolling stock has a NEM socket which allows any type of coupler to be simply plugged in and will be at the right height.

Currently just about all OO stock now comes with a tension-lock coupling plunged into a NEM socket.

This now opens up the possibilities of getting correct couplings for your era.

Real modern UK rollings stock use the Bukeye connector which is the same design as the standard US equipment but this didn’t happen untill recently.  Early rolling stock was coupled by a 3 link chain and hook as shown below.

Each end of the wagon has a cast hook and a 3 link chain.  Therefore each end is universal and either chain could be lifted and dropped over the hook.  This simple method worked well but rolling stock snapped back and forth as the train accelerated and braked. The buffers are sprung as to absorb the impact but as the trains got heavier it became more of a problem so the Instanter coupling was invented.

This new coupling, as shown below, is similar to the 3 link but the central link is a special cast iron shape.

The coupling can still be used as a loose fit coupling or can be rotated to make it a close coupled connection.  To do this the wagon brake is applied and the locomotive pushed up which compresses the buffers.  Then the chain can be put over the hook and the center link easily rotated.  When the locomotive releases the pressure the buffers release but not all the way as the chain is now much shorter and pulls tight.  Because of the rounded shapes at the top of the Instanter it cannot rotate and keeps the link short.  Now the rolling stock is close coupled and will not snap back and forth.  In the drawing below you can see the Instanter in its rotated position with the rounded shapes at the top.

The two horns at the bottom of the Instanter are there to aid the shunter when they want to release or connect the coupling.  Once the locomotive has pushed up and compressed the buffers the shunter uses a shunter’s pole, as shown below, to hook the coupling up and over.  The metal ‘pigs tail’ in the pole hooks onto the horns.  This makes the shunter’s job safer as they don’t need to go between the rolling stock to couple up.

Many modelers are switching over to Kedee couplings, one of the American standard couplers, as they now supply a coupling which fits directly into the NEM pocket.

However these can be fairly expensive and what if you model older stock which would have had an Instanter or 3 link coupling?  Well the answer is a 3D printed couplings.

My design for these has evolved over the last few months and have been tested thoroughly.  Now I have a set which works in a variety of situations and in next week’s post I will tell you all about them and share with you some photos and videos of the real thing.