OO Gauge Fixed Link Couplings – Part 1

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.