Link and pin couplings in miniature railway use.
A personal observation, By Colin Edmondson, 2018
At the present moment the only guidance offered to operators of miniature railways is
HSG216, which is in the process of being replaced. The following is an excerpt from that
“Couplings 77 The design of any coupling should be adequate for the purpose and ensure that the rolling stock is securely coupled in all circumstances, e.g. when propelling or in a derailment. You need to prevent over-riding in the event of a collision. There are a number of different coupling systems which may be appropriate including three-link with side buffers, bars secured with pins and combined central buffer/coupling. The use of scale couplings of the three-link, instanter or screw type are not recommended as in scale form they have insufficient strength to ensure safety in all circumstances.
78 Safety chains should be considered between the locomotive and tender or driving truck. Their use between other vehicles in a train could prevent a division even if the main coupling fails.
79 On miniature railways where propelling of trains takes place, you need to consider ensuring the lateral stability of the vehicles within the train during this operation.”
Link and pin couplings
A link and pin coupling bar, in its basic form, is a length of steel bar with a hole drilled near
to each end. Each end is placed in a coupling pocket mounted on the locomotive or rolling stock and a pin is inserted to give a fairly rigid coupling between items of rolling stock with a sufficient range of movement to allow normal operation of the railway without binding. If
rotational movement between vehicles is excessive, such as may occur in a derailment,
then link and pin couplings may assist in keeping the train together as a unit.
Points which need to be taken into consideration during design and build.
The design has to take into account that it will be used by potentially untrained persons, or
by trained persons in a way which you would never have considered possible.
The coupling bar cross section has to be suitable not just for the traction loads which place
it in tension, but for propelling and braking loads which place it in compression. The forces
imposed on the coupling bar are greatest by far during heavy braking by the locomotive or
during a derailment, forces which can easily reach a few tons.
The coupling bar has to accept the angularity caused by the sideways swing of the outer
ends of a locomotive or item of rolling stock on a curve due to overhang placing the
coupling off centre in relation to the track, particularly when successive right and left
curves follow suit without a straight section in between.
A short coupling bar of a given cross section is less likely to bend, but due to angularity
can impose side loads on the ends of the vehicles during braking, possibly of sufficient
force to push an item of rolling stock sideways to the point of derailment. A longer coupling
bar of a given section is more likely to bend, but reduces the effects of angularity.
The locomotive shown here is travelling right to left, and is braking heavily. The coupling bar is shown in red. The red arrows by their length show the resultant forces on the rear of the locomotive, or other vehicle. It can be seen that a longer coupling bar reduces the side loads imposed.