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

guidance.


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.


Ideally in order to take compression loads the coupling bar should be flat and straight, but

problems which have to be overcome include joining together vehicles which have their

couplings at differing heights. This is generally overcome by using a kinked coupling bar

with the ends bent at a slight angle to overcome the height difference. During acceleration

and braking this arrangement imposes vertical loads on the couplings and vehicles, known

as 'jacking loads', as well as horizontal loads.



To reduce the risk of a kinked coupling bar bending under compressive loads the bends should be as near to the ends as possible.


The coupling pocket and coupling pin

The coupling pocket, or the means by which the coupling bar is attached to a vehicle, has

to be firmly attached to the vehicle and has to provide a slot of sufficient size to allow the

coupling bar to swing to all vertical and horizontal angles required in normal operation

without making contact with the sides. If the coupling bar does make contact before it

reaches the angle required it imposes much greater levering loads on the end of the bar

and the coupling pin than those present in normal operation.


However, the slot should not be greater in the vertical dimension than is required for the

normal vertical movement between items of rolling stock, as the greater the distance

between the holes in the coupling the greater the bending loads on the coupling pin.


The pins used should be a close fit in the holes in both the coupling bar and the couplings.

A small amount of slack action in each coupling can build up and can result in shock loads

being imposed as a locomotive may have travelled a short distance before the rear

carriage of a long train begins to move.


My personal opinion is that it is better to use round steel bar for coupling pins rather than

screws or bolts, as the notches formed by the screw threads of the latter can form stress

raisers which can lead to the generation of fatigue cracks. These are caused by repeated

reversal of the bending loads and eventually lead to breakage. If screws or bolts are used

then they should be high tensile (marked by an '8.8' on the bolt head), a nyloc nut can then

be used on the bottom to prevent the pin coming out unintentionally.


It is important that the coupling pin does not ride up and disengage in normal use. If the

coupling bar pinches the pin it can lift it to the extent of the clearance in the slot of the

coupling. If the free length of the pin below the coupling is greater than this distance by

some extent then the pin should never ride up sufficiently to come out. A slightly greater

hazard is fiddling fingers, it is not unknown for a passenger to deliberately remove the pin

if it does not have a positive locking device. It is for the railway operator to assess whether

the gradients on the line could cause this to endanger the public in any way, such as a

train rolling freely down a gradient. It may be decided that a positive stop is required, which

can be in the form of a spring clip, such as an 'R' clip, placed through a hole drilled

crosswise in the end of the pin. This in turn may be fastened to the head of the pin by a

chain or wire to prevent it going astray.

Clevis couplings

It is common practice to use a clevis and spring pin in place of the coupling pocket. If this

is the case it should be ensured that sufficient play is provided between coupling bar and

the jaws of the clevis such that in normal operation no additional bending moment is

applied to the clevis. A bending moment added to the loads created during normal use

may overload the bolt holding the clevis to the buffer beam, and may cause cracking of the

jaws themselves. Because the clevis is only fixed to the locomotive or rolling stock by a

single bolt or screw this should be inspected at intervals, especially after a derailment

which could have imposed bending loads on the clevis and thus bending loads on the bolt.

The red arrow shows a downward force being applied to a coupling bar which is a close fit in a clevis used as a coupling. One effect is to prise the jaws of the clevis apart, resulting in a crack forming where shown by the green arrow. A second effect is to potentially bend and stretch the bolt, by effectively using the green spot as a fulcrum point a much greater tensile force can be applied to the bolt. For this reason a high tensile bolt (marked by an '8.8' on the bolt head) should be used and it should be checked regularly for tightness.


Safety chains

It is a frequent practice to add safety chains between vehicles as a backup in case the

main coupling fails for any reason. It is up to the operator to decide if the gradients and

layout of the railway require them to be fitted. It should be borne in mind that a locomotive

driven from a separate driving truck could continue on its own if the coupling between

locomotive and driving truck parted, possibly striking the rear of the train in front or the rear of its own train if a continuous route is used. A clevis coupling relies on only one bolt to

fasten it to the locomotive or rolling stock, so a safety chain between the vehicles may be

considered, again the operator knowing the ruling gradients of the line and method of

operation should decide if this is required. The mounting position and length of the safety

chain, if fitted, should not allow it to catch on the track if left hanging loose, which means in

practice that a separate chain probably needs to be attached to each vehicle. The means

of attachment to the buffer beam can be via a 'U' bolt, which has two fixing points, or an

eyebolt, which has one. The means of fixing the chains to the vehicle should be properly

engineered and positive, as the loads imparted to a safety chain if a coupling parts will be

shock loads. When sizing a chain the shock loadings should similarly be taken into account.


The means of joining the chains together also needs to be properly considered, as

quite severe shock loads need to be handled if it is called upon to do its job. Two simple means of joining chains to the mountings and together are by means of a 'D' shackle or a screw gate carabiner. The latter has the advantage of not requiring tools to lock it and is therefore possibly more suitable for a connection between locomotive and train which has to be regularly disconnected. A spring link carabiner may not be strong enough to resist the shock loads imposed on it.




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