Alex Jackson Coupling - page 3

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The electromagnetic uncoupler
To uncouple as we've just described, it is necessary for one coupling to be pulled down vertically. Wagon 'B' is moving over the electromagnet whilst pushing wagon 'A'. By pressing a push button switch on the control panel the coil is energised and a magnetic field is created between the rails. The dropper (or armature) attached to the shank of the coupling is of soft iron wire and is attracted to the magnet so creating the vertical movement downwards to the wheel axle. In this lowered position, sufficient movement has been made to disengage the hook, but the dropper must not touch the magnet face. If it does, the likely result is a large pile-up.

The position of the electromagnet on the layout should be indicated by some unobtrusive marker, especially if it is some distance away from the controller. The operator is then better able to see the relationship between the moving wagons and the magnet. If there is a location where all vehicles are required to be uncoupled, a permanent magnet of suitable size and power may be fitted instead of an electromagnet.

The electromagnet
In some of the published reviews of the book “Alex Jackson – The Man and the Coupling”, there is reference to a lack of information about the electromagnets. On page 48 I wrote: ”For model engineers of their quality and experience that was probably quite sufficient because, we must remember, that they truly understood this coupling and what was needed. Each a professional engineer they would have had an inherent understanding of magnetic circuitry.”

It is ironic that having expressed that as a reason that some details of the coupling had not so far been published, I fell into exactly the same trap. The uncoupler magnets have never ever raised a problem for me, but that may well be that for so many years of my professional life I was involved with many types of electromagnetic devices. In particular I usually had easy access to ex-GPO relays and still have a few dozen in stock!

Pages 16 & 17 of the book give clear information of how to adapt a typical GPO relay, and suggest that a 200 ohm relay coil supplied with 24V dc has proved to be suitable. If you wish to keep to 12V dc operation, then look for relays with 100 ohm resistance. These pages also give some information that would allow home winding of a suitable coil. Should you opt for operating the magnets on 12V dc, please use a different transformer-rectifier unit to that used for track supply. Actually, if you do need the above information, it seems to me that you would be well advised to seek the help of a friend who has a deeper knowledge of things electrical.

While the GPO relay is still my personal choice for an uncoupler, it has fallen out of use in present day electrical control panels, but should still be available in second-hand or surplus equipment shops - try a search on the internet. I adapt the relay by removing all the contacts, basically keeping only the coil on its core. The core diameter is drilled about 6mm deep and tapped 2BA and then a suitable length 2BA countersunk steel screw is threaded through a clearance hole in the baseboard surface on track centre-line, and then screwed into the core holding the magnetic coil exactly where it is needed This screw must be steel (a magnetic material) because it becomes an extension of the magnet’s core and, along with the dropper on the coupling, part of the magnetic circuit.

On my layout I use 55V dc for the uncoupler magnets. This may sound high but I had a suitable transformer-rectifier unit available and I had the knowledge to know that during the brief time that the coil is energised, the higher voltage would not cause sufficient development of heat to damage the insulation. Of course, with such a system, the switches controlling the magnets must each be “press to on” type, so there is little chance of the magnet being energised for longer than the time to complete the uncoupling move. Please though, do not feed such a voltage to your 12V rated coils, even intermittently, unless you have the electrical knowledge to know that damage will not result.

Unfortunately, because I personally have no need of other types of electromagnets for this application, I have no experience of using what our traders have to offer. However, I can see no reason why such as the ‘Seep’ electromagnet, those obtainable from Model Signal Engineering, or you can find one of your own choosing from electrical stores such as Maplins. If you prefer to use one of these readily available electromagnetic coils but have worries as to its suitability for this application then may I suggest that you contact the manufacturer directly, explain how you wish to use their product and ask their assurance as to its suitability.

The dropper (or armature)
The dropper can be made from soft iron wire obtained from an ordinary paper clip, one of which when straightened, will make several armatures (a new method is discussed later). Solder each armature in a position with the vertical arm on the inside of the axle, trimmed to a length which suits the rail height and does not allow the armature to touch the face of the magnet when in the lower position.

Hook setting gauge
The shank height - i.e. the normal height of the wire above the top surface of the rail - is, at 10mm one of the important dimensions which must be used at all times to ensure consistency of operation. Another important rule is that the wire must lie along the centre line of the track:it therefore follows that it must lie along the longitudinal centre line of the vehicle. Both conditions can easily be set and maintained by use of the simple hook setting gauge shown in Fig.13 and this is the only way that such a check should be made; setting one coupling against another will only lead to confusion (which is surely true of any coupling).

Ratio of movement - wagons
The drawing below shows that in relation to a 12mm diameter wheel there will be a permissible downwards movement of 3mm, the axle acting as a stop when the coupling is in the lower position. There should be a minimum of 0.5mm (0.020in.) clearance between the tip of the dropper leg and the magnet face.

You can also see that the distance 'D' of the dropper from the point of anchorage in relation to the distance 'H' to the hook, is in the approximate ratio of 2:3, so that for a movement of 3mm at the dropper, the vertical movement at the hook will be 4.5mm, which is ample for disengagement of the coupling hooks.

Ratio of movement - coaches
In this application, using 14mm diameter wheels, the movement is reduced to 2mm. With the ratio of movement between the dropper and the hook being approximately 4:7 the movement at the hook would be 3.5mm which is satisfactory. These dimensions vary with varying applications but it is desirable to have 3.5mm downwards movement at the hook to obtain satisfactory operation.

Locomotives
Because of fitting difficulties, it is our general practice to have the coupling at each end of a locomotive non-operative. No dropper is therefore required and uncoupling is performed from the coupling of the adjacent vehicle. This does create minor difficulties with double headed trains and if the fitting difficulties can be overcome, (somewhat easier with the new method later described), then an operating coupling should be fitted.

Coach sets
Coaches permanently coupled in sets can be arranged to have an operating coupling at each end of the set.

Minimum running radius
Simple pulling of a coupled train is permissible on quite small track radii. It will be seen, however, that two wagons being pulled around 30ins radius causes the coupling wires to be offset from the centre. This results in undesirable side force which increases as the radius gets smaller. For reliable running we keep our minimum radius curves at 30 inches . Pushing loose vehicles around a small radius is of course possible, however, the coupling cannot in any way act as a centre buffer, the vehicle buffers must fulfil their proper function on curves.

Minimum radius for coupling and uncoupling
Coupling will not occur reliably if the vehicles are brought together on a track radius of less than 48 inches, due to the relative angles of vehicles. If possible, therefore, always leave vehicles for recoupling on straight track.

Uncoupling should be arranged on a length of straight track if at all possible. It is certainly not desirable to have the uncoupling electro-magnet positioned on track of a radius of less than 48in.

Buffer stops
The precise nature of the setting of the coupling wires does not take kindly to harsh handling. and being rammed up against a solid object like a buffer stop is quite likely to disturb the setting. Rail built or other types of buffer stop should be made so that the cross beam is high enough to allow the nose of the coupling to pass under it.

Height bar
Though not essential with the hook fitting so far described, a number of modellers have adopted the fitting of a height bar, being a simple stirrup of 0.015 wire fastened transversely across the under-side of the vehicle and of such size that when the coupler shank is lightly sprung up to it, the required shank height of 10mm above rail level is obtained. If this fitting is adopted, care should be taken to see that the shank does not bear too hard on the height bar or the magnet will be unable to pull the armature down when required.

More developments
Most of the development of the Alex Jackson coupling took place in the 1950s and it was described in the Model Railway News in 1960. It may be thought that there is little that could be done to improve such a simple idea but modellers within the Manchester Model Railway Society and elsewhere kept coming up with more and more ingenious ideas that improved the coupling in several ways. The next page describes some of the more modern methods of anchoring the coupling to vehicles and those who read more specialist model railway magazines like the Model Railway Journal may learn of other peoples' ideas using Alex's original ideas.