Model train repairs

Mehano coupler holder repair

[coupler holder 1]

One of the common problems with model trains is a missing coupler. If the coupler is just broken, the replacement part can be easily obtained and the replacement procedure is usually straightforward. Indeed, in some cases the latter requires disassembly of the undercarriage, a rather tedious and complicated process.

[coupler holder 2]

In a more unfortunate case, just like for this Mehano undercarriages, a part of the undercarriage is chipped off. Here we see the pin that goes into the coupler hole, but the plastic part that keeps the coupler in place is missing.

[coupler holder 3]

The coupler holder is in fact a plastic strip, which is an extension of the undercarriage box. The location where plastic strip has chipped off is just barely visible on the box.

[coupler holder 4]

In order to make a repair, I created a strip of 0.5mm thick copper with dimensions 6.5mm x 20mm and bent it into the L form. On the other hand, I created a rectangular hole of dimension 6.5mm x 1.5mm on the undercarriage box at the place of the missing coupler holder. To create a hole, I drilled a couple of 1.5mm holes and connected them together.

[coupler holder 5]

Then I pushed the copper strip with a shorter end through the hole and glue it to the undercarriage box from inside using a two component adhesive.

[coupler holder 6]

After assembly of the undercarriage, the metal strip holds the coupler in the place. The Achiles heel of the undercarriage has become the strongest point.

[coupler holder 7]

The locomotive is ready for a ride!

Automatic switcher operation and improvement

In this part I shall explain the operation of automatic HO switcher operation and the improvement that could be made to an old HO switcher.

[switcher 1]

The automatic switcher switches the direction when the current is applied between the middle and one of the two remaining electrical contacts. Disassembling the switcher reveals the actuator, a device that creates mechanical moves in two opposite directions. It consists of two solenoids and the metallic plunger within it.

[switcher 2]

Originally, an old automatic switcher plunger was made of the soft iron, shown on the right side of the picture above. The efficiency of the switcher can be greatly improved by replacing it with a permanent neodymium magnet. One possible way of adapting neodymium magnet to original soft iron is by buying a slightly shorter neodymium magnet, shown on the left side of the picture above and glue a small metallic hook made by copper (or some other metallic) sheet, shown in the middle of the picture above. The replacement is more or less straightforward, despite the fact that the principles of operation, explained below, are a bit different. It is important to orientate the magnet in the right direction, which could be determined simply by trial and error test (before gluing the metallic hook).

In my case I succeed to drop the required switch voltage from 12V to 5V. Since required energy is proportional to the square of voltage, this means sixfold reduction in energy. Even more importantly, since most of the supplied energy goes into warming the solenoid, this means that the solenoid heating is sixfold smaller and the probability of overheating will be significantly smaller.

Let's explain the behaviour of the plunger within the solenoid. For that we have to understand the following facts:

  1. The lowest energy position is when solenoid magnetic field and plunger magnetic field have the same direction and the plunger is in the centre of the solenoid. The highest energy position is when solenoid magnetic field and plunger magnetic field have the opposite directions and the plunger is in the centre of the solenoid.
  2. Forces always push the plunger toward the lower energy. This means that if solenoid magnetic field and the plunger magnetic field have the same direction, the plunger will be pulled toward the centre of the solenoid. On the other hand, if solenoid magnetic field and the plunger magnetic field have the opposite directions, the plunger will be pushed away from the centre of the solenoid. Alternatively, if there is enough space, the moment of force will try to rotate the plunger.
  3. Soft iron is originally unmagnetized, but in the presence of the external magnetic field (from the solenoid) it is magnetized in the same direction.
[switcher 3a] [switcher 3b]

Picture above left shows the behaviour of the soft iron plunger. (a) When there is no external field, the plunger stays unmagnetised and no force is exerted on it. (b) Current flowing through the solenoid creates magnetic field within it and magnetises the plunger in the same direction. This means that the plunger is pulled toward the centre of the solenoid, regarldess of the current direction. (c) In order to have the plunger movement in both directions we need another solenoid pulling the plunger in the other direction.

Picture above right shows the behaviour of the permanent magnet plunger. (d) Even when there is no external field, the plunger is magnetised, but no force is exerted on it. (e) Current flowing through the solenoid creates magnetic field within it. If the field directions of solenoid and plunger are the same, the plunger is pulled toward the centre of the solenoid, however, if the directions are the opposite, the plunger is pushed away. (f) If we use another solenoid next to the first one, the directions are reversed.

Despite the fact that one could move the permanent magnet plunger only with one solenoid, I advice the use of both solenoids. Namely, if the field directions of solenoid and plunger are the opposite, moment of force can appear trying to rotate the plunger into the lowest energy position. This can create friction between the plunger and the solenoid and hinder plunger movements.

The final question that remains is why the producers did not originally used permanent magnets instead of the soft iron. I suspect that thirty years ago only permanent ferrite magnets were available, and those achieve smaller magnetization than soft iron, making the soft iron solution more efficient. With the appearance of the neodymium magnets, the situation is reversed.

Created by Marko Pinteric: feedback form.

Updated . Web page has been read by visitors since March 2017.

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