Improving the Jouef MT
pantograph (1)

Original page created on 24/03/2023.

How to hold the pantograph folded

Observation

The Jouef MT type pantograph is quite thin, but it has some simplifications. In particular, it lacks a double bar under the bow (red arrow). It also has a not very unobtrusive system to hold it in folded position (orange arrow).

Moreover, this device is not easy to use: the claws pointed by the orange arrow have to be inserted under the lifting spring brackets: they have to be positioned alternately by pushing to the right and then to the left, forcing a little. As a result, as I have removed the pantograph’s central fixing screw, which simply remains glued to its insulators, there is always a risk of it coming off.

The idea is to make the missing bars out of steel (piano wire), and to replace the mechanical hold by a magnetic hold, thanks to a magnet. This idea is not mine: it is used on a 1:32 model, whose references I unfortunately did not note, and was also given by a member of the Loco-Revue forum. The trick is to make something discreet enough.

Note: I have done previous attempts to do this, but it worked randomly. The article describing them is no longer directly accessible.

Realization

The bars will be fixed by drilling the side legs of the bow, just under the openings receiving the upper joints of the pantograph.

Sequence of operations

To facilitate the operation, the bow is separated into two parts by lifting one of the two tabs that hold them together. The part to be drilled is placed astride a wooden support 8 mm wide and more than 5 mm thick for optimal holding and without risk of deformation.

Two ⌀ 0.5 holes spaced 0.7 mm apart are drilled 0.7 mm below the existing oblong opening. The bars will be too low compared to reality, but I don’t see how to do otherwise. This will also facilitate the magnetisation that we will see later on.

Click on the picture to zoom in.

Two segments of ⌀ 0.5 × 10 (approximately) piano wire are prepared. Their ends are chamfered with abrasive paper to facilitate their insertion. They are then glued and adjusted in length with a diamond disc.

The claws that were used to hold the folded pantograph are cut off. Caution: the other claws must not be removed, as they are intended to keep the bow horizontal when the pantograph is raised. The operation is not very easy, as the metal used is quite hard. Therefore, it is necessary to use a sturdy pair of cutting pliers, or an abrasive disc (although in the latter case, the heat may damage the gluing: to be done preferably before the latter).

Finally, the two pieces of the bow are joined together again.

Mounting one (or two?) magnet(s)

I don’t need to tell you about all the experiments done, with different sizes of magnets, their height in relation to the chassis, their arrangement, etc. I simply show you the two solutions that gave the best results. The simplest one will be retained. The magnets chosen are cylindrical, ⌀ 2 × 2 mm.

A pole piece (a pompous term for a 0.5 mm thick small tin plate) is glued with CA glue in the centre of the frame, at the location of the fixing. This piece is wide enough for the experiment. We will see that it will be shortened later.

Note: In the following series of tests, the bow is equipped with a single bar. Only later, the two-bar solution was found to be more effective and more consistent with reality.

Test with two magnets

The idea is to improve the attraction force of the bow by obtaining a closed magnetic circuit, so that the magnetic flux is as concentrated as possible. Note that the two magnets are placed in reverse polarity (the blue and red markings indicate the north and south poles, respectively). A secondary advantage is to ensure the horizontality of the bow, which is not the case with a single magnet.

View in unfolded position.

Click on the picture to zoom in.

View in folded position.

Click on the picture to zoom in.

You may notice that the bow is not centred on the magnets. This is because they are not glued to the tin plate, which allows them to be moved around and makes it easier to adjust their position.

Test with a central magnet

This solution is a little less effective, but still sufficient. It has the advantage of being simpler. The lower plate will be kept, although apparently unnecessary, for two reasons:

• It contributes to the height positioning of the magnet: 0.5 mm less makes the attraction of the bow random.
• As seen above, it allows the magnet to be held in place during the adjustment phase, before final gluing.

View in unfolded position.

Click on the picture to zoom in.

View in folded position.

Click on the picture to zoom in.

This test being successful, the tin plate is replaced with a smaller one (about 3 × 2 mm). Gluing of the magnet (with CA glue, not very effective but sufficient given the low forces involved) is done once the best position has been determined. The folded position is very satisfactory, and the effort to unfold the pantograph is moderate.

In the folded position, the bow is almost perfectly level (this depends of course on the care with which the parts have been glued).

You can see in this photo how obtrusive the pantograph reinforcement bars are. This is what led me to try to improve the appearance. We will see this on the next page.