Alex Holden (alex_holden) wrote,
Alex Holden

Lachenal Cranked Lever Syndrome

The flattened rod and slotted pivot actions in Lachenal concertinas are quite ingenious and mostly work pretty well and reliably, but they do suffer from one potential weakness. Sometimes when a lever is cranked around an outer button to reach an inner button, it is subject to a twisting force every time the button is pressed. After decades of use this can lead to the slot and lever wearing in a way that allows the lever to partially rotate in the slot. This causes the affected button to feel sloppy and 'lumpy' and can cause the pad to not seal properly. Three of the buttons on my concertina were affected by this problem to varying degrees (interestingly there are several other cranked levers in the instrument that don't have the problem). There are various ways to solve the problem, including replacing the affected lever and pivot with new ones to the same design (I believe Steve Dickinson still makes them), or fitting a riveted action salvaged from a different brand of instrument. I decided to repair and improve mine as follows.

This shows how much a worn lever can twist in its pivot:
N.B. This is a macro photo: the pivot is less than 5mm wide and the slot is about 1.5mm wide.

The worn pivot slot:

Notch worn in the corner of the lever:

I silver-soldered (hard soldered) a thicker piece of brass onto each worn pivot, deliberately allowing the solder to partially fill the slot so the worn parts were filled with solder. On the first one I did I completely filled the slot with solder, but that proved unnecessary and made it more difficult to re-cut the slot.


Here's the pivot after re-cutting the slot with a jeweller's piercing saw and an escapement file. It's deliberately wider at the bottom so that it is possible to thread the curved parts of the cranked lever through the thicker metal of the repaired pivot without straightening it first. After the spring is fitted, the lever operates up in the top section of the slot, which isn't significantly wider than it was originally. The slot is cut in such a way that the section at the top that the lever pivots on is roughly the same size and shape as on the original pivot. The main difference is that the 'cheeks' of the slot are about three times as wide, giving much more support to the lever when it tries to twist in the slot.

I also built up some silver solder on the worn areas of the levers and then filed them back to the original shape. I don't know if the silver alloy solder is as hard as the brass, but it is certainly pretty hard (much harder than soft solder) and ought to have good wear resistance.

Here's one of the improved pivots fitted to the rebuilt action board between a couple of standard pivots:

Quite a slow, fiddly job, but the materials cost next to nothing (I already had the solder and flux in stock from my jewellery work), and the actions work as well as new ones again. The extra thick pivots should also make them much less likely to wear in the same way.
Tags: concertina, engineering, photos

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