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THE SILENT CURTAS MYSTERY

 
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RoMan



Joined: 28 Aug 2010
Posts: 7
Location: Italy

PostPosted: Wed May 17, 2017 7:59 pm    Post subject: THE SILENT CURTAS MYSTERY Reply with quote

In 1955-56 a new variant was added in the curta production: the silencing spring.
It is basically a spring which is wound around the groove in the 0 positioning disc with one end that is bent and passes under the non return pawl and enters its own specific hole (2mm) in the base (lower main casting).

The way it operates is simple: during the normal rotation of the main shaft, the spring keeps the non return pawl slightly lifted up, so that it loses its contact with the disc teeth and doesnít produce its usual ticking. Yet, if we try the inverse rotation, the spring backs up and the pawl starts meshing again with the disc teeth, thus preventing, as it usually does, such inverse motion.
We do not exactly know how many silent curtas were prepared (no more than a few thousands, however), but there is a strange fact: some curtas were made with the hole in the base, but without the spring. Therefore, in operation they produce their normal ticking, they are not silent, but their base has the specific hole for the silencing spring.
According to my experience, I would say that, considering the curtas with this hole, only one out of eight or one out of ten also has the spring.
At a certain point, they might have stopped adding these springs. But why?
Were they taken away in a second moment? And who did that? Why?
Here is the data I have collected from my direct experience. The gaps are so large because I have only recently been attracted to this issue.

CI:
30029 without hole
-----------
32036 silent, now on eBay http://www.curtaservice.it/1/silent_curtas_4906576.html
33728 silent
37625 hole without spring http://www.curtaservice.it/1/images/670_0_4906834_8022.jpg
41681 hole without spring
-----------
46509 without hole

CII:
501528 without hole
-----------
504094 hole without spring (poster on eBay) http://www.curtaservice.it/1/silent_curtas_4906578.html
506547 silent
507157 silent (photo by Sebastian Dammers in this BB) https://www.flickr.com/photos/131105513@N02/16404073206
511272 hole without spring
-----------
514051 without hole

I would like to invite all curta owners to add further information after checking their curtas. Besides mentioning the serial numbers of the silent curtas they may have, they should also check if, in curtas whose serial number is close to the silent curtas range, there is a hole in the base. Itís easy: you only have to remove the base plate unscrewing the two counter sunk screws; itís a 2mm hole and itís under the non return pawl.
It can also be seen when looking laterally, but in order to see it well, you can lift the non return pawl with the tip of a screwdriver.
I am confident that all together we will be able to define stricter limits and understand more about this variant which is very interesting for collectors.

Perhaps this silent curta was not appreciated by everybody and at Continaís they soon stopped putting the springs, even though they continued to mount the bases that had already been prepared with the hole?
Perhaps the springs were taken away by dealers upon requests of their customers who preferred to hear the typical ticking?
Perhaps the springs, whose correct operation depends on the right amount of friction between the spring and the disc, could no longer work correctly after some time or due to dust and so they were removed at the first problem or the first time the curtas were cleaned.


Last edited by RoMan on Thu May 18, 2017 4:00 pm; edited 14 times in total
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murff



Joined: 27 Oct 2009
Posts: 254
Location: Switzerland

PostPosted: Thu May 18, 2017 8:20 am    Post subject: Reply with quote

Interesting overview about the mysterious silent Curta!

To visualize the modified Lower main casting (the 2 mm hole) the following two pictures:


[ without the silent modification ]



[ with the silent modification ]


Is there a picture of the silent spring (not to confuse with loud summer Very Happy )

I will check some relevant numbers...
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RoMan



Joined: 28 Aug 2010
Posts: 7
Location: Italy

PostPosted: Thu May 18, 2017 2:46 pm    Post subject: Reply with quote

For meticulous people (as I am) I want to add a further curiosity that I have noticed.
In CIs the spring is wound around the disc in a gripping way, that is the friction on the disc during normal motion tends to grip the spring around the disc.
In CIIs the spring is wound in the opposite way, so that the friction tends to loosen the spring. (CI and CII silent springs here: http://www.curtaservice.it/1/images/670_0_4906580_8022.jpg)
Both ways of mounting the spring can cause problems.
In the gripping way, if the friction between the spring and the disc tends to increase in time, the spring might be drifted and finally come out of its hole in the base.
In the loosening way, the free end of the spring only needs to find the slightest unevenness in the disc groove to get stuck and come out of the groove (in order to avoid this, the free end of CII springs is bent outwards), but the same problem can arise as friction increases in time.
I prefer when springs are mounted in the gripping way, it is safer and its possible defects in my opinion are not so frequent and anyway can be avoided by slightly lubricating the disc groove.
They might have preferred it at Continaís as well, but they were obliged to use the opposite mounting owing to the different zero positioning disc.
In the CII zero positioning disc (smaller) the roller indentation is deeper than the groove and, with a gripping spring, when this indentation passes under the fixed end of the spring, this slightly enters the indentation and, at the end of the indentation, the spring derails every time outside the groove.
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murff



Joined: 27 Oct 2009
Posts: 254
Location: Switzerland

PostPosted: Thu May 18, 2017 7:46 pm    Post subject: Reply with quote

I can confirm the range of serial numbers, both Curta Type I and II - no new or tighter borders found...

And all my Curtas without the spring.

Do you recreate these springs? I would be interested in some tests...
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