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I am trying to repair a plissee blind, that does only fold up until half, then stops, no matter how high you push it. It has a metal-thing holding three metal spirals that can be connected to the plastic drum in the middle. The middle plastic drum has two square-shaped holes where the metal spirals can be connected (see pictures, it's hard to describe). It also has a kind of shuttle that is connected to the two strings pulling the blind. The shuttle is connected to the metal-thing via another string. The metal thing should strongly pull the shuttle to the right, so the blind will be kept up (this is the problem). If you pull the blind down, the shuttle should move left and should be kept left by the weight of the blind. So far I understand it. But now I have no clue how I can make the metal-thing have that strong pull to the right. I can pretty much guess how the left and the right metal spiral should be connected to the middle plastic drum, because I could see some dirty marks on the metal thing, caused by the metal spirals moving around. But I have no clue what to do with the middle metal spiral. Even theoretically, I have no clue about its function, because however I put it, it seems to "do nothing".

Is there anybody out there with a better technical understanding to imagine that spiral spring in practise? I am thankful for every hint or resource, as I could find absolutely nothing online (at least in German; I am missing the right research words in English, as these seem to be kind of special terms).

whole spring

connection to the middle plastic drum

metal basis

spring inside the blind rail

Edit: left or right spiral only have one connector left or right spiral and their spindle

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    $\begingroup$ The metal spring works like a tape measure. Both ends of each metal spring needs to be attached. The inside end to the spindle (or some fixed (not moving) piece) and the outside end to the part being extended from the spiral. Each spring is considered a constant force spring. If one is disconnected or broken then the system only develops half the force and will not wind up on its own. It looks like there are 3-springs: one on each side of the white pulley, and one inside the white pulley. $\endgroup$
    – Jim Clark
    Feb 28, 2021 at 17:55
  • $\begingroup$ Thanks. I cannot connect the inside ends of the left and right spring to their spindles, though. These springs end abruptly inside (I will attach a photo in my question), and only the middle spring has a connector on both the inside and the outside end. So it seems like the (left and right) springs can only be connected at the outside end. Does that exist? $\endgroup$
    – ema
    Mar 1, 2021 at 9:07
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    $\begingroup$ Maybe you should try contacting the vendor. Replacement parts may be inexpensive. A hand sketch of where this assembly resides, the components in motion, along with the spring attachments, and how the springs apply Force to the assembly, may help us understand the assembly and how it is supposed to function. Right now, I don't understand how it is supposed to work. $\endgroup$
    – Jim Clark
    Mar 2, 2021 at 21:44

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I came to the conclusion that one of the metal spirals must have broken in two, because I researched vendors' websites of these springs (e.g. these datasheets) and I don't think there exist any three-spirals-with-three-spindles springs. As I could only find industry-level vendors of these springs, and none that claimed to be specifically suitable for this plissee blind, I gave up.

For others researching this: it is called constant torque spring :)

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