Dear All, I am trying to work out how to make a small retracting cylinder, by which I mean, something along the lines of a roller blind, but that is drawn out, and then when let go, will return by itself. The dimensions are: Cylinder length 55mm, diameter 7mm.

I was thinking of a 7mm brass cylinder with a helical torsion spring inside, one end fixed to the brass 'rotating' drum, and the other to an inner steel axle, of about 1.5mm diameter.

I'm learning, so please forgive me if this sounds ridiculous, but I need to solve this one as I have to make 10-20 of these little blighters!

Thank you John and Michael for your quick replies.

Michael, you’re exactly right, it needs to be contained within the 7mm tube. It also needs multiple rotations, between 5 and 7.

Thank you Neil, having made balsa planes as a child that was my first idea, but as you say, it needs to be much more reliable than that. I remember years ago restoring a very old clockwork boat for a friend. That was powered by a long, approx 30cm slack spring. That’s the kind of power delivery this needs. I’m afraid I have no idea what torque it needs, but I don’t think that much, just enough to return about 5-7 turns of silk or similar.

Many thanks for the continued advice. Yes, I’d read somewhere about trying to make my own springs with guitar wires etc. I need the central, stationery axle to fix the unit in place. I don’t have any exact requirements for wall thickness on the outer tube, so whatever works is fine. I’d have thought, with the overall size of the unit, that anything over 1mm wouldn’t be necessary. Have considered trying to find a local engineering firm to make these up for me too, but have no idea what they’d charge.

I have managed to get hold of a couple of springs, so will have a go at a mock up tomorrow. Will post the results! Having explored the concept a little more, I think I will actually only need 3-3.5 revolutions.