Vague idea how to start....

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!

Try and have a look at an old plate camera, they had a shutter with this type if mechanism, the shutter being made of cloth . The return spring was from memory somewhat larger than the drum and mounted on the outside using a clock type spring.

I have a couple in the attic that belonged to my grandfather if you want photos send me a PM

Regards John

Interesting puzzle, Dominic

First question ... Am I right in assuming that the mechanism needs to be contained within that 7mm diameter x 55mm long envelope?

Second question ... How much does it need to rotate? fraction of turn, or multiple turns?

There will be more !!

MichaelG.

How much torque needed?

The simplest solution (if not the most reliable) is winding up a large rubber band...

Neil

Dominic,

I don't know their proper name, but; the style of spring that lifts an up-and-over Garage Door would probably do the job ... I'm sure such things must be available [somewhere!] in miniature sizes.

MichaelG.

Edit: Here are some useful illustrations

Edited By Michael Gilligan on 19/08/2015 14:23:49

Sounds like a long coil spring with a loose rod down the middle may be up to the job.

Neil

Dominic,

Unlikely to be practical, because you would need electrical power & control. but these are a "near miss"

MichaelG.

Hi Dominic

Once you have determined the wall thickness of your tube you will know the maximum outside diameter of your spring, you will also need to allow clearance.

I am guessing maybe 1.2mm wall for the 7mm tube? that would leave about 4mm for the spring. I think you will need to get an idea of the force required, hopefully not a large force to retract your material.

The thinner the spring wire is the better allowing more turns in the 55mm length you have available. this will reduce the stress on the wire as it winds up five to seven times.

Musical instrument shops sell spring wire for strings, maybe a good place to get short lengths of wire to experiment with. Or maybe you have a spring maker nearby.

Roslau make good quality music wire.

**LINK**

This book is worth having if you work with springs.

**LINK**

Regards
John

I would try with a long coil spring down the tube. Don't think you would need a rod down the middle. One end fixed to a mount and other end to the tube. Springs are quite easy to make if you have a lathe, pester some guitar playing friend for any single wire string and try a few.

They would probably charge £250 to £300 for a 10 off. What are they for?