retractable

[sfward7]

Hi there,

I'm looking for types of retractable springs. From what I've found, I'm a bit unsure of what springs are used in tape measures (not the tape itself). What I've gathered is that it could either be a; power spring, extension/constant force spring, or spiral torsion spring. What I'm looking for is a spring that will reel in 50 metres of thin string (0.3mm but very strong). Could someone please advise me whether this is possible and what spring (with outer diameter < 100mm) can do this?

Cheers

 

[fegenbush]

In order to provide some guidance, more information is required. First, the diameter and width of the spool needs to be known in order to calculate the number of revolutions and torque required.

 

[GregLocock]

Yes more specs needed but with a 100mm OD and 50m of line I think you are going to need a clockwork motor.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[sfward7]

Many thanks for both your replies. I just looked at clockwork motors Greg Locock and that seems like it could be a viable option.

There are 2 options for specs; the diameter of the spool can be no more than 50mm and width of the spool no more than 100mm. Or diameter < 85mm and width < 70mm. I believe they use these springs in retractable power chords/cables and because the string used is much thinner than the chord I thought this may be possible.

Cheers

 

[tbuelna]

Does your design require a relatively constant force on the string during deployment and retraction?

 

[3DDave]

The first item is an energy calculation - 50meter * X Newtons of drag = energy minimum to be supplied by the motor.
Next up, number of turns of the spool. The minor diameter will be the volume of the package - volume of the string. There's a packing factor to take into account and possibly a need for a traversing guide (like fishing reel)

You'll probably require a gear step up drive between the motor and the spool; just guessing based on the amount of string and the spool size limitation. You might also need a variable ratio drive as the largest torque applied to the spool happens at the same time the least torque is available from the motor.

I'll throw out that cycloidal teeth should be investigated for any gears. I read a great book on watch design. These have similar problems - small number of winds resulting in many times the revolutions require step up trains with low losses. Involute teeth are good at transmitting uniform motion, but there is a lot of friction due to sliding and so they are not used in watches. Instead cycloidal teeth are used. The motion is not as uniform, but it has more rolling contact than involute does.

 

[berkshire]

Have you investigated Negator springs

http://www.ametekhunterspring.com/P...ngs/NEGATOR-Constant-Force-Spring-Motors.aspx

To cover the length of coil you are talking about you are still going to need gearing, because I think the max spring length available is 210".
B.E.

You are judged not by what you know, but by what you can do.

 

[berkshire]

Oops I did not read your OP closely enough, you had already mentioned constant force springs in it.
My Bad.
B.E.

You are judged not by what you know, but by what you can do.

 

[IRstuff]

Tape measures use a clock spring mechanism:

http://www.ehow.com/how-does_4969895_tape-measure-retract.html

http://www.youtube.com/watch?v=ZBSJ4bIKeuc

Unless your windup spool is wide and you can get more than one wrap in each layer, a 100-mm diameter might be tough. You might need to have something like a fishing reel mechanism that lays multiple wraps per layer.

You might be able to shrink the retraction spring, since you say the string is light. You may need to shrink the retraction spring just to keep it from either snapping the string or causing it to jam by pulling in the line too fast.


TTFN
faq731-376

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[sfward7]

All the advice given is very much appreciated. I shall look into the ideas and considerations mentioned.

 

[sfward7]

And yes tbuelna, it does require a constant, slow and controlled force of retraction or even a force that decreases as it winds in.

 

[Metalfixer]

Here is a link to a "Hand line trolling reel", I use one for fishing on the Detroit River. The spring inside is probably available, or maybe you could use
the complete reel.

http://www.andersonsprobait.com/gallery/showimage.php?iid=51