Spooled wire device with adjustable tension?

[futnuh]

We're working on a museum exhibit where the user input device is a sliding block mounted on a guide rail. The position of the carriage is sensed via a wire-draw encoder mounted at the left end of the rail with the wire fastened to the left side of the carriage. Here is the specific encoder:

http://www.sikoproducts.com/viewProduct.cfm?productID=SGI

At the moment, the slidable block when released gets accelerated left by virtue of the tension in the wire. We'd like to balance this with a pull on the right so that the block, left untouched, remains in position. The above encoder isn't overly cheap, and more importantly, doesn't have a tension adjustment, so duplicating this on the right-side is not a good option.

Question then: does anyone know of a spooled wire device with adjustable tension? Need 0-4m of travel, probably around 20N of force.

As always, any help is much appreciated.

 

[alansimpson]

Spring counterbalance from

http://www.mcmaster.com/

 

[rb1957]

i don't understand how tension in a wire accelerates a sliding block ?

it sounds like the "user" positions the block on the wire, and the wire is drawn thru the encoder to tell the user where the block was. why not make the wire a loop ?

 

[futnuh]

@Alan, unless I'm mistaken, the spring counterbalances on the McMaster-Carr site don't have adjustable tension nor the 4m extension we need. We're really after a unit with a wire coiled on a drum. Just like the above SIKO unit but without the actual encoder and with adjustable tension.

@rb1957, suggest taking a look at the above link. The encoder is inside the sealed unit. The end of the wire gets fixed to the movable carriage. We need to counterbalance the natural tendency of the wire to retract into its housing.

 

[rb1957]

ok, so the encoder is a flexible tape measure. presumably the encoder is fixed and the user is pulling on the wire. you could add a loop from the end of the wire up as high as you want (as long as you have something to loop over), down to the floor (or whereever the encoder is mounted) and up to the wire again. an adjustable link (turn-buckle) would allow you to put a small amount of tension in the loop. i think the end of the wire should maintain it's position.

 

[mikek10]

Tensator (

http://www.tensator.com)

used to do something like a big spring loaded fishing reel but I can't find it on their website now - maybe send them an email?

 

[SincoTC]

In the UK, "Tensator" type springs are produced by George Emmott (Pawsons) Ltd and are known as Constant Force Springs.

http://www.emmottsprings.co.uk/

Hope this helps.


Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
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[unclesyd]

How about something like this.

http://www.vermontcountrystore.com/...iSubCat=760&iProductID=9332&searchid=inceptor

You could get one these and remove the pawl and get constant tension.

http://lubeq.com/categories/64

Additional areas of thought.

Are you may be able to find a retractable large dog leash that would have your length.

Bungee cord and few pulleys.

 

[mikek10]

The tensator product we used to use was more like this

http://www.reelcraft.com/whatsnew.htm#spring

 

[sreid]

How about a weight on a "Rope" through pulley blocks. The idea is to use a heavy weight on pulley blocks that has multiple loops of rope so a short vertical gravity force is converted into a large, low force horizontal motion.

 

[amorrison]

Mount your sliding block between two coil springs that are concentric with the guiderail. Preload the springs (by having the springs too long for the space 2@2 meters) to the "stiffness" desired. Springs will center block. If you want the block to be perfectly in the center make the left spring a little longer so it provides more force to counter the encoder force.

Depending on the OD of your guiderail you may be able to get home/fence gate springs from the local hardware store that are 0.6 m long - cut or stretch(beyond yield strength)as desired.