[gearsoup]

I am looking for a spring loaded mechanism, sorta like a seatbelt (minus the locking ratchet) You see them at banks, where they are used on posts to make aisles. I found the ones the banks use in mcmaster-carr; http://www.mcmaster.com/#retractable-belts/=8ktzsm
but they are $80 each. Not willing to spend that sort of money. Another option I could use is to use the wound spring inside of windo shades, but they are a bit on the weak side.

Any suggestions? I am not opposed to making my own mechanism, would just rather use an off-the-shelf item



GS

[Al_The_Man]

What about using the seat belt idea, you can pick them up at a wrecker for cheap.
Especially the ones that let you strip it yourself.
It should be easy to remove the locking device.
Al.

[gearsoup]

Tension. I don't think they have enough pullback for what I am trying to do, not to mention the size is more than I want. I mentioned the seatbelts because it would give people an idea of what I was looking for as far as mechanisms go.

[holbieone]

be CAREFUL if you use seat belts , some of them have the same propellant as airbags inside

[hitachibos]

what are you trying to pull back?

how about a hose or electrical cord retracting reel

[Al_The_Man]

Tension. I don't think they have enough pullback for what I am trying to do, not to mention the size is more than I want. I mentioned the seatbelts because it would give people an idea of what I was looking for as far as mechanisms go.

What is the actual physical size? If you want quite a bit smaller, have you looked at the coil springs in items like large measuring tapes etc?
If you want stronger spring but smaller, then you may have to look for descrete coil springs, I have some used in linear to rotary actuators for encoders, the internal spring is pulled under tension by a steel stranded cable.
Al.

[gearsoup]

okay, I might as well tell you what I am doing so maybe that will help with the answers. I didn't want to, cuz I was quite proud of the concept. Nonetheless.....


I am designing a 4'x4' CNC router. I want to have way covers. Collapsing way covers take up one inch per foot (collapsed); this is un acceptable, as I would have to increase the overall footprint of my machine by 8". It keeps getting bigger and bigger, and enough is enough.

This belt would be running parralel with the thomson shafting, and there will be a sheetmetal enclosure surrounding the ways. The belt would ride in a channel, keeping it in line. As the gantry moved back and forth, the belt would retract or extend accordingly.

So, it needs to have enough tension to support it's own weight, and apply pressure againts the sheetmetal producing a somewhat decent seal against sawdust. total length of the strap needs to be roughly 4 feet, and there would be one on each side, so if the X was home, one would be fully extended, and the other collapsed, and when the X was at the limit of it's travel, the opposite would occur.

[Geof]

Don't use springs rig up a stepper drive working in parallel to the axis drive.

[gearsoup]

well, that would work I guess..

Except, as it wound more way cover onto the spool, it would need to move faster. Plus, wouldn't it be more expensive to go that way?

[Geof]

If you use a stepper you don't need to worry about changing the speed, just drive it faster than necessary and it will lose steps but still keep the cover tension to the holding torque of the stepper.

If you can't find a suitable spring operated mechanism a stepper drive will probably be the least complicated and expensive.

[vegipete]

Keep in mind that as the cover extends on one side, it retracts the same amount on the other side. Can you see a way to just wrap the cover under the machine and back to the other side? Or replace the center section of this long cover with cables and a tension spring?

[Fabio1]

U need some tape measure springs.I've seen similar concept on small Tos boring mills.

[ger21]

Search constant force springs at Mcmaster Carr

[doorknob]

How about old-fashioned, spring-driven window shades?

You would need to modify the action so that they wouldn't latch, but instead would always retract or extend as needed...

[handlewanker]

Hi, most of the methods so far have been using existing roller type recoil mechanisms, with their decreasing/increasing tension etc, except for the stepper motor type by Geof, so why don't you just get the way covers mounted on a drum each end and couple up to cheap electric motors to keep constant drive pressure on the drum but allowing it to be wound and unwound against motor torque.

The motors can be controlled torque wise by something like a VFD or even a light dimmer switch if you go to brush motors, and ordinary light dimmers are good for about 500 watts at 240 volts.

You would only want enough power to make the motors drive and take up the covers when they go slack.

If you get really techy you could make the motor coupling a fluid or mag drive type clutch and allow the motor to run constantly against the slipping clutch, no controller needed.

Speed of the motors would have to be enough to allow rapid travelling of the table whilst winding up the blinds.
Ian.