First, my super scientific test bed:
First I checked the pull on the spiral coils.
Just like wikipedia said, they had fairly constant force that was independent of how far i pulled. Unfortunately, they only provided 0.3-0.4 lbs of pull, which is inadequate for the side masks, and definitely inadequate for the bottom. The funny part is I hoped I could just wind the spring tighter, but the whole “constant force” kinda means that will not work.
Next up: 1/8” bungee. It has an “elasticity” of 125%, which means 1 foot can stretch to 2.25 feet (i think, 100% means it can double its length).
Now, obviously when at rest, the force of 1 bungee is 0 lbs.
Time for some metrics:
#Bungees[/URL] Stretch(%) Force (lbs)
1 50 0.9
1 100 2.1
2 50 1.2
2 100 4.3
Okay, what did we learn about bungee tensioning properties:
1. They need to be stretched even at their least-stretched position to maintain a force. I’m estimating around 25-50% is a good minimum.
2. The pulling force increases as it is stretched. I’m not sure if this is linear or quadratic, but I probably don’t need to get into it.
So, say I have a 10 foot wide screen, and say I want to pull side masks on motorized rollers towards the center (5 feet each):
- I need bungee in a length that provides reasonable force over the 5 feet.
- I need the minimum length of the bungee to be about 1.25x it’s original (unstretched) length, and the maximum length (e.g. when the mask is fully wound back on it’s roll) to be 2x it’s original (unstretched) length.
This means 5 feet of “mask travel” needs a bungee that, unstretched, conforms to:
2x - 1.25x = 5 feet
.75x = 5 feet
x = 6.66 feet
(The BUNgee OF the BEAST!)
At full extension, when the mask is fully withdrawn, the bungee will stretch 6.66 * 2 = 13’4”
When the mask is fully closed, the 6.66 foot bungee will be stretched 6.66 * 1.25 = 8’4”
The next question that comes to mind: how do I keep that much bungee in a low-profile space? My first thought is using 1 or two fixed pulleys to wrap it up. Designs to follow.
PS- I know this looks like finger painting compared to stuff i read on the DIY subwoofer forum!
PPS- These stretch ratios are actually pretty convenient. A 10’ wide 16:9 screen is 5.625’ tall. At full extension, a bungee can travel half the width of the screen (5’), double back over a pulley (5’), and travel halfway down the height (2.125’). That 12.125’ is already really close to our desired 13’4” at full extension, and it is 5’ + 2.125’ at full withdrawal (with the mask closed)
LUCKILY, this means the SAME bungee should work perfectly for both sides of the mask, and the tension on the top and bottom of the mask will always be the same! Go arithmetic!