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Tension on AT-material to avoid bowing out on curved frame?

post #1 of 15
3/18/13 at 4:03pm
Thread Starter 
Hi! I`ve built a curved steel frame with AT-material. I attached the material to the back of the frame with staples into wood strips that I glued/screwed to the steel frame. The AT-material has rather much give, and it was easy to stretch the material without wrinkles. I hung the screen, and everything seemed fine. Later I have attached curtain tracks above and below the screen that will carry the masking system. Yesterday I mounted the aluminum straight edge for the masking fabric, and I noticed that the screeen material is bowing outwards about 10mm at the vertical midpoint. This was almost impossible to see without the straight edge in place.

I searched and found a thread by Don here, where he recommends not to staple the sides. Seems logical to not apply much tention sideways to avoid lifting the material. I took the screen down today, and removed the staples at both sides. This was not enough to avoid the bowing, so i need to remove the staples along one of the long sides, and give it some more tention.

I need some advice regarding how much tention to apply, or maybe someone here have a solution to this problem? Please let me know. I want to get the screen back on the wall so I can finally finish my DIY motorized masking:D


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post #2 of 15
3/18/13 at 9:43pm
There is no real hard or fast rule for how much tension except to say enough to get out all the wrinkles. The issue is the frame where the larger the screen, the thicker the material it is made from needs to be. The cross brace is a good idea so long as you treat them for light reflection. Because your screen is AT, light passes through it and if you have braces that reflect light, they will light up like a neon with the projector.
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post #3 of 15
3/19/13 at 1:35am
Thread Starter 
Thanks!
I don`t see the cross braces through the AT-material so far. They are painted flat black. I used square 25X25mm (1x1") tubes for the frame, and about 12x12mm for the cross braces. The width of the frame is 285cm 2,35:1.

I thought that to much tension to the fabric could perhaps degrade the audio properties?
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post #4 of 15
3/19/13 at 1:19pm
I always think its best to wrap the braces with stick on black velvet, many good manufacturers do this anyway.
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post #5 of 15
3/19/13 at 1:23pm
Thread Starter 
Quote:
Originally Posted by RapalloAV View Post

I always think its best to wrap the braces with stick on black velvet, many good manufacturers do this anyway.
I may do this as a precaution, since I still have the frame on the floor.
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post #6 of 15
3/20/13 at 3:33pm
Thread Starter 
Removed all staples along one of the long sides, and stapled again, applying more tension. Hung the screen again, but no change grrr...screen material stil bowing out. Is this he nature of curved screens?
Anyone care to check with something straight on their curved screen?
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post #7 of 15
3/20/13 at 6:24pm
Quote:
Originally Posted by Gunnar View Post

Anyone care to check with something straight on their curved screen?

Looking at your images again, and are you curving the screen as you install it or is your frame already curved?

If you have pulled the fabric too tight, you will get a concave bow top and bottom. When I did my curved curved screen, the screen itself was flat and I then curved it. Something I learned (the hard way) with mine was if the fabric was attached to the back (as the manufacture intended), the frame bowed mimicking the pincushion I was wanting to correct and this made the fabric sag. If it was attached to the front, it curved without issue. So I installed my screen in backwards and then had to make a new mask cover. I will changing my fabric out soon and know that it is easier to attach the fabric to the frame when it is curved rather than attempt to curve it once it is made up.

A while later I was able to inspect a Stewart Curved screen and noticed it too was attached to the front and hidden behind a mask.

I release this is not the answer you were hoping for, but maybe you need to re-think your design here.
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post #8 of 15
3/21/13 at 4:51am
Thread Starter 
Thanks!
The frame is already curved, and the fabric is covering the front of the frame, pulled around the frame, and fastened with staples at the back. By concave top to bottom, do you mean as on the attached sketch? (frame seen from the side, and fabric bowing outwards towards audience). i cut the excess material yesterday after pulling it rather tight, but hopefully there is enough left to release some tension.. I will mask off the part that touches the front of the frame anyway, so in worst case I can glue/tape the fabric to the front of the frame.
Edited by Gunnar - 3/21/13 at 5:18am
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post #9 of 15
3/21/13 at 2:18pm
Thread Starter 
I have now removed all staples at the bottom of the frame, with the frame still hanging on the wall. I then reattached the fabric temporarily with some tape at the bottom of the frame with very little pull. STILL ALMOST THE SAME BOW :-(
It seems that with this fabric with this amount of give, I can not avoid this.
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post #10 of 15
3/25/13 at 9:03am
I may be jumping in out of place, but I think with a curved screen you want a "lot" of tension vertically, and probably as little as you can get horizontally. If you put a lot of tension in the horizontal direction it will tend to pull the center of the screen material forward to be in straight line between the vertical edges, sort of like a Pringles chip.
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post #11 of 15
3/26/13 at 3:56am
Thread Starter 
Quote:
Originally Posted by stanger89 View Post

I may be jumping in out of place, but I think with a curved screen you want a "lot" of tension vertically, and probably as little as you can get horizontally. If you put a lot of tension in the horizontal direction it will tend to pull the center of the screen material forward to be in straight line between the vertical edges, sort of like a Pringles chip.

Yes, I agree. I have now reattached the fabric with rather high tension vertically, and as little as possible horizontally. The bow is still there.
I asked the shop/manufactorer, where I got the fabric, and they confirm that they see this as well on their curved screens, so I guess I have to live with this. I don`t really see any difference in focus between the center and the top/bottom areas.
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post #12 of 15
3/27/13 at 3:52am
When I did mine, the frame itself bowed horizontally with the fabric attached to the back. The screen was still flat at this stage and was fine whilst flat. The moment I placed into the rig to curve it, the horizontal parts bowed in and the fabric sagged. When I reversed the frame (cloth now attached at the front), the frame did not bow.

It sounds like the horizontal parts of the OPs frame have bowed and stayed that way.
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post #13 of 15
4/2/13 at 6:45am
Thread Starter 
Quote:
Originally Posted by CAVX View Post

When I did mine, the frame itself bowed horizontally with the fabric attached to the back. The screen was still flat at this stage and was fine whilst flat. The moment I placed into the rig to curve it, the horizontal parts bowed in and the fabric sagged. When I reversed the frame (cloth now attached at the front), the frame did not bow.

It sounds like the horizontal parts of the OPs frame have bowed and stayed that way.

The frame doesn`t seem to change at all while attaching the fabric, it is curved with a radius of about 11m before attaching the fabric.
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post #14 of 15
4/3/13 at 2:58am
So to further explain, if the ends were 1000mm, the centre became 950mm and the fabric sagged when attached to the back. Once turned around, the ends were 1000mm and the centre remained at 1000mm and the frame could be curved to cancel the pincushion.
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post #15 of 15
4/5/13 at 11:55pm
Thread Starter 
I think steel is a nice material to work with. No warping over time, rigid and relatively cheap also...
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