Elevated Bridge B/C

[sewforlife]

seems good. my superglue is.. VERY messy. and I hardly ever use it. maybe this year.

[dragonfly]

SIG.

Always.

http://www.sigmfg.com/IndexText/SIGCA010.html

The medium (green) works for just about everything.

Yes. Always.

[bob3443]

hey,
I was wondering what most of you guys used for glue when you build.
Most of the time, I use the balsa glue from PITSCO but lately, I've been hearing that other glue works just as well and is even lighter.
Any suggestions?
Thanks!

They have specialty balsa glue?
I use Loctite glue this one: http://www.loctiteproducts.com/products ... 4&plid=192, i've used this one before: http://www.loctiteproducts.com/products ... 4&plid=711. The second one just gives u some extra time to make sure the pieces are aligned and clamp it down and what not, although the regular one gives you enough time (for me anyways).

[Dark Sabre]

true its just that Nail polish remover is something that was easier to find than Buckets of acetone

and tell them to ask their Mothers thats where I figured out it worked

Lowes (and probably any paint store?) carries acetone by the quart and the gallon. fyi

[StampingKid]

On the JohnsHopkins Bridge Designer, when you load it you cannot load more than 100. But I can't find what the load number means in relation to the results. Say you load it from the top at the top two middle nodes balancing 50 and 50 on the nodes, does that mean that you are loading 50 percent on each? And when you calculate the results and get 150 on a compression member does that mean the member will bear 150%? The load forces have no units--so i think it can only represent a percent. And if it represents a percent, should you be loading 25 and 25 to get an idea of the actual bridge as the other truss will also be taking half the weight?

[andrewwski]

The load represents a force of arbitrary unit. Entering 100 could mean 100 lbs, 100 N, whatever.

The results will then tell you the forces in each member in the same arbitrary unit. So if you entered 100 lbs as your load and got a member with 150, that would mean that member is under a force of 150 lbs, which is 50%, yes.

Generally using 100 as the load is the easiest, as the numbers you get in each member will in fact be a percent then.

Nos, if you don't have a node in the very center, you can't enter a load of 100 there, in which case you have to split it up to two loads of 50 on the closest nodes. That will result in one of the center members having no load on it - obviously not what the real bridge will have - but the rest should be correct.

[13bridges]

They have specialty balsa glue?
I use Loctite glue this one: http://www.loctiteproducts.com/products ... 4&plid=192, i've used this one before: http://www.loctiteproducts.com/products ... 4&plid=711. The second one just gives u some extra time to make sure the pieces are aligned and clamp it down and what not, although the regular one gives you enough time (for me anyways).

Yeah, the balsa glue is in the blue bottle. Here's the link to it: http://shop.pitsco.com/store/detail.asp ... =0&t=0&l=0

[jander14indoor]

Be careful, by convention when people say balsa cement they generally mean solvent based glues like
Testor's Wood Glue http://www.testors.com/category/137220/ ... _Materials
Duco http://www.devcon.com/products/products ... N=78337610
Sigment http://www.centralhobbies.com/building_ ... igmnt.html
These glues are typically nitrocellulose based plastics dissolved in nasty solvents (the stuff that gives glue sniffers a buzz by destroying brain cells, use with ventilation). Some folks like it because it is easy to control for weight. Setting time depends on heat and thickness of glue, varying from minutes to hours.

CA glues or superglues are a liquid monomer in the jar and polymerize into a plastic when exposed to a catalyst. Typically moisture in the air, thus the reason some can be very slow to set in our overheated under-humidified houses in the winter. Or instantly by a store bought catalyst typically sold in a spray bottle so you can fill the air with this very powerful catalyst and set off the glue remaining in your bottle. Hint, use an eyedropper instead of the sprayer!
These can be a little messy as already discussed, and harder to use in small amounts unless you know the trick, a fancy CA applicator like this: http://www.gryffinaero.com/models/ffpag ... yaapp.html
As to brand or quality, freshness is probably more important. You also don't want any fancy additives as most reduce strength to get some other property. Most useful is either very thin or thin for bridges.

Epoxies are more complex short chain polymers that cross link into long chain polymers with special catalysts. Takes varying times to set depending on formulation and temp. Very strong, formulations can be found to stick to almost anything. BUT, hard to use effectively for this event. Had a place in boomilever. Problems, everything you mix will set in the specified time, so you typically have WAY to much or WAY to little. The lack of soaking into wood already mentioned can be fixed, but its a pain.

Almost forgot to list aliphatic resins. More commonly yellow or white wood glue. These are long chain polymers suspended in water. Set by tangling together as the water dissolves. Good strength, but long to dry for SO purposes.

Jeff Anderson
Livonia, MI

[StampingKid]

What glue is best for long beams--I or L? CA would seem to add too much weight. Wood glue thinned with water might be better as the water would evaporate. Any thoughts?

[jander14indoor]

I've had good success with balsa cement thinned with acetone or dope thinner. Apply a thin bead to one edge, touch to second piece, take apart, let dry. Repeat, but leave in place the second time with light clamping force, just enough to ensure contact along the length.

I've used CA, it worked, but like you said, hard to control weight gain.

I've also used wood glue thinned with water, wasn't happy with the weight gain.

Try different methods, check the weight gain with each and see what works best for you. Takes a little practice to develop good technique.

Jeff Anderson
Livonia, MI