Designs B/C

DoctaDave
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Re: Designs B/C

Postby DoctaDave » April 19th, 2015, 10:22 pm

sjwon3789 wrote:I always see every teams attempting the arch designs because it is efficient to some extent -- until they discover a more efficient design, although I have not attempted them because I hear how inconsistent they are for each testing. I believe the score in division C is around 1600-1800 for arch bridges, whereas the tower-like design has the potential to go above 2000.


Not quite true.

I've seen several teams getting well over 2500 with an arch in C div. They seem to have the advantage of stability, since the loading block usually sits on the bottom, so it's less likely to tip over than the "tower" design, although I do believe the tower design loaded from the top has more potential than an arch loaded from the bottom if built correctly. But more importantly, arch bridges are just absolutely gorgeous :D

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Re: Designs B/C

Postby DoctaDave » April 19th, 2015, 10:42 pm

dholdgreve wrote:It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.


Does CA glue actually get dry and brittle? I'm pretty sure CA glue does not dry with evaporation as many other glues do, but it actually reacts with the humidity in the air and then polymerizes for a permanent bond. So once it sets, I'm not sure taking away the moisture would really affect the performance of the glue, and remember that the strength of a joint is ultimately dependent on the face of the wood. I think even a joint that has been made over a year ago would still fail as a result of the wood face pulling off instead of the glue getting old.

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bernard
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Re: Designs B/C

Postby bernard » April 19th, 2015, 11:21 pm

DoctaDave wrote:
dholdgreve wrote:It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.


Does CA glue actually get dry and brittle? I'm pretty sure CA glue does not dry with evaporation as many other glues do, but it actually reacts with the humidity in the air and then polymerizes for a permanent bond. So once it sets, I'm not sure taking away the moisture would really affect the performance of the glue, and remember that the strength of a joint is ultimately dependent on the face of the wood. I think even a joint that has been made over a year ago would still fail as a result of the wood face pulling off instead of the glue getting old.

Would be interesting to do some tests and leave some joints for a while. I might do that if I ever get bored. I've had several bottles of CA glue this season and I've left some of them open for a while. Some have hardened and some are still as viscous as when I bought them. I'm not sure why, but just what I've noticed.
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Re: Designs B/C

Postby dholdgreve » April 20th, 2015, 9:01 am

bernard wrote:
DoctaDave wrote:
dholdgreve wrote:It sounds like you may have misinterpreted what I meant when I said that the cyanacrylate glue can get old and brittle... I meant that this can happen after he bridge is assembled, the glue has dried, and is now exposed to the atmosphere... Not that the shelf life of a bottle is only 3 to 4 months... I believe that, kept properly sealed, a bottle of glue is viable for a couple of years or longer.


Does CA glue actually get dry and brittle? I'm pretty sure CA glue does not dry with evaporation as many other glues do, but it actually reacts with the humidity in the air and then polymerizes for a permanent bond. So once it sets, I'm not sure taking away the moisture would really affect the performance of the glue, and remember that the strength of a joint is ultimately dependent on the face of the wood. I think even a joint that has been made over a year ago would still fail as a result of the wood face pulling off instead of the glue getting old.

Would be interesting to do some tests and leave some joints for a while. I might do that if I ever get bored. I've had several bottles of CA glue this season and I've left some of them open for a while. Some have hardened and some are still as viscous as when I bought them. I'm not sure why, but just what I've noticed.


I have no empirical data to support that claim that C/A glue will get brittle over time, but I have seen it fail multiple times after being set on a shelf for a few months, after having held without any issues 2 or 3 times when first built. This, with no additional sanding or any other modifications, and yes, the joint actually split apart with both piece of wood intact... totally glue failure.
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Re: Designs B/C

Postby AnonymousMouseRI » April 25th, 2015, 6:55 pm

How has the pratt truss worked out? We tried it at states (even though we got it together in about a day), and it only held 17 lbs. We have to finagle a new design for nationals. Any ideas? I am sort of at a loss.

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Re: Designs B/C

Postby someusername » May 6th, 2015, 8:46 am

AnonymousMouseRI wrote:How has the pratt truss worked out? We tried it at states (even though we got it together in about a day), and it only held 17 lbs. We have to finagle a new design for nationals. Any ideas? I am sort of at a loss.


I have found quite a lot of success using the pratt truss (because the way I think of it, since the longer beams are in tension rather than compression, you don't need them as big/dense, so you can save a little weight using them) but I would say just use what has worked for you in the past. I have learned the hard way the consequences of changing your design just before a big tournament.
simplicity is key...sometimes


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