losing weight

dnLeeK
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Re: losing weight

Post by dnLeeK » March 17th, 2011, 9:52 am

Our tower weighed 12.3ish and held all 15, which was good, but we're looking to improve that a bit. I don't want to mess with the cross-members, because the last time I did that, the tower buckled early and we got an efficiency of 4. -_- Anyways, we were looking into maybe giving the top half a taper to reduce mass, would this be prudent? I'm worried about stability, but theres not much else we can do to reduce weight at this point.

hogger
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Re: losing weight

Post by hogger » March 17th, 2011, 2:06 pm

It is hard to know what you mean without seeing a picture or more detailed description of your design, what kind of wood, glue, etc... An easy way to lose weight is to use lighter wood. With 12.3 grams, there should be plenty of places that you can use lower density wood especially if you are using balsa wood. Design of the tower is only half of the battle, the other half or maybe more is selecting the right dimension, density and quality of the wood for each piece in the design.

dnLeeK
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Re: losing weight

Post by dnLeeK » March 18th, 2011, 2:51 pm

Our design is a square pyramidal base, along with a top that has a slight (<half a centimeter) taper to it. We use 1/8x1/8 for the sides and 1/16x1/16 for the cross bracing. We have 1/8x1/16 connecting the feet at the very bottom. The dimensions are such that it just fits the opening when placed diagonally. Looks like http://gallery.scioly.org/details.php?image_id=3311 but with less cross bracing: we use 2 levels of x cross bracing on the bottom and 4 on the top. I'm not sure where we could use the lighter wood, because our tower broke literally right at the 15 kg mark at state. I try to pick sturdier balsa for the 1/8x1/8 because those members are under the most compression. Giving the top more of a taper would save some weight, but I'm worried it might cause issues with the loading block and also that would require a separate jig for the top section. I could also save around half a gram to a gram with more precise glue placement, but I don't think that's enough.

hogger
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Re: losing weight

Post by hogger » March 18th, 2011, 3:10 pm

dnLeeK wrote:Our design is a square pyramidal base, along with a top that has a slight (<half a centimeter) taper to it. We use 1/8x1/8 for the sides and 1/16x1/16 for the cross bracing. We have 1/8x1/16 connecting the feet at the very bottom. The dimensions are such that it just fits the opening when placed diagonally. Looks like http://gallery.scioly.org/details.php?image_id=3311 but with less cross bracing: we use 2 levels of x cross bracing on the bottom and 4 on the top. I'm not sure where we could use the lighter wood, because our tower broke literally right at the 15 kg mark at state. I try to pick sturdier balsa for the 1/8x1/8 because those members are under the most compression. Giving the top more of a taper would save some weight, but I'm worried it might cause issues with the loading block and also that would require a separate jig for the top section. I could also save around half a gram to a gram with more precise glue placement, but I don't think that's enough.
From your description, I can point out a few problems of the top. First 2 levels of x braces for bottom and 4 for the top is bad. That is probably why you broke with such heavy wood. The ability to withstand compression force of a balsa stick decreases exponentially with the distance between bracing point. Try to take a stick of balsa and try to break it by pressing down vertically, try it for 4 cm stick and then 7.5 cm stick. Look at the picture you attached, use a lot more bracing levels and much lighter wood (especially the bracing wood) and you will be more successful.

Good tower should be sub 7 grams, which means the balsa stick for the posts of the tower should be medium to light grade (about 1.2 grams per 36 inches 1/8 x 1/8 stick). The bracing sticks can be a lot lighter and depending whether it is compression or tension, you can vary each piece quite a bit.

SLM
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Re: losing weight

Post by SLM » March 18th, 2011, 6:53 pm

hogger wrote:
dnLeeK wrote:Our design is a square pyramidal base, along with a top that has a slight (<half a centimeter) taper to it. We use 1/8x1/8 for the sides and 1/16x1/16 for the cross bracing. We have 1/8x1/16 connecting the feet at the very bottom. The dimensions are such that it just fits the opening when placed diagonally. Looks like http://gallery.scioly.org/details.php?image_id=3311 but with less cross bracing: we use 2 levels of x cross bracing on the bottom and 4 on the top. I'm not sure where we could use the lighter wood, because our tower broke literally right at the 15 kg mark at state. I try to pick sturdier balsa for the 1/8x1/8 because those members are under the most compression. Giving the top more of a taper would save some weight, but I'm worried it might cause issues with the loading block and also that would require a separate jig for the top section. I could also save around half a gram to a gram with more precise glue placement, but I don't think that's enough.
From your description, I can point out a few problems of the top. First 2 levels of x braces for bottom and 4 for the top is bad. ...
I agree. The legs carry more compression force than the members in the chimney part of the tower. Therefore, assuming you are using the same size wood for the main compression members for the chimney and the base, the legs need to be braced more frequently than the main chimney members.

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