how much you can you remove and still be strong???

[LCHIEN]

so how much can material can you remove from a 1x2 to reduce weight and keep the same stiffness and strength?

Something done in metal trusses all the time: skeletonizing
Possible Main beam for a trebuchet...
Skeletonized by drilling 7/8" holes every 1.25" (32%) and the edges 3/8" rounded over, (10.6%) - 42.6% weight removed; still seems quite strong...

Where else can you take stuff off? I think two 1/4" holes off center between the big circles will save another 5% to make nearly 50% weight savings.

[Uncle Cracker]

Seems to me that the resultant strength would depend a great deal on the graining structure (and imperfections) in the specific piece of wood. I have no idea how one could work those factors into the equation.

[Wood_workur]

Unlike metal (as far as I know), wood tends to fail in a particular direction- with the grain. Seeing as you plan on using it for a trebuchet, I'd be very careful about removing more seeing as its going to have to support the weight (and motion) of the counter weight, I could see it splitting the 1x2 right down the middle.

Also, since the moment of inertia is directly proportional to the mass, how much of a performance increase is an additional set of holes going to add to say, running it through the planer, or making it shorter/narrower? I'd be willing to be planing it down to 5/8" would keep it stronger and would cut the mass to about 52.2%

[cabinetman]

so how much can material can you remove from a 1x2 to reduce weight and keep the same stiffness and strength?

It stands to reason that nothing can be remove and keep the same stiffness and strength.

Something done in metal trusses all the time: skeletonizing
Possible Main beam for a trebuchet...
Skeletonized by drilling 7/8" holes every 1.25" (32%) and the edges 3/8" rounded over, (10.6%) - 42.6% weight removed; still seems quite strong...

Properties of metals differ from properties of wood. What would remain after removal of sections IMO would be neither linear or exponential in comparison to unfettered wood. What remains may be subjectively calculated on certain properties that are generally applied to the specific species, and taking into consideration its immediate properties which may affect its performance or reaction to certain stresses. Those conditions and stresses may include, compressed, lateral, and torsional forces, shear strength, and load equivalencies. Other conditions and facts may include the specific MC (moisture content), and temperature. The details will vary among individual samples.

Where else can you take stuff off? I think two 1/4" holes off center between the big circles will save another 5% to make nearly 50% weight savings.

Where you take weight off, may be directly proportional to the loss of inherent properties. Situations that utilize skeletonized wood (as in perforation, suggesting jointless), are being used in applications where the size of the wood, either in thickness or width is appropriate as a spacing media, as in aircraft and marine bulkheads.
.

[waynedye]

Test it

I fear that the wood with holes is not as strong as you think. I MIGHT be okay with those holes gone and rounded, but that splitting is a common problem with overloaded wood.
Try clamping a piece at each end, then hang heavier and heavier weight on it. One way would be with a hanging scale and something like a duffle bag loaded with tin cans.
Do the same test with an untrimmed piece of 1x2 and a piece trimmed as you are trying, then let us know what you learned.

But be careful to get nice straight-grained pieces that are comparable. With a lot of the 1x2s I get at my building supply superstore, there are plenty of flaws that weaken it.

Wayne (once a civil engineer but now I forget stuff)

[cgallery]

I agree with the comments regarding grain. I'd laminate both sides with something like mica before removing the material. A possible alternative would be to use a high-quality plywood, but that may increase weight.

Interesting, though, I like where you are going with this.

[drillman88]

Would something like a Ibeam structure work. The ones they use for floor joists are really strong and light.
You may need to get creative to get the look you want but I would think the spine of this set up would be extremely resisistant to the types of forces that this will be subjected to.

[rnelson0]

Why do you want it lighter? A trebuchet gains power from size and the angular momentum it imparts to its load. If you are doing this in wood, an oversimplification of the relationship would be "greater weight = greater force". You want a big honking main beam. It has to hold the counterweight at one end and launch the projectile at the other. That is going to impart a lot of force along the length plus two distinctly sharp forces in opposite directions at either end, plus a significant force wherever your stop is. Three of those forces are going to be applied at nearly the same instant.

The only way I see a skeletonized beam withstanding these forces is to make sure that the beam is whole at the base, at the top, and where it will hit the stop. There is risk to removing parts anywhere (the torque between the stop and the top will be significant and can crack a solid beam), but I think you would have the best luck between the pivot and the counterweight, although that is a fairly short section. Not sure that the savings are worth the risk.

BTW, you *have* seen those videos where they use shaped tree trunks for the beams and they snap anyway, right?

[LCHIEN]

i'd only skeltonize it between the projectile and the axle, not the counterweight and the axle.

The weight of the beam to the projectile is a is larger than the mass of the projectile, we could transfer more momentum to projectile with a lower mass beam. It its too thin then it gets floppy, hence not wanting to go too thin - I was also considering ripping it in half. the tall way (becomes 3/8" x 1-1/2") - some plans call for a tapered beam, thicker towards the axle (base) and thinner at the projectile end.

[LCHIEN]

what about this? Seems pretty strong and stiff and only 34% of the weight of the solid 1x2.

According to the Treb modelling program a 50% reduction in beam weight can create a theoretical 20+ percent range improvement for my sized projectile and CW.

In weapons, range is everything. When fighting a battle do you want to be the side with the 100 yard bow and arrows or the 120 yard range bow and arrows?

RNelson: the Treb gains distance by converting potential energy to kenetic energy. and then getting the KE from the weights to the projectile. The bigger the beam mass the more KE goes into moving it rather thant he projectile. A zero mass beam is ideal but not doable, the max performance is to make the beam only as strong as it has to be and no more. A stiff beam transfers more KE to the projectile than a floppy one. So light and stiff are the keywords.

[pierhogunn]

can you wrap that in a single layer of fiberglass?

or what about laminating several 1/8" strips of ply together, and then removing material

ply / fiber / ply / fiber / ply / fiber / ply

[cgallery]

I think it should be pretty strong. I guess the weak point is the thinner wood between the holes, but I suspect that it will be plenty strong for its intended application.

[tseavoy]

Just try it. If it breaks, make it bigger.

Tom on Marrowstone

VIRTUES, n.pl. Certain abstentions.

[Kristofor]

First you go to your college physics store and purchase the massless, frictionless, deflection-free components, then....

How will the beam be mounted to the fulcrum? Perhaps you can taper (length & cross section) or otherwise have a non-uniform beam to minimize bulk/rotational inertia from areas that will be under less stress.

[LCHIEN]

First you go to your college physics store and purchase the massless, frictionless, deflection-free components, then....

How will the beam be mounted to the fulcrum? Perhaps you can taper (length & cross section) or otherwise have a non-uniform beam to minimize bulk/rotational inertia from areas that will be under less stress.

It's a two-piece beam, the part supporting the coutnerweights is stronger and doubled up. Some designs sport tapered beams. To do that you need to be able to calculate the width at the fulcrum and the width at the tip for the strength you need. Overall, my perforated beam will fairly light but stiff, I hope I don't need to taper it.