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Aia's method has worked well for me so far.

iwonder wrote:Aia's method has worked well for me so far.

So you've been sticking the tension members into the base?

ptkid wrote:So for attachment of the tension members to the base should I stick them on the two outer edges of the base or should I drill to holes inside of the base and stick the tension members in there? I have tried the sticking on the outside approach so far using Loctite Ultra Glue Control and almost every time it breaks there. The maximum weight I have gotten has been around 6.8 kilograms and it breaks in the same all the time. What has been working for you?

The situation/problem you’re running into – tension member pulling out/off the bolt attachment plate/block is the …..”biggie” for booms; the most common failure mode. The results iwonder posted on the Ongoing Contest(Scores) thread yesterday reflect the same challenge. Aia’s wiki/guide information highlights this challenge, too.

In almost all cases, if you look at the attachment plate/block, and the tension member, you will see bits of wood from the block/plate on the tension member, or wood from the tension member on the block/plate. This failure mode is called shearing; specifically shear parallel to the grain. Going back to towers last year, there is no joint in a tower with shear forces even beginning to approach what’s going on in the tension member attachment of a boom- a couple kg at most, compared to ~22kg each for a C-boom w/ 2 tension members (15.5kg for a 2 tension member B-boom). Double those numbers for a single tension member. Even going back to elevated bridges, on the bottom/main tension member in a truss, maybe 10kg. There are two key variables at work- things you can adjust/modify to make this joint hold up to a full boom load; the glue area (if the surface area where you have glue is larger, the shear force is distributed/reduced; e.g., double the area, cut the shear force in half), and a mechanical property of the wood; it’s shear strength parallel to the grain. Increasing glue area is a straightforward problem

So, some information on shear strength in wood, and a couple ideas to play with that may help solve this problem:

Mechanical Properties of Wood http://www.fpl.fs.fed.us/documnts/fplgt ... ter_05.pdf
A few shear (parallel to grain) strength numbers from this, in kilo Pascals (kPa). The units don’t really matter; it’s the proportions; the relative strengths for different woods that do:

Balsa- 2,100 (this value is shown for wood with a specific gravity of 0.16- in pounds per cubic foot, that’s about 10- a bit below the middle of the range commonly available (5-20 lbs/cu ft). 20 lb/cu ft is probably on the order of 4,000 kPa shear strength)
Bass- 6,800 ( 1.7 to 3x that of balsa; the density associated with this value is a specific gravity of 0.37. For a 3/32nds” square bass, this would be 1.3gr 24” stick. That’s a bit below the average density. With some searching/weighing/selecting, you should be able to find some 24” sticks up toward 1.7 to 1.8 gr; their shear strength is probably on the order of 7,000 to maybe 8,000 kPa)
Oak- 12,000 to 14,000 (pushing twice that of bass; maple and hickory are in the same range)
Douglas Fir, Spruce, and Pine- 8,000 to 10,000 (a bit stronger than bass.

For comparison, shear strength for CA glue is typically in the 10,000 to 12,000 kPa range- significantly stronger than balsa or bass; in the range of oak.

How to use this information to improve your tension attachment joint will depend on what you’re using, and where you’re at.
First, if your attachment plate/block is thin- a 16th, an 8th, even a ¼, try taking it out to a full ½”. If you’re using balsa in the plate/block, it should be “end-grain oriented”- the grain running parallel to the bolt or tension member, or in between; shear strength perpendicular to the grain is a LOT less.

Second, if you’re using balsa in your attachment plate/block, and bass for the tension member(s), premature shear failure will be in the balsa (pieces of balsa attached to the tension member). Higher density balsa can help, but it won’t get you to the shear strength of bass. Higher density balsa, of course, would mean more weight. Very low density balsa, in an ‘end-grain sandwich’ (with thin –like 1/64th –high density walls, against the attachment wall, and on the bolt head/washer side) will get you the stiffness needed to hold the tension member(s), at a low weight; the question is, how do you get one or more of the faces in the hole for the tension member in that block/plate “lined” with wood with a higher shear strength than the low density balsa? If you make the hole bigger, and insert, say 1/64th “liner” pieces, the weak shear plane will just ‘move out’ to the outside of the liner pieces- the balsa outside them will shear away. However, if you do two things, you can overcome that problem. You should already have the tension member(s) running as close to the washer as possible, with the tension member just touching/clearing the washer. If you put “liner” pieces on the tension member, so the edge of the liner “catches” on the back face of the washer, the washer will hold/block it- prevent it from being pulled- in the direction of the tension; prevent it from being sheared at the balsa. Given the shape (when you look closely) of the edge of the washers, a 1/64th” liner piece will probably not fully/cleanly engage the face of the washer. However, if you make it tapered, so the thickness is….a 32nd, or maybe 3/64ths at the end that engages the washer, it will firmly “lock” against the washer. The steel will hold it in place against the tension/shear force, and the strength of the joint in shear becomes the shear strength of the liner plate (or the tension member, whichever is less).

Of this gets you close, but not to full load capability, two options. First is adding a shear pin- a very small diameter (like 1/32nd”) hardwood pin; you can make by sanding/filing down an oak splinter; drill through liner plate(s) and the T-member after they’re glued together, put glue on the pin and in the hole, push it through (get a nice tight fit). Second option is going to hardwood for tension member and liners. We’ve been able (as in have successfully tested) a single, hardwood tension member; it and the attachment joint tested to 125 pounds; the T-strip is ¼ wide x 1/32nd thick, at a very competitive weight.

So, food for thought….

Admittedly I've only had time to read through your post once, but I was wondering, why would you use end grain balsa? Wouldn't end grain be weaker in shear in this application, and isn't there a buckling type force on the member(if you can get the tension members close enough to the washer this goes away) that end grain also couldn't handle? Right now my base has grain running parallel to the ground when the boom is mounted, it seems like this would hold the 'buckling' force better.

iwonder wrote:Admittedly I've only had time to read through your post once, but I was wondering, why would you use end grain balsa? Wouldn't end grain be weaker in shear in this application, and isn't there a buckling type force on the member(if you can get the tension members close enough to the washer this goes away) that end grain also couldn't handle? Right now my base has grain running parallel to the ground when the boom is mounted, it seems like this would hold the 'buckling' force better.

Go back and look at my post to HenryHscioly on 11/30- where he was using a thin plate, and having bowing problems on either side of the bolt. When you say "buckling force on the member", I assume you mean bending force on the attachment plate/block- from the tension members pulling on both sides of the bolt- the same he issue he was seeing. It has to be in a sandwich to work- to get the increased stiffness- it's the outer thin sandwich layers, and the distance between them, that give you the stiffness- the balsa just gets/holds the distance between the plates.
By having the balsa in end grain orientation (grain running perpendicular to the wall), the tension member is running essentially parallel to the grain, giving you max shear strength (actually, depending on how you orient the grain), it may be fully parallel (if you line up the grain with the angle/plane of the T-member), or slightly off (if you line up the grain with the bolt). Either way, much more shear strength (on the order of 3x) than perpendicular to the grain (if you have grain running parallel to the wall).

If your base is laminated, you could also experiment with grain direction by making one piece horizontal and the other vertical. If one piece is thicker than another, I would suggest making the thicker piece have the horizontal orientation. I think Balsa Man's suggestion should work the best in theory, but this is another possible route. So many variables to test in this event!

Has anyone thought about using Sitka Spruce instead of balsa? It's significantly heavier but it's also crazy strong. They sell it on National Balsa.

It's really just a question of strength to weight ratio, all you can do is try it and find out

iwonder wrote:It's really just a question of strength to weight ratio, all you can do is try it and find out

Yup, structural efficiency is all about strength to weight ratio.
Trying and finding out is one way to go, but not the only; you may save time and effort by some research time on wood properties (see link I provided recently). Looking at relative strengths (for tension and compression) and relative densities will help you decide if a wood selection makes sense; twice as strong, at twice the density is an.....even tradeoff (if you go at half the cross section for the heavier option, except, your glue surface area goes down- which may, or may not be a problem.

The other thing - major variable - you need to consider/take into account is grain structure; if grain planes run parallel, and are continuous (instead of slightly diagonal, running out to edges of a piece), it will be significantly stronger. Ordering online, you may find some of the pieces are "good'and others not so good. The other grain aspect that makes a difference is the number of grain planes (as in growth rings)- more means more stronger/continuuous fibers, means more strength. I mention this regarding spruce because of something we ran into. The time before the last time S-O ran booms, my older son used spruce for tension members (~1/4" x 1/32nd)- worked great. Got them from Hobbytown. Next time, my younger son got some- we did some tension testing- significantly weaker. Looking at the couple of leftovers from older son's, we saw they had 4-5 grain lines/growth rings. The newer ones were 2-3; much more.....open-grained.

I've read through most of this and found it all very useful and interesting, but there is one thing I haven't seen addressed (and if it has I missed it), but what are the average distances you guys have been using between your two long trusses on a tension boom? I am looking for a starting point, so even if you've nothing to back up your chosen distance, it'd be a useful starting point. Thanks!