That's a good question. Let me throw out a few things to think about. I'm not coaching a team this year, so these thoughts are not from actually building/testing. This is coming from thinking about how to translate something that worked last year (with bolt and washer), to the J-hook of this year.iwonder wrote:Here's another question, I was thinking about trying a tube boom this year, and I fully understand how they could've been made last year, but I could never figure out how to handle a base this year. The only thing I could think of is a dowel and you would have to have two tension members the went into the distal end as one member, but split and attached to either side of the base. Is there some other method I'm not thinking of?
What we used last year was a "lock block" on the underside of the tension member. It provided a way to "hook" the T-member on the top of the washer. T-memb was 3/16ths" er 1/4" wide by about 1/32nd thick. Lock block 1/4" wide x thick x 1/2" long. The face of the block contacting the washer is cut at an angle, so that when the T-member is sloping down toward the distal end, the face is vertical. That means it's in full contact with the backside of the washer. The washer is held vertical by a 1/2" thick, end grain, light balsa. Tightening the bolt against the washer and block be clamped the washer vertically in place. A sloped 1/4" wide slot in the top of the balsa block allowed the tension member to sit down in the block so that the top edge of the washer was almost in contact with the underside of the tension member. Tension member done in hickory, lock block in oak. Two 1/32" diameter oak pins through the tension member and lock block.
The factors you have right for a joint like this to work are the shear strength- parallel to the grain - of T-member and block and glue area. It's all about force per unit area, e.g.pounds per square inch; the larger the glue area, the lower the
shear strengths can be. These woods and glue area worked for a C-boom; tension load about 90 pounds; about 3/4 of that for a B- boom. The reason for a flat/strip T-member was to increase glue area.
So, finally, to how might one adapt this to a J-hook??
What have you got to lock against?
The most obvious thing is the inside of the J.
Hmm. to do that, you would have to turn the T-member on edge- where the 1/4" dimension is vertical, rather than horizontal. I don't see a reason that can't be done; might just be worth checking out. The face of the lock block against the inside of the J would have to be curved to match the radius of the J curve; that's not a problem. Probably would have to file a 1/8th" radius groove; to keep the block from slipping sideways.... that's not a problem.
A nice thing is it's pretty easy to test T-member+lock block and joint- hang it vertically.... don't need to do whole structure testing.
and yeah, I was planning on having to buy a new one each time :/