Boomilever B/C

[fleet130]

If you go to the store and ask for "1/4" washers"- the OD is 3/4". The hole in them - the ID - is actually a bit larger than 1/4" (~5/16")

Bolts, nuts, washers, etc. are manufactured to meet various standards. I don't have the actual standards on hand, but you can see the sizes in this chart. You can find similar info for nuts, bolts and other hardware.

[Balsa Man]

If you go to the store and ask for "1/4" washers"- the OD is 3/4". The hole in them - the ID - is actually a bit larger than 1/4" (~5/16")

Bolts, nuts, washers, etc. are manufactured to meet various standards. I don't have the actual standards on hand, but you can see the sizes in this chart. You can find similar info for nuts, bolts and other hardware.

Thanks for the link. Interesting. A little follow-on on this subject - for both competitors and event supervisors..

The rules call for "3/4 inch diameter flat washers"
From the table in the link, USS spec "1/4 inch" washers are not quite 3/4" OD; at 0.734", 0.016" smaller. Note, too, that SAE spec washers are a lot smaller- 1/8th" smaller; they are obviously not the correct/intended one. Don't buy them. It might be possible at a fastener specialty shop to find a 0.750 washer, but it seems reasonable to assume USS spec washers are what is intended.

You'll see when you go to Home Depot, they have, in bins, both 1/4 cut washers, and 1/4 flat washers. They're USS. They also have little bags of SAE 1/4.

Anybody bothered to look closely at a washer?
Interestingly, when you throw the caliper on the cut and flat, the flat is slightly larger; 18.6 mm, vs 18.7mm; 0.0039 inches larger; does that 4/1000th of an inch make a difference? I don't think so; can't imagine a design/build where if you used one, and at the competition had to use the other, it would be a problem. But the difference is there.

There's something more interesting, that could have... a small but measurable performance implication- if you're trying to be seriously competitive; playing with small variables, looking for little increments of improvement. Take a close look at your washer(s). Both the "cut" and "flat" washers are not flat. Get a good straight edge; steel ruler, razor blade; put the flat sides against the edge, and look with a bright light behind. On one side, there are tiny lips, around the OD, and around the center hole; maybe 1 or two thousandths above the flat surface. It appears that the washers have been made by stamping them out of a sheet of steel with a die. On the "front/upper" side, the edges (outside and center hole) are pushed down/rounded; let's call this the rounded side. On the "back/lower" side, the metal is pushed out, forming sharp edges, and the noted "lips" let's call this the sharp side. If you put the sharp side against a straight edge, and assume the little sharp lip will push into wood if it's tightened against an attachment plate, the "contact diameter" is 18.6 (or 18.7)mm. If you flip it over, though, the contact diameter is only 15.4mm. Not a lot, but significantly less.
So, how does it matter that the sides are different?

First, looking at the tension member attachment to one bolt; conventional attachment plate held to the wall by bolt/washer; tension member(s) glued into the plate. Could be one tension member, could be two on either side of the washer. With the tension member(s) out beyond the edge of the washer, when loaded, there will be a force trying to bow the plate-pull the side(s) with the tension member(s) attached away from the wall. With the rounded side of the washer against the plate, it will be able to bow more. How much, depends on how far beyond the washer the T-member is mounted, and how stiff the plate is...? 1/2mm....2mm. That'll cause two things; effective lengthening of the member(s), letting the boom/compression member droop, and a twisting force applied to the T-member, and it's glue joint in the plate. It will fail at a lighter load than it would if the washer were mounted with the sharp side to the plate.
Second, if you're using 2 T-members mounted on 2 bolts, an additional factor- if the two washers are mounted with opposite sides against the plate, in addition to the distortions at the one mounted rounded side to plate, the asymmetric pull of the two tension members will put torsional (twisting) force into the compression member(s), again, leading to earlier failure.

Enough on washer minutia.....

[retired1]

I tend to use stainless as it is smoother and flatter.
If you are a purist, you might be able to find a 19mm washer.

[JimY]

Today I looked up and found that the winning boom at nats in 2008 weighed 6.53 grams and did go to full load. Efficiency was therefore was just shy of 2300. I did not see it, so don't know if it had one tension arm or two. I did see the boom that ended up getting 2nd in 2007. It had two tension arms and rather unusual upside-down Pratt or Warren trusses for the compression beam. I recall that it finished well behind first place in efficiency score and just ahead of third.

[Balsa Man]

I tend to use stainless as it is smoother and flatter.
If you are a purist, you might be able to find a 19mm washer.

True on both counts, but....
A point of my discourse, that maybe I didn't highlight enough - in competition, you have to run with what the ES provides.
The....reasonable expectation for that is "1/4 flat" (USS) washers from Home Depot/Lowes, etc. Given that, you want to design and test using them.

Today I looked up and found that the winning boom at nats in 2008 weighed 6.53 grams and did go to full load. Efficiency was therefore was just shy of 2300. I did not see it, so don't know if it had one tension arm or two. I did see the boom that ended up getting 2nd in 2007. It had two tension arms and rather unusual upside-down Pratt or Warren trusses for the compression beam. I recall that it finished well behind first place in efficiency score and just ahead of third.

Thanks for the info, JimY.
Still curious, of course, if the winners - or others that did decently, managed to run a single T-member. Hopefully, there's someone on the board that saw/remembers... RJM?

[juicemanman]

So, just to clear it up, there has to be 3/4" between each tension member on the base right? Because of the 3/4" washers?

[fanjiatian]

Probably, although I don't see why you'd have a boomilever that narrow.

[fanjiatian]

Is there a way to draw designs on Microsoft Word to scale?
(I don't have Autocad or similar software)

[Rath4o1]

I use AutoCad however I think you can draw things on Microsoft publisher?

[retired1]

So, just to clear it up, there has to be 3/4" between each tension member on the base right? Because of the 3/4" washers?

For many, that would be a minimum, but you could go above the washer and have it a bit closer together. Right now, the main advantage would be a slightly better angle (vector) than dead abeam the bolt. I guess that you could reduce bracing by only having 1 - 2 cm separation on the compression members.