Boomilever

Overview
A Boomilever is a device built to hold a specified weight a given distance from a vertical surface. Rules include a maximum vertical height for the device, how far the device must hold the weight from the vertical surface, the max weight, and the mode of attaching the device to the wall. It falls into the same category of building events as Elevated Bridge and Tower.

2012-13 Construction Parameters:


 * Constructed only of wood and bonded by glue
 * No limits in the cross-sections of wood pieces used
 * Attachment to testing wall by attachment base and 3 bolts
 * Distance between left and right bolts: 20 cm
 * Distance of center of loading block from wall: 40-45 cm
 * Maximum vertical distance between mounting holes and loading block: 25 cm
 * Maximum vertical distance between attachment base and points of contact on the wall: 20 cm (Div. B) or 15 cm (Div. C)

Additional Rules:
 * Loading block is a 5.0 cm x 5.0 cm square x 2.0 high with a hole in the center for a 1/4" eye bolt with washer/nut, from which a hook and the plastic bucket will be suspended.
 * Three 1/4 in x 3 in bolts with 3/4 in washers and wing nuts will be provided for attachment of the device to the testing wall

The definitive rules are located in the official rule book. Know your rules front to back. You can never read the rule book too many times.

Boomilevers undergo many different stresses than a tower or a bridge because of the single attachment point. Usually built of a combination of balsa, bass, and/or spruce, the structures have been known to be very competitive when weighing 20 grams or less while holding the max weight.

Scoring
Objective: To build a boomilever with the greatest efficiency. Efficiency is calculated as Mass held/Mass of boom Broken down into 4 tiers as follows:
 * 1) Boomilevers that meet all construction parameters with no competition violations.
 * 2) Boomilevers with 1 or more competition violations.
 * 3) Boomilevers with construction violations; may also have competition violations.
 * 4) Boomilevers that cannot be loaded for any reason.

Boomilevers in the first three tiers will be ranked by their efficiency score (Load Supported (grams) / Mass of Boomilever (grams)); those in the last tier will receive participation points only. Ties will be broken by (in this order) 1. greatest clearance from contact depth line and 2. lowest boomilever mass.

Basic Design
There are two basic types of boomilevers. First is the tension boomilever, where the tension chord is longer than the compression chord.



Then there is the compression boomilever, which is the opposite of the tension.



It is generally accepted that the tension boomilever is inherently better than the compression. This is because the strength of wood in compression decreases per length unit the longer the piece of wood is. However, wood holds the same strength in tension no matter the length of the piece. In the compression boomilever, the compression chord is longer than in the tension boomilever.

Height
The taller the boom, the less force is put onto the top and bottom chords. This allows you to decrease the weight of these chords. However, the taller the boomilever, the longer the truss pieces must be, which increases the weight. The goal is to find a balance. My suggestion is to not go any shorter than 3-4 inches. I do not, however, believe you need to use the entire 20cm. Plug your design into the Bridge Designer and see how the numbers change as you increase the height of the boomilever. Another person (bah) says that you should use as much of the 20 cm as possible. You save a negligible amount of weight by doing otherwise.

Trusses
Most boomilevers have what is called a truss. There are three standard trusses, the Warren, Pratt, and Howe. Each of these trusses have different ways of spreading out the load, and it is worth playing with the bridge designer to see how each of these work. Remember the KISS principle, and keep the truss simple.

Warren Truss


The basic idea behind a truss goes back to the tension verses compression boomilevers. A piece of wood loses strength as the length increases. Thus the truss serves to divide the compression chord up into smaller sections, greatly increasing the amount of load before buckling. Basically the truss keeps the compression chord from bending and twisting. As a starting place, I would try and break the bottom chord into sections 2-3 inches long.

Pratt Truss


A Pratt truss includes vertical members and diagonals that slope down towards the center, the opposite of the Howe truss. The interior diagonals are under tension vertical elements under compression. They are statically determinate bridges, which lend themselves well to long spans.

Howe Truss
The Howe truss is the opposite of a Pratt Truss. The diagonal web members are in compression and the vertical web members are in tension.

Lenticular Truss
The rarest type of truss among the boomilevers, a lenticular truss includes a lens-shape truss, with trusses between an upper arch that curves up and then down to end points, and a lower arch that curves down and then up to meet at the same end points. One type of lenticular truss consists of arcuate upper compression chords and lower eyebar chain tension links.

The Tower Crane Design
bah is fond of the tower crane design. Instead of the trusses connecting the tension and compression chords, the tension chord is free to hang all by itself; the only connections are to the distal end and the attachment base. The trusses instead form a box compression chord design serving to resist the twisting tendency of the compression chord due to the loading. Basically design a truss for the compression chord that would reliably resist the twisting and bending of the compression chord due to load. I like a box 5 by some number of cm, with only one side of lateral bracing.

Base
Most boomilever bases I have seen through the years are way overbuilt. Anybody should be able to make a base under a gram, and less than .5 of a gram if you are good. Obviously, you will not be able to achieve this if you are using both bolts to attach your boomilever to the testing wall.

I honestly cannot see any reason to use all three bolts. If you are worried about stability, I say look at the bridges of the past couple years. Most were no wider than 5cm and had no problem with stability. You will cut a lot of weight by only using one bolt, not only on the base but also with the lateral bracing.

A nice attachment base snugly accommodates your tension sticks and the attachment bolt and washer. Making your own laminated balsa sheets work well in this. Use three 1/16 inch sheets and cross grain them, or even use like five 1/32 inch sheets. A nice glue for attaching the tension sticks to the attachment base is the preeminent Gorilla Glue manufactured in the great country of Denmark. Don't forget to wet the wood so the glue expands nicely.

Lateral Bracing
Don't let the term turn you off. Lateral bracing is simply a horizontal truss connecting the compression chords of your boomilever. Like any normal truss, there are various designs you can use. And the goal is the same for lateral bracing as is a normal truss, to keep the compression chords from bending and twisting.

You generally do not need a lateral truss connecting the tension chords of your boomilever. The tension chords do not bend, and only twist under very extreme force. If anything, you can use a couple perpendicular pieces to connect them.

Glue
Glue weight can be a major weight factor, but it doesn't have to be. You can easily use 0.1-0.5 grams of glue (or less) on a 10-gram boomilever. A lot of people use CA glue, or ambroid. I use an interesting glue called Weldbond found at Ace Hardware. Find a glue you can use well and stick with it. Make sure not to use too much.

A certain person (bah) likes to use the thin CA glue and he uses insulin syringes to precisely dispense it. One drop is sufficient for a lot of joints. If you feel unsure, you can double up or triple up. Two drops from the insulin syringe is .01 grams, I think. Gorilla glue is nice for the attachment base, as mentioned in the base section.

Wood
The two varieties most heavily favored are basswood and balsa wood, both which can likely be purchased at a hobby store. You should proudly march into your favorite hobby store that stocks balsa and basswood armed with a hundredth gram precision scale and weight all the size sticks you desire.

Lightest balsa sticks by the yard: 1/16 * 1/16 sticks are around .3 grams. 1/8 * 1/8 sticks are around .7 to .8 grams. 1/4 * 1/4 sticks are around 3 grams

Expert building for the compression chord with the lightest possible balsa from the store will yield very competitive boomilevers.

I do not have the lightest basswood stats on hand. Using the lightest 1/16 * 1/16 basswood sticks for the tension sticks may result in premature failure. Please experiment.

It is often a good idea to buy sheets of balsa/bass and cut strips/sticks to a size using a balsa stripper instead of buying sticks of sizes because it gives greater flexibility with sizes, its also generally cheaper than buying sticks and sheets are more consistent in terms of density than sticks unless the sticks are carefully sorted.

Basically, everything should be made of balsa except for the tension sticks which should be basswood. This shouldn't imply that this is the only way to do things. We don't intend to stifle creativity. However, do remember that balsa wood is better at compression then tension.

Tools
The design of the boomilever is usually very simple. The problem is that a lot of people do not know how to build well. Building well is imperative to being competitive at the boomilever event. A suggested way to build, from bottom to top: Your table -> corkboard -> Paper template -> polyethylene plastic sheet (from a plastic bag) -> the wood pieces to be joined -> thumbtacks Straightness is key. Check straightness with a long straightedge, like a T square. I also draw my templates with pencil on paper. I do not use CAD software to do so, but you are free to do so if you want to Have the wood pieces at the correct places then apply glue on it. Really make sure that its right before putting glue on.

The Easy Cutter from Midwest products is very useful for balsa events. Midwest Products Easy Cutter So is the X-Acto knife. A nail file is also important. You cut a little longer than you need, and you file it down to the exact length you need.

Gluing
Lap joints are great for the boomilever, or for any balsa event for that matter Explanation of Joints

Gaps in glue joints are EVIL. Use like the eraser on a pencil to push down on the wood when you glue so there are no gaps. Minimizing Weight Gain While Maximizing Strength When Gluing Balsa

The Dreaded Distal End
According to the esteemed Bob Monetza: "You'd want to get the point of support as close as possible to the center of the block. If the center of the block is beyond the connection point between the tension and compression chord, it will cause bending of the compression chord, like a diving board."

The laws for competitive boomilever building.
You need less wood than you think. You need less glue than you think. Repeat

Testing
It is generally a good idea to test your boomilever before competition. Of course, you never test unless you have enough time to build another one before competition. Testing a boomilever does not weaken the structure in most cases. The key to know if your boomilever is damaged is to listen while you test. If you hear any pops, cracks, or groans then the boom is probably in need of repair. However, if you do not hear anything, your boomilever should be as good as new.

One method is to test the boomilever to the point where you like the efficiency score and stop without breaking the boomilever. I definitely think it is a good idea to test some boomilevers to failure, but you can use this method if you are nearing a competition.

Testing to failure lets you know where the structure is weak, and gives you the knowledge to make the boomilever better. Typically, the weak spot on boomilevers is the connection to the base.

Testing at Competition
When you are at the competition, your testing methods are important. Don't waste time pouring sand. You should be able to get all 15kg of sand done in less than 3 minutes. The longer the boomilever has to hold weight, the more likely it is to break. Basically, the faster you can pour the better in most cases, although with pouring devices, take care not to pour too quickly initially, as that can cause a huge amount of initial stress, since the flow rate adds additional force when it hits the bucket, not simply its own weight. Avoid spilling sand. This is an annoyance to the event supervisor (ES). There have been cases where the ES makes the testers pick up the sand off the floor and put it into the bucket. This gets old fast, remember that the clock is ticking. Your best bet is to not fill the pouring container up to the brim, or worse, over the brim.

It is possible that the ES will let you bring and use your own pouring cup. This is definitely something to think about, since sometimes what the ES brings is less than nice. I had to once pour sand out of those 1 gallon ice cream containers. If you do bring your own cup, clear it with the ES beforehand. They may let you use it under the condition that it is available to everyone else.The ideal pouring container would probably be a plastic, 2-cup measuring cup with a large open-end handle.

It does NOT matter if the sand is not even: It is a common myth that it will damage or lower the amount of weight the boomilever can carry. The physics behind it are slightly complicated if you were to write out the derivatives, but think about it this way: The force exerted by the boomilever is being pulled down by the force of gravity by a single point of contact- the chain and or hook of the loading black. Since gravity will always remain constant, no matter where the bucket is or how it is leaning, it will ALWAYS pull down at the chain with the same force, since as previously said it is connected at a SINGLE point.

Boomilever is a fun event; make sure you don't get caught up in trying to win so hard that you stop enjoying it. Build a lot, test a lot, and document everything.

Useful Links

 * Boom Specs 07
 * How to build a boom


 * Aia's Boomilever Guide