Unofficial specifications and supplies for 2007 Science Olympiad Div. C Boomilever Event

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What is a Boomilever?
The Boomilever event requires construction of a cantilevered boom, hence the name. A cantilever is a horizontal structure rigidly secured at one end, such as a balcony, diving board, or crane, that will bear weight along its length or just at its unsecured end. We present here a discussion of basics and one proposal for a solution. Try it! Let us know how it goes! (ed@turnertoys.com)

THE PROBLEM: Basic Engineering Concepts
A Boomilever could be made as a simple plank rigidly attached to the vertical test fixture. The application of significant weight at the far end would bend the plank until its limit of elasticity is exceeded, and it breaks. A solid plank occupying the entire allowed dimension of 20 cm vertical by the required horizontal projection of 40 or 50 cm would be very stiff and easily support a great weight, but be so heavy it would be very inefficient (strength/weight). Even cutting the weight 50% by using a triangular solid would have this problem.
The solution is to provide just those parts of the solid triangle that are needed to handle the anticipated forces. A triangle is the stiffest shape there is. What we end up with is one or more projecting members on top and on bottom of the triangle (like a line-drawn outline).
**NOTE: THE FOLLOWING DESIGN IDEAS ARE HYPOTHETICAL. This Hypothesis is the result of **deductive logic based on an intuitive (inductive reasoning) understanding of structures based on years **of practical experience. It is up to you to test and verify or refute this hypothesis by building and **modifying many versions of it. You should produce an accurate drawing of each embodiment of the **solution, to calculate angles and member lengths. Dimensions of your model will reflect the **actual contest rules. There is no reason to use wood species other than balsa, which has the **highest strength/weight ratio of any wood.

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The top members will be under tension when load is applied to the far end, and the bottom members under compression. That is, the load is trying to pull the top members out of the test fixture wall, and push the bottom members to try to make them shorter. If enough load is applied, the top members may pull out, depending on the strength of the joints.
The bottom members will be under compression when load is applied. The forces will be trying to reduce the distance between the load and the supporting wall. This will occur when the bottom members flex, either vertically or horizontally, until they break. We try to maximize stiffness of these members by bracing them with struts and using stronger materials. They should also be as short as rules allow, since shorter pieces flex less. Thus, the bottom extension should be horizontal.

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The Solution The upper structural elements projecting from the wall attachment do not need stiffening. A device like this could just as well use string in place of balsa for these members. They do need to be anchored very strongly to the backing piece, which is used to bolt the device to the test fixture wall. Let's start with this backing piece. The requirement is to provide an element that will safely support the concentrated forces of a 1/4" steel bolt anchoring it to the wall. A piece of 1/4" Lauan Mahogany (any lumber yard) will be strong enough. Optionally, for a fail-safe joint, you may glue a 1/2" square piece where the bolt holes go, and the same at the points of spar attachment. Since the boomilever structure requires anchoring to the wall only at the the top, this backing board need be no larger than is required for bolt holes and points of attachment for the top spars.
Upper Extension - "Hangers" They should be mortised into the plate to maximize glue joint area, not merely glued. Use 1/8" balsa, or you can experiment with thinner pieces. Drill a hole about 1/32" less than the thickness of the hanger, at the angle at which the hanger will enter the backplate. Drill all the way through. This is best done on a drill press, to minimize wobble which will enlarge the hole, and because accuracy counts. Square the holes with a jeweler's file as wide as the stick you are working with, so that the hanger is an easy press fit (ask a machinist what that means - there are slip, press, and drive fits).
File accurately so there are no voids. ALWAYS TEST DRY BEFORE GLUING!!! If you make the holes too big, you will have to scrap the backplate and start over, so go slow. Use yellow carpenter's glue (Titebond, etc) for this joint. Apply glue to inside of square hole and tip of hanger. Push hanger through. Wipe excess with paper towel or small rag and warm water. It may be best to cut hangers overlength, cut/sand flush with backplate, and trim to length when they are attached to bottom extension. Once there is any pressure on the joint, the glue is starting to set, and should not be adjusted further. Allow 24 hours before loading, although construction can continue immediately.
	Lower extension structure and point of load attachment This substructure must be stiff. Hangers should prevent vertical flex; lateral flex can be minimized with triangular bracing. Drawing suggests placement of hangers and supplementary struts; You will have to build and test to determine exact number and placement to build the structure that attains design goals with greatest efficiency and a margin of safety. The design goals are specified by the official rules. Point of load attachment should be framed out with balsa for strength, like framing a doorway in house building. See lower photo: How could it be done (much) more simply? Top-most hanger should be mortised into load-point structure exactly in line with force of load. If additional vertical stiffness is needed for the lower substructure, you may glue a "railing" along its top surface. Make sure the joint is flat and accurate, and joint accurately 90 degrees. Apply pressure with assembly pins. A wide thin piece is ideal. This can be laminated from 1/6" sticks.
NOTE ONCE AGAIN: FEW OR NO DIMENSIONS ARE GIVEN. YOU WILL DETERMINE THESE BASED ON THE OFFICIAL RULES AND YOUR EXPERIMENTATION. DRAWINGS ARE NOT TO SCALE, NOR INDICATE EXACT PLACEMENT OF ANYTHING.