Engineering Projects/StrandBeest/Howard Community College/spring2012/p1550MWSY/Model Leg Assembly

Materials
The original designs were made of PVC piping and plastic strapes, but the strandbeest can be made of anything. The main consideration of the materials are that they are light and strong enough to hold their own weight while moving. Some groups have used legos to create their beests. Others have choosen light woods.

Here I intend to provide all instruction of building a model strand beest. The materials needed are cardboard, wooden dowel, wire, sissors, tape measure, and wire cutter.

I have choosen these materials for the model leg assembly due to the fact that they were economical and readily available and because my wife is crafty some we already had. A couple of quick points; Measure twice, cut once. While one of the triangles in the leg assembly looks to be a right angle, it is not. More on this in the golden numbers section.

Design
The first thing you need to know about the design of the leg assembly is that it is based off of known ratios of lengths that have been well tested... so here they are.

a:38.0                   f:39.4                    j:50.0 b:41.5                   g:36.7                    k:61.9 c:39.3                   h:65.7                    l:7.8 d:40.1                   i:49.0                    m:15.0 e:55.8

These are ratios so the scale of the project can be in whatever units you choose and multiple of these numbers. For example, my group did this project in inches due to the softwear of the CNC router we were utilizing and at 37.5% of listed values. Where as my leg assembly model was done in centimeters at 25% listed values.

One thing that is not readily apearant in the design is the values l and m from the golden numbers. The value "a" corresponds to the design as a bar connecting one leg assembly with another so that they move in unison. "l" is a small notch coming up from the point at which the two "a" bars connect that connects them to the crank shaft. "m" is the radius of the crank shaft.

Assembly


As mentioned above, please measure twice so you need to only cut once. The dowel will need to be cut slightly shorter than the listed values this is due to the way I have choosen to assemble them. The wire holding the dowel to the cardboard will create a small gap extending the length slightly. Unfortunately the extent of the gap will depend on how well your wire will bend and hold so a little bit of trail and error will be in order here. Once the dowel is cut it will need to be primmed on the end. SLIGHT preasure from the wire cutter should do the trick. Slight pressure being realitive to the hardness of the dowel you procured. The intention of this is to leave an indent along the rim of the end about a quarter inch from the end. This allows the wired a seat to settle and not move around.

I had attempted to drill a fine hole through the ends of the dowel, but regardless of how small the hole. The dowel would always end up cracking.
 * short aside*
 * end aside*

Once a small bit of wire has been attached to each end of each dowel length, the pieces are ready to start being combined together. There is no particular order this needs to be done in with the exception that lengths "j" and "k" should be assembled last. This is due to the fact that they are held together by a single bit of wire with a loop in the center of the spacing between them. This loop is where the crank shaft would connect. Additionally, this leads to the complication of spacing. Because the design to meant to move the leg assembly by applied pressure from each other piece while the crankshaft is turning. Assembling this piece last allows for us to correct any small errors in measurement or assembly in the previous pieces. (each error changes the sweep of the leg more from the ideal) And congradulations you now have a model leg assembly to help you understand the greater design of the STRANDBEEST!