Truss designs in arms

[Max Lobovsky]

Most arms on FIRST robots use some sort of large extruded aluminum box. I have often seen some sort of truss design cut into these boxes to apparently make a more efficient shape. I am trying to design the truss to be cut into our arm member (4x4" 1/8" thick square aluminum tube), and would like to know how other teams have designed theres in the past.

I have seen three main designs. Holes, equilateral triangles (leave no struts perpendicular to arm direction), and right triangles (leave struts perpendicular to arm direction).

I currently like the equilateral triangle design simply because it "looks" more efficient, but I have no real good reason. This design would remove just a bit over 50% of the material:


I know I am opening an enormous can of worms here, but I'm looking for rules of thumb. (radii of fillets, ratios of widths of different parts of it, and of course an answer to which truss design is the most efficient).

[Ianworld]

Well the thing to understand about cutting out holes in your large aluminum extrusions to understand what your team is capable of. The last two years my team has just taken a 3" or 3.5" hole saw to 4 x 2 aluminum extrusions. That works pretty well. If you have access to a mill and a lot of time you can use something like what you have above. As for the material you leave i guess it depends on what size extrusions you're using. I like to leave enough material that i could drill a hole in it and bolt to it.

[Max Lobovsky]

I plan to manufacture said design with pretty simple tools. Just drill three holes for the corners of each triangle then use a jig saw to cut the lines. The jig saw will go through it in seconds, though the ~5*4*4*3 holes may take a good bit...

[Ianworld]

thats a pretty good idea. I might look into doing that this year with my teams arm. Thanks for the tip.

[Odlaw]

There is this West Point Bridge Design Competition every year. You get a program, available on line http://bridgecontest.usma.edu/index.htm, and build a simulation of a truss bridge. I have done this for several years, and I found the best truss design was the equalateral triangle. It evenly distributes the forces and holds up under strange cercumstances. If you use the right triangle the perpendicular edge takes a lot of compression and fails much sooner. Download the program and test some different ideas. It's a fun contest.

[Elgin Clock]

Here is a twist on your design Max. We used this on not only that part of the extrusion, but also the part on the left. This is pre hole pattern on the left square tubing. In total, 3 pieces of square aluminum tubing were cut this way (shown in picture 3) on a 5 axis Prototrac*(?) lathe..

[dhitchco]

Good discussion here........I'm learning a lot (as a mentor and former engineer).

Some questions from "real life"

1) Why are bridge trusses made mostly with "triangular" openings and why is a bridge built as a square "box" with trusses on either side instead of a triangle? Yes, that would make for a one-lane bridge and "short" trucks traversing the bridge. But why aren't bridge trusses UNDER the span triangular tubes?

2) There is a construction product called a "Gradall". It's boom is TRIANGULAR in shape, but it has CIRCULAR holes cut-out to save weight. How come they didn't choose to use triangular cut-outs also?

3) Forklift truck, backhoe diggers, and most cranes use SQUARE box elements instead of triangular....why? Seems like a articulated arm made of triangular components would be stiffer; although it makes for a tougher method to make the "hinge".

[ngreen]

We built our truss last year opposite of what you want to do. Instead of cutting out of a solid piece we took pieces and built them into three trusses for our boom and gib. That robot has more rivets than you can imagine, but was super strong and light.

[Max Lobovsky]

Our particular arm design has a good reason to be cut from one piece. The arm needs to be pretty straight and flat because it will have a slider running on the corners.

[Alavinus]

Overall equilateral triangles are probably the best way to go if you want to remove that much matierial. But are you sure you are going to need it? My former team built a 12 foot arm last year using roughly equal pieces of 2*2, 1.5*1.5 and 1*1. After cutting the peices the arm was under 20 lbs, so we ddin't need to reduce weight. (Although it didn't look quite as cool)

Here are some possible answers

1) Bridges are built tall in order to accomodate tall trucks. Having a triangular opening at the top would force the bridge to be either unecessarily tall or have issues with tractor trailers moving their cargo. The supports underneath a bridge are not triangular because it is fairly hard to find triangular stock. Most places that I've seen have a majority of square and rectangular stock.

2) The Circular holes leave more matierial to attach to I believe. Also, when designing construction equipment, you need to have a very large saftey margin because of the things they do. (Ever heard of bumper cars with pans or bulldozers?)

3) Again triangular stock is much harder to come by. Square tube is much easier to order cheply and probably easier to make. Also, if you look at boom cranes, I have seen those as an open truss triangular design, as well as square with triangles running down the center.