THE BRIDGE II

"DESIGN FOR ASSEMBLY"
STUDENT

WHY DOES THIS PROBLEM MATTER?

This is a challenging project that deals with one of the problems associated with any product sold in the world -- assembly. One of the specifications a design engineer must consider is how much a product will cost when it reaches the salesroom floor. Often, a very large portion of the cost is due to the assembly time. If a product is designed with the factory worker in mind and can be assembled at a rapid rate, the company can retail its product at a substantial discount.

What we have done with this project is to introduce one more important trade-off that most engineers must take into account when designing products. Our list then grows to three:

to carry the most weight possible
to be as light as possible
to be assembled in the least amount of time possible.

Today, companies often use one armed robots to assemble their products. This may make it even more important that there are easy ways to manipulate and attach the components of the product.

Figure 1.

WHAT ARE WE SUPPOSED TO DO?

Prior to the formal testing, you may design your bridge, manufacture your pieces, and test them to see how well they fit together. Your bridge will be evaluated in 2 phases, the assembly phase and the testing phase.

The assembly phase will begin with your bridge pieces separated on the top of a table. No two pieces of your bridge may be touching at this point. When the instructor is ready to time your group, your assembly procedure may begin. To make this interesting process (as well as to mimic several one armed robots), each member of your group may only use one hand to put the bridge together. Note, besides having a well designed bridge, it is imperative for your team to practice the assembly process and work together as a team -- as though you are a group of one armed robots being manipulated by one computer. Each time a team member fails to keep one hand behind his/her back. A minute is added to the assembly time. The instructor will stop timing when the team members have placed the bridge on the abutments and are ready for the next phase.

The testing phase may takes place once the bridge is assembled, and must meet the following requirements: Your bridge must touch only the tops of 2 wood platforms that are 6"x6" and spaced 2' apart (see figure 1). Your bridge must rest on the platforms during the entire testing process. You may not nail, screw, drill, or otherwise modify or attach to the platforms.

In order for you instructor to test your bridge, you must attach the eye hook provided to the bottom, center of your bridge. Only the hook can extend below the 2 foot level (top of the abutments).

WHAT EQUIPMENT AND MATERIALS CAN WE USE?

Your bridge may consist of the following:

5 blocks of styrofoam (.75 x 10 x 5) inches
5 rubber bands
10 paper clips
1 piece of wood (3.75 x 3.75 x 3.75) inches
10 nails
16 inches of clear tape, max width 1 inch
all the spaghetti you want
2-2 liter soda bottles (empty or full of original contents or water)
1 foot wooden dowel rod, diameter _ 1/2 inch
4 feet of string
1 hook

You may use any tools at your disposal (typically - hammer, pliers, etc.)

HOW WILL WE KNOW HOW WELL WE HAVE SUCCEEDED?

After you construct your bridge, in the assembly phase, the instructor will stop timing and attach a basket to the eye hook and add weights gently in small increments until the basket touches the ground, (whether your bridge fractures or deflects when the basket hits the ground the test is over).

The Basic Performance Requirement for the bridge is that you build a structure that is able to hold the basket.

The Superior Performance Index (SPI) will be calculated by dividing the weight you support by the weight of your bridge minus the time it takes your team to assemble your bridge.
SPI = [weight supported (grams) / weight of your bridge (grams)] - assemble time (minutes)

WHAT ELSE MIGHT BE USEFUL TO KNOW?

You will need all the information given in the Bridge I handout as well as the information you learned during that project. Recall which designs worked well and how they could be improved. Keep in mind which designs were easy to assemble. Remember, if two hands are required for a specific operation, it will require two people to do it, and it will probably be two dominate hands (i.e. two right hands for two right-handed people).

SAFETY CONSIDERATIONS UNIQUE TO THIS PROJECT

It is important that no part of your body is under the weights while your bridge is being tested. Also remember to exercise caution when using any tool while you are constructing your bridge.

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