Tuesday, September 3, 2013

Tinkering (Starting the build)

Even though we went through concept finalization I wanted to tinker/toy with the pull back car design. This was to exercise more of an engineering method where one builds a prototype to justify ones conclusions. This was more of a small scale proof of concept to see what one could achieve without investing too much time is the design didn't work out; so more breadth than depth. My initial design was basically directly driving the wheels with a 8:40 (1:5) gear ratio which basically had the travel of about 6 inches or less before the spring reversed and stopped the car like I knew it would.

The next thing I tried was to add a more ratio to the gears having a 24:40 and then a 8:24. When doing the math I figured out that it was a ratio of 3 to 5 multiplied by the ratio of 1 to 3 which is again 1 turn to 5 turns which is exactly what I had before. At that point I realized how dumb I was and quickly researched gear ratios. Another thing I also noticed was that because there was so much force on the chassis from where the spring was mounted that the chassis itself and the axles began to bend and torque a bit.

So after I figured out was configuration I figured out what the correct gear ratio should be. So now with the gearing changed to 40:8 x 24:8 the overall ratio was now 15:1 or 15 revolutions of the drivetrain for every one turn of the spring! Although I can only turn the spring about 1 to 3/4 before the axle starts to bend I think this is a great start and shows promise in the pull back powertrain. This was great proof of concept but bears some greater challenges if this design is going to go any farther. A couple of things to note are how important weight is on the drive wheels for a system like where every ounce of energy counts. Also that since this system stores no momentum because when the spring runs out the car stops immediately which will be a huge concern aiming for anything higher than about 3 feet. Lastly the biggest challenge will be trying to strengthen the legos. Although this prototype seemed okay it really pushed the limits and stresses of the legos. I will either have to revamp the design or move it into another building material.

All in all a great day with much success but more than that a lot of lessons learned.

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