Group 20

Stress Analysis Design Project 2012

Robert Cross, Gaurang Poddar and Gautam Poddar

Photos of Our Project:

Brief Description of our Mechanism

In order to life the two pound weight, our design used a simple lever arm mechanism attached to the front of our truss. In turn, the truss was attached to our base which put the whole mechanism five inches off the ground in order to pass through the obstacle. The main structure of our design can be seen as two parts, the base and the arm. The arm is a triangular truss system, which is made up of bent aluminum strips. We chose the triangular system because we felt that it would be strong enough to prevent bending along the moment arm once the weight was applied to the end of it, but would require less material than a rectangular truss. This arm was the first part of the design that we built and as a result is very sturdy. The base of our system is a rectangular truss system, which extends out to connect to the arm. There are diagonal supports that extend from the lower portion of the rectangular base to the bottom of the triangle truss

Interesting and Original Features

The part we are most proud of is the triangle truss that extends out from our base to the weight. It took a lot of effort to build since we had to bend the length-wise beams by exactly 60o. Due to our design, and the support beams we placed on the truss, deflection was almost completely eliminated.

Theoretical Predictions

i)                    Torque Capability of Mechanism

WWeight = 1lbf (d = 4in)

MServo = 72 oz-in = 4.5lb-in

Thus the mechanism can theoretically rotate counter clockwise due to the positive moment and lift the weight. Also, it takes 4/4.5in = 89% of the servo’ capacity to lift the weight.

ii)                  Displacement of Weight

Thus the mechanism should theoretically be able to lift the weight by at least 2 inches.

Problems with original design:

The most significant problem with our prototype was that the base was not sturdy. As a result, when we attached the truss to the base, it noticeably bent forward. This was mainly due to two reasons: firstly, we had to use the 0.5” strips to make it since we used all the 0.75” strips. Secondly, we attached the truss to the base before we added the cross beams to the base.

Another problem was that the piece that connected our lifting mechanism to the truss broke at the last second, and we did not have time to fix it before the first design review. Additionally, when the mechanism would rotate, the truss would get in its way, which prevented the mechanism from rotating the full 90 degrees.

Lastly, the truss was too long which is why its end was almost touching the pole. As a result, there was little room for the mechanism to lift the weight.

Changes we made and how they improve our design:

The most significant change we made to our design was that we completely rebuilt the base. We made the entire base slightly taller so that if it deflected while lifting, the arm would no longer hit the bottom of the hole that it was passing through. Because the arm was now higher, we moved our mechanism to the bottom of our truss, so that it could contact the weight at the proper point. We also added a small piece of aluminum to the front of our base so that it can more easily lock into the game board. The most important change we made to our base though, was simply taking our time while building it and making sure it was properly supported before attaching the truss arm to it. Our new base is sturdy and has very little deflection.

The other area in which we changed our design was in the lifting mechanism. As mentioned above, we moved our lifting mechanism to the bottom of our crane arm, as it attaches more simply to that surface and better contacts the weight. We also stripped down and simplified the arm of our mechanism, as we realized that a bent piece of aluminum was strong enough on its own to support the weight. Other than that, we kept most of our mechanism the same, as our calculations from our first design indicated that it should be able to lift the weight two inches. The only issue we didn’t take into account was the deflection of the rest of our structure. The arm raises two inches, but the entire mechanism lowers, which keeps the weight below two inches. This will be our primary consideration for our final design. We plan on increasing the length of the mechanism and adding a counterweight to the end of it, so that it can lift the weight by a larger distance.