24-262 Stress Analysis Project: Group 4

Daniel Miller, Cooper Barth, Richard Musgrave

---

How the Device Works

The mechanism was built from the start to be an extremely sturdy truss like structure that could withstand a great amount of torque on its end.  All of the aluminum beams used in the structure were bent to provide extra support.  It has a square base that goes straight up and out to the larger of the two holes to pass through.  Our arm is another truss like structure with two bent beams connected at top and bottom with diagonal supports along the length.  At the end of the arm we mounted the motor.  We then constructed a lifting mechanism which was loosely attached to the arm which would rotate a second lifting arm that protruded out from it.  This second lifting arm rested just under the weight in a perfect location as to lift it completely vertically without any wasted horizontal force being applied to the weight.  Then, to add extra torque to the arm we added a counter weight equivalent to all the extra weight we had to spare under 20 ounces to the back side of the lifting mechanism.  To get even more torque from this counterweight we extended it from the back of the mechanism using an additional aluminum beam.  When we finally tested our structure, the combination of the sturdiness of the structure, the low friction loss due to the loosely attached lifting mechanism, the counterweight extended from the back of the mechanism, and the purely vertical lift, allowed the motor to turn 90 degrees and lift the weight to a height of 2.9 inches.

---

Predicted Lifting Capacity

The basic design principles used in our structure is a basic rigid box type with rigid cross braces all made of L members to prevent moments causing bending in the structure.  The arm out to the weight is a traditional boom type used on high rise cranes, but we simplified the boom to a more 2 dimensional structure because there were minimal amounts of torque acting on the structure compared to the rigidity of the structure.

The lifting mechanism allows us to apply the entire moment from the motor in a vertical direction to the one pound weight.  We use a rectangular structure with one side attached to the servo and one side mounted to the boom.  Protruding from the vertical side closest to the weight is the lifting arm.  This is strategically located to always apply a vertical force to the weight.  The horizontal sides are originally at an angle of -20 degrees to the horizontal, and once the servo applies the 100 degree rotation, ends up at an angle of +80 degrees to the horizontal.  This angle change is what results in the weight being lifted vertically approximately 6 inch.   Theoretically we would only achieve this height in an ideal system with no friction losses.  With the friction between members the actual lifting height will be much lower. The energy supplied in the torque by the motor will be reduced by the friction forces that oppose rotation motion on the entire lifting arm.
After the first build, there are some design improvements that we want to consider.  First adding a counter weight to the lifting mechanism would ease the work load of the servo and allow us to reach a higher maximum height.  Second, we would like to rebuild our base structure making it smaller to reduce the overall weight.

---

Interesting and Original Features

• Extremely Sturdy, no deflection – The strapping design of our structure allowed us to use whatever we wanted add the end of the lifting arm without any deflection.
• Bent Beams(L-shape members)– Provided added support throughout the entire structure, and make the beams much more resistant to bending.
• Diagonal Supports allow for the transfer of bending stress that would deform the bar, into much weaker axial stresses that cause little to no deformation to the beams. The L-shaped members are much stronger axially than they are in the longitudeal sense.
• Loosely attached lifting mechanism – The lifting mechanism being loosely attached to the end of the arm allows it to rotate about the arm without friction losses, thus applying more torque to the weight and lifting it higher.
• Extended counterweight – Because torque is force multiplied by distance, the farther away that the counterweight is from the spot that the torque is applied (@ the weight) then the higher the mechanism will be lifted.
• Purely vertical lift – By placing the lifting arm directly under the weight we were able to lift it completely vertically without wasting force.
• Weighed 19.9 ounces – From the start, we decided that we were going to go for highest and not lowest.  Therefore we were devoted to using all the weight allotted to create an extremely sturdy structure that could withstand large amounts of torque.
• One thing that we were very proud of was our ability to adapt our structure to unforeseen problems that we did not originally take into account in our first design.  For example, we did not plan to have our base entirely in the square in which it was supposed to be contained.  Thus we had to make some serious adjustments to our structure to get it to fit in the square.  Another example of us adapting our structure was that we did not take into account the length of the weight sticking out from the pole when we designed our arm.  This length then had to be subtracted from our arm.