The lift mechanism 
Jeremy at work on the base 
Mike with a small splinter 
Mark in the lab 
Phase I and II lift mechanisms 
Structure on the playing field 
Phase III Lift Mechanism 
Arm under tension 
Our Truss 
Mr. Happy 
The base plate with holes 
View of the arm 
Phase III design on field 
Phase III Lift mechanism 
View of Phase III from above Mechanical Engineering 2009 Jeremy designed an built the lifting mechanism and base plate used in the final design. His experience with building musical instruments allowed him to precisely machine and assemble these complex pieces. Jeremy also designed the website.
Mechanical Engineering 2009/ROTC Mike designed and built the suspension arm setup that the group uses. He was responsible for reducing the weight of our final design while still maintaining its strength. Mike's experience with CAD software allowed him to create the CAD models for the group.
Mechanical Engineering 2009/ROTC Mark assisted with the design and building of the entire structure. He created the powerpoint presentations for both design review one and two.
Our design went through three distinct phases, corresponding with the two design reviews and the final competition.
Although most groups chose to use a trusses to compose their structures, Group 16 chose to create a structure based on the same principles as a suspension bridge for the first design review. With weight being a critical factor in everyone's design, the use of a suspension system makes sense for a structure that will undergo only certain types of stress. Using extruded aluminum wire allowed us to create a structure that was strong in tension, while not wasting materials on making it strong in compression as well. Our Phase I design weight in at a total of 14 oz.
Our first lifting mechanism was based on a rack and pinion system. A servo was attached to a gear box with a 2:1 gear ratio, allowing for 180 degrees of rotation, and 3.14 inches of lift. However, the design was plagued with problems. The gears meshing in the transmission created power loss, and the attachment between the servo and the gearbox was weak (this was later rectified by using a special screw provided to all groups). Mike hand cut the teeth on the rack because of a rule against using off the shelf parts. This rule was later revoked.
For the second design review, the group focused on lifting the weight at all costs, then worrying about the weight of the structure. For this reason, our second structure is somewhat over-engineered. However, due to the continued use of the suspension bridge idea, we still weighed in at just 16oz. One of the main changes to the structure is the addition of a truss arm that reaches from the suspension base to the lift mechanism. This truss deflects very little and allows our base to take most of the stress. Unlike most other group's trusses, this truss is very thin, making it strong in only one direction. Our group felt that it would be better to design our structure to apply stress in only that one direction, instead of creating a larger, heavier truss to take loads and moments in multiple directions.
Our Phase II lift mechanism is very different from the Phase I design. The design was conceived during a brainstorming session when Mike wondered aloud "Could we make a moment arm that uses the servo as its own counterweight?" Jeremy set to work sketching and calculating, and soon a design working on that very principle was built and working.
The design is a double moment arm. This allows the servo to move the arm it is attached to by changing the angle of the bar connecting the two arms. This design also eliminates some of the power lost to horizontal thrust in a single moment arm, because a double moment arm always pulls vertically, instead of circularly. Its maximum lift was 2.12".
Before the final competition, our objective was to reduce the weight of our design without compromising the stress, as well as improving the aesthetics of our design. This meant judiciously removal of supports and repeated testing.
Major changes from Phase II include a new base that is much stronger than the old base plate, at 2/3 of the weight. We also replaced many of the steel screws with aluminum rivets. 25 screws weight 1.0 oz, whereas 25 rivets weight less than 0.2 oz. Many beams were trimmed to use the absolute minimum metal necessary. Through these small improvements, we were able to reduce the weight from 16oz to 12.5oz, even though the structure was stronger than our Phase II design.
Our Phase III lift mechanism closely resembles our Phase II mechanism. However, it is completely rebuilt. The newer mechanism has a lower
factory of safety, but a much greater lift of 2 3/4" in actual testing. It also reduced deformation in the pivot connecting the lift to the main arm.
Theoretically, our lift is capable of 3.54" of lift, using a maximum of 24 oz-in of torque, which is a factor of safety of 2.0 for the theoretical maximum torque of 48 oz-in the servo can provide over 90 degrees. In real life, the torque provided by the servo is considerably less than the theoretical value, but the range of motion is closer to 120 degrees. There are also power losses due to friction, torque, and bending of the structure under stress.
We have a lot to be proud of in our design. Our lift mechanism is innovative in that it uses the servo as the counterweight, as well as a dual moment arm system to reduce extra stresses. Imitation is the sincerest form of flattery, and our lift design was certainly the most copied (although many groups were unable to replicate our design's success without our calculations).
We are also proud of our unique frame. By making our design strong in only the directions in which it would be under stress, we were able keep our design very light weight, but very strong. Our unique suspension system is lightweight and very strong. For proof, check out the photo gallery to the left for the picture from above. You can see our base bowed in tension from the cables.
Our Phase I design was able to attach to the playing field and support the weight without touching the walls. The lift mechanism functioned without the weight, but was not able to lift the weight.
Our Phase II design was again able to sit in the playing field and support the weight (with less than 1/4" deflection!) without touching the walls. Our lift mechanism was able to lift the weight repeatedly to a height of 1 3/4".
In its final form, our Phase III design was able to lift the 1lb weight 2 3/4" high, 8 times in 30 seconds (the requirements were for 2 inches twice in 30 seconds). See the Media section below for a video of our final design lifting the weight 4 times in 14 seconds.
At our final design review, despite many last minute problems, our design lifted slightly over 2 inches multiple times, passing the test. Our official weigh was at 12.5 oz.
Our group won the prize for the lightest design to successfully complete the final requirements.
Below is a collection of media created in association with this project. This includes powerpoint presentations, videos, and CAD designs. All media is owned by Jeremy Ozer, Mike Harrison, and Mark Bradneau.
Design Review I Powerpoint Download CAD Drawing of the Playing Field Download Design Review II Powerpoint Download CAD Drawing of the Phase III Structure Download Video of Phase III design lifting 1lb weight in lab Download
