Team Vision for Detailed Design PhaseTeam MUSIC's Detailed Design Goals:
- Legs Design and Analysis
- Hinge Redesign and Analysis
- Wing Platform Design and Analysis
- Exhibit Design
- Lift Assist Sensitivity Analysis
- Phase I Testing
- Accurate Bill of Materials and Cost Estimate
Engineering Analysis and Simulation
Various analyses were performed on the Murphy Stage to verify the current design is acceptable.
Stage Structural AnalysisLoading conditions analyzed
- 150 lb/ft^2 over the entire top surface of the stage.
- Forces exerted on the stage from the lift assist system.
- A person's heel on the stage (a lot of force over a small area).
Metrics to consider
- Deflection: The stage should not be too "spongy".
- Stress: To make sure any of the components will not break under loading conditions.
Below is a summary of the analyses performed during this phase and the results.
Resulting from this summary, we have decided to change the very top surface from 1/8" hardboard to 1/4" plywood to mitigate the risk of puncture from a small object such as a heel.
Below are the plotted results from the finite element simulation for deflection. The finite element tool that was used is ANSYS Classic.
In an effort to optimize, each loading scenario was run using using a honeycomb thickness of 1" and 3/4"
The first set of pictures is the engineering spec for 150 lb/ft^2 over the entire surface. The hinges were accounted for which restrict motion in the x, y, and z directions as well as the legs which restrict motion in the z-direction.
The next plots show the deflection from the loading scenario of the lift assist system acting on the stage as it is in transition. 340 pounds act on the stage per gas spring (3 total), the hinge restricts motion in the x, y, and z direction and a simulated person at the edge of the stage who is lowering it restricts motion in the z direction.
From these results, we have decided to use 3/4" thick honeycomb in an effort to reduce cost. Using 1" honeycomb does not add much more structural benefit to the stage.
Lift Assist AnalysisAdditional analysis was done on the lift assist mechanism to better understand how it would respond to changes in weight of the stage.
Drawings and Schematics
Since the previous design review, more consideration has been given to how the stages mate with the support structure, as well as how the stages are best constructed and integrated with feet upon deployment.
Stage Surface ConstructionThe stage surface is created using a sandwich style construction. The conventional approach would be to use 3/4" plywood. The decision to use a sandwich style construction over the conventional approach was made in an effort to reduce weight. Using the conventional method, plywood contributed to over half the stage's total weight. Reducing weight decreases the complexity of other subsystems including the lift assist mechanism, legs, and locking mechanism. It also results in a safer design in case something were to fail while the operator transforms the stage. The graphic below shows the construction on the stage surface from a front on view.
Top Layer Plywood was chosen as the top layer because it is light and relatively stiff and hard. This will provide the hard surface of the stage which people will stand on. The goal is for it to resist the impact of small surface area objects such as the heel of a high-heeled shoe. The chosen plywood has three layers of wood with fibers in a [0,90,0] angle orientation.
Middle Layer Polypropylene plastic honeycomb with an 8mm hexagonal cell size was chosen as the middle layer because it is lightweight while still maintaining high strength and stiffness properties. This will allow the stage to still hold up under extreme loading conditions while still being light enough for the stage to be manually folded up against the wall.
Bottom Layer Hardboard was chosen as the bottom layer of the composite because it was able to be bought at a thinner thickness than the plywood while still maintaining similar mechanical properties. The hardboard provides added strength to the composite while also keeping it light.
Framing Supports The composite material will rest on top of an array of 2 x 1” hollow aluminum extrusion framing. This framing underneath the composite will allow the composite to span a large area while limiting deformation due to the sheer weight of the composite. The framing will provide some areas of the composite that will limit the degrees of freedom of the deformation of the composite.
Bill of Materials (BOM)Link to bill of materials is here
Test PlansA link to the test plans can be found here. During this phase, we completed point load testing, to understand the kinds of loads that could break the stage surface if applied at a small point.
We ended up undergoing 8 tests between the two surface types, varying the loading conditions. All tests were done with 6" x 6" samples of the surface materials.
Tests 1, 2, 6
Tests 1, 2, and 6 were done by pressing a 1/2" x 1/2" square punch into the material as the material was sitting on a 1" diameter hole in a piece of aluminum.
- In test 1, the hardboard had a "soft failure" of a dent where the load being read at the same distance kept going down at 300 lbs.
- In test 2, the plywood had a "soft failure" at 575 lbs.
- In test 6, the hardboard had a "soft failure" at 340 lbs.
Tests 3, 4, and 5
Tests 3, 4, and 5 were done by pressing a 1/2" x 1/2" square punch into the material as the material was sitting on two 1/4" thick parallels 3" apart.
- In test 3, the hardboard saw 1/4" of deflection at 140 lbs, and further testing would have been just pressing the hardboard against the platform.
- In test 4, the plywood saw 1/4" of deflection at 540 lbs, at which point a crack developed along the back of the sample. Notably, the sample cracked parallel to the grain even though it was being bent against the grain.
- In test 5, the plywood saw 1/4" of deflection at 230 lbs.
Tests 7 and 8
Tests 7 and 8 were done by pressing a 5/16" nut (1/2" flat-to-flat) against the material that was sitting on a 1" diameter hole.
- In test 7, the hardboard saw a "soft failure" at 310 lbs.
- In test 8, the plywood saw a "soft failure" at 350 lbs.
The previous design iteration used hinges that attached to both the stage and the frame with mechanical fasteners. Due to the availability of hinges, as well as the distinct difference between intended use and the use in our design, this idea was revisited. The team decided on an 3/4" 1018 steel axle mounted to the frame and supports, instead of the surface-mounting hinges.
This new design was easier to analyze, allows for more simple geometry, and will result in less concentrated stresses.
The next design innovation the team created was to solve the problem of attaching feet. Leaving feet permanently attached and visible was an unappealing idea, so the design required they be removable or hidden. Hidden feet would have required more time to design for stability and ease of deployment.
The determined solution is a series of removable rows of legs, designed for ease of placement and stability.
Each row of legs attaches with four pegs. Each peg is a common fastener with a spacer, a washer, and a nut.
These pegs first allow proper location of placement, and then lock the stage in place when slid 2 inches towards the hinge axis of the stage. The slot is at first 5/8", and narrows to 3/8".
Risk AssessmentA link the the updated risk assessment can be found here.
Design Review Materials
Design Review presentation can be found here
Design Review meeting notes can be found here