Blog #4

Work Period of April 13 to April 26

During the work period of April 13 to April 26, our team made significant progress toward developing the final concept for the Mechanical StairGlide Luggage project. We narrowed our design direction down to two main mechanical concepts that could potentially meet the project requirements for stair climbing, compact storage, structural safety, and airline dimensional limits. For both designs, we completed CAD drawings to better visualize the geometry, folding/retraction behavior, wheel placement, and overall integration with the luggage body. These CAD models helped us compare how each concept would fit within the suitcase envelope and how the mechanism would interact with standard stair geometry.

In addition to CAD development, our team began analyzing the forces acting on each design. We are currently working on free-body diagrams, force calculations, and structural feasibility checks to determine how the load will transfer through the wheel arms, pins, supports, and attachment points during stair climbing. This analysis is important because we need enough engineering evidence to decide which design is stronger, safer, easier to manufacture, and more effective at reducing pulling force. At this stage, the main work still in progress is completing the force analysis for both concepts, comparing their advantages and weaknesses, and using the results to select the best final design for continued development in Capstone II.

Final Design Description

The final design selected for continued development is a retractable mechanical stair-assist mechanism mounted to the lower sides of the suitcase. The mechanism is based on the second concept, which uses a folding arm system supported by an outer shaft housing, locking shaft, support plates, center fixed arm, ball locking pins, and a spring-loaded push rod assembly. When the mechanism is deployed, the folding arms extend into the stair-climbing position and allow the suitcase load to transfer through the wheel/arm assembly instead of requiring the user to lift or drag the full suitcase over each stair edge. When the mechanism is not needed, the arms fold back into a compact stowed position so that the luggage can still function like a normal checked suitcase.

This design was chosen because it provides a more compact and realistic integration path than the curved tri-claw wheel concept. The selected mechanism has a simpler axle and bracket layout, fewer large protruding parts, and better potential to fit within the airline dimensional limit when retracted. It also allows the main load-bearing components, such as the locking shaft, support plates, and folding arms, to be analyzed and fabricated using standard mechanical design methods. The final design will continue to be refined in CAD to improve part spacing, pin locations, locking reliability, and suitcase attachment points before fabrication in Capstone II.




Key Analytical Results

The main analytical result from this work period is that the second concept appears more feasible than the first concept based on the design matrix and first-order force analysis. In the design matrix, Concept 2 received a total score of 85, compared with 66.5 for Concept 1. Concept 2 scored higher mainly because it is more compact, lighter, less expensive to fabricate, and easier to integrate with the suitcase body. These results support the decision to continue developing the folding-arm mechanism as the final design direction.

The force analysis for the selected design modeled the suitcase and mechanism during stair climbing using force equilibrium and moment equilibrium. The analysis considered the suitcase weight, mechanism weight, stair height, wheel/arm geometry, pulling angle, and contact forces at the stair edge. Using a 70 lb suitcase load, 5 lb mechanism weight, 7 in stair height, 4.5 in link length, 60° pulling angle, and 30° frame angle, the calculated pulling force was approximately 32 lb, or 142 N. This value is below the project target of 300 N, meaning the selected design has the potential to reduce user effort compared with the baseline stair-dragging test.

The structural analysis also identified the axle shaft as a critical component because it experiences both bending and shear during stair climbing. Since stair contact may not be perfectly symmetric, the analysis used a conservative load case where one axle carries 70% of the applied load. The selected shaft material is 6061-T6 aluminum, which provides a good balance of strength, weight, machinability, and cost. Further analysis is still needed in Capstone II to complete the stress calculations, verify the factor of safety, and confirm that the shaft, pins, folding arms, and support plates can withstand repeated stair-climbing loads without permanent deformation.



Future Plan for Capstone 2

In Capstone II, our team will move from concept development into detailed design, fabrication, integration, and validation testing. Before the start of the fall semester, we plan to finalize the selected design by completing the force analysis for both current concepts and using the results to choose the most feasible option. The selected design will then be refined in CAD with more accurate dimensions, part interfaces, pin locations, support features, and folding/retraction geometry.

After the design is finalized, our team will prepare the bill of materials, select the final materials, and determine which components will be machined, 3D printed, or purchased. The next major step will be fabricating the prototype and integrating it onto the suitcase. Once the prototype is assembled, we will test its deployment, locking reliability, stair-climbing motion, pulling force, stability, and structural strength under load.

The main goal for Capstone II is to prove that the final mechanism can safely support the suitcase, reduce the effort required to move luggage on stairs, remain compact when folded, and meet the project constraints. Testing data will be compared with our analytical calculations to verify whether the design performs as expected. If problems appear during testing, we will revise the design and improve the prototype before the final report and presentation.


Figure 1: Force Analysis on curved tri-wheel

Figure 2: Stair-Climbing Force Analysis with weight





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