Blog #2

Technical Problem Statement

The objective of this project is to design and validate a purely mechanical, retractable stair-climbing mechanism that can integrated into a standard checked suitcase. The system must safely transport a 70 lb load over typical 7–8 inch stair risers while reducing user-applied pulling force to below 187 N. The mechanism must fully retract within airline dimensional limits (L + W + H ≤ 62 inches), maintain a minimum structural safety factor of 2 under worst-case loading, and remain stable during stair ascent and descent.

The major challenge is transferring the suitcase’s weight through a deployable tri-wheel with a retractable/folding mechanism (figure 2) without structural failure, tipping, or excessive user effort. During stair climbing, concentrated reaction forces occur at stair edges, generating bending in the axle, shear in locking interfaces, and moment loading on the hinge structure. The design must balance mechanical advantage, packaging constraints, durability, and stability within a compact geometry suitable for airline baggage handling.

Key Physical Constraints and Design Challenges

Constraints	Requirement	Justification
Supported Load	50-70 lbs vertical load	Standard checked baggage is 50lb, but 70lb is used as the maximum design load to ensure structural safety under worst-case conditions [4].
Stair Geometry	Riser height: 7–8 in; Tread depth: ~10 in	Typical stair dimensions are used to determine the required wheel size and ensure smooth step-to-step movement [2].
Airline Dimensional Limit	L + W + H ≤ 62 in	Common airlines checked baggage regulations. Requires full retraction within suitcase envelope [4].
Retraction Requirement	No component extends beyond suitcase outer boundary when stowed	Prevents interference with conveyor systems and reduces exposure to impact damage during handling.
Operation Type	Manual	The design is restricted to a fully mechanical system with no electrical assistance.
Stability Margin	≥ 38°	Angle after which luggage stability is compromised. Calculated using the tipping angle equation.
Minimum Safety Factor	≥ 2.0 under peak loading	Standard mechanical design practice for manually operated load-bearing systems to prevent yielding
Durability	≥ 32 km equivalent rolling distance	Standard lifespan for normal luggage wheels [3].
Maximum Device Weight	≤ 5lbs	The threshold for check-in bags before being charged extra and categorized as overweight is 50lbs. 90% of suitcase weight should not be affected by device.
Packaging Envelope	Compatible with base suitcase ~ 30 × 19 × 11 in	Defines allowable internal volume for the retractable mechanism.
Deployment Reliability	No unintended lock disengagement under full load	Ensures structural integrity and user safety during ascent/descent.
Pulling Force	≤ 187 N	Defines applying force and involves overcoming gravity while pulling up and down the stairs [1].

Technical Analysis Plan

To address the identified physical challenge, structured analytical and experimental evaluation process before finalizing the design. Firstly, free-body diagrams will be developed for both stair ascent and descent conditions. These diagrams will model the suitcase as it rotates about the stair edge and will allow to determine reaction forces at the contact points, the required user-applied handle force, and the internal loads transmitted through the axle, hinge, and locking interface. These calculations will identify the worst-case loading conditions.

Using the forces obtained from the free-body analysis, closed-form stress calculations will be performed to determine bending stresses in the axle, shear stresses in locking components, and bearing stresses at contact surfaces. Each structural component will be sized to maintain a minimum safety factor of two under peak loading conditions. This ensures that no permanent deformation or yielding occurs during the run.

A tipping stability analysis will also be conducted to evaluate system behavior under eccentric loading. By analyzing the moment balance about the stair-edge pivot point, tracking the location of the center of mass during stair transition to calculate the minimum tipping threshold angle, and verify that the stability margin remains above 38 degrees. This analysis will directly inform wheel spacing and deployment geometry decisions.

Additionally, Finite Element Analysis (FEA) will be performed on critical components such as the axle shaft, hinge mechanism, and locking pin. The loads used in the simulation will come directly from analytical calculations. Hence, identify stress concentrations, confirm safety factors, and refine component geometry before fabrication.

Finally, experimental validation will be conducted on fabricated prototype. Pulling force during stair ascent will be measured using a calibrated digital force gauge under a 70 lb load. Structural behavior will be inspected for deformation or instability, and deployment reliability will be tested under repeated loading cycles. Durability will be evaluated through repeated stair engagement equivalent to a 32 km rolling distance using Stair Stepper Machine or with any regular stair. Measured data will be compared with analytical and simulation results to confirm that the system meets all structural, stability, and force requirements.

Soft Challenges Identified

Reliability: The retractable mechanism must not unintentionally disengage under load, as this presents a safety hazard.
Fabricability: Complex linkage geometries increase tolerance sensitivity and fabrication difficulty.
Weight Addition: The device must remain under 5 lbs to avoid significantly increasing total luggage weight.

Ergonomics: Handle angle and user posture directly affect applied force and perceived effort.

Figure 1: Tri-Wheels placement

Figure 2: Retractable mechanism components conceptual

Reference

[1] An objective set of guidelines for pushing and pulling. (n.d.). https://dam.assets.ohio.gov/image/upload/info.bwc.ohio.gov/forms/PushPullGuidelines.pd

[2] DMillworkEditor. (2024, November 29). Standard Stair Dimensions & How to calculate sizes. Dutchess Millwork & Machine Shop. https://dutchess.com/standard-stair-dimensions/

[3] How we test our luggage – jekyll & hide durability standards. Jekyll & Hide. (n.d.-a). https://www.jekyllandhide.co.uk/pages/luggage-testing?srsltid=AfmBOorv4BXmtZiWza34s03lX1v_sgkQb4Qckh0CVC-YiYP_Bt7eLza1

[4] United Airlines baggage allowance 2026. My Baggage. (2026, January 4). https://www.mybaggage.com/shipping/us/united-airlines-baggage-allowance/