Materials requested for projectWe decided to begin prototyping in SolidWorks with a fourbar mechanism that would give us the motion we wanted to relocate a reversing wheelchair onto a curb. The aim was to have the mechanism vertically elevate and horizontally translate a wheelchair in a continuous, smooth manner. Because we anticipated that the wheelchair user would operate the mechanism most conveniently with the wheel, the fourbar mechanism was designed so that the crank or input link would have full rotational movement, i.e. a Class I Grashof linkage. Taking into account approximate locations and dimensions of the parts of a wheelchair, the relevant linkage lengths were varied until the desired motion was captured. The CAD concept drawings are shown below
This project had two main prototypes prior to our final presented prototype. Images and a video of the first prototype are shown below.
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The first iteration had a few issues that required improvement. One primary problem was that upon retraction, the pushbar link interfered with the curb and was not able to reset back into its initial resting state. Another issue is that the current design for the pushbar raised the wheelchair too high above the curb. Upon returning to the ground, the wheelchair user would be subjected to a high incline angle which introduces a safety hazard and discomfort to the user. To address these issues, a second iteration was designed. The images and video are shown below.
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The second iteration is inspired by a crank-slider mechanism, providing a smooth and controlled motion to translate the wheelchair. The pushbar is redesigned to align with the ground during its period of contact so as to prevent the wheelchair user from tipping excessively. The guide slot link provides a path for the pushbar so that it is introduced when needed but does not interfere with the curb during retraction and the motion is relatively compact. This design fulfills the basic requirements of the curb scaling mechanism concept but still required a couple of improvements. Firstly, a separate mechanism is anticipated to engage the curb scaling mechanism via the wheel. Because the wheel would have to lock with the crank link throughout full rotations, the current design of the guide slot component is inadequate and interferes with this possibility. Consequently, the slot will have to be shortened at the top and lengthened at the bottom to further accommodate for a full range of motion by the pushbar. Furthermore, a new pinning location for the guide slot link is required.
Shown below is a list of materials requested for the project and final prototype.
Part | Price | Quantity | Notes | Status |
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| 8 mm retention rings | 20 | From Slider-crank mechanism | Ordered; Received | |
| 8x16x5 mm bearings | 20 | From Slider-crank mechanism | Ordered; Received | |
| Aluminum plate stock (12” x 12” x ¼” plate stock) | $30.51 | 1 | 8975K142 (McMaster-Carr Part No.) | Ordered; Not Received |
| Aluminum stock (¼” x 1” x 48” bar stock for links) | $12.28 | 1 | 8975K596 (McMaster-Carr Part No.) | Ordered; Received |
| Rotary shaft 200 mm x 8 mm | 2 | From Slider-crank mechanism | Ordered; Received | |
| Rubber lining (1” x 18”) | $6.47 | 1 | 9023K82 (MacMaster-Carr Part No.) | Ordered; Received |
Above are concept drawings for the mechanism. In reality, the curved pushbar will have two points of contact with the ground and will cause the wheelchair to not be perfectly level through the motion. Two cycles of the motion may be needed to get the wheelchair fully on the ledge.
An issue to consider for the project is the scale of the final prototype or presented class product. A life-size scale for a wheelchair presents difficulties in obtaining the required materials for construction and testing, and we may look into a reduced-size model for a prototype. Another issue is the separate mechanism for engaging and operating the curb scaling mechanism, which would be necessary for a complete, functioning attachment. The scope of the whole attachment may conflict with the class timeline, but because the two mechanisms are fairly independent, a single presentable mechanism with the conclusion of the class can be manageable. A final prototype might be the curb scaling mechanism, which would demonstrate translating a wheelchair or model from the ground onto an elevated platform.
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By testing our first designed model with a scaled down curb, we encountered a critical problem when going through the motions of the mechanism. As seen from the videos above, once the mechanism was over the curb and the mechanism needed to be retracted, the coupler interfered with the curve and thus preventing the desired motion to be completed. This led to the necessity of redesigning the mechanism.
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