Dan Gabioud, Joe Klopotic, Tyler DeBussey, Mike Strazishar, Mike Seidel University of Wisconsin-Madison
In today’s market there are two primary options when a person using a wheelchair needs to negotiate a staircase: ramps and powered lifts. Ramps require space (because of requirements on slope) while powered lifts are expensive. Given these limited options, we set out to design and fabricate a different alternative. A solution was achieved by strategically combining a platform lift modified with rollers, and stair-mounted rails, with modified chain hoists, braking systems, and safety gates into one complete system. This system is powered solely by the individual using the wheelchair without necessitating other expensive power systems. Additionally, both powered wheelchair users and manual wheelchair users need to be able to utilize the device. The new device, called the Funicular, has all these abilities and is a compact, safe, and cost-effective way to transport a wheelchair-user up any number of stairs.
Among the challenges a person using a wheelchair faces every day, is the challenge of stairs. The solutions for stairs currently existing can be lumped into two main categories: ramps and powered lifts. There are, however, limitations that arise with these two solutions. At a meeting with the Madison Spinal Cord Injury Group, it was discussed that traditional ramps, while providing a solution to go up a flight of stairs, can be a problem for wheelchair users due to their length as a result of allowable slope regulations set forth by the ADA (1). As our discussion with the Madison SCI Group progressed, the concept of electrically-powered lifts was brought up. Almost immediately, however, a member of the group mentioned the high cost of this type of system. Powered lifts solve the “footprint” problem associated with ramps, but can cost from about $3,000 (2) plus installation to much more than that ($10,000 and beyond). Our problem statement then became: Produce a device that can transport a person using a wheelchair up a flight of stairs while remaining compact (footprint approximately the size of wheelchair footprint) and cost-effective ($500 production cost).
DESIGN AND DEVELOPMENT
Referring to the figure below, to use the Funicular, a wheelchair user backs onto the platform via the front safety gate/ramp and continues to back up on the platform until the rear wheels of the wheelchair sit between the two rollers. Once the user’s wheels are on the rollers, they continue to rotate their wheelchair wheels, which spin the rollers. The platform then moves up because the rear roller is attached to two cogged belts which are attached to the chain hoists. The chain hoists are also connected to chains (not pictured for clarity) which are attached on the rails. The chain hoist then moves up this stationary chain moving the wheelchair platform and wheelchair occupant up the staircase. The reverse, but similar, process is followed to come down the lift by spinning the wheelchair wheels the opposite way.
For our design and development process, we began by setting design goals. Students from previous years started the groundwork on these goals and our team reviewed and added to them as needed.
These numerical specifications were derived from analytical and experimental values for a wheelchair user going up a standard ADA compliant ramp. In addition, the Funicular was required to have the capability to transport manual wheelchairs and powered wheelchairs.
For a manual wheelchair user, we were designing for an input force of 5lbs at the manual wheelchair push-rim. It was necessary to multiply that force throughout the system so that the platform would lift the combined weight of the user and platform. Initially, we believed that our team should use an “off-the-shelf” gear reducer due to the ease of install, easy adaptability between gear types, and wide range of gear ratios. While readily available, we found that commercial gear reducers were not compact and were expensive ($300 to $600 each). Because of this, we focused our attention to a garage-style chain hoist through our concept evaluation process.
In order to achieve our design goals of 5lbs input force at the push-rim and 3.3 inches rise per revolution, the size of the gears in the modified chain hoist had to be determined.
There were essentially three variables (D1, D6, and D10) that we had control over as the internal ratio of the hoist was fixed. Using a system of equations, we established optimal sizes for each gear that would accommodate our design specifications. However, due to packaging limitations, we had to increase the theoretical input force to 6.5lbs in order to maintain the rest of our design goals. This can be seen in the following two graphs.
Another modification to the chain hoist system was that the two strands of chain link were replaced with roller chain and a toothed-belt. This was done for many reasons including making the system run smoother, reducing production cost, increasing safety, and being more aesthetically pleasing.
Another sub-system that is essential to the functionality and safety is the locking/braking system. The ability to provide controlled braking for both up and down movement of the platform was addressed from the beginning of the design process. This subsystem must be able to withstand the forces imposed by the weight of the user and platform, and hold backlash and jerking motions to a minimum. Additionally we wanted this subsystem to be automatic without any additional effort by the user. Some of the early concept generation ideas for the braking system included simple devices like a saw-tooth ratchet and a ladder-style catch. Both of these concepts, however, lacked the ability to provide controlled braking when the funicular platform was descending. Another concept that was a possibility to provide locking/braking was the use of a Weston-style brake (3).
These brakes are usually implemented within chain hoist designs and would provide controlled raising and lowering. After going through concept evaluation, a design review, and discussing with our professor and peers, our team decided on the use of a Weston-style brake system.
Another component of the Funicular system was the attachment of the roller chain on each rail. The attachment needed to be reliable under the maximum expected load of 650lbs (combined weight of powered-wheelchair, user, and platform), which translated into a chain tension of 230lbs. It was also important that the attachment method did not interfere with the functionality of the system and be flexible to allow the platform to move. Initially, two concepts were generated and considered. One concept was a specialized attachment chain link that has plates for bolting, and the other was a hook that would be designed and machined. Both of these concepts needed to be considered in concept evaluation before a final decision was made. Stress calculations were conducted for both concepts. The chosen concept was the double-bolt attachment links because of its strength and flexibility. Finite element analysis was then used to determine the maximum stress more accurately with a 300lbs load applied for additional safety.
The attachment location was required to be centered on top of the rail given the attachment type. The top attachment would be flexible in vertical location and the location for the lower attachments was chosen to allow room for the bolts.
The addition of a gate system was imperative to ensure the safety of the wheelchair user and the safety of the system as a whole. The gate system needed to be safe and easy to use. An important factor for the system was that it would automatically engage and require no additional user input. Similarly, the system had to be intuitive to the user. Both gates double as ramps when at the end of the Funicular’s travel.
The front gate utilizes a weighted ramp that swings to a vertical position after the platform has risen off the ground. This system again requires no user input and occurs automatically during the ascent of the platform. When the platform is at its lowest position, the gate also serves as a ramp.
After assembling our prototype, our design team evaluated the ability of the system to meet the design requirements through testing. It was seen that bending was occurring between the chain hoist assemblies and platform. To fix this, two additional structural bolts were added between each chain hoist and the platform. It was also noted that the alignment of all the subsystems was crucial in order to have the platform traverse the rails smoothly. Therefore a good amount of time was spent on aligning the chains, rails, chain hoists and platform.
Once testing was completed, it could be seen if our prototype met our design goals.
All of our design goals were met with the only minor drawback being that the input force needed to raise the platform was just above our target goal. The major contributor to this is friction along the rails. Thus, for future versions of the Funicular, rails with smoother surfaces will be used. Also, different types of power subsystems will be tested in order to minimize user input force including the implementation of a small motor, fly-wheel, hydraulics, or spring-assistance. Now that we have confidence in our properly-working and safe prototype, we are beginning to test the device with actual people who use wheelchairs to get their feedback as well.
We would like to thank a number of people for their help in this project. First we would like to thank previous students who laid the groundwork for this project. Next we would like to thank Bruce Goeser and Gale Pence from Global Precision Industries for aiding in the fabrication of custom parts. Additionally we would like to thank Jim Kraus for giving his insight for the filming of the video clip above. And finally we would like to thank our Professor, Jay Martin, for his dedication, excitement, and encouragement from day one of this project.
- American Disability Association. (2006, November 29). Ada standards for transportation facilities. Retrieved from http://www.adanet.org/
- AmeriGlide Accessibility Solutions. (2013, April). Vertical platform lifts. Retrieved from http://www.ameriglide.com/Vertical-Platform-Lifts.htm
- Harrington: Hoists and Cranes. (2008, November 12). The Weston Brake: Description and operation. Retrieved from http://www.harringtonhoists.com/tech_support/edocs/EDOC 0467 rev00.pdf
Info for Dan Gabioud
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