Scissor Lift
Ankit Gupta, Jillian Haac, Nabil Khan, Vishal Rao, Richard Goldberg, Kevin Caves (University of North Carolina at Chapel Hill)
Abstract
Our client,“Zoe,” is an 8-year old girl who uses a wheelchair due to a spinal cord injury. She is able to transfer onto a platform at the same height as her wheelchair but two people are needed to assist her in moving between her wheelchair and the floor. Once on the floor, Zoe has ample arm strength to easily move around. Our client would like to be able to transfer onto the floor and back up onto her wheelchair with the help of just one assistant. This would allow her to participate in classroom activities on the floor and play with her younger sister at home.
Introduction/Background
Spinal cord injuries result from damage to the nerve roots that reside in the spine and brain. The severity of spinal cord injuries depends on the classification and position of the trauma (1). Our client, whom we will call “Zoe,” is an 8-year-old girl who sustained a spinal cord injury two years ago that left her with paralyzed lower extremities and mild respiratory difficulty. Zoe did retain mobility in her upper body, which has enabled her to operate a manual wheelchair and maneuver on the floor with her arms. However, due to the paralysis of her legs and her increasing weight as a result of normal growth, two assistants are needed to safely lift her from the ground to her wheelchair and vice versa. Unfortunately, Zoe does not always have access to two assistants at the home or classroom setting that can help her in and out of her wheelchair. This constraint limits her independence, inhibits her ability to participate in classroom floor activities, and discourages her from getting out of her wheelchair at home. There are no specific products on the market that can currently assist Zoe with getting to and from the floor more independently. While there are devices that are utilized to move loads vertically, most are designed to lift cars or industrial crates and exceed the force needed to lift a person; this renders current products both cost-prohibitive and unsafe for human use.
Project Goals
The goal of this project is to create a device that allows the client to move between her wheelchair and the floor with the assistance of only one other person. The device must be lightweight, compact, and portable so that Zoe’s caretakers are able to easily transport it between school and home.
Design and Development
Our device (shown in Figure 1) has a platform that moves up and down to transport Zoe between her wheelchair and the floor. Its principle of operation is similar to that of an adjustable office chair. A lockable gas spring inside of an office chair maintains its position when the lever is not depressed. If the user depresses the lever while sitting in the chair, the gas spring can be compressed and the seat lowers. If the user removes their weight from the seat and depresses the lever, the gas spring will extend and the seat will rise.
The top platform of our device will be controlled in the same manner. At its raised height of 22 inches, Zoe will place her wheelchair alongside the platform. She will perform a lateral transfer to move from the seat of her wheelchair onto the middle of the platform. Once she is comfortably seated, Zoe will push a button that unlocks the gas spring, which allows the platform to gently move down towards the floor. Once it has descended to its lowered position at five inches, Zoe can easily transfer onto the floor from the platform. To get back into her wheelchair, Zoe will position herself between the two armrests, and use them to partially lift her body from the platform, a process we have termed “de-weighting.”During this process, the client or assistant will press the button, unlocking the gas spring and allowing the platform to rise to the client’s current position. The button will then be released, and Zoe will be able to fully place her weight on the platform. Then, she will adjust the vertical position of the armrests to a comfortable height and perform the same maneuver again. This will be repeated until the platform is at its raised position. Zoe can then transfer from the platform onto the seat of her wheelchair.
Zoe on the Scissor Lift
Scissor Lift
Our device is based on a scissor lift design, which is often used in an industrial setting in order to lift heavy objects [2].It consists of two sets of diagonal arms attached to each of the corners of the top and bottom platform. When a force is applied horizontally to a bar connecting two of the arms away from the edge of the platform (shown in Figure 1), they move away from the edge of the bottom platform in order to lift the top platform. When this force is released, the arms begin to lower back towards the ground and the platform returns to its lowered position. The scissor lift mechanism is very efficient because it provides the most vertical height displacement for the smallest mechanical input, as used in industrial and hospital settings.
In order to implement a scissor lift, we used 1-inch square aluminum tubing with a thickness of 1/8 inch. This provides us with adequate strength but is also lightweight. On one side of the bottom and top platform, the arms are mounted to heavy-duty drawer slides. This allows these ends of the arms to move horizontally along the top and bottom frames, which is necessary in order for the platform to be raised and lowered.
Gas Spring
We used a lockable gas spring as our horizontal force to control the raising and lowering of the platform. The gas spring sits inside the frame of our bottom platform and is used as the force to push the arms of the scissor lift up, as described previously. When the button is not depressed, the release valve of the gas spring is closed and it remains in a locked position. While our client is sitting on the platform, she will push the button, opening the release valve and allowing the gas spring to compress. When the gas spring compresses, the top platform will lower.
Armrest System
We created an armrest system to allow Zoe to de-weight herself from the top platform. This is necessary in order for the gas spring to extend and the top platform to rise. We secured two 30” high aluminum bars to the bottom platform. We chose this height so that the bars would always remain above the surface of the top platform. These bars are located to the left and right of where Zoe will be positioned on the platform. The armrests have a rectangular cutout at the end so that they can move up and down these stability bars. When Zoe tilts the armrests at an angle, she will be able to move the armrest location vertically to move them to a comfortable position. When she applies a downward force on the armrests, they will remain in the current location because of the friction between the rubber-metal interface between the stability bar and armrest. When the armrest system is supporting Zoe’s weight, the gas spring will be able to extend and the platform will be raised. The theory behind this process is shown in Figure 2. One of the armrests will also have a removable pin so that Zoe can easily move it out of her way during transfers on and off the platform.
Platform
Aluminum angle-strips were used to create a rectangular frame for the top and bottom platform. A steel wire frame manufactured for cabinet shelving was used to create the surface top platform. In addition, a yoga mat was attached to the top platform in a double layer in order to ensure comfort. The surface of the yoga mat allows Zoe to slide on it while she is transferring onto the platform but it also will not allow her to slip off.
Safety
There is a short ramp, about 5 inches long, extending from the platform, which will direct Zoe’s legs away from the scissor lift mechanism when in use. This will avoid any possible injuries that could occur if her legs went under the platform while it is lowering. This same ramp will assist Zoe in moving onto the platform from the floor when the device is in its lowered position. There is also a backrest made of a lightweight wood attached to the top platform between where the armrests are positioned. This ensures that Zoe will not fall back off of the platform while it is moving or during transfer.
Portability
In order to achieve portability of the device, wheels are attached to the short side of the bottom platform. A strap is attached to the opposite side so the user can pull the device while it is rolling on its wheels. The device weighs thirty pounds, so an adult can lift it in order to put it in a car for transport or to lift over obstacles such as stairs.
Results
A full-scale, working prototype of our device was tested with Zoe to evaluate the design, as shown in Figure 3. Though the raised platform has a height of 22”, and rested 3” higher than the 19” bed of the wheelchair seat, the lateral transfer was still comfortably performed, and left the option of accommodating a wheelchair with a higher chair bed for future use. The platform remained very stable during this transfer, confirming that the locking mechanism of the gas spring held its integrity when loaded. Zoe’s physical therapist pressed the button in order to lower the platform to its bottom position. Zoe was able to transfer between the bottom platform and the floor with little assistance. Our client was also easily able to use the de-weighting system in order to raise the platform.
By testing the device with our client, we were able to show that it provides a way for Zoe to transfer between her and the floor with the help of just one assistant. This will allow Zoe to take advantage of the mobility that she has once sitting on the floor. However, the device does not have the appropriate damping to moderate its speed on the way down. This results in the slamming of the platforms due to excess speed, which we have attempted to solve by using shock absorbers, gas springs and hard stops attached to both frames. Though we have not finalized a solution to mitigate or eliminate this problem, we are currently testing many possibilities.
Despite this issue, the responses of the client’s mother and physical therapists were positive and hopeful. Zoe’s physical therapist commented that she would be able to use the device repeatedly and comfortably.
The materials used to build our device cost a total of $544.44. Although it is important for this device to be affordable for possible users, it is necessary to use materials that have the strength required for safety.
Acknowledgements
We would like to express our sincere gratitude to our client Zoe and her mother for their patience, enthusiasm, and support throughout this semester. It was a wonderful and unique opportunity to work with such a spirited and energetic client. We would also like to thank Zoe’s physical therapist, Charlotte Hughes, for her valuable insights on the device’s needs, as well as her unwavering encouragement towards completing our goal. A very special thanks goes out to Steve Emanual for offering his invaluable skills and experience in fabrication, and moreover, allowing us to exploit him for these skills to the tune of more than a hundred hours in front of the mill. Finally, we would like to thank Professors Richard Goldberg and Kevin Caves for their guidance, dedication, and encouragement in seeing this project through. Funding for this project was provided by NSF grant #0453339.