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Our goal for this project is to create a custom spinning chair for a young prematurely born child with cerebral palsy. This chair will help the parents give the child visual therapy from home.
Our assigned child was diagnosed with Cerebral Palsy at birth, which has also caused them a visual impairment. We have been asked to design a chair that can roll, spin, and recline so that the child’s visual therapy can be done more easily and effectively at home. We also have been asked to include a massage functionality for the child’s comfort, and a harness to restrain the child’s movement so that they do not lean forwards and fall out of the chair. Due to the patient’s current height and weight, the chair has to be relatively small; however, at the family’s request, we will also design it for them to use as the child grows for the next three to four years.
– Assist patient on visual aid therapy
– Chair with harness and if possible reclining positions
– Manual mechanism
– Portable structure
– Dark colors
As part of our background research, we analyzed the chair finished from the previous semester. We concluded that it was not safe and did not fit under other requirements. Therefore, our group started the project from scratch. One of the ideas was to design something similar to a hanging sensory swing but adapt a chair to it. Another option that the group analyzed was to build a structure based on the base of a welding chair where it rotates 360 degrees and have wheels which can be convenient to transport, and adapt the upper section of the structure with a chair such as therapeutic chair with straps or seatbelts, or a vibrating “rocker”.
“Portable Hanging Chair”
The initial concept design 1 is to have a hanging chair that can spin around. We would build a frame out of aluminum, and design it to fit a chair on the center of the structure. This structure would have small dimensions considering the size of the patient and could be placed on top of tables or the like for ease of access if necessary. In addition, wheels would be added to the bottom of the frame to facilitate transportation. The chair would have a harness to hold the child upright.
“Spinning Plate Chair”
The second concept design is to create a base similar to that of an office chair, but smaller and more sturdy for the child. This would involve adjusting or replacing the structure beneath the chair to better fit the design parameters appropriate to our family’s request. In addition to this, an adjustable seat would be designed on top of the base, which could be easily changed for another to better fit the patient as he grows up.
“Permanent Hanging Chair”
A metal brace would fit around the top of the frame of a sturdy door. From this brace, a rotating hook would hang on a sturdy bearing. This hook would be attached to the chair so that the chair could spin freely within the doorframe. This would make the system very small and portable, though may require custom-machined parts, raising the price and chance of error. The brace could be designed to tighten onto the doorframe by various means, likely by unscrewing/screwing the pinching elements on either side (similar to a vice or clamp).
We have decided to go with the spinning plate chair. It would require the least material, and few to none of its parts would need to be custom-machined, so it is the cheapest option. Not only is it the least likely to fail of our options, but if it were to fail, it would be the least dangerous for the patient, as they will not have anything suspended over their head. Since this choice consists of the least material, it would also be more lightweight and therefore more portable than the other options. This option is likely more stable than the portable hanging chair would be, and though the permanent hanging chair may be more stable, this option does not prevent people from using a doorway while it is in use. It also does not risk marking or damaging doorframes, as the permanent hanging chair could- something that matters to our client, as they intend to move in the near future. This spinning plate chair design also meets all of the family’s needs and would be the easiest to manufacture, as it could likely be constructed entirely from existing, easily purchased parts.
The selected design will be the spinning plate chair. It will consist of a child’s car seat attached to a plate suspended above the ground on a rolling bearing assembled in such a way that the car seat spins freely when the child sits upon it. We believe this design will be the safest, the easiest to manufacture, and the best option for the child and their parents.
The design which provided the best results for reliability and longevity required some higher quality parts that were recommended by ME shop technician, Jeff Randolph, Automotive Design Technician, Mark Davis, and Research and Development Engineer inside MET shop, Kevin Goodwin. With their help we were able to create a SOLIDWORKS model that achieved all aspects of safety and functionality for our assigned child. The following items and processes that will be needed in order to create this desired design is:
– Corrosion Resistant Swiveling Level Mount
– 0.5″ Aluminum Plate (18*18)”
– 0.25′ Aluminum Plate (18*18)”
– 2.5″ Aluminum Round Bar
– 2″ Tapered Bearing Race
– 1 Tapered Bearing Outer
– 1 Tapered Bearing Inner with Seal
– Alloy Steel Shoulder Screw (1″ Diameter, 3-1/2″ Length, 3/4″- 10 thread)
The above items are more than capable of handling the two year old child who will weigh no more than 60-70 pounds in next four to five years. It is also designed to be accessible for the parents (aluminum features) which will allow them to place on top of a counter, and allow for car transport and easy transitions from room to room. Our final concern when considering the sizes of the parts was making sure the chair would not be resting too high off the ground when connected to the plate. With modifications to the design of the stand we ended up with a great product.
The first analysis was done to determine whether the steel shoulder screw used in attaching the bearing was at any risk of failure with expected loading. It was determined that for a 250lbf weight on the chair, the factor of safety for this screw will be n = 41.7. This will be very safe for the child.
The second engineering analysis was done to determine if the feet at the bottom of the structure will be at risk of failure under expected loadings. It was determined as shown below that for the load from a 250lbf force distributed equally across all 5 feet, the factor of safety for the shaft of the foot will be n = 79.4, an even higher factor of safety. This will be very, very safe for this child.
Finally, the third engineering analysis was done to determine how sturdy the base plate of the chair will be. This analysis was done in Solidworks and consisted of a 500N distributed force being loaded onto the base plate. Solidworks solved this situation and declared the lowest factor of safety to be 40 at the eyelets of the screws. This will definitely be sturdy in our case.
At Prof. Canfield’s request, a 4th engineering analysis has been done to analyze the chair’s response to non-axial loadings. A 250lbf load was placed at a distance corresponding to the furthest possible point from the axis of rotation. This caused two reactions- a vertical force, which falls well below the bearings’ maximum loading, and two forces radial to the bearings, responsible for counteracting the moment caused by the load and its reaction. The factor of safety in this situation was found to be 3.77- definitely still safe for the child, especially considering how unlikely this situation is, but considerably lower than the other factors of safety in this design. The image for this engineering analysis can be seen below the images for Engineering Analysis 3.
Through the development of our design, the main goals were to make the final result as close to the SOLIDWORKS models as possible.
Both the base and plate seen in the design process images were water-jetted by the Research and Development Engineer inside MET shop, Kevin Goodwin. The water-jetting machine is very new to Tennessee Tech’s campus and provides fast and precise cutting to the aluminum sheets used to create these two components of the chair.
The bearing assembly was machined and press-fitted by Lucas, Christian, and Michael with the help of Automotive Design Technician, Mark Davis, in the Formula SAE shop.
After the base and plate were completed it was time to give it some color, which also took place under the supervision of Mark when these pieces were powder-coated to our preferred color.
Once the base and plate were finished curing it was time to determine the best way to mount the chair on top. This required some experimentation; however, eventually, we were able to find an easy process. By tapping holes into the plate and chair, we were able to create an opening that could fit a 1/4 bolt, and the only thing required by us was to place the chair in the desired position on the plate. When inserting the bolts through the chair and plate, lock washers with a large surface area were included to prevent an extreme amount of stress on thin plastic pieces when the child is sitting in the seat or during transport.
One last step was required to make sure our chair met all of our safety standards. An angled aluminum backing was mounted to the underside of the plate and connected to the back of the chair. This angled support was mounted to the base plate using 1/4 bolts with locking nuts and then was mounted to the back of the chair using 10-24 bolts and locking nuts. This backing was cut and ground down to our desired shape. This would keep the chair’s back upright. The car seat had originally been designed to be placed on an existing seat with a backing, so without this support, the seat’s back was liable to fall back if spun too quickly, which would likely frighten the child. A rifton small butterfly harness was also integrated to keep the child in place when sitting, to prevent them from leaning forward and falling out of the seat, as well as to limit panic attacks from instability.
For our testing, we were able to meet with the mother and child who would be using the spinning chair daily and get the mother’s opinion as to what she would like changed or adjusted. With her comments, we were able to purchase a better harness that would keep the child better stabilized while in his seat, and tightened the bolt that holds the bearing assembly together to reduce the speed of the chair’s spinning capability.
Another issue discovered during testing was the rigidity and lack of support a car seat has when there is not a structure supporting the upright section. Since a car seat is designed to be placed into a car, it is not designed for the back to remain upright under the weight of a child, as the car’s own seat will hold the back in place. In this case, however, the car seat was not in a car, and so did not have the support it needed, and fell backward when the seat was spun too quickly. This required the fabrication of an angled support (as mentioned in the fabrication process) that could be bolted to the underside of the plate and then riveted onto the back of the car seat. No further issues could be observed after this. This support can be further observed in the images.
For safe use of this spinning chair, the speed of motion has been limited to rid the base of any instability caused by an off-balance center of gravity.
Overall, it has been a great semester for all of us. It was nice getting to meet and get to know the guys within the group, because in the end relationships with those who share the same interests are some of the most important. Having experience with design tools such as SOLIDWORKS and MATLAB really sped up the process of design and allowed us to start the fabrication process relatively early in the semester. What was nice about the early start was that we were able to discuss more efficient processes of production with the technicians and shop managers, which allowed for easier advancement of the fabrication of the parts. With the small issues we ran into during the assembly of the chair itself, it took nothing more than some thought by the group to resolve the problem and completely finish the spinning chair for a happy child.
