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Our team was tasked with creating an arm-powered swing for a child with spinal muscular atrophy.
The parents of a two-year old child with SMA (spinal muscular atrophy) asked us to make an arm-powered swing that their child could use. They asked that it be portable and that she could continue to use it as she grows. the goal of the project is to give the child a way to improve her upper body strength while using the swing.
– Safety: Safety should always be the number priority especially when dealing with projects that children will interact with.
– Ease of Use: It should not require significant preparation time or upkeep
– Sturdiness: The design should be able to last a long time
– Add arm-powered mechanism without compromising swing functionality
– Portability: The family wants to be able to move the swing easily
– Have the swing be able to accommodate her as she grows
“Spinal muscular atrophy (SMA) is a genetic disease affecting the central nervous system, peripheral nervous system, and voluntary muscle movement (skeletal muscle).” (https://www.mda.org/disease/spinal-muscular-atrophy)
We learned that SMA causes the person to lose nerve cells. Some of these nerve cells are “motor neurons” which control muscles. Without these nerve cells the person has limited mobility and strength due to their muscles not being able to move. As they get older, the condition of the disease causes various effects. The most common is restricting them to a motorized wheelchair with little mobility in the arms and close to no mobility in the legs.
When we met with the family we learned that the recipient of our project actually has surprisingly good strength and mobility for a SMA patient. The parents wanted to build on the progress she already made by letting her work on upper body strength using a swing.
We tried to find examples of “arm-powered swings” on the internet. There were not many arm-powered designs that we could find.
One works by the user shifting the center of Gravity of the seat frame and the other is a glider swing.
A rigid bar with connected 90 degree handles is connected to a suspended chair. The user moves themselves with respect to the handle/rod resulting in an almost pure arm movement swing
This design would utilize a normal child seat with two bars that rise from the arms of the chair and extend through the bottom. A four bar would be formed underneath the chair with weights attached. The child would perform a rowing motion with her arms using the bars to move the weight underneath the swing causing the swing to move.
Similar to design 2 this design focus is on a regular child chair except the child uses their full body with extra help from their arms. Unlike a regular child swing, pulling in the arms would cause the leg rest to rise and the back support to lean back. This alow the child to mimic a more natural swinging motion and possibly develop the proper muscles to use a regular swing in the future.
We selected Concept Design 3. A couple of our members held a Zoom meeting with the therapist and the family where we went over our concepts. The family showed the most interest in concept design 3 as it should give the best overall workout for the child.
Our selected design has handles on the front which when pushed/pulled on rotate the back and leg rests forward and backward. The seat consists of a base and two sides with a rotating backrest. The backrest is connected to binary links forming a four bar with the handlebars.
We chose two different materials for the swing. We will use hollow, square aluminum 6061 rods for the mechanism and yellow pine wood for the seat, leg rest, and backrest. The frame is made out of galvanized steel. The chains are steel.
The seat consists of four parts: the base where the user sits, the two sides that act as arm rests, and a T-bar that acts as a safety feature. The pieces are rigidly connected to each other by screws. All of these pieces are made out of wood. There is also a safety strap that goes over the child’s waist.
The backrest is made of wood. It rigidly connects to the back rods. It connects to the seat base with revolute joints to allow rotation, when the locking pin is disengaged.
The leg rest is made from wood. It lays on top of the two handlebar rods in the front of the swing. It will be rigidly connected to the handle bars so that when the handles move the leg rest moves with it.
The mechanism consists of three binary links, forming a four bar, and a ground which in this case is the swing seat. There is a front rod where the user grabs on and initiates the motion. The backrest is included in the mechanism. The mechanism’s purpose is to translate rotational input from the handles into rotational motion in the leg and back rests.
We decided to buy a frame and attach the swing to it. Finding a portable frame without a online that was from a reputable seller, looked sturdy, and had good dimensions and weight tolerance was difficult. Eventually we settled on this frame https://funandfunction.com/portable-swing-frame.html.
In the first engineering analysis we calculated the max velocity of the swing based on angle with respect to the normal line to the ground.
For the second engineering analysis we estimated how drag force gets reduced as the swing leg/back rests extend.
We wanted to see if the sum of the weight of the child along with the centripetal force from the swinging would break the chain or frame based on the restraints given from the producers. We used the estimated max velocity from analysis 1. The max strength of the chain is 700Ib while the frame is 200Ib. Both of these values are more than double the expected max forces applied.
We purchased wood from Grant Cedar Mill. We decided to use red cedar for all of the wooden parts. We purchased stainless steel hardware from Lowes, McMaster Carr, and Highland Hardware. We cut the wood to size and drilled holes in the correct places. There is a hinge in between the backrest and seat that was probably the trickiest part of fabrication. We ended up making cutouts in both pieces that fit together and fastening them using bolts.
We ran into a couple of issues with the wood having lower than ideal quality. We ended up using shorter bars for the mechanism than we planned. We also decided not to use the leg rest piece.
After the pieces were all done we sanded all the edges. Then we applied 3 coats of a polyurethane varnish to each piece.
We put together a prototype of our swing to test it’s functionality as well as different attachment points for the chains.
We built the prototype out of 2×4’s and plywood. We made it big enough so that we could sit in it and test it.
We determined that the arm-powered motion of the swing worked as intended.
We tested multiple attachment points for the chains on both the swing body and the handle/backrest pieces. We determined that attaching the chain to the backrest itself as opposed to the armrest allows the swing to return to an upright position easier. The attachment point on the front side was left unchanged.
Adult supervision is recommended.
We delivered the swing and assembled the frame. Unfortunately, due to weather conditions we did not get to see a full demonstration of the swing.
This project was difficult because there is no reference for a project like this. As far as we know this is the first swing of its kind ever made. This project being in uncharted territory made it difficult to get it exactly right. There were many issues that we ran into that we solved as a team. We believe given the resources and time we had we made a very good product. We hope the family gets a lot of use out of it.
