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A child with CP is in need of a device that he can be rocked in from side to side as he lays on his side while wrapping his arms around something the size of an arm. Findings and outcome to be added…
A child with CP is in need of a device that he can be rocked in from side to side as he lays on his side while wrapping his arms around something the size of an arm. This device should include features that maximize its safety, portability, comfort, and durability throughout its intended service life of about 5 years.
1. Must be portable and lightweight. Ideally, should fit in a car for transport
2. Needs to be able to support a load of at least 140 lbs.
3. Needs to accommodate the child as he grows from 30″ to ~66″ in height.
4. The swing assembly must be hand-movable (<40 lbs).
5. Basket-liner material needs to be durable and removable for washing, as well as comfortable and non-abrasive to skin.
The child we are working with has been diagnosed with CP and, as a result, has difficulty with muscular control (body movement/coordination) and detailed vision, among other things. We have been informed that the way he focuses and processes inputs from the outside world is through a steady rocking motion. According to the therapist, the rocking motion mimics the way our eyes naturally process information.
Beyond this, we aren’t sure to what extent his CP affects his other abilities. According to cerebralpalsy.org, the scope of CP can include a wide range of symptoms such as developmental, sensory, and mental impairment.
There are several swings currently available that are used for for comforting therapies. They come in a variety of shapes and sizes, some with and some without frames. Many of the options are just modified hammocks designed to wrap around the individual and swaddle them. This type of hammock-swing is widely available in the $40-$300 price range, and is generally used to relieve anxiety in children with autism. Taking into consideration our client’s needs, no swing currently found on the market offers the components needed to provide a positive experience for the child. He likes to be held and swung in a unique motion while in a very precise position. The mother has also requested that the swing be portable and somewhat mobile, which further eliminates some of the commercially-available swings. Our product aims to be easily disassembled and transported unlike some of the existing stationary swings.
Table 1: The preliminary swing design concepts are compared below using a decision matrix. A scale of 1-5 (5 being the most favorable score) was applied to each important characteristic needed in our design. The design earning the highest total score was chosen as the candidate.
| Characteristics | Upper Hinged Swing w/ Basket | Scissor Frame Swing | Revolute Armature Swing |
|---|---|---|---|
| Portability | 3 | 4 | 2 |
| Durability | 4 | 2 | 4 |
| Comfort | 4 | 1 | 2 |
| Ease of Use | 3 | 3 | 3 |
| Affordability | 2 | 4 | 2 |
| TOTAL | 16 | 14 | 13 |
As shown above, the Upper Hinged Swing w/ Basket [Concept #1] received the highest score within the decision matrix. While it will be more costly to produce, the design maximizes comfort and durability while remaining fairly portable.
Proposed Deliverable Production Schedule
After selecting Concept #1 above, we performed preliminary calculations in order to specify our primary frame material (shown below). We settled on 1.5″ square structural tubing made of 6005A-T6 Aluminum Alloy. The specific tubing we selected had the unique characteristic of being telescopic, which was an important design consideration for our adjustable frame legs. The inner telescopic legs themselves are constructed of the same material, but with 1.25″ outer width. The majority of the design adhered closely to the ideas presented in Concept #1, including the folding hinges and the 4-bar swing basket. The frame of the basket (6061-T6, 1″ Round Tubing) was simplified in the final design to ease the manufacturing process. Additionally, the electronic components were excluded due to the added complexity/weight that we felt did not contribute to the end-goal.
Hand calculations were performed on the top crossmember to allow us to specify an appropriate material for the frame. A few different sizes of telescopic square tubing were considered before settling on the 1.5″ variety in order to balance cost, weight, and strength.
We wanted to visualize the motion of our chosen swing design, so GIM was utilized to generate a kinematic model of the swing basket and ropes. The resulting data showed that the basket rotates 2 degrees for every 10 degrees of rope rotation from equilibrium. The basket also translates about 4 inches in this same period. While these relationships aren’t strictly linear throughout the travel of the swing, they provide a generalized expectation of the basket movement.
After completing the 3D model, we evaluated the true performance of our upper crossmember design by applying a static load equal to an occupant weighing 150 lbs (same weight used to perform preliminary hand calculations). The upper crossmember was chosen for this analysis because it is subjected to the greatest amount of bending stress, making it a critical area to observe. Given the intended use of the device as a therapy swing, the associated displacements/accelerations of the moving basket were deemed to be negligible relative to the force of gravity acting on the loaded mass, so dynamic FEA was not performed. The results below indicated a max stress of about 2.307 ksi and a max displacement of 0.04186″. These values closely agree with our simple hand calculations. Assuming ideal welds that have only a marginally lower yield strength than the base material, these values imply a factor of safety of about 10.4.
| Total Weight | Frame Weight | Basket Weight | Total Volume | Frame (Folded) Volume |
|---|---|---|---|---|
| 36.82 lbs | 26.79 lbs | 7.78 lbs | 751.42 cubic inches (61.78″ x 66.00″ x 55.95″) | 278.51 cubic inches (52.24″ x 46.83″ x 6.05″)
|
As shown below, the center of gravity still falls within the footprint of the frame when the basket is at its furthest extent of travel with a 150 lb occupant. This implies that under safe/normal operating conditions, the swing will not experience any tipping behavior.
Testing the device was fairly simple and can ensure that it is safe and sturdy. The swing was designed for a child weighing only 25 lb, however we continually placed combined loads of close to 400 lb on the assembled frame. The basket was able to support the weight of 185 lbs easily. Needless to say the swing is sturdy and can easily take the weight of a 25 lb child. The swing has several locking tabs and pins that must be manipulated in order to properly secure and fold the frame.
The final product will be delivered to the client in their home.
The Swing assembly is made up of 8 main weldments:
Folding Procedure*:
*Reverse the Folding Procedure to properly unfold the Swing
(Starting from a folded configuration)
Step 1: Unlock the pin directly under the carry handle and remove the limit cables from the pin.
Step 2: Unfold the legs away from the upper cross member to a standing position.
Step 3: Located at the top of the frame, insert a locking pin into the tabs above each set of legs. It may be required to manipulate the legs to find an optimal position as the tolerances are tight to prevent movement.
Step 4: Unlock the telescopic legs located at the bottom of each leg to that the extensions may slide freely.
Step 5: Extend the legs to the desired position and reinsert the lock pins to secure them.
Repeat steps 4 & 5 for all legs.
Step 6: Once the frame is in a standing position, place the basket assembly under the frame such that the chain attachments are directly under the tabs on the frame.
Step 7: Lift the basket by the top link and attach it the the carabiner located on the frame.
Repeat step 7 for all four corners.
The swing is now set up and ready for use. To fold the sing back to a portable state, follow the steps in reverse order.
This project has offered great insight into the world of product development and fabrication. Taking a design from a purely conceptual stage all of the way to a finished and fully-realized item really exposes the challenges involved with practical engineering. Regardless of how diligent everyone is, it is very likely that hiccups will be encountered when converting a digital design into a physical product. In unplanned situations such as these, we learned the importance of critical thinking and adaptability and were able to overcome the obstacles that we experienced. Being such a large-scale group project, completing the swing required us to communicate effectively with each other, and the therapist representing our client.
Overall, we feel that the entire project was a success. We provided the client with some interesting design concepts, thoroughly modeled the chosen design, and fabricated it accurately through the use of many computer-generated 2D drawings. The entire project went relatively smoothly due to the cooperation and hard-work shown by all teammates. This enabled us to have an impactful learning experience.
