Motorized Swing from Suspended Rings

Designers: Austin Derfus and Greg Garbos
Client Coordinators: Debbie Reinhautsen
Supervising Professor: Dr. Larry N. Bohs

The client is an eight-year old girl with cerebral palsy, spastic quadriplegia, blindness and half-hearing loss. Due to her limitations, swinging is one of the few stimuli she can enjoy. Her family desired a motorized swing, because she often desires to swing for hours at a time. She is now 56 pounds, 42 inches tall, and has long outgrown her commercial motorized infant swing. The goal of this project was to design and construct a device to swing the client from rest for extended periods of time, and which will support her through adulthood. The swing is suspended from two rings, which support the swing’s driving mechanism as well as the child. An offset crank converts rotational motion to oscillatory action. Two speeds are available to change the degree of swinging.

How this project helped
Few toys provide positive stimulus for the client due to her cognitive and physical limitations. Since swinging for hours is known to be an enjoyable stimulus, a motorized swing allows her parents to more easily accommodate this need. Additionally, the swing gives her a new seating option. After five months of use, the client’s mother said: “She has so few options for enjoyment, the swing gives her another choice for stimulation that she didn’t have before. She uses it all the time. Other children with disabilities, including those with Down’s syndrome, have visited and enjoyed the swing as well.”

The swing suspends from two rings for compatibility with both the client’s porch and her  school. This design eliminates the need for a frame and increases the swing’s portability.

The swing’s driving mechanism consists of a variable speed motor with an offset crank, and a drive channel attached to a drive shaft. The offset crank moves in the channel and oscillates the drive shaft, which is mounted with pillow block bearings to the swing suspension system. The driving mechanism is mounted rigidly to the swing suspension system, which uses eye-bolts (‘rings’) screwed into overhead attachment points such as ceiling beams. A solid square steel rod slides through both eye-bolts, and custom brackets clamp onto these to prevent the rod from rotating. The swing driving and suspension systems are then mounted to this stationary square rod.

The suspension system includes two swing arms, which act as lever arms to push the swing. These arms are connected together, and attached to the solid square rod by three pillow block bearings. These bearings suspend from the square rod and permit smooth rotation of swing’s drive shaft. Adjustable-length chains connect the swing seat to the swing arms, allowing the swing to gradually obtain the speed of the driving motor. This design is compatible with any seat that can be attached to two chains, providing customization for a given user.

Switching seats involves detaching two caribiners. The swing is powered using standard 110V AC power, with a GFCI (ground fault circuit interrupter) attached for electrical safety. A variable speed control (Dayton 4Z827) converts the AC input to 90V DC for the 1/8 HP, 24 RPM DC gearmotor (Dayton 4Z130), and allows adjustment of the motor speed to match the natural frequency of the swing. Two variable settings in the offset crank allow for two different swing angles.

Safety of the client played a large role in the design. The attachment to two suspended rings prevents the possibility of tip-over. The final swing design was tested using no weight, 25 lbs., 50 lbs., and 170 lbs. For each trial, the swing started properly from rest, and swung to the predicted arc. Human trials were conducted to confirm the weight limit, and to observe the smoothness of the swinging motion. Additionally, a finite element analysis using Pro/MECHANICA was performed on the offset crank, which was considered the most crucial component of the design, to predict its maximum von mises stress and insure it was within the limits of the materials used. While this device was made for a specific individual, it can be used in any location with two suspended eye bolts, used with any seat, and can swing individuals up to 150 pounds.

Materials for the swing cost approximately $435; machining costs were approximately $500.

Comments are closed.