Before this project began, there were no prosthetic devices available for shoulder disarticulation amputations that could provide the user with the proper range of motion required to swing a golf club. Available prostheses for this level of amputation are rigidly attached to a frame, making the shoulder joint inflexible. The design of a successful shoulder joint requires a good range of motion that can be controlled to remain within the capabilities of a human arm.
Before the start of the 2012/2013 school year, Mr. Bruce Sarkin approached the Department of Mechanical Engineering at the University of Nevada, Reno searching for a graduate student that could help him design a prosthetic shoulder joint that could allow him to play golf. Instead of a single graduate student, Mr. Sarkin’s proposed project was accepted by a team of five senior-level undergraduate students.
Team Fore Arm’s design focused on the shoulder joint and important interfaces, since the other important arm components were commercially available.
For more information on how Team Fore Arm created a prosthetic shoulder joint for Mr. Sarkin, please use the navigation tees at the top of the page.
Team Fore Arm owes the success of their project to many people, for details, please see the Acknowledgements section.
Before the formation of Team Fore Arm, Mr. Sarkin had attempted to manufacture a prosthetic shoulder joint to interface with existing athletic prosthesis. Mr. Sarkin’s prototype had limited mobility, was bulky, and caused the arm to collide with his torso. Team Fore Arm then developed a list of specifications for the ideal prosthetic shoulder joint:
1. the device must not hit the user’s torso during the swing
2. the device needs to interface with existing prosthetic devices
3. the device must weigh less than eight pounds
Since range of motion was the largest priority, the team first focused on finding a joint design that would give the most control over motion. Initially, three shoulder joints were considered. One design was a disk and hinge system. The second design used a spring for the shoulder. The third design was a ball and socket joint. The spring design was dismissed due to issues with constricting motion. The disk and hinge system was disregarded after analyzing the motion, which did not have the fluidity necessary for a golf swing. The ball and socket joint was ultimately chosen due to the fact that it offers smooth motion and a large range of motion. However, this design required a difficult manufacturing process to achieve the desired tolerances.
PROOF OF CONCEPT
The first prototype was fabricated using the 3D printer in the DeLaMare Library. This proof of concept prototype was intended to prove that the initial design would function correctly. Because the material used by the printer was quite dissimilar to the planned material of the final prototype, it was not possible to perform any useful structural testing on the proof of concept. The proof of concept was useful for exploring a variety of possible swing paths and helped to predict and analyze potential problems of the design.
The ball used in the design is a 2” diameter fully hardened stainless steel, chosen for the extremely high tolerance and smooth surface. A flat spot was ground on to the ball using a belt sander and a hole was drilled at the center of the flat spot with a 1/4” carbide drill bit. A 1/2” threaded steel rod was lathed to create a 3/4” long section section with a 1/4” diameter on one end. The reduced diameter portion of the rod was then inserted into the hole in the ball and welded to achieve a secure fit. The rod extends 4” from the ball and interfaces with an aluminum connecting rod that that attaches to other prosthetic devices.
Initial designs involved the machining of delrin, but the socket design was too complicated to create on a 3-axis CNC machine; no 5-axis CNC’s were available. Team Fore Arm decided to have the socket and cap 3D printed in a high-strength nylon plastic by Shapeways.com. An additional cap was machined out of black delrin using a 3-axis CNC. This cap was designed to have a higher coefficient of friction than the 3D printed cap, which helps to restrict the motion of the arm.
The first exploratory tests mapped professional golf swings to refine the shape of the cap design. Average arm positions at various points in a swing showed where the cap should constrain the prosthetic to move along the desired path. A finite element analysis was performed on a solidworks model of the socket that used solid nylon material. It was shown that the design could take forces of impact in the range of 300 lbs. This finite element analysis did not take into account how the nylon was layered in the 3D printing process; it is assumed that the layering would negatively affect the strength of the joint.
Once the final prototype was manufactured, the ease of assembly was tested. It was determined that manufacturing tolerances were sufficient for everything to assemble correctly, with enough friction and weight in the joint to limit erratic motion in the apparatus. The overall weight of total arm is slightly less than eight pounds, which is the average weight of a human arm. The prototype was presented to Mr. Sarkin for feedback, resulting in the following video. During testing with Mr. Sarkin, it was confirmed that the interface points of the joint properly connected to the other prosthetic components.
There are many people that we, the members of Team Fore Arm, would like to thank.
Firstly, we would not have made it this far without the love and support of our families. We would also like to thank all of the instructors of the Department of Mechanical Engineering at the University of Nevada, Reno.
Specifically, we would like to acknowledge Dr. Emil Geiger for leading our Capstone course and Mr. Johnson Wong, Mr. Jacob Reid, and Mr. CJ Dudley for serving as teaching assistants. We would also like to thank Mr. Russell May for serving as our course Writing Fellow.
In addition to our direct classroom advisors, our team would be lost without the teachings of other University professors like Dr. Candice Bauer for our communication skills, Dr. Eric Wang for our knowledge of system dynamics, Dr. Kam K. Leang for our ability to understand and control those systems, as well as Dr. Shen-Yi Luo and Dr. Yanyao Jiang for our comprehension of 3D modeling and strength analysis.
Special thanks to Mr. Douglas Stadler and Mr. Tony Berendsen for helping us manufacture the fabricated components of our design in the Palmer Engineering Manufacturing Lab.
In order to simplify the project as much as possible, many components of our design utilized McMaster-Carr parts that were modified to our exact specifications. Shapeways.com was used to print 3D parts that were otherwise very difficult to machine. The DeLaMare Library allowed us to 3D print a miniature model our design for the initial proof of concept prototype for free.
We are also thankful that Biodesigns Inc. supplied Mr. Sarkin with a custom frame and that Mr. Sarkin was able to acquire commercial prosthetic components such as the TBDRES and the Eagle Golf TD from TRS Inc., which allowed us to focus on the design of the shoulder joint.
And finally, we owe immense gratitude to Mr. Bruce Sarkin for proposing our project and working with Team Fore Arm throughout the year to develop a prosthetic shoulder joint that suit his needs.