Jessica Jeffrey, Jacob McCarthy, Warren Roser, John Sweet
Abstract
The objective of this project was to improve upon an existing device developed by a Rochester Institute of Technology (RIT) Multidisciplinary Senior Design Team. The redesigned system is an accessory adaptable to manual wheelchairs that offers users the ability to stop on a hill via hand controls. The system includes a three-pawl engagement mechanism to prevent rollback and bicycle braking system. To ensure all three pawls engaged simultaneously, an experiment was performed to inspect the interaction and evaluate accelerated life testing. The final result of this experiment indicated that the system would function properly for over 5 years if engaged for 10% of the time.
Background
Users of manual wheelchairs naturally encounter problems every time they traverse inclines and declines. First, when traversing uphill, wheelchairs tend to roll backwards impeding forward motion. Second, when declining, users do not have the ability to steer or slow their speed without endangering their hands. In both scenarios, users that may otherwise be self-sufficient require an aid.
Between 2013 and 2014, RIT MSD Team P14007 began designing and prototyping a solution to these problems. The system included an anti-rollback device based on a ratchet and pawl structure. It could be installed onto manual wheelchairs to prevent rollback by removing and modifying the current wheelchair shaft. In addition, a bicycle braking system could be mounted to the shaft and wheelchair in order to give users the ability to slow or stop on a decline. One drawback of this device is that it was not easily adaptable to the variety of manual wheelchairs in today’s market. It was custom designed and permanently modified the wheelchair. The wheelchair could not revert back to original form.
RIT MSD Team P15007 was assembled to redesign and improve the anti-rollback device. The team was tasked with maintaining the current ratchet and pawl concept, but also address a new series of customer requirements. The goal was a redesign such that the system was more desirable to consumers, adaptable to a wider variety of manual wheelchair without permanent modification, and manufacturable on a large scale at a low price.
Methodology
The driving force behind this project was to create a feasible assistive device that prevents rollback and facilitates braking to meet customer requirements and deliverables. The customer’s main focus was ensuring that the system was attractive to consumers including high priority attributes such as, light weight system, adaptability without compromising function, low cost, and easy installation/maintenance. After discussing customer requirements, the team detailed a series of engineering requirements depicted in Table 1 below that acted as goals throughout the design process. The scale within Table 1 begins at 1 implying lowest importance and peaks at 9 signifying highest importance. In addition to the engineering requirements, three deliverables are anticipated at the conclusion of the project cycle: functioning prototype with documentation, scaled-up manufacturing plan, and installation/maintenance manual.
When redesigning the device to meet the criteria, the initial constraint was to retain the ratchet and pawl concept for the anti-rollback system. As a result, the main concept was adapted from the alpha design including the braking system and bearing block concept. However, given that engineering requirement S13 implies that the wheelchair must be reconfigured to its original state, the device’s mounting components needed to be revised.
Analysis
In order to best understand ways in which the system could be used, three conceptual use scenarios were defined to analyze the physical requirements of the components. They included a client reaching an incline/decline, aid reaching incline/decline, and client/aid interaction. From this brainstorming, the following five technical loading components were identified to analyze components when the system is loaded:
1. General Loading Scenario
2. Hill Loading at angel θ
3. Stress Elements
4. Level Ground
5. Tipping (one wheel elevated)
After evaluating these potential scenarios, a Design Test Life analysis and Shaft analysis were performed to estimate the material requirements. First, research revealed that the average life of a standard manual wheelchair was five years. Since many insurance companies covered a new wheelchair every five years, it was reasonable to design the system to last five years, as well. Assuming that individuals utilize their wheelchair approximately for one mile per day, it was estimated that in five years the wheelchair would travel 1.5 million revolutions. After further analysis, it was determined that the majority of clients would face a scenario requiring them to engage the ratchet and pawl system approximately 10% of the time. This led to a designed test life of the system, referenced by engineering requirement S8, to be 150,000 revolutions. Otherwise known as the 90/10 use scenario.
In addition to the Design Test Life analysis, the Shaft analysis calculated what material principles were required for the shaft to sustain the design test life assuming user weight, potential tipping, and material wear. Figure 1 illustrates the free body diagram used as the basis for this analysis. Note that x5 denotes the location of the center of gravity of the wheelchair and client. The purpose of the analysis was to choose a material that would yield a factor of safety greater than 1.3. The second figure below displays a range of potential shaft materials and the chosen material, 4140PH in purple. As can be seen, the 4140PH has a factory of safety greater than the desired 1.3 bold line.
Testing
To ensure that all engineering requirements have been met, several tests were conducted. The biggest risk to the system was the ability of the pawls to engage simultaneously such that the anti-rollback functioned properly. This was directly related to engineering requirements S1, S3, and S8. The Ratchet and Pawl Test lead to the development of a test fixture, depicted in the figure below. The test fixture represented a constantly engaged anti-rollback system. A motor spun the ratchet while the three pawls remained stationary. The goal of the test was to spin the ratchet at a low, constant speed for at least 150,000 revolutions which was representative of life testing. Throughout the tests conducted, the ratchet teeth were consistently measured in order to evaluate wear patterns.
The first test performed, Test A, utilized three identical pawls. These pawls were of the first designed and can be referred to as Pawl A. In this test, the springs were anchored to the fixture with an approximate spring force of 1.5 Newtons. The test ran for approximately 100,000 revolutions before it was turned off for data collection. At this time, it was noticeable that the pawls were no longer engaging the ratchet. As was performed before the test, the ratchet was evaluated under an optical comparator in order to determine if the teeth were being worn away. It was concluded that there was no statistical difference in the size of the ratchet teeth; thus, only the pawl material was reduced. Under further review, it was clear that the pawls were deformed to the point of ultimate failure. Given that the hardware was analyzed at 62,000 and 100,000 revolutions, it was estimated that the pawls failed around approximately 75,000 cycles. As a result of these test results, the validity of the test, spring force, pawl design, and component material was reviewed in order to determine next steps.
Knowledge gained from Test A resulted in one main change administered in Test B. A brief analysis demonstrated that the spring force in the actual assembly would only be about 0.5 Newtons. In Test A, the spring force exerted on the pawls was three times this force. As this alone could have highly contributed to the material wear on the pawls, the test was ran again with new pawls, still of Pawl Design A. Test B ran for over 180,000 revolutions before all three pawls stopped engaging.
In addition to evaluating and implementing the new spring force in Test B, the pawl design was analyzed for potential improvements. As a result, Pawl Design B was developed which is in essence just a thicker pawl design. The concept driving this new shape is that regardless of the design, there will still be wear as a result of the interaction between ratchet and pawl. Thus, if there is more material to be worn away before ultimate failure, then the device will have a longer life.
Results and Discussion
The final prototype was evaluated according to the engineering requirements derived from customer needs and preset deliverables previously described. Upon completion the delivered prototype successfully addressed all engineering requirements within a reasonable margin. Table below depicts the results of each test associated with the engineering requirements. In addition to these requirements, the functioning prototype with associated documentation such as the CAD and bill of materials, Scaled-up Manufacturing Plan and Installation and Maintenance Manual has been delivered to the customer at RIT.
Specifically in regards to Engineering Requirement S8, the test resulted in surpassing the pawls’ designed life expectancy of 150,000 revolutions. Similarly, the ratchet was tested for a combined total of over 280,000 cycles. Thus, it can be concluded from this test that the ratchet and pawl sub-system would function properly for over 5 years if engaged for 10% over this period at average utilization.
Conclusions and Recommendations
This project was intended to redesign a previously functioning hill-holding prototype to better appeal to consumers while simultaneously prepare the design to potentially launch to market. The P15007 Anti-Rollback prototype has met customer requirements by successfully addressing 100% of all engineering requirements. The device has retained the ratchet and pawl concept that functions as an anti-rollback system. The braking system is refined to include locking functionality in addition to original slowing and braking abilities.
In order for the system to be brought to market, there are a few recommendations that could be explored to refine the design and present it as an even more appealing option for clients. These recommendations include:
- Maintain quick-release functionality of wheelchairs by adding a pin in manufactured shaft
- Review design to reduce variation in size and length of fasteners
- Weight reduction through component design
- Material selection
- Alert system to ensure client is aware of system reaching end of life
Acknowledgements
Team P15007 would like to acknowledge and thank the following individuals and organizations for their support, guidance, and assistance throughout the project.
- Rochester Institute of Technology
- Edward Hanzlik
- Dr. Elizabeth DeBartolo
- Jack Cholette
- Dave Hathaway
- Jan Maneti
- Rob Kraynik
- John Bonzo
- Adjuncts: Pat and Mark
- Monroe Wheelchair
- CP Rochester