Bamboo Crutch Design for Developing Communities in Zambia Abstract Introduction

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1 Bamboo Crutch Design for Developing Communities in Zambia Failure Analysis, Static Analysis, Static Testing and Manufacturing Plan Neil G Syal, Panos Y Papalambros Dept. of Mechanical Engineering, University of Michigan, Ann Arbor, MI Abstract The purpose of this project is to develop a crutch that can be built from local materials in Zambia and manufactured with basic hand tools. According to a joint paper by U.S. Aid and The World Health Organization, providing adequate funding and improving affordability should be one of the primary strategies for improving accessibility to mobility aids in lesser resourced areas. By working with Non Governmental Organizations (NGOs) that currently distribute aluminum crutches in Zambia, the unique needs of mobility aid users in the region. This report provides the failure analysis, structural analysis, static loading test results and manufacturing plan of the crutch designed in the report Bamboo Crutch Design for Developing Communities in Zambia Research, Ideation and Design. Based on the results, the crutch was most likely to fail at the crutch handle via the support reed buckling. From the analysis and tests, the crutch design was able to support up to the 95th percentile of male Zambians without showing any signs of buckling. Lastly, this report presents a detailed pictorial manufacturing plan to teach people of a non technical background how to manufacture the crutch and inspect the product to maintain high quality. The future of this project is the implementation phase which the NGOs will review the design and decide to implement the design to better satisfy the demand for crutches in Zambia. Introduction Bamboo Crutch Design for Developing Communities in Zambia Research, Ideation and Design identifies the status of mobility aid distribution in Zambia as a design opportunity [1]. There exist a staggering proportion of disabled Zambians that lack access to a mobility aid: barriers to access being both economic and geographic. Current efforts by Non Governmental Organizations (NGOs) collect and distribute aluminum crutches, though admirable, are not sufficient to satisfy the demand [2]. Through local manufacturing, the NGOs are able to reduce cost and eliminate the need for shipping, both of which will improve accessibility. Large variations in the material properties can exist between two pieces of bamboo. This variation, combined with the large dimensional tolerances associated with the use of hand tools, equate to a project which is difficult to model. Failure Mode Effect Analysis (FMEA) was performed to identify failure modes and the risk associated with them. Based on the FMEA results, the appropriate test was conducted to determine the strength of the most at risk failure mode. In addition, to enable future fabrication, a simple manufacturing plan has been developed to enable a person of a nontechnical background to fabricate the crutch. Lastly, possible applications of these techniques and their further developments are presented in hopes to improve living conditions in rural areas of Zambia. This purpose of this paper to is to report the FMEA results, perform structural analysis on the most at risk failure mode, design and perform a test to validate the calculation and present the manufacturing and future work of the design. 1

2 Design Overview Design requirements: Traditional aluminum crutches are highly durable and are the ideal solution. Unfortunately, their high retail and shipping costs make them inaccessible to many people in the Solwezi District. The homemade crutches built from local materials are the most accessible since material cost is low and shipping is not required. These crutches have a far lower expected lifetime due to their lack of standardized manufacturing plan or quality control [3]. After identifying a significant need for crutches in the Solwezi District, six design requirements were developed to determine the success of the project show below in table 1 [1]. Table 1: Any design concept generated must fulfill these requirements. Design Requirements: Support dynamic loading from a 160 lbs person [4] Last the user at least one year Built from materials available in Solwezi District [1] [3] Verification Method: Static load test Cyclic loading test* Material research, ethnography Built using only hand tools Prototype, manufacturing plan (Appendix. A ) Fabrication and distribution cost 10% of aluminum crutches Fabrication time less than 3 man hours *Cyclic loading tests has yet to be designed or performed Price estimates from CCZ interviews [1] [3] Follow manufacturing plan, time with stopwatch Final Design; The final design originated from one of the earlier concepts design 3 T junction crutch design due to its simplicity to manufacture [1]. The final design implemented joint designs and truss support at the underarm and handle to support the necessary loading and bending moment. The tip design is integrated for protecting the bamboo shaft. Figure 1 below shows a detailed picture of the final design. Figure 1: Prototype photo demonstrating final design 2

3 Failure Mode Identification To determine what static analysis would give meaningful insight to the crutch behavior during use, Failure Mode Effects Analysis (FMEA) was performed. FMEA is a design tool used by design and manufacturing engineers to identify possible failure modes of a product and evaluate the risk of the product failing. (table 2) The failure modes were identified by evaluating the design of the product. From the evaluation, three possible sources were identified as functionality failures: failure in the underarm support, failure of the crutch handle and failure in the main crutch shaft. Each failure mode was then further evaluated to understand the causes for the failures. Table 3 below shows each failure mode and the reasons for each failure. Each cause was then evaluated based on three parameters: likelihood of occurrence, likelihood of detection and severity of failure. Each parameter was evaluated on a one to ten scale with one defined as the best and 10 being the worst possible scenario. The parameters for each cause of failure were then multiplied to produce the Risk Priority Number (RPN). The RPN is used to rank the causes of failure and rank the failure mode with the highest amount of risk. Tables 4 and 5 below show that the fracture of the support reeds had the highest RPN and the handle joint of the crutch had the highest total RPN. Table 3: The RPN ranking for each cause of failure. Cause of Failure Occurrence Detection Severity RPN Failure in handle support reed Failure in underarm support rees Yield, slip or damage of lashings Handle Joint Crack propagation Fracture in crutch handle Slip in handle main shaft joint Bamboo degradation Yield, slip or damage of lashings Underarm Joint Fracture in main arm rest

4 From the FEMA analysis, failure at handle was determined to have the highest RPN. In addition, the support reeds had the highest RPN compared to the other possible causes for failure. With these results, analysis and tests were performed on the support reed of the handle joint. By analyzing and testing the support reed on the handle, meaningful results were gathered that give insight to how much weight the product can support. Structural Analysis Simple buckling calculations show that maximum loads in the main shaft can withstand loads of 4000N. For this reason, analysis will focus on the crutch handle as the cause of failure. The first step of analyzing the crutch handle is to represent the system as a simple statics model (figure 2). For the purposes of this analysis all of the joints are pin joints because they do not constrain any moments. Additionally, all joints on the main shaft are considered grounded because of the significantly larger second moment of area of the main shaft. With these constraints one can do some basic static analysis to determine the force in the reed as a function of the weight and handle geometry. This allows one to connect loading of the handle to the maximum force in the reed (Eqn 1). F reed = 1 2 W /cos(θ) (Eqn.1) Due to the slender nature of the reed it s reasonable to conduct analysis for buckling. Fortunately, research has been performed on the buckling of bamboo pieces for the purpose of construction [5]. The method used in Column Buckling of Structural Bamboo is similar to a standard calculation for the buckling of columns with a slightly different method for handling material variation in bamboo (Eqn. 2,3 and 4). λ = r L (Eqn.2) r = I/A (Eqn.3) f cr = π 2 2 E b,d/λ (Eqn.4) Where λ is slenderness, L is length, I is second moment of area, A is area, f cr is elastic critical buckling strength, is the design modulus against bending. E b,d After determining the critical buckling strength, the design method calls for using the Perry Robertson design method with design constants from experimental testing (Eqn 5 and 6). 4

5 f cc,d = f cr f c,d / (ϕ + ( ϕ 2 1/2 f cr f c,d ) ) (Eqn.5) ϕ = ( f c,d + ( 1 + η) f cr ) /2 (Eqn.6) Where f cc,d is the design compressive buckling strength, f c,d is the design compressive strength, η is the Perry Constant [5] [6]. By relating Eq 1 to Eq 5, the handle will buckle at a load of 1100 N or 250 lbs. It is worth mentioning that the method described above is inherently conservative in nature. The compressive strength is given a safety factor of 1.5, the truss actually does constrain a moment and the modulus of bending is likely higher in a reed than bamboo. Exploration of Handle Geometry: A key point brought up in this paper is that both material properties and the physical size of bamboo can vary widely. This is large important when constructing a handle because the geometry of the handle is largely dependent on distance between nodes. Using the same analysis shown in the previous section, a plot was created to explore how geometry affects buckling behavior. The analysis varies some of the key factors in the previous equations to see their effect (Eqn.9). θ = arctan(l handle/ L internode ) (Eqn.7) L reed = 2 (L ) (L ) (Eqn.8) 2 handle + internode Where L handle is the length of the handle, L internode is the height of the handle, L reed is the length of the reed undergoing buckling.this investigation shows that from a buckling perspective it is most important to choose pieces of bamboo that are short. This sort of behavior is somewhat expected because both handle length and internode length affect the length of the reed. The longer a reed is the more slender it is and more prone it is to buckling. Another interesting feature of this graph is that there does become a point that having too short of an internode length starts to negatively impact the design. This feature likely occurs because when theta become too large the reed will need a comparatively large force in order to hold up the reed. From this plot, it s easy to see that the slenderness of the reed is an incredibly important factor for the buckling of a beam. One can postulated what would happen if there was a requirement that the reed length must be at least 9.5 mm as opposed to 9 mm (figure 3 and 4). 5

6 (3) (4) Figure 3 & 4: Maximum force on handle before buckling occurs in a reed of 9 mm (3) and 9.5mm(4) diameter On average, the increase of diameter by 5% increased the max load by 22%. This is to be expected because this increased the moment of inertia by 1.05^4 or roughly 22%. So by increasing the moment of inertia of the reed, in this case by increasing the diameter of the reed, the force withstood before buckling is greatly increased. Another potential way of increasing the moment of inertia would be to tie an additional reed to the load bearing reed. Such a solution would also greatly increase the force that the handle is able to maintain. Lashing Forces: The forces in the lashing are highly dependent on the angle, theta, and load on the crutch handle. By constructing a free body diagram, the force in the reed can relate to the force that the lashing must withhold. F lashing = F reed sin(θ) (Eqn.9) For the purposes of this analysis, the worse angle case of 45 degrees and a maximum load of 1300 N on the handle was used. In this case, the lashing held 650 N total. The Zambian community is likely to use a number of different material to lash the crutches together. For the purposes of this project, a relatively weak fibrous material with a maximum load of 90 N was chosen. Each lashing will be able to hold a maximum of 180 N because it is constraining the handle in a loop. The last assumption is that each lashing has a preload of around 10% of its maximum load. With these consideration the lashing will be able to hold with a safety factor of 2.1. It would likely be difficult for the individual to properly test the strength and safety factor on the lashing that they use. A simplified way of determining this is that the lashing must be able to hold the maximum weight that they wish for the crutch 6

7 to hold. The people making these crutches should continue to add additional lashings until the lashing can hold the weight that the craftsman wants the handle to hold. Once they determine the number of times the crutch must be lashed they should add one additional lashing. The justification for this method is that the lashing holds about half the weight on the crutch which would lead to a safety factor a little below two and one additional lashing will increase it over two. Static Load Testing The goal of the test is to determine if the failure of the crutch matches the FMEA evaluation and if the static analysis performed could be used to predict the behavior of the design. Two prototypes were tested in the experiment. Methods: The crutch main shaft was constrained to a fixed pillar to isolate the crutch handle (figure 6&7) as the only component that will be under a static load. Once secured, the crutch was loaded with 25lb weights until failure. To record the experiment, video recording devices were used to film the crutch failure at different angle. Results: The first prototype s first failure occurred at a load 150lb by the handle slipping out of the notch (figure 8) on the main shaft and ultimately fell apart at 200lb. From the video recording, the handle slipped due to insufficient notch depth. This would cause the handle to slip and transfer load onto the lashings. With the high load on the lashing, adding more weight causes the lashings to break and cracked the support reed shown below in figures 9. From the first test, the failure of the crutch was determined to be due to poor product quality and manufacturing practices. In addition, the FMEA and static analysis was not invalidated and another test with a better made crutch would be needed. Taking what was learned in the first test, a second prototype was created with a deeper handle notch and better manufacturing practices to avoid accidental cracking. Using the same procedure, 200 lbs were loaded onto the crutch without any joint slippage or visible cracking. Additional weights were not available but with a static load of 200 lbs, a 142 lb person would be able to put all their weight (a very rare circumstance) on the crutch statically and dynamically without fear of the handle crutch failing. Since the 95th percentile of a the weight of males in Zambia is 160lbs [2], almost all of Zambians will be able to use this crutch without fear of failure provided that they avoid the very extreme circumstance that all of their weight is placed directly on the handle. 7

8 Manufacturing To ensure that the NGOs and local craftsmen will be able manufacture the product with the intended quality, a manufacturing procedure and quality checklist that can be used with basic hand tools, dimensions from the user's body, and non destructive testing was created. These plans were created to address as many communities as possible by designing the manufacturing plans and quality check for some of the lowest resourced areas in Zambia. Fabrication: The manufacturing plan is designed to help even the lowest resourced areas in Zambia to create the crutch design. From the needs assessment, local craftsmen would almost always have basic hand tools such as a carving knife, hand saw, and file [1]. In addition, the manufacturing plan will not require standard measurement tools and can be made using lengths from the end users bodies. Figure 10 below shows an example of using body lengths for measurements for the bamboo crutch. To communicate the manufacturing steps to the NGOS and local craftsmen who may not speak English, manufacturing will include sketches of the manufacturing procedures. These sketches will help avoid any language barrier and ultimately aid in the transfer of information (figure 11). Through timing the actual creation of the prototype, the total time to manufacture the crutch was 2 man hours. Please refer to appendix A for the entire manufacturing plan including the pictorial sketches. Quality Checklist: The goal of the quality checklist is to ensure that the user will receive a quality crutch and to help the builder save time by identifying material defects and mistakes that would lower the strength of the material. Appendix B contains the checklist that will be given to the NGOs and local craftsman. The quality checklist is divided into three parts: before, during, and after manufacturing. For the before manufacturing phase, the craftsman would need inspect the raw material for any defects and the correct shape. These defects include cracks in bamboo or signs of rotting. The manufacturing stage focuses the quality of the holes created and what to do when the bamboo cracks during manufacturing would. To prolong the life of the bamboo, all the holes and slots will need have rounded edges to avoid stress concentrations. In addition, if any cracks occur during the manufacturing process, they should be dealt with depending on the type of crack in the bamboo (Appendix B). Lastly, after manufacturing, one will test the fully assembled product to ensure the user will be able to use the product correctly. The handle joints must hold the weight of the user with a static load safety factor of 2 and must not show significant wiggling after light usage. Lastly, there is a requirement that the end user must feel safe. This soft requirement is need to ensure that the end user will use the product correctly and associate the crutch with high quality. 8

9 Distribution Plan and Future development The successful implementation of the design eventually requires that the crutches be distributed to their intended users. The following section outlines two potential distribution strategies. It is possible that the distribution plan that works well for a particular location and NGO may differ from that which works well for another scenario. NGO Volunteer Fabrication and Distribution: Currently, NGOs like Crutches4Africa raise funds and enlist volunteers to purchase or collect aluminum crutches and distribute them in regions of need [3]. The design costs less than an aluminum crutch from a materials standpoint, and requires zero shipping costs. With the money that Crutches4Africa uses to acquire and ship crutches, they could potentially build several more bamboo crutches, allowing them to reach more users. This model of volunteer fabrication is likely one of the first steps that will be implemented, as it can double as a training tool to teach locals how to manufacture crutches. This model is also advantageous because it does not depend of the successful information transfer required to set up local manufacturing. Local Fabrication: This model requires that locals with basic carpentry or construction training manufacture and sell the crutches. Manufacturing plans can still be distributed by NGOs, but afterwards the fabrication is entirely local. This plan is advantageous in that, if successful, it is a sustainable solution which no longer requires NGO intervention. It also can provide a potential source of income for local craftsmen. Local fabrication requires a number of criteria in order to be successful. First, it must be economically feasible that fabricators could profit from the sale of the product. At this point it is difficult to know whether people in the Solwezi District would be willing to pay for the product. Second, it requires that the fabricators have access to the required tools and materials. Lastly, it requires that NGOs are successfully able to train craftsmen in the proper manufacturing of the crutch, including quality control to ensure that the end product can be trusted not to fail. While these criteria are far more difficult to achieve, the reward is a sustainable solution to the mobility aid problem in Zambia. Any NGO that makes use of the design described in this report will know the best distribution plan for their particular situation. As the project continues, feedback both on the design and the distribution of the bamboo crutch is expected and will hopefully give way to more successful designs. 9

10 Further Applications of Technology The design was intended not only to deliver a solution for NGOs to allow more people with needs to access low cost, sustainable, and easy to make crutches, but also encourage further application of manufacturing bamboo products. The end users, who are disabled and may have difficulty finding a regular job, may now become manufacturers after exploring the manufacturing techniques, and they may be able to develop a source of income. Possible applications of the technology include stools and wheelbarrows (figure 13 & 14). (13) (14) Figure 13 & 14: A design concept for a stool that employs the designed joints(13) and a design concept for a wheelbarrow that employs the designed joints (14) Conclusion The bamboo crutch design presented in this report has the potential to be an affordable and accessible mobility aid for rural areas in Zambia. The design was able to achieve the design requirements (table 1). The structural integrity of the final design was validated via Failure Mode Effects Analysis (FMEA), buckling calculations, and static load testing by being able to hold 200lbs. The crutch can be fabricated entirely from local materials, without the need for power tools, and within two man hours by using pictorial steps and developing techniques to make high quality version of the design. The distribution of the proposed design will likely involve Non Governmental Organizations (NGOs) currently involved in mobility aid distribution. These organizations may choose to fabricate the crutches themselves, or distribute manufacturing plans so that local craftsmen and entrepreneurs can fabricate and sell the crutches themselves. With this validation and manufacturing plan, there is support to believe that the crutch design can make help meet the demand for mobility aids in the Solwezi district in Zambia. 10

11 References [1] Whalen, E., & Papalambros, P. (2016). Bamboo Crutch Design for Developing Communities in Zambia Research, Ideation, and Design. Retrieved April 22, [2] Eide, A. H., & Loeb, M. (Eds.). (september 2006). Living Conditions among People with Activity Limitations in Zambia. SINTEF Health Research Report. Retrieved April 18, 2016, from og tjenester/zambialcweb.pdf. [3] David Talbot, Crutches4Africa, Personal Communications [4] Yamauchi, T. (2008). Growth and Nutritional Status of Children and Adults Living in Contrasting Ecological Zones in the Southern Province of Zambia. Graduate School of Health Sciences, Hokkaido University, Hokkaido, Japan. Retrieved from [5] W.K. Yu, K.F. Chung, S.L. Chan, Column buckling of structural bamboo, Engineering Structures, Volume 25, Issue 6, May 2003, Pages , ISSN , (02) ( [6] Mechanical properties of bamboo. (n.d.). Retrieved April 18, 2016, from aachen.de/eng/reports/mechanical_properties/referat2.html 11

12 Appendices A. Manufacturing Plan 12

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17 B. Quality Checklist The following checklist is intended to aid the manufacturer in identifying good building practices during the fabrication of a bamboo crutch. Inspection Checklist Before Manufacturing Y/N Is the bamboo vertical (± 15 )? Does the bamboo have any cracks? Does the bamboo have any signs of rotting? Discolored sections? Infections (insect, fungus etc)? Water Damage? Weak Spots? During Manufacturing Do all the holes have rounded edges? Are there cracks in the bamboo due to manufacturing? Surface Cracks? (where and how many) Through the surface? (where, how many and can you lash the crack) Through Node? ( throw away) Final Inspection Can hold 150 lb without failure or major cracking? Travel 15 meters without lashes slipping? Travel 15 meters without major discomfort Feels stable when using a pair? Customer feels safe and comfortable using the product? 17