Andrew Morlock, David Mast, Paul Johnson, and Jacob Welch
Abstract
The objective of this project is to demonstrate the need and present a solution to stabilize a wheelchair in the lateral direction. Stability is achieved by designing a cambered axle which replaces the original straight axle of the wheelchair. Current products available to wheelchair users, which achieve the same function, are limited and expensive, thus cost is an important focus throughout the design stages. A few concepts are generated and rated using a Pugh Chart for aid in selecting the final design. A concept of cambering the rear wheels is chosen and developed from the concept to a prototype design. Finite Element Analysis (FEA) is used to ensure the final design’s structural integrity and also for material selection. Throughout the designing process, our time is managed with the use of a Gantt chart.
Introduction
In many cases when a wheelchair is used outdoors, it is subject to unpredictable, rough terrain. Standard indoor wheelchairs aren’t designed to be used on rough terrain and all-terrain wheelchairs are very expensive and it’s usually not feasible for an individual to afford. If accessories were available to modify the standard wheelchair to conquer a rougher terrain, it would be less expensive compared to buying an all-terrain wheelchair.
Safety for handicapped individuals is of high importance because the users may not have full function of their appendages in order to assure their own safety. According to statistics from 2003, 65-80% out of 100,000 wheelchair related injuries were due to tipping of the wheelchair [1]. With this data it becomes apparent that the lateral tipping of wheelchairs is a major problem and it will only increase if the users were to take normal every day wheelchairs and try to be more adventurous with them. This was a problem that we came across at Camp Brim Shire in St. James, Missouri when we visited [2]. Therefore we set out to correct this problem guided by our objectives which are explained in detail in the next section.
Background
The performance of the equipment designed is expected to be easy to use, allowing for an untrained person to operate it in a safe fashion. The equipment cannot cause harm to any person in the vicinity, under normal conditions, whether it is in use or not.
The objectives for the final design are outlined here
- Wheelchair Stabilizer Objectives
- Compatible with most standard wheelchair models
- Safe
- Increased stability from current models
- Not to hinder the patient
- Not to decrease mobility
- Meet ADA standards
- Inexpensive
Based on our center of gravity calculations and experimental data from the wheelchair in our possession, we found the center of gravity to be high. This high center of gravity allows for easier tipping and would become worse on uneven ground. Since one of our goals is to keep this option affordable it is designed to fit on most standard wheelchair models without any or minimal modification to the chair itself.
Current State of Art
In the market, many options concerning stability for wheelchair users are available to consumers. These options are however, in most cases, very costly. The motorized wheelchair for example, achieves more stability and provides the user with an easier propulsion method among many other features, but is usually a costly investment. Another option might be to buy a cambered athletic wheelchair however this also doesn’t appear to be as desirable as initially expected according to our analysis below.
Figure 1, Cambered Athletic Wheelchair (4)
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The camber wheelchair is sold as one unit and is used in the same fashion as any other wheelchair. The user will sit in it and wheel themselves to their destination. The cambered wheels allow the for a wider wheel base which means that a larger angle is needed to tip the chair over compared to a normal chair. This design also protects the user’s hands from obstructions such as walls because the top of the wheel is tilted in further than the bottom. Since the camber wheelchair is sold as a unit, the cost is quite significant. The “Top End Terminator Titanium” from Invacare is $3,218.00 and other chairs of this type are in a similar price range [4]. When this is compared to a standard wheelchair from Care Medical Source which costs $139.00 [5], the cost of the cambered wheelchair is quite significant. This is a very expensive endeavor for someone who only intends to use it part time.
Methods
The main idea for preventing lateral tipping was to increase the wheel base. This could be done in several ways. One is to use longer axles so that the wheels sat further away from the chair but that provided new problems such as the user not being able to reach them. Hand crank mechanisms were suggested but that would deviate from the goal of low cost. Extra wheels could be added to the outside as well but that would become cumbersome and may restrict a person’s passage through a door or hallway. For these reasons finding a way to convert a normal wheelchair to one with a camber was the chosen design path.
New Design
Camber Axle
Figure 2, Camber Axle
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One of the main goals was to increase the lateral stability of a standard wheelchair and this is achieved by putting a camber of 6° on each wheel which increases the wheel base by 5”. Since the wheel base is wider a larger slope will be necessary to topple the chair. The user’s mobility and comfort has been discussed in the ‘Journal for Rehabilitation Research & Development’. According to this paper, not only do we gather that physically it is a value to have cambered wheels but also that an angle of at least 6° is preferred amongst their test groups[6]. With the bottom of the wheels being the widest part of the wheelchair, the hands of the user are protected from obstacles such as door frames.
The parts that will be needed to convert a normal wheelchair to one with a camber will be inexpensive compared to buying a wheelchair with cambered wheels. The new parts will cost approximately $67.00 for a pair of axles and a detailed analysis of this can be seen in the “Cost Estimation” portion of this paper.
Considering this device only adds 5” to the width of the wheelchair, a standard wheelchair with original width of up to 27” will still fit through ADA standard doors thus meeting our design criteria for width [7]. The axle will be made from 17-7 PH stainless steel which offers us the strength and corrosion resistance that we need in order to meet the weight specifications for the wheelchair.
Figure 3, Final Concept Assembly
The operation of this device requires the original axle to be removed. This is done simply by lifting the respective side of the wheelchair up to remove the weight from the wheel being removed. The quick release button on the axle is pressed and the wheel will slide off. The new axle slides into the original fixture of the wheelchair and clasped onto the frame. The wheel is then mounted onto the new axle by sliding the original bearings onto the outside part of the new axle. The wheel is secured with a pin to prevent it from sliding off and a pin is inserted into the other end as well to keep the axle from leaving the wheelchair.
Figure 4, Final Concept Exploded View
Cost Analysis
Table 1, Cost Analysis
For total engineering cost, the assumption is as stated below, only with an added cost per hour to reflect expenses and other incidentals incurred. An estimation of manufacturing 100 parts per day was made and including vacations and down days, an estimated 23,000 parts could be manufactured or 11,500 sets of axles. The estimated manufacturing time is about 18 minutes per part. Assuming $60 per hour for the required machining operations the total manufacturing price would be $40 per set of cambered axles. Material cost has already been determined to be $23.51, so total cost for manufacturing, materials, and hardware comes to $67.01.
Design Analysis
The final length of the new axle effectively places the top of the wheel in the same location as before, thus all calculations below account for angle of 6°.
Results of tipping hand calculations
- Increased wheelbase width 5”
- New wheelbase width 28.25”
- Center of gravity from origin (see sketched picture for location) (x, y, z)=(13.83, 11.625, 27.89)”
- Increase angle of lateral tipping 4.16° 18.33%
There are also calculations for deflection in the shaft so that the manufacturer’s design can be compared to ours. The calculations predict that there will be a deflection of 0.0225 inches when a load of 350 lbs. is applied directly to one axle. This is an acceptable amount of deflection since the wheelchair is only rated for 350 lbs. which would normally be distributed over the entire chair. In order to ensure our design a factor of safety study is done by appling the maximum load to one axle to make sure that it would not fail in the worst case scenario.
Robustness and Sensitivity, New Design
The material used in the new design is 17-7PH Stainless Steel
Figure 5, New Design Factor of Safety Study
Figure 6, New Design Factor of Safety Study (Magnified)
Figure 7, New Design Deflection Study
Force applied: 350 lbs.
Material 17-7 PH Stainless Steel
According to this study the factor of safety for our new design is a minimum of 2.07 with 350 lbs. applied. With the new material the factor of safety exceeds that of the manufacturer’s part allowing a safer product. The maximum deflection of this part with the 350 lbs. load is 0.00218” which is less than the original manufacture’s deflection. The deflection study resulted in a number which is close to the calculations we did by hand showing that this analysis is reasonably accurate.
Conclusions
The design is intended to replace the original axle with an axle which is angled and since it uses the original wheel and attaches to the same frame as before, little or no modification is needed for the wheelchair to allow this device to be attached. The attachment will camber the rear wheel 6° effectively adding 5” to the wheel base.
A prototype of this design was tested for lateral tipping similar to the original testing. The results of this post design testing showed that the angle at which the wheelchair tips has increased by 16.5% which was expected from our estimation at the beginning of the project.
The angled axle requires more ‘loose’ parts compared to the standard axle. This is considered to be an inconvenience and is more difficult to assemble than before. However, we believe that the added benefit of stability for an inexpensive price outweighs this inconvenience.
A cost effective design was the main goal of this project. The price for the set of axles is estimated to be approximately $67.00 which is considerably less expensive than buying a new wheelchair of that type.
Citations
[1] http://injuryprevention.bmj.com/content/12/1/8.abstract
[2] http://www.campbrimshire.com/
[3] http://resna.org/
[4] http://www.invacare.com/cgi-bin/imhqprd/inv_catalog/prod_cat_detail.jsp?prodID=TEDTI
[5] http://www.caremedicalsource.com
[6] Perdios, MSc, Angeliki, Bonita Sawatzky, PhD, and A. William Sheel, PhD. “Effects of camber on wheeling efficiency in the experienced.” Journal of Rehabilitation Research & Development. 44.3 (2007): n. page. Print.
[7] http://www.access-board.gov/ada-aba/ada-standards-doj.cfm#a1009
[8] http://matweb.com
The team from left to right: David Mast, Jacob Welch, Paul Johnson, Andrew Morlock
The team with prototype