This Instructable will show you how to use the engineering design process to design and create lever system to help control and operate a wheelchair with less force required than standard wheelchairs. We want to create this to help people with weaker upper bodies operate their wheelchairs for longer amounts of time.
Before we start to tell you what any of this means, you should first understand what the engineering design process is and how we will use it!
Step 1: Engineering Design Process
The engineering design process is a technique used to ensure that the right steps are taken when developing a design. The problem is first defined and possible constraints are identified. These constraints are researched and identified. The process continues circularly to brainstorming possible solutions to the identified problem. After possible solutions have been made the circle continues with the best possible solution being chosen. A prototype is made of the best possible solution to determine if the solution will work effectively. The prototype is then tested and analyzed. After being tested, the prototype is improved upon and communicated to others.
Step 2: Defining the Problem
The first thing to consider is, why are we building this lever arm in the first place?
People who need wheelchairs in developing countries don’t have equal or adequate access to education, employment, or medical care. This poses a burden to families and the countries and states that try to provide resources. (source) Additionally, those with upper body extremities face additional challenges navigating through rough terrain. (source) Estimated 65 million people in need of a wheelchair Only 5-15% of people have access to wheelchairs in low-income countries because resources simply don’t exist.
Since developing countries are the most affected by the inaccessibility of afforable wheelchairs, our target audience would be people in living in these areas.
According to "The Impact of the Free Wheelchair Mission", countries such as Vietnam, Chile and India were in need of these wheelchairs. Furthermore, common medical diagnoses which necessitated the use of wheelchair were stroke (40% Vietnam) or muscular dystrophy (Chile 21.8% India 21.6%).
Market Value Proposition
“A way to address the current limitations of wheelchairs in less developed countries that will allow wheelchair users with upper extremity weaknesses to use their wheelchairs with ease for longer periods of time than standard chairs.”
Step 3: Developing Possible Solutions
One of the important aspects of the lever arm was creating a propulsion system that could not only go forward but backwards. When considering and developing possible solutions In addition, the lever arm system needed to provide enough room to fit through a doorway and enough torque and strength to propel the wheelchair forward in rough terrain. Another important aspect was using parts that can easily be replaced and fixed. Overall, the intent was to focus on bicycle parts to allow individuals to easily fix and replace all parts on the wheelchair. It was also important to create a design that can easily attach and dissemble from the wheelchair.
Step 4: Best Possible Solutions
The best possible solution was chosen using many key factors. Our original intent was to use all bicycle parts to make our device easy to fix if anything broke; however, when creating the lever arm, it was difficult finding optimal parts that would allow for reverse motion.
Here's a list of some options we explored and why/why didn't they work
- Sprockets were ideal finding a solution but moving backwards created problems because we needed to find an easy way to switch the motion
- Planetary gear system optimized the lever arm by allowing both forward and back while maintaining a small enough size to fit through a doorway. In addition, the planetary gear and ratchet system also allowed for easy assembly and removal when attaching the lever arm to the wheelchair. Additionally, it allowed us to toggle between backwards and forwards motion easily.
Step 5: Design Each of the Parts
After identifying the best possible solution and researching wheelchair lever arms, the next step is to begin designing parts for the prototype. We begin by discussing what specific parts of the design are needed and measuring the wheelchair that the lever arm will be attached too. Drawings and parts were brainstormed on whiteboards and discussed during meetings. Eventually, we were able to settle on main design parts and begin using SolidWorks to create the parts.
List of parts:
Below is a list of all the parts you will design in the upcoming steps. Each component plays a critical role in the functionality of our lever arm, which we will explain as we go through each step!
- Planetary Gears
- Gear Carrier
- Ring Gear
- Ratchet System
- Connectors (Plates and screws)
Step 6: Planetary Gears
The planetary gear system provides our lever arm driver with a way to move forward and reverse through the use of a simple clutch. The internal planetary gears allow for the ring gear to rotate in the direction opposite to that of the input and also allow for a gear ratio making the system easier to turn. We selected our gears by checking McMaster Carr to find parts that would be small enough to keep our mechanism within a reasonable size limit, however, we needed to change the material from nylon to a steel in order to ensure that our system would rotate smoothly and be strong enough to withstand the forces needed to smoothly operate the chair. So, we chose medium carbon steel because of its light weight and relatively low cost
Part Link: https://www.mcmaster.com/#57655k53/=1clpvm8
- Click on the link provided
- Scroll down until you see the engineering drawing and the option to download as a Solidworks file (seen in the image above)
- Once downloaded, save the part and it's now ready to use!
Step 7: Gear Carrier
The gear carrier is a plate in which the small planetary gears previously described would be placed. The plate would have 3 shafts that extent out and each gear would be placed around it.
- Start with a base sketch of a circle extending out from the origin with a diameter of 2.5 inches.
- Extrude the circle out .15 inches to make a circular plate.
- On one side of the plate, create a new sketch and make a circle that has its center 1.02 inches above the origin and has a diameter of .24 in (this is to give a little clearance between this and the gears).
- Click circular sketch pattern from the toolbar and select this circle from the sketch and change the number of instances to 3.
- Next, select extrude boss and extrude this sketch out .68 inches to create your final part.
Step 8: Ring Gear
The ring gear encircles the planetary gear system and in our mechanism serves as the output, connecting to the hub of the wheelchair. When the ring gear is allowed to be reactionary, it rotates in a direction opposite to that of the input giving the chair a reverse motion. However, if the ring gear is placed in direct drive with the input by moving a clutch, it rotates in the same direction as the input giving us a forward motion.
The video above will explain what you need to know!
- First boss extrude is .90 inches
- Second is .15 inches
Step 9: Base Plate
The purpose of this plate is to connect the wheels of the wheelchair with the entire lever. It acts as a connect where it allows the movement of the ring gear be transferred onto the whee. By doing so, it allows the wheel cahir to move forwards or backwards
Step 10: Planetary Gearbox Assembly
Now that each individual component of the planetary gearbox has been made, we need to put it all together. This video will guide you through the process of the planetary gearbox assembly.
Step 11: Ratchet System
The ratchet is a mechanism that consists of a bar or wheel with angled teeth in which a pawl, the small metallic part, engages to allow motion in one direction only. The pawl is able to move freely through the angled teeth, and it is locked in the straight teeth so rotation can be achieved ( as seen in the images above)
This was an off the shelf part bought from Amazon, thus no parts need to be created in Solidworks.
Step 12: Lever
The lever arm is the location of force applied by the weight of the person driving the wheelchair. This motion drives the movement of the whole mechanism, and therefore the wheelchair wheels, backwards and forwards. The lever arm has an attached bike brake, and is directly attached to the pawl and ratchet mechanism. The cheap and durable material used allows for repeated motion under considerable stress and friction. Additional rubber grip handles for the user’s comfort, grip, and hand location on the lever.
Open up a new and empty part.
- Create a centerline through the origin, and then on one side of it, sketch one half of a lever.
- This half will include a quarter circle at the top for the knob, then a straight line that is about 18 inches long.
- In two places in the middle of the line, draw two, separate, closed arcs that will represent the rubbery grips of the lever arm.
- Then, revolve the whole thing around the centerline created earlier. This action created an extruded object that looks like a stick with two grips.
- Then, using a new sketch, make a small handle at the top of the lever to be used as a break.
- Next, a small circle, 0.25 in in diameter will be created using extrude cut, so that a screw can be used to connect it to the pawl.
- Also, two circles of the same size will be made through both of the grips so that the bike brake wire can be lead from the top handle of the arm to the bottom of the lever.
- Then, to make a second part, the elbow (or connector), open up another empty document.
- Start at the origin by making an arc, with a radius of 1.26 inches and is 0.63 inches, and then making a connected straight line that is 1.61 inches long.
- Using sweep, choose a circular profile to sweep the created sketch, and create a curved structure that looks like an elbow.
- Now, to attach the elbow to the lever arm, create an assembly and insert both components needed.
- Then, by clicking on the circumference of the bottom part of lever arm and the circular top of elbow, use a coincident mate to connect the parts together.
- This will create a part of the total assembly of the whole lever mechanism.
- Now, you are on your way to making you sketches and design come to life in 3D using Solidworks. Awesome!!
- In the prototype and final design, the elbow will allow the lever arm to attach to the ratchet and the rest of the mechanism. The prototype used a PVC pipe and PVC connector, and cloth for the grips. In the end, the whole lever arm and elbow are made of aluminium alloy with the grips made of rubber for comfort.
Step 13: Clutch
The clutch is created based on the design of the ring gear. Its function is to deliver the energy from the motion of the lever arm to the planetary gear to allow smooth movements. When the clutch is aligned to the outside teeth of the ring gear, by sliding the clutch to the ratchet, it makes the planetary gear be intact to the ratchet and allows the direction of the movement of the planetary gear and ratchet to be the same (forward movement of the wheelchair). When sliding the clutch away from the ratchet, it allows the ring gear to move in opposite direction of the ratchet (backward movement of the wheelchair)
Step 14: Creating the Parts Assembly
In just a couple of steps we will learn how to assemble our lever from all the different parts we already created.
Before creating the assembly, we first need to begin our assembly using the best practices.
- The base part, which is the first component, must be fixed to the origin and it must not move. Therefore, we need to carefully choose which part does not need to move in our design. In this case, we will choose the part called Connect.
- To start the assembly, we must go to the file named Connect, click File >> Make Assembly from Drawing. This will automatically fix the part to the origin.
- Import all of the different parts into the assembly using the button Insert Components. Browse each file and drag it into the working area. The parts we will need are Ball Bearing, Lever+Elbow, Ratchet, Click, pipe2, the bracket, the hinge and hinge connect, the Final gearbox assembly, and the input shaft.
- Put the parts together by using mates. Click on Mate, which has a paper clip as an icon, and click on the surfaces that you want to mate together. When the parts are not oriented properly and you want to move them, you can use the button Move Component.
- Mate the ball bearing to the lever. You must click Mate, and choose the inside circle of the ratchet, and the elbow part of the lever arm. Solidworks will give you some options, and may sometimes choose one for you. In this case, we will choose concentric mate. Then, to completely mate the parts, we will click on the same surfaces and choose a coincident mate.
- Mate the ratchet to the lever and to the ball bearing. We will first put the Ratchet into the Lever+Elbow by choosing the smallest circle in the middle of the Ratchet and concentric and coincident mating it to the elbow part of the lever. Then, we will choose the small circle in the Rachet again and will coincident and coincident mate it to the inner circle of the Ball Bearing. We will also coincident mate the flat outer surface of both the Ratchet and the Ball Bearing.
- In the next step, we will add the Click to the Lever+Elbow to complete the ratchet mechanism. We will use the Pipe2 and a .25 diameter screw and nut from the Design Library to connect these two sections. First you choose one side of the pipe and tangent mate it to the Lever+Elbow. Then, you must coincident mate the other side of the pipe to the click. Finally, to make the ratchet work, we must use a Limit Distance mate between the side of the click and the side of one of the teeth in the ratchet. You go to Advanced Mates, click limit distance, and choose the distance to be from 0.01 to 5.00 inches..
- To add the screw and nut, go to Design Library >> Toolbox >> ANSI Metric >> Hex Bolt. Choose the dimensions which is a .25 inch diameter and 3-inch length. The process is the same for the nut but you would choose Nuts >> Hex Nuts >> Hex Nut. To mate it to the assembly, click on the threaded part of the screw and concentric and coincident mate it to the small circle in the Click. Then, you do the same with the inside of the nut and the long part of the screw. Finally, click on the side of the bolt and tangent mate it to the lever arm.
- To add the Final gearbox assembly we will use the Connect, which is actually the part that is fixed to the origin. Select the wider side of the connect and the inside circle of the ratchet and we will mate them using concentric and coincident mates. Remember that when using concentric mates we must decide to click Lock Rotation if we don’t want the parts to rotate with respect to each other. Then, we select smaller side of the connect and concentric and coincident mate it to the smaller circle in the planetary gear.
- Finally, we will add the hinge connect, the hinge, and the bracket. You will begin by tangent mating one side of the Hinge connect, which is a tube, to the lever. You will next coincident mate one side of the bracket to the other side of the hinge connect. Furthermore, coincident mate front side of the hinge to the back upper side of the bracket. Make sure that the hinge extends to the ratchet and that it fits between one of the teeth.
Step 15: Materials
Once all the final parts were developed and designed through solidworks the next step was to obtain materials for each of the parts and analyze the potential costs. The material families consist of metals, polymers, elastomers, ceramics, glass, and hybrids. Ideally for the purpose of the design, the lever will be rotating and withstanding forces and pressure which can be further examined through the free body diagram. Since the design is constantly moving it needs to be ductile and strong to withstand the forces acting on the gearbox and lever. It is evident that the material must be ductile, and strong, but also needs to maintain hardness with a low wear-rate. The wheelchairs are targeted toward rural areas with rough terrain and needs to be able to withstand tough conditions. With all of these properties in mind the materials can be narrowed down effectively. The following diagrams and analysis was used to determine the final materials:
- Low wear-rate
- Easy to machine
- Cost efficient
- Low production energy
Figure 1: Compares the fracture toughness of the materials with the value of the constant toughness increasing as the lines are displaced upwards and to the left. The tough materials lie towards the upper left corner wit the brittle materials towards the bottom right. From the following figure it can be found that ceramic and glass are brittle and can fracture easily. For this reason ceramics and glass can be negated as a possible material for the design. Metal has a high toughness and does not fracture easily, this material satisfies property 1.
Figure 2: Shows that metals have the highest amount of strength with polymers lacking strength. It is important for the material to be durable and strong to withstand pressures and forces. It can thus be determined that the most efficient material is metal satisfying properties 2 and 3.
Now that material has been determined as the major material for the final design it can further be analyzed what type of metal can best optimize the design.
Figure 3: shows the relative prices of the materials and offers a way to consider price when determining what material is best. As seen below under metals Titanium alloys are a viable option but the cost makes it a prohibitive option. Stainless steel is also relatively expensive. Thus due to cost efficiency many of the materials that are over 1 ($\kg) are not viable. The materials analysis are mainly aluminum alloys, low alloys steels, carbon steels and cast irons which satisfy property 6.
Figure 4: further shows production energy with respect to the materials. Specifically, metals such as aluminum alloys, low alloys steels, carbon steels, and cast irons are the materials being chosen from for the design. Looking at figure 4 carbon steels have a relatively low production energy. Carbon steel satisfies property 9.
With the possibility of carbon steel being the best viable option more research was done examining the material. Carbon steel has a good strength to weight ratio and is easily able to force fit, as well as is plentiful and readily available satisfying property 7.
When analyzing and researching carbon steel four different types are available. There are four different types of carbon steel that are based on the amount of carbon present in the steel. The first type is low grade carbon steel, it is easy to shape but is not as strong as the other types of steel. The second type is medium carbon steel that is ductile and strong and consists of long-wearing properties. Type three is high grade carbon steel and holds shape memory well and is extremely strong. Type three is commonly seen in springs and wires. The fourth type is extremely strong but brittle requiring special handling. Due to the properties listed above, type two, medium carbon steel, is most compatible with our design and efficient.
For the reasons stated above and analysis regarding properties and costs, medium carbon steel was the deciding material for all of the parts in our design except the lever. Aluminum alloy was chosen for the lever and a foam rubber was decided upon for the handle grip on the lever for efficiency. Aluminum alloy was a great choice and followed almost all of the properties and has relatively high performance as well is considerably light weight. The foam rubber is commonly used in hand grips and is cost effient and is easily available to use.
The link provided will allow you to easily access the material chart referenced in the figures:
Step 16: Cost of Final Design
The examples above are used to show how each of the parts in the assembly were calculated. In the materials discussion of the instructable's one of the figures showed that the cost of carbon steel is approximately $0.80 per unit mass, $/kg. The above examples show how some of the parts were calculated. First, you open up the respected part and click on tools at the top. Then scroll to evaluate and click mass properties. The mass is provided usually in pounds, with some being in grams. The majority of the smaller parts had a negligible weight and were not included. The cost of the material is a rough estimate with an approximate cost of $1.56 per lever in material. This cost does not include manufacturing or labor. Here is how it was calculated:
- Ring Gear= 0.95 pounds
- Plate=1.46 pounds
- Lever component=0.73 pounds
- Gears=0.078 pounds per gear, there was 4 gears total per lever
- Gear connect=0.60 pounds
All of the following parts were converted to kg and multiplied by 0.80 to calculate the estimated cost for materials for the final design.
Step 17: Result of Final Design
The overall result of the design was to effectively create a quality lever arm that can propel a wheelchair forward with high performance through rural terrain. The design is cost effective, consist of lightweight materials, and is durable. Due to the weather conditions, the overall materials of the design are able to stay strong and maintain hardness with low wear-rates, as well as manufacturers with a low production energy level. The design is able to use a planetary gear system, a clutch, a ratchet system and other smaller components to propel the wheelchair forward with the motion of the lever arm. The overall motion and propulsion system decreases the amount of force needed when acting on the wheelchair to allow an optimal and easy motion forward. The lever arm also maintains the same arm movements but propels the wheelchair backwards to allow for easy reverse.
Step 18: Prototype Design-Materials
When creating the following design parts the prototype is created to ensure that the solution worked effectively. The prototype is used to analyze and test the performance of the design without manufacturing the finalized lever arm system. The prototype uses cost effective materials and alternative methods towards testing all the parts and the performance without spending numerous amounts of money. Specifically, it serves as a preliminary model to test the design and improve upon the model. The prototype design uses materials that are cost efficient and uses cheap manufacturing methods. Many parts were taken off the shelf-ordered from online- and custom parts were 3D printed. Prototypes consist of cost effective ways of replicating the design to test the overall performance.
To all the prototype to be as cost efficient as possible and created in a short amount of time the materials used for the prototype were carefully chosen. First, the sun and planet gears consisted of a plastic organic polymer that was efficient, cheap, and off the shelf. The part was purchased from McMaster-Carr as an off the shelf part. Another optimal material was 3D printing. A resin 4 3D printer was used to print the connector, clutch, and ring connect. The resin was cost efficient, beautifully colored, and efficient in manufacturing time. The quality of the 3D printed resin was strong and relatively easy to drill holes through. The ring gears were also 3D printed with a standard 3D printer and were cost efficient and efficient in time. The lever was made with PVC pipe which was strong enough for prototype purposes, cheap, and readily available. The ratchet and pawl were off the shelf ordered for a relatively cost effective price and was used to attach to the lever and gear box. A door hinge was used as a cheap method of preventing the ratchet from slipping and a screw was used as a makeshift clutch. The ball bearings were purchased as an off the shelf part which was readily available and cost effective. Overall the materials for the lever arm components were cost efficient, timely, and easily available.
Step 19: Pros and Cons of 3D Printing - Prototype
One of the materials and processes for manufacturing custom parts discussed in the previous step is 3D printing. Two different types of printers were used, as pictured above. The first printer used ABS Plastic and created the part layer by layer. The second printer, the Form 2, used a resin which was continuously cured by a UV light similarly going through each layer
- The design was matched all of our specifications and came straight from the SolidWorks design, allowing for it to have all of the dimensions and correct sizing.
- Printing 3D components also allowed for rapid prototyping and was efficient in usage of time
- Reduced costs, was easier to store replacement parts, and was overall adequate for prototype.
- The resin 3D printed components were very strong and durable as well as lightweight, cost efficient, and timely.
- There was a limited material choice, limitations in product sizes and mass production
- Many of the more detailed 3D printed mechanisms and parts broke easily and were difficult to get precise due to the support material getting in the way
- Since the parts are moving the material was not as smooth and did not allow the optimal amount of movement for the gears.
- Parts would occasionally fail and would need require a reprint
Step 20: Cost of Prototype
The following excel goes through the materials used when creating the prototype. An excel sheet was created that carefully goes through the items purchased, the quantity, and the costs to determine the overall total costs. It is necessary to always be adding the parts to the excel document throughout the design process and prototyping process to make sure the budget is not being passed and maintaining knowledge of what can best be afforded. It is also necessary to determine what parts should be off the shelf and what parts custom made. The 3D printed parts were much cheaper than the off the shelf parts but were less durable, not as precise, and did not always work. The more important parts such as the planetary gear box was off the shelf with simpler parts consisting of 3D printing. The 3D printed parts were cheaper and allowed the prototype to maintain a relatively cheap cost. In addition, all 3D printed parts were free with the help of fellow teammates who were able to produce parts. The cost of our prototype was cost efficient and cheap. However, the excel sheet provides the cost of all prototype parts, materials, and glues in full without the negation of the free 3D printed parts.
Step 21: Step by Step Instructions on Assembly of Prototype
The following steps go through the instructions on how to assemble the prototype.
- Gather all the parts needed to build a lever
- Start off by building the lever: take the 3/4 in. PVC cut it to an approximate length of 2 ft. using a pipe cutter
- With the leftover pipe, cut the pipe to ~1 inch and attach it to the end of the lever arm (~2ft PVC pipe) with an arm joint.
- Attach the ratchet to the ball bearing with JB Weld. The center hole on the ratchet is concentric (share the same center) to the center of the ball bearing. To further stabilize this connection, epoxy putty is used on the outer edge (wait for at least one hour before further use)
- Then take the pawl and align it with the ratchet.
- Take the lever arm align it with the hole on the pawl. The 1 inch PVC pipe that connects the lever arm should be inserted into the center hole of the ball bearing. For this insertion to be secure, double-sided tape was wrapped around the pipe.
- Take the screw (4 in X 0.25 in) and insert it through hole of the pawl until it hit the lever arm. The point on the lever arm the screw hit will be marked.
- Find a bit that matches the size of the screw and drill the a hole through the pipe at the point where it was marked in step 7.
- Take a PVC pipe and cut ~ 1.5 inches (distance from the pawl and lever arm)
- Place the cut PVC pipe from step 9 between the lever arm and the pawl Insert the screw (4 in. X 0.25 in) through the hole of the pawl, then the pipe, and through the hole drilled on the lever arm. Next take a nut and screw it onto the long screw until it is tightly screwed and touching the lever arm.
- Spin the ratchet and move the lever arm to make sure the pawl is working properly.
- Use hot glue and the PVC pipe made from step 9 with lever arm to solidify the connection
- Take more PVC pipe and cut ~4 inches. Then align it where the arm joint is so that the pipe’s circular part is touching some of the lever arm and some touching the arm joint. This will have the pipe parallel to the ratchet
- Place the hinge on half of the bracket. Align the other half of the pracket to to the other side of the pipe from step 13. In this case, the half of bracket is touching and perpendicular to the pipe and the hinge is on top of the other half of the bracket.
- Check see if the hinge is align with the teeth of the ratchet and it can stop the movement of the ratchet (if so continue to next step, if not adjust the location of the the pipe and hinge)
- Use JB Welds to glue the bracket and hinge together. Use gorilla glue to glue the pipe with the lever arm and the bracket to the pipe. Also use gorilla glue to stabilized the connection between bracket and hinge. Then check if the hinge is working properly.
- Take the input shaft and drill through the shaft in vertically so that when inserting a thing screw about 4 in X 0.16 in it will freely move.
- Drill a hole at the arm joint of the lever arm. Hole size should be similar to that of the hole drilled in step 17
- Take the 3D printed input shaft and insert it through the hole of the ring gear. Note: the smaller end goes through the ring gear.
- Input the plastic gears gotten from McMaster Carr onto that smaller end of the input shaft - this gear is the sun gear. (extruding part of the gear should be pointing outwards)
- Take the gear connect and put 3 plastic gears through the small rods sticking out at 3 positions (the extruding part of the gear should be facing the gear connect
- Once three gears are connected to the gear connect, place them into the ring gear where the 4 gears are able to turn when turning the gear plate
- Now connect the planetary gear to ball bearing but inserting the input shaft that is extruding out from the gear into the center hole of the ratchet.
- Insert a screw through the hole of gear connect, the sun gear, input shaft, ratchet, lever arm’s arm joint then secure it with the nut and add epoxy putty and JB Weld epoxy and hot glue to secure it.
- Drill a hole horizontally through the extruding part of the plate
- Take the plate and place it on top of the ring gear. Make sure the teeth of the plate is going in between the outer teeth of the ring gear. To secure this, gorilla glue is added between the teeth of the ring gear and teeth of the plate.
- Wrap Duck Tape around connecting points of the lever: the pipe between pawl and lever arm, pipe connecting the bracket and lever arm, the arm joint where the a screw and nut came out of the hole, and etc. (Just any where the lever seems insecure
- Attach the lever to the wheelchair by inserting the the extruding part of the plate into the force fit shaft (make sure the hole drilled on the extruding part is aligned with the hole on the force fit shaft.
- Insert the a M4 screw through the holes and tighten it with a wing nut
- Repeat steps 2-29 for the other lever arm.
- PVC pipes, ratchet, pawl, hinge, planetary gears, clutch, gear connectors, plate, screw, nuts, epoxy, drill, ball bearing, hot glue, bracket, input shaft, gear connect
Step 22: Iterative Testing Method of Prototype
For our prototype, we used a multi step testing process. First, we drew out a rough sketch of the part which we wanted to design and wrote down any relationships and interactions that there would need to be. Next, we inputted the design into solidworks with any relevant mates in order to see if our design would work in a simulation. Finally, if all the pieces fit together in solidworks, we would print or model them in the real world and repeat the process again if changes needed to be made. Using this method, we went through a few different design options. For example, the phase 1 design utilized a chain and sprocket system so that the lever would be placed in a more natural location, however after testing this design in solidworks, we decided that it would overcomplicate the design and drive the price up. In addition, our final design switched the output, making the ring gear the output instead of the gear carrier because after when we tested the first physical model, we found that it reacted slightly differently than how we expected. Our final prototype worked fairly well, however we did still have some problems keeping certain parts stationary with relation to each other since we could not find an epoxy that would hold to the plastic. However, after some tinkering and trial we were able to achieve the desired motion.
Free Body Diagram
The free body diagram is attached below. It provides information on the necessary movement and forces that will be acting on the wheelchair to help you better understand how the lever arm would work and whether or not our design makes sense. If we can account for all forces, we know that this could potentially be a viable solution
Step 23: Results of Prototype
As seen from the video the overall result of the prototype was satisfactory. The overall mechanism worked successfully but had a hard time staying together due to the liquid epoxy. Liquid epoxy did not successfully hold together the prototype and caused some problems. Despite the difficulty keeping the parts together we were able to finally allow the liquid epoxy to dry and use duct tape to secure the prototype. We then were able to test the prototype on the wheelchair as seen in the video. The wheelchair is propelled forward by the motion of the lever arm.
- Use different fabrication methods to create the gears such as laser cutting. During the project, we found that while the plastic gears did work, their size was too small for traversing long distances
- We used wing nuts to connect our attachment to the wheelchair, which worked fairly well but found that larger wingnuts may get interfere with parts of the wheel
Step 24: Contributions
Step 25: Work Cited
- “GRIT – United States of America.” Zero Project, zeroproject.org/practice/mountainbike-wheelchair/.
- Steel, O'Neal. “Carbon & Alloy Steel.” O'Neal Steel - The Metals Company, www.onealsteel.com/carbon-and-alloy-steel.html.
- Material and process charts Mike Ashby, Engineering Department Cambridge CB2 1PZ, UK Version 1