Introduction: Custom Sprocket Driven Wheelchair
Our company, Jesus Take the Wheel, was asked by the Free Wheelchair Mission to design a lever driver mechanism for their wheelchairs. Our group decided to make a design that is more similar to a hand-pedaling mechanism than a lever arm. Our pedaling mechanism is designed almost similar to a bike pedaling system with its multiple gears and chain. Our main sprocket is designed to have custom holes, which act as our multiple gears. Our intention is that the different gears will allow our wheelchair to function through different terrains by using a handle that is allowed to transition between gears using a key and locking system.
Step 1: Addressing the Market and Population Needs
Current Market: In the market today, there are multiple designs of lever arm wheelchairs that are already being put to use. According to the Journal of Medical Engineering & Technology, “[l]ever-propelled wheelchairs have been described as more efficient and less physically demanding than hand-rim propelled wheelchairs” (van der Woude, Veeger, et al.). However, there are many problems and constraints with the designs and use of lever arm wheelchairs in the market today, like the inability to spin in place efficiently.
Population Needs: Our wheelchair design would be going towards “people in the world living with disabilities, many of whom do not have equal access to medical care, education, and employment. This is particularly true for those living in low and middle-income countries” (Shore and Juillerat). Also, people living with disabilities in third world countries are faced with the challenges of living around rugged terrain and varying doorway sizes where it may be difficult for them to maneuver around. According to the Medical Science Monitor, “[f]or many of the disabled, a wheelchair is a critical source of mobility which aids independence and integration into society, including their ability to earn a livelihood. For disabled children, a wheelchair aids their cognitive and psychosocial development” (Shore and Juillerat).
Market Value: From the wheelchairs provided by Free Wheelchair Mission,the “cost to produce, ship, assemble, and deliver the chair to recipients was $59.20 worldwide” (Shore and Juillerat). However, recipients were able to receive “local and national fundraising efforts,” and the “wheelchairs are provided free of charge to recipients” (Shore and Juillerat).
Market and Value Proposition: As a way to address wheelchair users who may have upper extremity weakness and need to propel themselves through rough terrain in third world countries, our company has designed a mechanism with hand pedaled sprockets, allowing them to go farther and faster in their wheelchair without having to use the push rim.
1. van der Woude L. H. V., Veeger H. E. J., de Boer Y. & Rozendal R. H. (2009) Physiological evaluation of a newly designed lever mechanism for wheelchairs, Journal of Medical Engineering & Technology, 17:6, 232-240, DOI: 10.3109/03091909309006331
2. Shore S., Juillerat S. The impact of a low cost wheelchair on the quality of life of the disabled in the developing world. Medical Science Monitor : International Medical Journal of Experimental and Clinical Research. 2012;18(9):CR533-CR542. doi:10.12659/MSM.883348.
Step 2: The Game Plan
In order to figure out how we were going to try to solve the problem we had multiple brainstorming session. As you will see our final design was different from the sketches in beginning. As we tested we encountered obstacles that didn’t anticipate, and that’s great because that’s how we learned.
Step 3: Designing the Small Sprocket in Solidworks
The sprocket located on the release axle of the outside of the wheelchair tire is positioned so that it moves in relation to the main sprocket where the handle is attached. As the consumer applies force to the handle, the main driving sprocket will move causing the chain to move, and lastly, the smaller sprocket on the tire to rotate. After researching gear ratios and understanding how bike gears work, we decided to create a 3:1 ratio between our sprockets. This is the average gear ratio for bikes and we believe that it would be the most effective for this project. Therefore, the smaller sprocket has ten teeth while the larger has 30 teeth.
Through much effort and design changes, the driven sprocket was designed to be able to attach to the custom push-fit shaft using nuts and bolts. A driven gear on McMasterCarr was used as a basic template for the final design. The original part contained two different holes, and a key hole on the driven gear. All of these components were deleted using Solidworks in order to design this part with the necessary personalized components for our project.
First, I sketched a circle with a diameter of 1.100 inches on the flat side of the driven sprocket. I then used extrude cut to create a cylindrical hole that went through the whole sprocket. This step is necessary in order for the driven sprocket to slide over the custom shaft part. Two symmetrical circles were then sketched onto the driven sprocket using hole wizard. These circles were 180 degrees apart from each other and had a diameter of 0.26 inches. These holes were then aligned to the holes created on the custom shaft in order to secure the two parts together using a bolt and a nut.
The first figure above showcases the multiple steps taken to create the smaller sprocket in SolidWorks. Step 1 presents the original downloaded SolidWorks gear from McMaster Carr. The product can be found on McMaster Carr using the product ID number 6280K323, this product is a sprocket with ten teeth and can be used with an ANSI 35 Chain. After downloading the part, I deleted the extra components that it contained; such as, the keyhole, screws, and extra holes. After doing so, in Step 2 I changed the dimension of the inner diameter to 1.1000 inches. Following this, in Step 3 I used Hole Wizard to successfully create two holes aligned 180 degrees away from each other. Each hole containing a diameter of 0.26 inches.
The STL file will give you the ability to 3D print the product, if you so choose. Driven gear is another name for our small sprocket.
Step 4: Designing the Custom Shaft
The custom part was designed to act as a shaft in order to secure the driven sprocket to the tire of the wheelchair. The design mimicked the custom shaft that we had made in class together. For this specific part, I used a top down assembly design in order to create the custom shaft in regards to the hexagonal bolt at the center of the wheelchair tire. First, I used the solidworks file that contained the assembled tire and created a reference plane parallel to the center bolt. Second, I used convert entities to replicate the outline of the hexagonal bolt and the inner circular shape of the release axle onto the reference plane that was previously created. This outline of the circle and hexagonal bolt was then used as my base sketch for the custom part. In order to add volume to this sketch, extrude boss was used to create the cylindrical shape of the part. This part was extruded 3.28 inches from the base. Lastly, I sketched two symmetrical circles with diameters of 0.26 inches onto the shaft. I used hole wizard to cut through the whole shaft and create two holes exactly 180 degrees apart. These holes will later be aligned to custom holes in the driven gear allowing the two parts to be secured with a bolt and nut. The holes of the shaft were created so that they were 1.57 inches away from the end that is sticking away from the release axle.
This custom shaft is secured onto the hexagonal bolt of the tire using a push fit. This means that it will need a small amount of physical force to shove the shaft onto the hexagonal bolt. After doing so successfully, the custom shaft will be able to mold to the area it is shoved into and act as a bridge between the tire and the driven sprocket.
This figure showcases the steps taken to achieve the custom shaft. Step one, we utilized convert entities to replicate the outline of the hexagonal bolt along with the circle of the release axle of the wheelchair tire to a reference plane. Step two, we used extrude boss to add depth to our part. Finally, in step three, hole wizard was used to create two symmetrical holes with a diameter of 0.26 inches. These holes were placed 1.57 inches away from the end opposite of the release axle in order to allow the proper alignment of the smaller sprocket to the larger main gear with keyhole.
Step 5: Designing the Butterfly Bar and Support System in Solidworks
To begin with, create a sketch from the Top View, showing the direction of the pipe and adding dimensions of 60 mm and 279.40 mm, perpendicular. From the Front View, create another sketch by using the endpoint as the center of the circle with diameter of 27.85 mm. This might take a couple of tries because there are two different sketches.
Once you completed this task, you use the Sweep Extrude feature. The blue profile should be the circle sketch and the pink profile should be the direction of the pipe. This will happen naturally, as the Sweep feature will use the blue profile as a reference, following the path of the pink profile.
From here, use the Shell feature to remove the interior mass of this part. For this part, Shell, 1 mm to create the illusion of a pipe. To cut a hole in this pipe that will allow a screw to go through, start off with a plane.
Go to Reference Geometry and click Plane. Using the Front Plane as a reference, set the distance to be 60 mm. From here, you can sketch a circle, and using dimensions, 137 mm, 13 mm, and a diameter of 8 mm. Extrude cut, through all to get the cut. You can play around with the edge, by using the Fillet feature to round out that ‘ugly’ edge, with maximum fillet of 13.00 mm. You are finished with the butterfly for your wheelchair!
Step 6: Designing the Sprocket With Keyholes in Solidworks
The larger sprocket is attached to our wheelchair using the butterfly bar, and that is designed with our multiple “gear” holes. Our sprocket also contains keyholes and a locking system that will allow our handle to transition between gears as well as keeping the handle in place while the gear is in motion. For the base feature sketch of our main sprocket, a flat sprocket from McMaster-Carr was used. The part downloaded from the website has a product ID number 2299K55 or you could search “sprockets” and click on Roller Chain Sprockets, then Flat Chain Sprockets for ANSI Roller Chain and then choose the appropriate sprocket that contains 30 teeth and corresponds to the ANSI 50 chain.
Once the part is downloaded into SolidWorks, we had to customize the sprocket so that it was designed to have our gear and key holes, as well as a hole for the screw that will connect the latches onto the sprocket. If we were to cut the sprocket in half and we use that half circle to sketch our gears, keys, and screw holes on there.
First, we wanted to sketch three circles that will act as our “gears” and we made each circle to have a diameter of 15.2400 mm. Also, we wanted those holes’ center points to be vertically aligned to the sprocket’s center point, and the bottom circle’s center point to have a distance of 25.4000 mm from sprocket’s center point. Next, we chose the extrude cut feature and set it up as through all so the holes cut through the whole sprocket.
Second, we created the transitions for the handle onto the sprocket by first sketching a rectangle that starts from the top circle’s center point to the bottom circle’s center point and it has a height of 36.3568 mm and a length of 5.0800 mm. Then we also used the through all extrude cut feature for the transitions.
Third, we created the key holes that will be attached to the handle by sketching a circle with a diameter of 6.3500 mm, and where its center point is horizontal to the center point of the circle that was created in the first step. Also, we made sure the center point of the circle was touching the left, outer edge of the original circle. We repeat this step for each of the three circles that was created as our gear holes. From there, we also used the through all extrude cut feature.
Fourth, we created holes for the screws that will attach our latches for our locking system by sketching three small circles with a diameter of 2.9000 mm and we sketched those above and to the left of the key holes that were created in the previous step. From there, we used the through all extrude cut feature.
Step 7: Designing the Latches in Solidworks
Making these latches are fairly simple! Start of with a sketch on the front plane. Use the Slot sketch feature. However, you will need to click the drop-down arrow to see the Centerpoint Arc Slot. Use the origin as the centerpoint that will allow this part to be fully defined easier. Now it is time to add dimensions! Dimension the arcs with a radius of 3 mm, the centerline of 9 mm and the inner radius, closer to the origin, of 11 mm. Extrude this sketch by 0.5 mm. Now, you either choose Hole Wizard or manually, sketch and a circle. Either way is correct, just make sure you dimension the hole with a diameter of 3 mm, and use the concentric relation to fully define this sketch.
Step 8: Designing the Handle (Key) in Solidworks
Our handle is designed cohesively with our gear box as a ‘key and lock’ system. This part essentially has two “keys”, each with its own distinct purpose. Two inches into the handle resides the first key, who functions primarily as the entity permitting the user to switch in and out from different gears. The second key at the base of the part enables the handle to be initially inserted into the gear box, however it’s 45 degree offset from the first key ensures that when our user may be switching from gear to gear, the handle simply is not completely removed from the gear box.
This part was designed off of a series of small, individual boss extrudes in order to implements each feature. We started by sketching a .4” diameter circle concentric with the origin and extrude bossed blindly 2.375”. After doing so, we sketched a .27” by .22” rectangle, with one .17” side coincident and midpoint with the origin, on the base of the cylinder. This was then boss extruded .125” with its direction facing into the preexisting cylinder. This “key” permits the user to shift in and out of desired gears.
From there, we sketched a .12” diameter circle , concentric with the first cylinder, on the base of the current handle, and extrude bossed .1” outwards. With this additional small cylindrical region, users are able to transition from gear to gear without having to physically remove the handle during movement.
Finally, to finish the handle, we needed to create the base “key”. Sketching again from the base of our last created transitionary cylinder, we created another circle .4” in diameter. We then created another rectangle, with the same dimensions, however laying perpendicular and to the left of the first. Since both the sketched circle and rectangle are overlapping, the trim entities tool was utilized to remove the any unnecessary pieces, leaving only the perimeter of our sketch. This piece could then be extrude bossed .125” outwards from our part to create our base “key”.
Step 9: Designing the Cover in Solidworks
For safety purposes, we decided to include a cover around the driving gear to prevent our uses from hurting themselves. To design the cover on SOLIDWORKS, we first have to open a new part and create a sketch. The first sketch is a circle with its center at the origin and have its dimension be 190.5mm. Next, you use the feature Extruded Boss/Base and extrude the circle by 6.35mm. Then, you create another sketch with the dimensions shown in the first figure above. After that, you use the Revolved Boss/Base feature for 180 degrees. Next, you create another sketch on the design for the screw to enter. The dimension of the sketched circle is 8.89mm. Then, you use the Extruded Cut feature to cut the hole. The design should look like the second figure above.
Step 10: Assembly in Solidworks
Butterfly Bar with Clamp-on Frame Fitting
We are about to do one of the major assemblies that is needed for your wheelchair. This next part can be obtained at www.mcmaster.com. Search this part called Clamp-On Frame Fitting or the part number 2534T210. Click on the part number ID and it will take you to another page, where it shows you the Engineering Drawing of this part. The page should look like this. Download this part into your folder and open this part into SolidWorks. Next, click on the drop-down menu and click Make Assemblyfrom Part. You also need to Insert Component and add the butterfly bar that you created prior to this assembly.
Now it is time to mate! On the Assembly menu, you can find the Mate option. First, mate the outer surface of the bar with the inner surface of the clamp. This will be mated by a concentric mate. Now, this mate is a bit tricky! You will mate the outer edge of the bar with some edge in the clamp. (Do not worry, this is just a section view to show you this mate!) This will be mated by the tangent mate.
You are done with this assembly!
Gear Assembly Mechanism
We are about to do another assembly that will be a little difficult, but not impossible. You will also need this for your wheelchair, as it will provide the accessible to move your wheelchair.
We start of with a Reference Assembly. This is more of a sketch, so we, as the designer, can know where to place the gears, in our assembly. Sketch an arc on the Front Plane, making sure that the origin of the arc is coincident with the origin in SolidWorks. Sketch a line from the endpoint of the arc to an arbitrary distance. Connect this line with another arc, and finish it off with a line. Have the bigger arc with a dimension of radius of 80.26 mm and the smaller arc with a radius of 29.19 mm. The lines should have a length of 426.58 mm. You will probably have an under defined sketch, by this point. To make this fully defined, use Add Relations to make the arc and lines tangent to each other. Also, have the origins of the arcs, horizontal to each other. Once this is fully defined, you are ready to Make Assembly from Part/Assembly.
Insert Components: Main Sprocket and Small Sprocket made in the prior. Mate the Main Sprocket using both Concentric and Coincident. For the concentric mates, use the arcs and in the inner radii, and for the coincident mate use the origins. You will see that this is still not ‘fixed.’ To avoid this, mate a line to the top plane.
We are a step closer! Go to your Assembly menu and click on the drop-down arrow of Assembly Features, as you will see an option to add a Belt/Chain. For your Belt Members, make sure you click on one the crests of the both gears. You will see a preview, in yellow and bold. Click the green check and you are ready to add your links. To find these parts, search part number ID, 6261K194 and 6261K244, on mcmaster.com. Download these two parts and Insert Components.
To make the chain, there is a feature under Linear Component Pattern. In this drop-down menu, click the Chain Pattern Feature, that will do the chain with two simple links. For the Pitch Method, click the third button because this will make sure that the chain will not have any gaps between links. For Sketch Path, use the sketch that was created in the Belt/Chain and click on Fill to Path.
These next few steps are going to be a bit tough, and a little bit of trial and error. For the Chain Group 1, click on of the Adding Link. Then, click on the inner surfaces, as SolidWorks will automatically move down. For the Path Alignment Plane, for both links, just make sure you are clicking the front plane of the links. Do the same for Chain Group 2, just make sure you are clicking on the outer surfaces of the Connecting Links. Remember this will take some time because you got to make sure that the chain will be between the outer faces of the gears.
Once you are good to go, you will see that the chain will not move with the gears. To fix this, mate the surface of the Adding Link to one of the edge in the gear. This will, of course, move this Adding Link, to that destination. However, there is an option, under Mechanical Mates, that will fix this problem, called Gear Mate. Once you click the green check, you are done with this major assembly!
To finish off our assembly of the gear mechanism, we need to add and secure our latches for the keyholes, ensuring that the handle key will not be accidentally removed during operation. After inserting the latch into assembly, you concentric mate the edge of the hole of the latch with the the edge of the keyhole of the main sprocket, aligning the two. Next, you coincident mate the base face of the latch with the outerface of the main sprocket, now placing the latch directly against the sprocket. From there, we need to secure the latch and sprocket with an appropriate screw fastener. We can choose a suitable McMasterCarr screw (92314A110) and start by aligning it to our latch and sprocket by adding a concentric meet between the bottom edge of the screw and the edge of the hole of the latch. Then, we lay the screw flat on top of the latch by a coincident mate between the face of the base of the screw head at the outer face of the latch. Finally, we restrict rotation of our screw by mating its top plane parallel with the top plane of the the assembly. Our screw can then be secured in place with its corresponding McMasterCarr nut (91841A005). We again start by aligning it with the screw with a concentric mate between the outermost edge of the nut hole and the bottom edge of the screw. From there, we add an essential screw mate, which is a mechanical mate. To do this, we mate the axis of our screw with the outermost thread of the nut, and set the revolutions per mm to 20 to replicate the rotation of the nut. To create an axis in our screw, we can click on reference axis in our features, and choose the right and top planes, since these are perpendicular. Finally, we mate the base of the nut coincident with the outermost face of the sprocket, locking the nut in place. Repeat for the additional two latches.*
Now that you have your Gear Assembly Mechanism, you can then insert that subassembly to the wheelchair. From there, we start with a concentric mate between the inner round face of a screw hole of the small sprocket of the Gear Assembly Mechanism with the inner round face of a screw hole of the Custom Shaft, aligning the two pieces. Next, we center the Custom Shaft inside the small sprocket with an additional concentric mate between the innermost round face of the sprocket with the outer round face of the Custom Shaft. The small sprocket must then be secured to the Custom shaft with an appropriate fastener. Here, we can use McMasterCarr part 91253A550. After creating an axis in the screw using the two plane method explained above, we can insert the component into our assembly. The bottom edge of the screw is first mated concentrically with the inner face of the screw hole of the small sprocket. We then insert our corresponding McMasterCarr nut, part 95462A029. Here, we can concentric mate the outermost edge of the nut hole and the base edge of the screw aligning the two. Furthermore, a screw mate is applied using the axis of our screw and the outermost thread of the nut, at 20 revolutions per mm. To lock the nut into place, we implement a tanger mate between the outermost round face of the small sprocket and the innermost flat face of the nut. Finally, we can prevent the rotation of the nut and screw by applying a parallel mate between both of their right planes.
Next, we can input our second subassembly, the Butterfly Bar with Closed-Clamp. To assemble the closed clamp on the armrest of the chair, we begin with a concentric mate between the innermost round edge of the top clamp and the edge of the wheelchair armrest indicative the start of its gradual taper, aligning the two pieces. Then, in order to prevent the undesired free movement of the Butterfly Bar around the armrest, we can coincident mate the front plane of the assembly with the top plane of the Butterfly Bar with Closed-Clamp assembly. Next, we need to set the Butterfly Bar assembly position correctly on the armrest. We can do so using a Distance Mate between the outermost round face of the top clamp with the round face of the Gen 2 Left Frame/Frame of the wheelchair, and set it to 127.186mm.
From here, we can begin with adding some finishing touches to the main sprocket. We start by inserting two abrasion resistant cushioning washers from McMasterCarr, part 90131A102. Once inserted, the inner edge of the washer hole is concentrically mated with edge of the hole of the main sprocket. We can then coincident mate the innermost face of the washer with the outermost face of the main sprocket, thus aligning the washer and laying it flat on the outside of the sprocket. We can additionally restrain its rotation by creating a coincident mate between the top plane of the Gear Assembly Mechanism and the top plane of the washer. This same process can then be repeated with the second washer, however using a coincident mate between its outermost face and the innermost face of the main sprocket. Next, we need to add a fastener to connect our main sprocket to the butterfly bar, specifically McMasterCarr part 92620A553. With this screw, we can create a concentric mate between the bottom edge of the screw and the inner round edge of the washer, aligning the two. The screw can then be placed flat on the washer with a coincident mate between the bottom face of its head with the outer face of the washer. We can finally restrict screw rotation with a perpendicular mate between the top plane of the assembly and the top plane of the screw. With our screw in place and through the main sprocket, we can add a nut, part 94895A029, in order to secure the placement of the sprocket. With this nut, we will need to align it with the screw using a concentric mate between its outermost edge and the base edge of the screw. We can similarly implement a screw mate with the outermost thread of the nut and the axis of the screw, and setting it to 20 revolutions per mm. The outermost face of the screw can subsequently be coincidently mated with the exposed face of the innermost washer, laying it flat. Next, we proceed by inserting the Driven Gear Cover. The inner round edge of the hole of the cover will be concentrically mated with the base edge of the screw, aligning the two. Afterwards, we can utilize the outermost face of the nut we just previously added, and create a coincident mate between that and the inner face of the Driven Gear Cover, laying the two flat. We then attached another 94895A029 screw using the same process just previously stated, however using a coincident mate between the outermost face of the nut and the innermost face of the Driven Gear Cover. Now, we need to attach our screw holding our main sprocket, Driven Gear Cover, washers, and nuts to the butterfly bar. Due to the fact that the hole position of the butterfly bar was created due to a series of trial and error, it would be inconvenient to use the typical concentric mate between the hole and the screw. We however instead could use a distance mate between the inner round face of the butterfly bar’s screw hole and the very tiny flat edge of the thread of the screw, and set it to 0.05mm. Due to the clearance of the butterfly bar’s screw hole, the screw is set inside, perhaps just slightly off center. To finish off the securing of the screw to the butterfly bar, we add one final 94895A029 nut to the end of the screw against the butterfly bar using the same processes of concentricity and screw mates. However, it becomes a little bit difficult to create a coincident mate between the flat face of the nut and the round face of the butterfly bar. So instead, we implement a distance mate between the outermost face of the nut and the innermost face of the Driven Gear Cover and set to 36mm, placing it at the edge of the butterfly bar. Now that all our components are in place, we must restrict the rotation of the Driven Gear Cover, ensuring that it’s doing its job of protecting our users. Using an angle mate of 65.9 degrees, between the right plane of the Driven Gear Cover and the top plane of the assembly, we can maximize protection from any possibly harm from the main sprocket. Last but not least, we insert our handle into the assembly. From here, we can apply a concentric mate the large inner round face of one of the three handle holes in the and the smallest cylindrical face of the handle. We can then apply one final coincident mate between the innermost small rectangular key face and the outermost face of the main sprocket. With only these two mates in place, the handle can be twisted mimicking its movement between the keys and keyholes that would occur for the user when wanting to switch to a different location on the main sprocket
After repeating this long process for the opposite side of the wheelchair, we can come to our final design!
Step 11: Prototype Designing
After you are done with everything in Solidworks you get to start prototyping! This is the fun and frustrating part. Things begin falling together and falling apart all at the same time. You need to decide to materials, analyze your costs, and make some tough decisions.
Look at the images for our specific parts choices and their information.
We needed a locational clearance fit for our ¼”-20 screws on our holes for the PVC pipe, therefore we created holes with a tolerance of approximately 0.05in. We also needed a force fit on the release axle on the wheelchair, therefore we used a 1” PVC pipe with a tolerance of approximately 0.005in. Using washers between the hex head screw and the keyhole sprocket allowed us to lower the tolerance and make the assembly a tighter fit.
For building our prototype, we had to use cheap and durable materials. Since our design revolves around a bike design with sprockets and chains, we decided to order these parts off-the-shelf since our assembly in SOLIDWORKS used the off-the-shelf parts. However, we still had to alter the driving gear sprocket, so it can contain the desired transitions for the handle. We cut holes into the metal with a 7/16” and 7/32” cobalt drill bit. Some other materials that we bought that were off the shelf included screws, metal washers, nuts, bolts, PVC pipes, and metal clamps. We used steel and PVC because they were cheap material that we believed had the durability of spinning the sprocket which would result in spinning the wheelchair wheel. Although we bought many of our parts off the shelf, we also had to 3D print one of our parts because it was custom made for the wheelchair and our design. We 3D printed our driven gear with ABS plastic and made it with 30% infill with 3 shells so it can be strong enough to handle the chain.
Step 12: Building the Sprocket System for Presentation
Due to some unforeseen problems, we were not able to present our original design on the day that our project was due. Some of the materials and parts that we made are not in our original design and they are not our ideal manufactured design.
First, we attached the smaller sprocket to the wheelchair by force fitting a 1” PVC pipe onto the release axle of the wheelchair wheel. We decided to use PVC for the force fit so it can mold around the release axle and be fixed in place. We then inserted a metal rod that has a 1” outer diameter inside the PVC pipe by bolting a 2” length 6/32”-20 screw and nut. We then attached our 3D printed gear to our metal rod by screwing a 2” length ¼”-20 bolt with a compatible hex nut.
To create the bars that will attach the driving gear to the wheelchair, we used PVC pipes and elbows along with metal clamps. To create the assembly of the PVC bars, you must connect a 1” outer diameter and 2 ½” PVC pipe to another PVC pipe that is 9” in length with an elbow. Then connect a 3” length PVC pipe to the other end of the long PVC pipe with an elbow and connect another 2 ½”. The pipes should look like the first figure.
Then, to connect the PVC pipes onto the wheelchair, put one end of the PVC pipe near the armrest of the wheelchair and connect them using a 1” metal clamp. Lock the clamp and pipe into place on the wheelchair with a nut and bolt that comes with the metal clamp packaging. To attach the other end of the PVC pipe, to the wheelchair, you must use a routing clamp with one mounting point. To attach the routing clamp onto the wheelchair, you must drill a hole onto the PVC pipe and have it attached to the hook with a nut and bolt like shown in the second image. Then hook the routing clamp onto the wheelchair.
Drill a 7/16” hole onto the bigger sprocket with a cobalt drill bit to make holes for shifting gears and drill a 7/36” holes along the sides of the bigger holes to create the keyholes on the sprocket. The sprocket should look like the third figure.
To create the handle for the gear, you must drill a total of three holes onto the PVC pipes that go completely through the pipe. Two of the holes should be the same distance as the two top holes on the gear and connect them with a rubber washer, nut, and bolt. There should also be a hole at the end of the PVC pipe handle that would have a 3” length bolt that would work as a handle. To create some comfort for the tester, wrap the bolt with duct tape thick enough for your comfort. The handle should look like the fourth and fifth figure.
To connect the sprocket with the keyholes, to the PVC pipes, drill a hole on the long PVC pipe that is half the length of the PVC pipe. Then insert a metal washer on the 3” length ¼”-20 bolt and then insert the sprocket with the keyholes along with another metal washer behind it. Next, insert three hex nuts onto the bolt to create a larger distance between the sprocket and the PVC pipes. After that, insert the bolt onto the PVC pipe and lock it into place with another hex nut at the end of the bolt. The connection between the sprocket and the pipe should look like the sixth figure.
Then, add the 4” chain along the smaller sprocket and the sprocket with the key holes.
Repeat these steps for the other side of the wheelchair.
Step 13: Iterative Testing, Methods, and Results
Our testing methods can be best described as trial and error.
The original design did not account for the main sprocket and the chain being so heavy, so we had to add an additional part to the pvc piping system. We decided to add an extra clamp to the bottom in addition to the clamp on the top of our pvc piping system to stabilize the top and bottom to the wheelchair. Once we added the new support we found that it was still not enough so we added duct tape thinking that would fix the problem. To our dismay, we then realized that the pvc was too ductile to support such a large gear. You can’t mix materials of such different strengths. The pvc pipe tended to rotate towards the ground. This caused our sprocket to be displaced at an angle which was inefficient to our overall gear and chain system. Then, we thought if we can laser cut a wooden gear would that fix our problem since it is lighter. Laser cutting is not expensive, and it is very efficient. Unfortunately, in order to get that done we needed to be trained, and we just didn’t have that kind of time. This would have been an big improvement to our prototype since the wooden gear would create less strain on the pvc pipes.
The main sprocket was thicker than anticipated and created quite a problem when we tried to drill holes in it.. We created our own holes in it by drilling through the gear. This was a difficult task because of the material and thickness of the gear, but we somehow managed to properly create our intended design.
Another Sprocket Surprise
When you 3D print the object tends to shrink and reshape a little bit because the plastic starts out hot and then cools down. This caused the gear teeth to shift and made it harder for the chain to grip. It was also very rough increasing the chance of misalignment in our system. We also didn’t realize that the more teeth you have, the easier it is for it to grip the chain. In hindsight we should have make the small sprocket with more teeth and not 3D printed it. The small sprocket was designed on SOLIDWORKS to have a 3:1 gear ratio with the larger sprocket. We thought this ratio would be ideal because we had modeled it off of a bike.
In order to account for keyhole's odd shaping, we specially made a handle from wood. While our whittling intentions were pure we did not realize that it’s not smart to mix materials with such different chemical properties as the handle could snap like a sad little twig.
We wanted to get our wheelchair to work somewhat, so we did some last minute changes. We ditched the specially made handle because it didn’t create enough torque. then, we bolted a 10in pvc pipe to our gear with a bolt for a handle. This created a lever arm that, unfortunately, would not have the ability to change torques, but it did allow use to move the wheelchair. In order to support the pipes we threw as much duct tape on our wheelchair as humanly possible. The chain was not tight enough because of the lack of tension, but miraculously it did move slowly forward (as long as no one was in the chair).
It wasn’t until the end when we started doing the physics equations, where we found our biggest error. In order to have a mechanical advantage with a lever arm you need a long lever arm length because the moment of the arm is proportional to the torque. In other words, the longer your lever arm, the easier it is to move.
Here are the equations and values:
L = moment arm
T = torque
F = force = 130N (estimated through research on the Physics forums, it is roughly the force an average person must use to move themselves in a wheelchair) T = F*L
Lever arm on race day
L = 10in = 0.0254m
T(LEVER ARM) = (130N)(0.254m) = 33.02Nm
Handle (Has multiple positions) L1 is the length closest to the center
L1 = 0.5in = 0.0127m L2 = 1in = 0.0254m L3 = 1.5in = 0.0381m F = 130N
T1 = 1.651Nm T2 = 3.302Nm T3 = 4.953Nm
As you can see the torques in the original design were a lot smaller, than the torque for the lever arm. If we wanted our original design to work we would have needed to make the sprocket a lot bigger or create a long handle that was parallel to the sprocket to utilize the torque.
Step 14: Lessons Learned
1. The outside diameter of a metal pipe is measured while the inside of the pvc pipe is measured. For example, if get an 1in pvc pipe and an 1in metal pipe they will be different sizes.
2. Be careful using a 3D printer because when it prints the plastic is hot, and then when it cools down its shape will change, which may affect your assembly.
3. Always have a drill buddy. (see Julianna’s hand)
4. When using a power drill, it’s important to put enough weight on it that you begin drilling, but no too much, as it could compromise the integrity of the drill bit.
5. Duct tape is a useful tool when you’re in a pinch.
6. You can’t mix material of greatly different molecular strengths.
7. Laser cutting is cheap and easily accessible on the UCI campus.
8. When you are in your brainstorming process it’s a good idea to use physics to backup your assumptions before you begin your CAD modeling.
Step 15: Cost Analysis and Results
Total Price of Materials: $ 4.20
Zinc Alloy for Sprocket with Keyhole, Latches, Small Sprocket, and Handle:
Zinc Alloy is often used to create gears and automotive parts. This material has a lower melting temperature and it requires lower pressure for casting. These traits allow a zinc die to last longer than most other metal dies. A zinc die is a term representative of the process it takes for Zinc Alloy to be molded into the intended shape of the manufacturer. Because we require the zinc alloy to be molded into an articulate gear shape, this would allow our product to last longer when in use. Plus, zinc alloy is known to contain reduced corrosion effects. Along with this, because zinc alloy has a low melting point then it allows for a hot chamber casting. A hot chamber casting is less expensive and a faster process when compared to cold chamber casting. This material is also recyclable which would allow it to be re-melted and reshaped into other products or the same product when needed to be replaced. Because Zinc Alloy is recyclable it reduces pollution, saves resources, and reduces waste overall.
Malleable Cast Iron for Custom Shaft:
We are choosing to use Malleable cast iron as the material for our custom shaft. This material is often used for shafts that are meant to mold into a specific shape with enough pressure. This is important to our custom shaft due to the fact that it will be secured with a push fit to the head of a hex bolt and should mold into that shape. This material will allow our custom shaft to mold into the area where it will be push fitted thus increasing the stability of the shaft overall. Plus, this material is strong and able to withstand heavy weight. It is a perfect middle ground material because it exemplifies the strength of cast iron while also being malleable. This material is also recyclable which would allow it to be re-melted and reshaped if needed be for other materials. This assists in reducing pollution, saving resources, and it reduces waste.
Low Alloy Steel for Bike Chain and Bar:
We chose to use Low Alloy Steel as the material for our chain. This material is within the same family of Alloy Steels used for most bike chains. Because we are essentially using a bike chain for our wheelchair, this product is easily accessible if it needs to be replaced. Plus it is fair priced and durable.
We also chose to use Low Alloy Steel as the material for our bar. This part acts as a holder for our main gear. This gear needs to have a base that is strong, and able to sustain heavy weight due to the gear being on the heavier side. Low Alloy Steel is fair priced and affordable, and has a high young’s modulus - meaning it is stiff and will not change its shape under pressure.
Low Alloy Steel is also recyclable, has great abrasion resistance, and it is not particularly energy intensive to create.
Polyethylene for Cover:
Polyethylene is the material chosen for the cover part of our wheelchair. This material is most often used for household products and containers. As a result, this material contains thermal stability and can be molded to whatever shape we would like. The cover would work best with this material because it would be able to prevent any injuries done by the gear, and also it would be able to sustain weather conditions and last long. This material is very inexpensive and it is also recyclable.
Step 16: Contributions Page
Everyone put in 100% effort to help make this project come together. We are all happy with how great this team worked together.
Project Manager: Anastasia Karnaze 54437048
The project manager, Anastasia, was responsible for organizing meetings, making sure everyone was doing their work at the right time, and doing any random thing that was need. She designed the cover for phase 1 that was cut from the final design, and outlined and compiled the instructables.
Manufacturer: Cynthia Ilynn Perez Partida 85388301
The manufacturer, Cynthia, was responsible for choosing the materials, picking them up, and altering materials. She also designed the cover for the sprocket with keyholes.
Materials Engineer: Julianna Bordas 10256482
The materials engineer, Julianna, was responsible for designing the small sprocket, deciding the final product materials, and the cost analysis for everything. She also took part in the iterative testing, and did lots of drilling.
Tester: Anastasia Karnaze 54437048
The tester, Anastasia, was responsible for testing all of the different parts at each phase of design, racing on race day, and coming up with fixes when things did not work.
Lead Designer: Leilani Camara 34405517 and Jose Martinez 57524771
The lead designers, Leilani and Jose, were responsible for making engineering drawings for each part, communicated an changes, and full assembly drawing for the entire wheelchair. Leilani designed the handle key and Jose designed the support system and butterfly bar.
Researcher: Jazmin De Los Santos 41677160
The researcher, Jazmin, was responsible for designing the sprocket with keyholes and doing the market value research.
Printed Name: Anastasia Karnaze
Sign: Cynthia Perez
Printed Name: Cynthia Perez
Sign: Leilani Camara
Printed Name: Leilani Camara
Sign: Julianna Bordas
Printed Name: Julianna Bordas
Sign: Jazmin De Los Santos
Printed Name: Jazmin De Los Santos
Sign: Jose Martinez
Printed Name: Jose Martinez