Introduction: Clamp-on Lever System for Wheelchairs

Created by: SuperAmazingDesignerWorks (SADWorks)

  • Project Manager: Prima Donna Abrantes
  • Manufacturer: Grant Barnes
  • Materials Engineer: Melany Yeung
  • Tester: Nathan Chiang
  • Lead Designer: Ilho Moon
  • Researcher: Evan Ranario

Step 1: Design Section (Overview)

  1. Existing Research and Objective (Introduction)

  2. Step-by-Step design of parts using Solidworks

  3. Step-by-Step assembly of parts using Solidworks

  4. Cost Analysis and Results

Step 2: Existing Research and Objective

Market Proposition:

A way to address the disabled population in third world countries using a lever wheelchair arm design that can ease transport even through the toughest terrain and at the same time be affordable

Research for Wheelchair:
Inspiration of design from Grit freedom chair, reasons:

  • Easy to place lever in our wheelchair
  • Can only focus on designing the ratcheting mechanism in one area
  • Has variable torque due to the chain and sprocket system
  • Doesn’t increase width of wheelchair

Free Wheelchair Mission Paper (all information taken 2012):

  • Overall the wheelchair improved quality of life
    • Statistics:
      • Initial: 46.6% never left home, 52.3% hospitalized >1 days/months, 70.3% reported daily pain, 48.7% reported negative mood
      • After 12 months wheelchair use: health & mood increased 20%, pain significantly diminished, level of independence increased 11%
  • WHO info:
    • 1 billion in world living with disabilities many in developing countries
    • Lack of resources in developing countries makes it difficult for disabled people to live
    • Inability to access to work drives person to poverty and reliance of the state
  • Common medical diagnoses for wheelchair use:
    • Stroke: Vietnam 40%
    • Muscular Dystrophy: Chile 21.8%, India 21.6%
    • Before wheelchair: 78% not owned wheelchair due to money

Step 3: Cross-Connector System (Parts)

Clamps

Bottom Half

  1. On the front plane, sketch a semi circle of R0.75" and length 1.50". Use revolve boss to create a sphere.
  2. On the top plane, sketch a circle of ⌀1.25". Extrude boss this sketch by 1.975".
  3. Back on the front plane, sketch and extrude cut a rectangle through the top half of the sphere (the vertices and edge of the rectangle should be coincident/tangent to the upper half of the sphere, so it should measure 1.50" x 0.75").
  4. Now sketch and extrude cut a circle of ⌀0.625" in the center of the sphere.
  5. On the top plane, sketch one circle of ⌀0.6875" on either side of the part (where the extrude cut doesn't run through), making the centers of the circles coincident with ends of the diameter of the sphere. Extrude boss these circles by 0.25" in the same direction as the first extruded cylinder.
  6. Draw centerlines of length 0.25" along the origin of the plane (and diameter of the circle). From the open vertices, draw circles of ⌀0.25". Extrude cut these circles all the way through to create holes for bolts.
  7. Create a new plane at the bottom of the part.
  8. Sketch and extrude cut a circle of ⌀0.625" through the cylinder (length of 1.975").

Top Half

  1. On the front plane, sketch a semi circle of R0.75" and length 1.50". Use revolve boss to create a sphere.
  2. Sketch and extrude cut a rectangle through the bottom half of the sphere (the vertices and edge of the rectangle should be coincident/tangent to the upper half of the sphere, so it should measure 1.50" x 0.75").
  3. Sketch and extrude cut a circle of ⌀0.625" in the center of the sphere.
  4. On the top plane, sketch one circle of ⌀0.6875" on either side of the part (where the extrude cut doesn't run through), making the centers of the circles coincident with ends of the diameter of the sphere. Extrude boss these circles by 0.25" in the opposite direction as the first extruded cylinder.
  5. Draw centerlines of length 0.25" along the origin of the plane (and diameter of the circle). From the open vertices, draw circles of ⌀0.25". Extrude cut these circles all the way through to create holes for bolts.


Cap-Screws

  1. On the front plane, sketch a quarter of a circle with R0.38".
  2. Then sketch a horizontal line 0.09" inwards, and a vertical line 1" down from that.
  3. Draw a vertical line starting from the open vertex of the quarter-circle all the way down until its endpoint is in line with the endpoint of the other vertical.
  4. Connect the two endpoints with a horizontal line.
  5. Use revolve boss to start the base of the cap screw.
  6. Sketch and cut-extrude a rectangle o.38" x 0.10" through the cap to create a flat head cap screw.

    Cross-Connector

    1. On the front plane, sketch a circle ⌀1.25". Then extrude boss this circle by 3".
    2. Sketch and extrude cut a circlular hole of ⌀0.75" through the existing cylinder.
    3. Repeat steps 1-2 to create an "X". Create a new reference plane resting on top of the X.
    4. On top of the new plane, sketch and extrude boss (using midplane instead of blind) a circle of ⌀1.15" by 0.45".
    5. Sketch and extrude another circle of ⌀1.15" on top of this circle (switching back to blind) by 1".

    Pipe Nipples (Note: the handrims on both sides are different diameters, so two pipes of different lengths were made)

    1. On the front plane, sketch a circle of ⌀1.25". Then extrude boss this section by 8".
    2. Sketch an inner circle of ⌀0.75". Extrude boss this circle by 9.5".
    3. Add external threading to each end of the pipe nipple.
    4. For the smaller pipe, extrude boss the outer circle by 6.2" and the inner circle by 7.7".

    Internal threading

    • Hole where pipe nipple is inserted (for all clamps, cross connector, and box mentioned later):
      • Length: 0.75"
      • Type: Inch Tap
      • Size: 0.3750-16
      • Thread Options: Right-Hand thread
    • Hole where cap screws are inserted:
      • Length: 0.25"
      • Type: Inch Tap
      • Size: 0.250-28
      • Thread Options: Right-Handed Thread

    External threading

    • Cap screws:
      • Length: 0.25"
      • Type: Inch Tap
      • Size: 0.250-28
      • Thread Options: Right-Handed Thread
    • Pipe nipples:
      • Length: 0.75"
      • Type: Inch Tap
      • Size: 0.3750-16
      • Thread Options: Right-Hand thread

    Step 4: Box-Connector System

    Socket Bar

    1. For the socket-bar, first a ⌀1.7" circle was sketched on the front plane, and then extruded 0.1".
    2. Next a plane was created on the face just extruded, and a ⌀1.5" circle centered on the other circle was sketched there and extruded by 1.25".
    3. After that on the front plane a 0.5" x 0.5" square was sketched centered on the circle, and then extrude cut for 0.75".
    4. After that a new plane was placed on the end of the cylinder, and on that a ⌀0.75" circle was drawn centered on the other circles and extruded by 5.65".
    5. Next three filets were made: a R0.25" one on the end of the second extrusion, a R0.03" one at the beginning of the bar (by the square hole), and a R0.1" one at the end of the bar.

    The Box

    1. First, I sketched 1.75 x 4" rectangle on the plane 1 (top plane).
    2. Boss-Extrude the rectangle to make a 3-D rectangular model.
    3. (Sketch 3) Sketch a circle with 0.75" diameter on the right plane and use extruded cut through all(Cut-Extrude 2).
    4. (Sketch 4) Sketch a same-sized circle in the step 3 on the top plane and use extruded boss by 1" (Boss-Extrude 2).
    5. (Sketch 5) Sketch a same-sized circle in the step 4 on the front plane and use extruded boss by 1 inch(Boss-Extrude 3).
    6. Use Thread 1 and 2 to create 0.06" on the extruded cylinders on planes.

    Shaft Collars (directly downloaded from McMaster Carr)

    Clamps (the clamps are identical to the one made previously except for the inner diameters of where the clamp latches onto the handrim; the cap screws are the same as before, though)

    • Diameter 1: 1.00"
    • Diameter 2: 1.27"


    Step 5: Sprocket and Chain Drive System (Parts)

    Sprockets (1-1/2" diameter shaft sprockets were directly downloaded from McMaster.)

    ------------------------------------------------------

    Chain (Chain modifications made to create the chain in assembly. Shown below.)

    1. Reference plane 4 was made and used convert entities to copy shape of chain. It was then moved over one of the chain holes and extruded to create a part of a chain as shown in the picture below.
    2. The same thing was done with the other side using plane 5 as a reference plane.
    3. Plane 6 was created so that we can sketch a circle to represent the connecting pins. It would then be extruded into a cylinder as shown below.
    4. All measurements would be in McMaster since no modifications were made to its size.

    Step 6: Brake System (Parts)

    Brake

    1. Sketch a circle of diameter 1.79”, and another circle of diameter 1.4” inside of it. Extrude this by 4.5".
    2. Add a reference plane on top to sketch another circle of diameter 1.79”, which has a concentric circle of diameter 1.6” in it, on the reference plane, and cut extruded that sketch.

    Brake Cap

    1. Draw a circle with a diameter of 1.93”, then extruded it by 0.5”.
    2. Fillet this extrusion.
    3. Draw a circle with a diameter of 0.97” and then extrude it as well.
    4. Add M1.9x1.5 external threading to the face of this new extrusion.

    Wire

    1. Sketch a circle of diameter 1.7” and do a circular sweep with radius .1” .
    2. Draw two more splines that have also had a circular sweep with radius .1”.

    Step 7: Lever Arm System (Parts)

    Socket

    1. Draw a 0.31” x 0.61” rectangle and a circle of R0.4”. Then do loft on the two sketches.
    2. Draw a circle of diameter 1.8” extruded by 0.25” and another circle with a diameter of 1.32” extruded by 24”.
    3. Draw a circle of diameter 1” on the face of Boss-Extrude2; then cut extrude using this circle by 1.84”.
    4. Thread Cut-Extrude1 to create M16x1.5 internal threading.

    0.5" Drive

    1. To sketch the square drive head, sketch a 0.5 inch square around the origin of your chosen plane (center of square should be at origin.
    2. Sketch a circle with diameter 0.71 inch that is concentric with the square (same center) and connects all four corners.
    3. Boss extrude (0.5 inch) only the square’s area 0.5 inches.
    4. To form the part that will be attached to the socket wrench hole, boss extrude (0.70 inch) the whole circle area in the opposite direction as the extruded part in step 3.
    5. To get a curved face of the extruded circle, we are going to fillet the outer part of the circle 0.045 inch.
    6. To eliminate the sharp corners on the square, click on each of the corner lines (4 items total) and fillet corners 0.02 inch.
    7. Create a new sketch to start the top most outer circle part.
    8. Draw a 0.5 inch square and 0.5 inch diameter circle centered at the origin.
    9. Cut-extrude into the square block 0.02 inch to create a slight indentation.
    10. Create a new sketch to start the the top most inner circle part.
    11. At the origin, draw a 0.2 diameter circle. Cut extrude 0.1 inch into the previously extruded part.

    Ratchet Gear

    1. Go to McMaster-Carr.com and search “6283K240”. Click on the product detail and download solidworks 3D file.
      • This is a close version of what we want -> a 24 teeth gear so we can just adjust it. The mcmaster carr version is a little too thick and the angle of the teeth have to be adjusted.
    2. Delete the “Pawl” and “Ref” sketches. F
    3. irst to change the thickness, open the sketch under the first boss-extruded part. Change the bore diameter to 0.71 inch and the diameter of the circle to 0.7505 inch.
    4. To fix the teeth angles, we need to redraw lines. In the teeth sketch, there should be a center dot and draw two lines connecting the bottom line to the center dot. Delete the two triangle side lines.
    5. The last two step we will leave alone. CirPattern takes the sketch of that one tooth and patterns it around the outer edge of the circle to create a gear pattern
    6. The chamfer, just makes an edge on to the inner circle of the gear.

    Socket Wrench Handle (Head)

    1. Create Sketch for “Outer Ridge”. It might be most helpful to refer to the engineering below to make an accurate sketch. This will make the outermost edge of the ratchet.
    2. Boss extrude the outer edge 0.61 inch.
    3. Boss extrude tool’s inside’s bottom 0.10 inch.
    4. Create a new sketch for “the inner ridge” that is slightly raised from the bottom.
    5. Use below engineering drawing to make an accurate sketch of the inner rim.
    6. Boss extrude the sketch from step 4 0.01 inch.
    7. Create a new sketch to make the handle attachment.
    8. Make sure the sketch is closed or else the sketch will not be fully defined and not be able to be extruded.
    9. Boss extrude 0.61 inch.
    10. Create an another sketch for the knob hole with a 0.15 inch circle.
    11. Cut extrude the circle.

    Socket Wrench Cover

    1. Make an accurate sketch of the cover using the below engineering drawing.
    2. Boss extrude 0.1 inch and leave the circles un-extruded to create the holes.

    Lever Final

    1. Sketch the lever using lines, and tangent circles. The left straight edge is 0.65 inch. The bottom circle has a radius of 0.07 inch. Other dimensions can be seen in the engineering drawing below.
    2. Boss extrude above sketch 0.01 inch.

    Control Knob

    1. Create Sketch for “Outer Circle”.
    2. Sketch a Circle of diameter 0.05 inch centered at the origin.
    3. Boss extrude 0.07 inch.
    4. Chamfer outer circle 0.10 inch at 49 degrees.
    5. Create a Sketch for “Attachment” on the other side that is not chamfered.
    6. Draw a 0.13 inch circle centered at the origin.
    7. Boss extrude 0.10 inch on sketch from step 5.

    Knob

    1. Sketch the general shape of the knob. Dimensions can be seen in below engineering drawing.
    2. Boss extrude 0.1 inch.

    Step 8: Lever System (Sub-Assembly)

    Knob Assembly

    1. Insert knob and control knob into assembly.
    2. Fix the control knob as this will be the part that will not be moving.
    3. Concentric mate the top curved edge of the knob to the circle of the control knob.
    4. Coincident mate the face of the knob and the face of the control knob.
    5. Distance mate the right bottom corner of both parts to prevent the knob from moving when the control knob is not moving.

    Socket Wrench Assembly without handle or cover

    1. Insert all components: “socket wrench handle”, “0.5 inch drive”, ratcheting gear, lever (x2), knob assembly.
    2. Make sure “socket wrench handle” is the only one fixed as this will be our base part.
    3. Concentric mate the “socket wrench handle” edge of big hole and the “0.5 inch drive” main 0.5 inch circle.
    4. Concentric mate the “socket wrench handle” edge of big hole and the ratchet gear.
    5. Coincident mate the face of the “socket wrench handle” and the face of the ratchet gear. I made a Layout sketch to help easily place the lever and knob, but this is not necessary if this if you just want to vaguely draw the part. (This was done by measuring the physical ratchet that bought from Amazon.)
    6. Concentric mate the circles of the levers and circles in the layout sketch.
    7. Coincident mate the face of the levers and the face of the “socket wrench handle”.
    8. Concentric mate the small circle edge on the “socket wrench handle” and the cylinder on the knob assembly.
    9. Coincident mate the face “socket wrench handle” and the face of the control knob part of the assembly.
    10. Limit angle mate to Right Plane and Right Plane of Levers on the “socket wrench handle” to prevent rotation for a more realistic movement of the ratchet. (25 degree max, 13 degree min)
    11. Limit angle mate to Right Plane and Right Plane of assembly to prevent rotation for a more realistic movement of the ratchet. (18 degree max and -18 min)

    Full Socket Wrench Assembly

    1. Insert components: “Wrench part”, Socket Head Assembly, and Socket wrench cover.
    2. Make sure that the wrench part is the fixed part. Coincident mate a long edge of the wrench to long edge on the socket head.
    3. Coincident mate a short edge of the wrench to the short edge on the socket head.
    4. After these two coincident mates, the lever and socket head should move together just like a normal ratchet.
    5. Concentric mate the top curved edge of the Socket Head Assembly and top curved edge of the cover.
    6. Parallel mate the left straight long edge of the Socket Head Assembly and left straight long edge of the cover.
    7. After these two mates, the cover will move forwards and backward but will fit perfectly on top of the ratchet head assembly when the z axis aligns correctly.

    Attaching the Connectors + Support System to the Socket Wrench

    1. After attaching the socket to wrench part, (Concentric 1) use concentric mate to align the center of socket bar with the center of edge of the circular face of socket handle.
    2. (Coincident 1&4) Use coincident mate to attach the socket bar to socket part.
    3. (Parallel 1) Use mate to make the top plane of socket bar and sprocket parallel.
    4. (Concentric 2) Use concentric mate align the center of sprocket with the center of socket bar and (Coincident 2) attach them together.
    5. (Concentric 9) Use concentric mate to align the center of brake with the center of circular edge of wrench and (coincident 22) use coincident mate to attach them together.
    6. (Concentric 11 & coincident 24) Same mates(brake cap) as step 5.
    7. (Parallel 10&11) Mate the planes of brake cap and socket-wrench assembly parallel.
    8. (Concentric 5) Use concentric mate to align the center of hole of the box part with the center of circular edge of socket bar.
    9. (Parallel 5) Mate the top plane of new_box and the top plane of socket bar to prevent the box to rotate.
    10. (Concentric 13&14) Use concentric mate to align both centers of the extruded cylinder on the plane and the hole of clamps(Clamp-1.00 and Clamp-1.27) on the same line.
    11. (Parallel 13) Use parallel mate to fix the box. (Parallel 14&15) Use parallel mate to fix the clamps.
    12. (Coincident 32&33) Use coincident mate to attach the clamps to the box part.
    13. (Concentric 15&16) Use concentric mate to align both centers of shaft collars and the circular edge of socket bar on the same line.
    14. (Coincident 35&36) Use coincident mate to attach the collars to the box part.
    15. (Parallel 16) Fix the shaft collar not to rotate around the socket bar.
    16. (Concentric 17) Use concentric mate to put brake wire on the sketched arc(sketch 3) around the brake part.

    Step 9: Cross-Connector System With Chain (Sub-Assembly)

    For clamp-pipe assemblies, there are some repetitive because cross connector has 4 arms to connect 4 pip.s

    1. First, use concentric mate to align the center of clamp assembly, the circular center of pipe, and the center of hole of cross connector on the same line.
    2. All coincident mates are used to attach the clamp assembly to pipe and also, attach the pipe to cross connector.
    3. Parallel and perpendicular mates are used to make the clamp assemblies and pipes not to rotate.
    4. After these steps, we attached another gear to connect the clamp-pipe assembly with the socket-wrench assembly.
    5. Concentric mate is used to align the center of the gear and the center of clamp pipe assembly on the same line.
    6. Tangent and coincident mate to attach the gear to the cross connector.
    7. (This is how to mate chain with gear.) On plane 2, (Sketch 2) sketch the outline of belt whose length is 51.43"
    8. Use the chain pattern to create the belt and attach it to both gears by using belt mate(BeltMates 1).

    Step 10: Full Assembly

    Notes

    • These design(s) for the assembly are the ideal, final ones made after iterative testing with the prototype. The prototype (which will be discussed next) will slightly differ regarding some of the parts connected to the lever, but the overall idea of using a chain-drive system is the same.
    • Zipped Master Folder of Solidworks Parts/Assemblies/Drawings also included

    Attachments

    Step 11: Cost and Material Choice Analysis

    Initial Designs/Thoughts:

    With the goal to improve or revise Freedom Wheelchair’s lever system in mind, our group was inspired by the simple 2 gear system on a bike. We wanted to use a similar system to allow for more mechanical advantage so that people with disabilities would be able to move more easily through rough, non-ideal terrains. We choose to use a sprocket chain system to achieve this. When planning out our final design alongside prototyping, we went through a list of possibilities.

    With limitations of not having a fabrication lab to customize our parts, we looked into Mcmaster Carr and Amazon to fulfill our prototyping needs. We had three main requirements in mind: durability, heaviness, and cost. Initially we wanted to use PVC because it is a lightweight, durable material, however in the final design we realized that it would not be a suitable choice because of the harsh environments these wheelchairs could be performing in. Soon we decided to transition into designing using mainly metal parts.

    In our engineering designs, we decided to use Alloy Steel because it was an affordable steel material that is stronger and harder than normal steel through the application of heat treatments. This material, however, does rust easily so, a top coating of stainless steel should allow for the wheelchair from any oxidation. We choose to use carbon alloy steel sprockets, sprocket chains, pipe nipples, and clamps for our prototype because these would represents our gears in the lever system. Most of the parts we ordered for the prototype were steel because it was durable, cheap, and easy to find. The lever system design was a seemingly perfect idea because it was based off efficient, daily use objects: bikes. When we received our ordered parts, we soon found out that steel is very, very heavy. This posed a problem because we had 3D printed parts that would not be able to compete against the force of steel parts.

    Race Day:

    After attempting to race with our assembled prototype, all our 3D printed (plastic) parts broke almost immediately. We realized the importance of choosing types of materials for prototyping and designing. We should have chosen to use all metal parts or all PVC instead of a mix of everything.

    Choosing aluminum over steel: After doing our final assemblies, I looked into the material properties to find the total weight. For one side, it was about 37 pounds. 74 pounds was too much for a person to have to push (and a reasonable reason for why our 3D parts broke so easily). Even the mechanical advantage of using a two gear system with ratchets would even out with the sheer weight of all the lever equipment.

    We went back to the drawing board to improve our design. To have a more lightweight lever, we decided our final design would be in aluminum rather than steel. According to the Wenzel Metal Spinning articles on different metals, steel is about 2.5 times denser than aluminum. This would decrease the weight of the lever system significantly. We did not think to use aluminum initially because it is expensive. While steel is harder than aluminum and alloys of aluminum are easily dented and scratched, the metal is corrosion resistant after being spun. It would be very sustainable in outdoor areas because the aluminum would not oxidize as easily as steel.

    Calculations:

    From SolidWorks, it stated that each side’s assembly would weigh about 37 pounds if the material were alloy steel and alloy steel (SS) with preset density settings.

    However, after looking up the density of aluminum (0.1 in/cm^3), it seemed that aluminum was the obvious choice for the parts that did not need to be super durable.

    • Volume of Small Rim Assembly = 135.62 cubic inches -> 13.56 pounds of aluminum
    • Volume of Large Rim Assembly = 134.00 cubic inches -> 13.40 pounds of aluminum

    By redesigning with aluminum parts, the wheelchair would be more robust and more durable in outdoor areas where oxidation is unavoidable. Steel gears and chains would mostly recommend to be used over aluminum gears as it is under a lot of force. If we were to build another prototype, we would have more lightweight materials and smaller but more efficient gears.

    Designing Brake:

    We thought that a stainless steel brake would be the best because it is extremely durable and would take a lot of force to break. While it is expensive, only a small amount of this metal would be used to manufacture the brake wire.

    Material Options:

    This is an overestimate on the budget of the wheelchair because ideally, the parts would be
    both made out of aluminum and steel depending on its function. Even though the steel is slightly cheaper than the aluminum option, having a heavier wheelchair made of steel for people with disabilities would be ultimately worse.

    References:

    Step 12: Prototype Section (Overview)

    1. Assembly Choices (including fasteners and tolerances)
    2. Step-by-Step Instructions for Building Prototype
    3. Iterative Test Method and Results (including force-body diagrams)
    4. Contributions Page

    Step 13: List of Materials and Tools

    Materials (bought from Lowes or McMaster Carr)

    • Off-the-Shelf
      • (8) clamps
      • (4) 6.2" pipe nipples
      • (4) 7-3" pipe nipples
      • (2) 3/4" cross connectors
      • (2) roller link chains (~50")
      • (2) 1-1/2" diameter sprockets (19 teeth)
      • (2) 1-1/2" diameter sprockets (28 teeth)
      • (2) 24" x 1.25" PVC pipes
      • (2) socket wrenches
      • (2) 10" x 0.5" hex bolts
      • zipties
      • string
    • 3-D printed parts
      • (2) "socket-bolt connector" (to hold front sprockets)
      • (2) "zip-tie" boxes (to affix front sprocket system to the wheelchair)
      • (2) brakes
      • (2) 11" x 1/4" PVC hollow dowels

    Tools (Lowes)

    • epoxy (Gorilla Glue)
    • epoxy putty for metals (SteelStik)
    • silicone caulking
    • duct tape

    Step 14: How to Build the Prototype

    1. Take a cross connector and screw in all four smaller pipes (for the smaller push-rim); then screw in to each pipe the clamp converter and then the clamp itself.
    2. Take a back-sprocket-plate and fit the sprocket on it (tightening with the set screws in the sprocket).
    3. Use the holes in the back-sprocket-plate to zip-tie the plate to the cross connector (put two zip ties in a hole and pull them out the nearby hole, making it so that the open ends face away from the sprocket—then close around the bars).
    4. Use the clamps to attach this assembly to the push-rim.
    5. Repeat steps 1 and 2 but for the larger pipes for the large push-rim.
    6. Zip-tie the box to the wheelchair frame at the part of the bar that bends down to the foot rests (using same method as in step 2: push in zip-tie and pull through so that the ends face away from the box, and tighten around frame). Do this for both sides of the wheelchair.
    7. Take the socket wrench, and attach the socket. Fill the bottom of the socket with silicone caulking, and then insert the bolt into the socket. Leave to dry overnight.
    8. Apply epoxy to the sides of the socket and bar. Slide the socket-bar connector over the bolt until flush with socket and in contact with epoxy. Let dry for ~3 hours.
    9. Wrap duct tape around socket wrench handle until it fits the interior of the pvc lever arm. Apply epoxy around the socket wrench handle and slide it into the lever arm. Let dry for ~3 hours.
    10. Repeat steps 5, 6, and 7 for the lever system on the other side.
    11. Attach the gear to the sprockets on the wheel and lever arm, and then slide the lever arm bar into the zip-tie box. Wrap duct tape around the lever arm bar on the inside next to the box (stopping the box from sliding out).
    12. Repeat step 9 on the other side of the wheelchair.

    Step 15: How It Works: Force Body Diagrams

    (1) n = number of teeth | d = diameter | ω = angular velocity | τ = torque

    In our prototype, the gears of our chain drive are the same size, so they do not make the wheelchair move more efficiently. They are there so that the lever can be placed at a more convenient location for pushing and pulling. However, in our final design we have made the front sprocket larger than the back sprocket. The sizes fill out chart as seen in Eq. 1.

    When using the lever, the torque resulting from the force applied to the lever will be 2 . This means that τ1 will be ~1.47 times larger than τ2. While this has the unfortunate consequence of making the wheelchair move slower, it will make it easier for the user to move the lever.

    (2) Variables


    τ = torque

    r = radius from axis of rotation to point of application of the force

    F = applied force

    θ = the angle between the r and F vectors when drawn from the same origin will usually be about 90 degrees so sinθ=1 therefore, as long as r>1 the torque produced will be larger than the force applied, thus more efficient.

    Step 16: Iterative Testing (1)

    Iterative Design Process

    After identifying the problem, it took several ideas before a solution (the lever arm system) could be designed. The initial design used for the prototype was chosen because its feasibility of manufacturing of given the lack of access to tools, machining, welding, and because it would be cost efficient (also known as the design and financial constraints). Throughout the process, the design was continually scrapped and modified to fit constraints that was previously not foreseen. In Step 5/6 (when the prototype was constructed and tested), several issues were encountered regarding the design of prototype, which led to another re-design The most difficult issue faced was the overestimation of the ability of zip ties to hold parts in place and the strength of 3D printed parts. In response, the design was again altered to be more robust. However, there was not enough time nor resources to counter these issues, so makeshift parts and support systems using duct tape were used for race day.

    Specific Issues Encountered on Race Day

    (1) MANY zipties were needed to keep the lever and chain taut.

    (2) The silicone caulking did not have enough time to set (2-3 weeks). It was only given 1-2 nights.

    (3) The 3-D printed sprocket disk broke and had to be replace using zipties and a spare metal pipe.

    Step 17: Iterative Testing (2)

    Alterations to Design after Testing

    (1) The original design for the clamps looked like the ones above but has since been redesigned to the one on the left. This was done because because the new clamp is less expensive while also easier to clamp onto the handrims.

    (2) Our original plans for the part that connects the lever system to the wheelchair was like the figure above. However we changed the design to our current final designed to make the lever arm system stick out less from the wheelchair, make the system stronger, and to make production cost cheaper. Our final design, compared to our first design, has the socket shifted over closer to the socket wrench (to minimize protrusion size), it removes one shaft collar (per side) (to reduce cost), replaces the hexagonal bar and socket, with a socket bar (removing the need to use epoxy and sealant to bind the parts together), and it shrinks the size of the box drastically, from 5”x5” to 1.75”x1.75” (reducing material cost).
    The box (pictured in the assembly above) has also been changed further, our original design had the box be a part that was zip tied to the wheelchair. However, to increase strength and durability, we replaced the zip tie design for clamps. It was later elongated to add more support to keep the lever upright.

    (3) The original design for the sprocket-disc and cross connector is pictured above, and was originally attached to the cross connector with zip ties. To increase the durability and strength of the overall system, the sprocket-disc plate was merged with the cross connector as pictured. It was changed again so that the pipes and cross-connector could lay flush against each other.

    Step 18: Contributions Page

    Step 19: Bibliography

    • “Leverage Freedom Chair.” Grit Freedom Chair, www.gogrit.us/lfc/.
      • The design inspiration of our wheel chair.

    • Shore Susan and Stephanie Juillerat. “The impact of a low cost wheelchair on the quality of life of the disabled in the developing world.” Med Sci Monit, vol. 18, no. 9, 2012, pp. 533-542
      • Information on the company and the population.

    • “SPROCKET AND CHAIN GUIDE". www.revrobotics.com/content/docs/Sprocket-Guide.p... Accessed 4 June 2018.
      • Gives information about sprockets and chains. It shows how to pick them and the physics behind them.

    • “Torque Calculation.” Hyper Physics, hyperphysics.phy-astr.gsu.edu/hbase/torq2.html.
      • Gives physics behind lever arm.

    • Winter, Amos G., et al. THE DESIGN, FABRICATION, AND PERFORMANCE OF THE EAST AFRICAN TRIAL LEVERAGED FREEDOM CHAIR. 2010, static1.squarespace.com/static/54a1bb27e4b07419f3973e5e/t/56a800d21f403984cf9 8285/1453850835910/2010+ASME+IDETC+LFC+Draft+Paper%2C+Final.pdf.
      • Shows physics behind the freedom wheelchair

    • Winter, Amos G., et al. THE DESIGN AND TESTING OF A LOW-COST, GLOBALLY MANUFACTURABLE, MULTI-SPEED MOBILITY AID DESIGNED FOR USE ON VARIED TERRAIN IN DEVELOPING AND DEVELOPED COUNTRIES. static1.squarespace.com/static/54a1bb27e4b07419f3973e5e/t/56a800ba1f403984cf9 8192/1453850812070/2009+lfc+winter.pdf. Accessed 4 June 2018.
      • Shows what parts they used in the freedom wheel chair to give us ideas about our parts.