Introduction: Arms for Wheelchair

Introduction-( Market Value Proposition)

We were given the challenge to make the Freedom Wheelchair better by adding a lever arm mechanism. After thorough research, our group discovered that lever mechanism-powered wheelchairs provide a better and easier physical motion for the user. Typically a lever-powered wheelchair costs anywhere from $1,000 to $8,000. Instead of designing a 3-D printed lever arm our group came up with the idea of a making the entire mechanism using old bicycle parts, which in fact are quite easy to get access to and repair in third world countries. Our entire attachment costs about $100 to manufacture, which is quite less than the manufacturing cost of its competitors on the market as stated above.

This Instructables is made specifically for high school students who have no previous knowledge of the design-build-test process and aims to provide them with information about our prototype design, thought process and assembly.

Step 1: Design and Procedure of the Assembly.

To make sure our design was specifically built for the third world countries our group ensured that all our design decisions involved parts that were inexpensive and from a bicycle. Most of our design contains parts from used bicycles bought at a thrift shop. The following bullet points will guide you through our prototype assembly.

  • We first removed the hub which included ball bearings and the pedal attached to the crankshaft from the bicycles.
  • The second step in our assembly was to measure all the dimensions of the force fitted PVC shaft provided to us by the university to mount our mechanism on.
  • We then bought chains and sprockets according to the shaft dimensions. The chain was around 10ft long so we had to cut it into two parts.
  • Finally, we bought a clamp holder and a steel duct clamp to fix our crankshaft hub on our wheelchair arm. The clamp holder is adjustable as it has U bolt screws and can be moved according to the convenience of the user.
  • We then adjusted the positions of both the crankshaft and the chains in such a way that they aligned perfectly with our sprocket attached to provide the circular motion which in turn moves the wheel.
  • The pictures above are somewhat a step by step visual aid for our procedure.

Step 2: Step by Step Design of Each Part.

As stated earlier our mechanism includes parts from a bicycle. The entire mechanism is quite simple and consists of four major parts.

The manufacturing design contains-

Sprockets with integrated ball bearings (Major Part), Chain(Major Part), Crank Set (Major Part) and the Clamp Holder(Major Part). The Crankset contains both the pedal and the "driver" gear. The set is mounted on a Clamp holder which is then connected to the wheelchair arms using U-Bolt screws(Minor part). The chain is then connected to the "driver" gear and "driven" sprocket which is force fitted into the hub of the wheelchair wheel. All the parts used in our manufacturing design can be bought from The engineering drawings for each major part and minor part have been included above.

Step 3: Design Advantages and Disadvantages.


People using a wheelchair are limited to the use of their upper limbs for most of the daily activities. The traditional manual wheelchair propulsion requires the use of upper limbs in such a way that it exposes the upper body parts to a combination of load and repetition which leads to the presence of injuries and pain.

Our ergonomic prototype design optimizes mobility while decreasing mechanical loads. The muscle groups involved in the motion are limited to forearms, shoulders, and wrists. The mechanism does not put stress on chest muscles. The mechanism is similar to a bicycle gear mechanism and hence is energy efficient.

Disadvantages (which we would like to solve in future)-

One of the main disadvantages of our prototype design is that the crankshaft limits the range of motion for the users' wrists. From a biomechanics point of view, this causes pain and potentially might injure the user's wrist. Other disadvantages include not including a disc brake, one revolution of the sprocket in our prototype design gives the wheelchair enough force to move at a high speed and hence a disk brake would have ensured timely brakes and safety.

Step 4: Materials Used in Prototype Design

One of the main decision our group faced while making our prototype was whether to 3-D print our parts or not but after researching a lot we figured the two main disadvantages would make our prototype design inefficient.

The first disadvantage was that the 3-D printers at our university couldn't provide both strength and size at an affordable price. The parts in our prototype design required both immense strength and large size to move the wheelchair.

The second disadvantage was the limited access to a 3-D printer in third world countries. The main focus of the project is to design a lever arm for the Freedom Wheelchair which is to be used in the third world countries. Replacing or Repairing 3-D parts in third world countries is a futile task.

Our entire prototype design consists of common materials which are either used to make bicycle parts or are easily available in a mechanical shop like-

Steel Alloy- (chain, crankshaft, sprocket,clamps).

The main reason bicycle parts are made of steel alloy is because of its strength. Steel is more than two times denser than aluminum and it is harder as well. Other alloys scratch and dent easier but due to its properties steel is very strong and is less likely to bend or dent under heat or force. These properties make it an ideal material in our prototype design. Steel is the principal material in the automotive industry and one of the most common materials in transportation and infrastructure due to these properties.

PVC- (force fitted shaft).

The force fitted shaft on which the entire sprocket assembly was attached to using a metal pipe was made of PVC. PVC's lightweight, strong mechanical strength, and abrasion resistance properties made it an ideal force fitted shaft for this project.

Step 5: Motion Analysis and Calculations. (Iterative Testing)

Equations Used-

The law of the lever

MA= FA/FB = a/b

Speed ratio


Gear trains




Chain and belt drives



MA=TB/TA=NB/NA (N=# teeth; B=output, A=input)

MA=TB/TA=rB/rA (friction belt drives)

A chain or belt drive can lose as much as 5% of the power through the system in friction heat, deformation, and wear, in which case the efficiency of the drive is 95%.

Calculations for Our Prototype-

The wheel is 26 inches in diameter. (13 inches radius) Crank: 36 teeth, 6 inches diameter (Right side) Crank: 28 teeth, 4.5 inches diameter (Left side) Sprocket: 60 teeth, 9.84 inches diameter

Right Gear ratio-“Gear inches” = (Wheel diameter in inches) x (number of crank teeth) /number of sprocket teeth = (26*36)/60=15.6

Left Gear ratio-“Gear inches” = (Wheel diameter in inches) x (number of crank teeth) /number of sprocket teeth = (26*28)/60=12.13

Right Distance traveled per one pedal turn=15.6π= 49.009 inches

Left Distance traveled per one pedal turn=12.13π= 38.118 inches

Without the gears, the gear inches are decreased since the crank teeth and sprocket teeth are the same number since there is only one gear. Our lever arm invokes a gear ratio to double the mechanical advantage. This allows the user to travel twice as far using the same force or use double the force to maneuver hills and other obstacles more easily.

Number of revolutions per minute is between 60 and 80 – depending on individual body.

Use 70 for calculations

Distance per minute=distance/turn*turns/minute

3.249 mph Right ; 2.527 mph Left

Actual speed (efficiency is ~95%)

3.086 mph Right ; 2.400 mph Left

Step 6: Cost Analysis and Market Scope.

At this moment the cost of manufacturing our mechanism is around $85. The cost of our mechanism could be decreased even further by using a recycling business model where the majority of parts being used are bought from recycling units. This business model would not only be beneficial for the manufactures but also the environment. The market for this attachment is immense especially in third world countries with an abundance of steel( cheaper manufacturing costs, parts can later be exported). Our prototype was designed specifically for the Freedom Wheelchair but can be modified to fit any wheelchair.

Step 7: Contributions

Team Name- Arms for Wheelchair (BME 60C)

Project Manager: Mayra Contreras

The project manager, Mayra, was responsible for overseeing project progress, meetings, etc.

Manufacturer: Mayra ·Contreras

The manufacturer, Mayra, was responsible for taking the 3D printer training.

Materials Engineer: Maisha Hassan ·

The materials engineer, Maisha, was in charge of budget & materials/parts choices during the design process.

Tester: Christopher Glaubensklee·

The tester was responsible for the iterative testing process and assisted in engineering drawings.

Lead Designer: Sohum Waikar ·

The lead designer was responsible for the assembly of everyone’s parts.

Researcher: Aditya Bhandari

The researcher was responsible for the documentation, research, addressing market &

value and making the Instructables.