To make the user as independent as possible it’s important that he is able to store the wheelchair in his car. To provide comfort it has to be big, so there has to be some kind of mechanism to reduce the dimension. For an independent person it’s very important to not need any help to get the wheelchair in and out of his car, which gives us the duty to make our construction as lightweight as possible.
As shown in Appendix 2: Engineering Specification List our wheelchair has to fulfill the geometric requirements to be collapsible (back rest folds 180 degrees) and to fit into a hatchback trunk (max size: 30.25" X 35" X 32"). Under force and torque we considered the weight topic and set the goal for the maximum weight to 35 lbs.
Our main goal was to make it easy to use, so we gave it in our Pugh matrix the highest weight (5). We also considered long durability (4) and lightweight design (4). A point contributing to the Ease of Use is the amount of necessary tools (3), which was important enough for us to treat it as its own criterion. To keep our ideas realistic, we put in the category difficulty to create (2). As it doesn’t have any direct benefit to the user, we gave it the smallest weight.
We had a closer look on four ideas:
● Screws (total weighted rating: -6.57) ● Clips (10.40)
● Spring loaded pins (18.86) ● Folding (21.43)
The screws had despite in the weight column no big positive rating. The idea was to screw parts together and open undo them for moving or disassembly of parts. This would cause a lot of trouble for the user.
Clipping parts together seemed to be a pretty good idea, but failed our expectations concerning durability, especially of the parts that have to be moved or disassembled often. It will still be a good way to attach optional parts like oars to the chair.
The spring-loaded pins are not a complete system; they help the other systems work. By pulling out a pin against the force of a small spring the user can easily undo a connection between two parts. We will use it to attach some parts that need a tight connection to the mainframe, like the pushing module in the back to the wheelchair. This idea leads to higher efforts to construct and build it, but sticks out regarding the ease of use.
Our favorite idea is the folding system; the exact rating is shown in Table 4.1. As easily seen it performed excellent in the ease of use section. As it has no real weak points, it became our clear favorite. It’s an easy technology which offers the user the highest possible comfort.
Wherever we want a part to be moved and not completely disassembled, which use a folding system. When we want a part to be taken away from the frame out of weight reasons we will use spring loaded pins. Those two concepts provide clearly the highest satisfaction for our users and will fulfill our targets.
Table 4.1: Collapsing and Disassembling Pugh Matrix for Folding Mechanism.
Criteria Weight (1-5) Rory Alex Sam Joshua Marco Marvin Max Benedikt
Difficulty To Create 2 1 1 1 1 0 1 0 0 Ease of Use 5 2 2 2 2 2 2 1 1 Durability 4 1 0 0 1 1 0 0 0 Weight 4 2 1 2 1 1 1 1 1 Amount of necessary tools 3 2 0 2 1 0 0 1 1 Total - 30 16 26 23 18 16 12 9 AVG Total 21.43
Drive Systems
As can be seen in the green column, the ratcheting lever design has the most positive attributes and least negative attributes compared to all the other columns. However, the Toothed Belt Drive system excels in some important areas that the Ratcheting Lever design does not, namely the water and sand endurance criterion. If the ratcheting system were made out of metal, the sand would increase wear on the parts, and seawater could corrode it. Small intricate parts that could be hard to replace would wear the fastest. Whereas the Toothed Belt drive could be used in the water and sand without corroding or having major wear damage.
If the pulleys that will receive the belt are made of plastic also, then the parts will easily function in sand and water with little issue. The plastic chain can be considered for the same reasons as the belt, however it would be a little more complicated to assemble, and more expensive to build. The major advantage to the ratcheting lever over the toothed belt is the ease of assembly.
The belt might be difficult to put on to the pulley under tension, whereas the ratcheting gears will be attached to the wheel directly and the only other part to assemble would be the pawl, which fits on a pin. Furthermore there may be a need for some lubricants for the pulley bearings, whereas the ratchet pawl needs only to be strong enough to withstand constant cyclic loading
However, I feel as though the functionality of the system in water and sand is more valuable an attribute than the ease of assembly and lack of lubricants in the ratcheting lever. I feel like a belt drive can be designed to compensate for these drawbacks such as adding a spring tension release lever, and sealed roller bearings. Despite the chart indicating that the lever ratcheting system has the most positive qualities and least negative qualities, I would still choose the toothed belt system.
Table 4.2: Drive systems decision matrix for Belt Lever and Hand Crank Drive.
Criteria
Weight (1-5) Rory Alex Sam
Jo sh ua Ma rco Ma rvi n Ma x Difficulty To Create, Cost 2 -1 -1 -1 0 0 0 -1 Easy of Use 4 1 2 1 1 1 2 2 Durability 4 1 1 1 0 1 1 0 Comfort 4 2 2 1 2 1 2 2 Speed 4 2 1 2 2 2 2 1 Adjustability 3 0 1 0 1 0 1 0 Weight 2 -1 0 -1 -1 0 0 -1 Total - 20 25 16 21 20 31 16 AVG Total 14.9