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control vehicles. The Open-Aid will subject this motor to large amounts of torque relative to its
required size. Finding a motor that would fit in a small appliance and still produce large amounts
of torque was difficult. Eventually it became obvious that only a geared motor or rotary plate
motor would produce 1,400 oz-in of torque with a low amount of power, given the space
constraints imposed on both the motors and gears.
After assigning the scores based on these considerations, the relative importance of each
category was rated in relation to the others. Normally, a survey of customers, retailers, and
manufacturers would be used to help determine these weighting factors. Instead, the weight of
each category was determined based the group’s judgment. The two most important categories
were cost and torque, since they determine the competitiveness and functionality of the
automatic jar and bottle opener. If the Open-Aid lags in either of these areas, it is essentially
worthless. The weight, diameter, and power of the motor comprised the categories of middling
importance, while the length and angular velocity were considered relatively less important.
4.3.5 Impact on Design
The weighted decision matrix used to evaluate this design problem is located in Appendix
5.4.3. According to the matrix, the best option was to use motor No. 2, a Buehler 12V DC
Geared Motor from Electronic Surplus Inc, and a 6:1 gear ratio. It is pictured below (Fig. 4.7).
Figure 4.7. Motors used in Open-Aid.
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