Page 107 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

88 Making Things Move

FIGURE 4-12 Free body diagram of Graffbot spool (left) and original Graffbot spool drawing
(right) (credit: Mike Kelberman)

In order to choose the correct motor, we need to take this one step farther and draw
a free body diagram of the spool, as shown in Figure 4-12.

We can neglect the weight of the spool and motor, because the holding force of the
screws that mount them to the wall cancel it out. That leaves just the force from the
rope and the torque from the motor that is in line with the spool shaft to resist the
spool from being unwound. Remember that torque is just force times distance? The
distance here is from the edge of the spool to the center of the spool. If the diameter
of the spool is 4 in, then the radius is 2 in, and we can solve for the unknown motor
torque:

Torque (T) = 7 lbs × 2 in = 14 in-lb
We now know that we need a motor that can turn with at least 14 in-lbs of torque to
get this Graffbot moving. Each motor needs to be this strong to control the spray
paint can platform.

How to Measure Force and Torque

You can measure your weight (the force you exert on the ground) by standing on a
common bathroom floor scale. But what if the object you need to weigh doesn’t fit
on a scale, or you need to measure the pulling force from something like a rope? And
how do you measure torque?