Page 96 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 4 Forces, Friction, and Torque (Oh My!) 77

torque is 1.4 ft × 1 lb = 1.4 ft-lbs. That’s why it’s easier to hold the can lower than it is
to hold it straight out.

Let’s take this example one step further and assume that you want to make a
human-sized puppet that can raise and lower a can of soup. If you put a motor at its
shoulder, how strong would the motor need to be?
In order to design mechanisms that move, you first need to understand how to
estimate if something will break when it’s not moving, or static. Most of the time, you
can isolate a static problem that represents the worst case of a dynamic (moving) one.
In this case, it’s when the weight of the soup can is farthest from the motor shaft,
thus requiring the greatest shoulder torque. Since the weight of the soup can doesn’t
change, the highest torque will be needed when the distance from the can to the
puppet shoulder is the highest. This happens when the puppet arm is parallel to the
floor, as in Figure 4-2. Since we already solved for this maximum torque of 2 ft-lbs,
you know to look for a motor that is at least that strong, and it will be able to handle
all the other angles just fine.

Friction
Friction occurs everywhere two surfaces are in contact with each other. It’s what makes
door hinges squeaky and mechanisms noisy. High friction is sometimes a good thing
when your mechanism interacts with the environment, such as the way friction allows
your car tires to grip the road. However, friction is usually your enemy when it comes
to making things move. It can rob your mechanism of power and decrease efficiency.
Low friction is what we strive for inside mechanisms to make things run smoothly. Low
friction is what makes nonstick cookware slippery and causes you to slide on ice.

So what is friction, and how can you design
FIGURE 4-5 Forces on a box at rest
projects and choose materials to minimize it?
Friction is actually a force, just like your
weight is a force. In fact, the force of
friction is a percentage of any object’s
weight. Suppose you are trying to move a
50 lb box across a hardwood floor. At first,
when the box is at rest, there are two forces
acting on it, as shown in Figure 4-5: