Page 102 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 4 Forces, Friction, and Torque (Oh My!) 83

7. Break out your calculator. To find the coefficient of friction between the wood and
clay, take the tangent of 45° (remember μ =tan ). You should get 1. This means
that if you tried to push a clay block across the floor (as we did earlier with the
box example), you would need to push with a force equal to its weight!

8. Do the same calculation with 12°. You should get 0.21. Since the iPhone is
slipperier than the clay, it slides more easily. If you push an iPhone across a
wooden floor, you need to push with a force equal only to 21% of its weight
to get it to move.

Reducing Friction
Now that you know that friction is the enemy, let’s look at a couple of ways to
decrease friction: clearance between parts and lubrication.

Clearance
In Chapter 2, we talked about tolerances of materials and parts. So now you know
that a 1/2 in shaft won’t fit in a 1/2 in hole very well if they are both 0.50000 in. You
need to leave a little clearance between parts that move relative to each other.

Clearance is just a fancy word for space. You need to leave space around your 1/2 in
shaft for it to move, so you may want to drill out a hole that’s around 0.515 in to give
it some room to spin. There is no magic correct amount of clearance—it will depend
on the size of your parts, their surface finish, and whether you want them to spin or
stay put. For example, think of LEGOs. Some parts, like the axles, slide right through
the holes in other parts. But the little gray stoppers, gears, and wheels you put on the
axles are harder to slide on. Also, once you slide them to the right spot, they generally
stay there. This is because there is less clearance between the axle and the gear than
there is between the axle and the hole through the LEGO piece.
These differences in clearances between the parts allow you to constrain the motion
of your LEGO parts just enough, but not too much. This follows the principle of
minimum constraint design we talked about in Chapter 1. Using clearance
appropriately is an excellent way to practice this principle.