Page 226 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

204 Making Things Move

In the Servos & Accessories section, ServoCity (www.servocity.com/html/servo_
shafts___couplers.html) has at least six different couplers you can use to extend the
shaft, attach other parts, or attach another shaft. ServoCity also offers all kinds of
arms, pulley wheels, gears, and sprockets (for chains) that attach directly to the spline
on the shaft. If you can use hardware ready-made to interface with a servo, definitely
do it.

Even if you don’t use one of the ready-made servo accessories, you can still take
advantage of the fact that the motor shaft is threaded and use a screw to attach
something to it. You can also glue the flimsy plastic servo arms that come with most
hobby servos to something more durable. Many hobby servo suppliers will sell you a
small amount of screws that work with your motor, so you don’t need to figure out
what size they are and buy a box of 100 from McMaster. Screw size 4-40 (screw sizes
are covered in Chapter 3) is common to standard servos, but it’s worth checking to
confirm before you try a random screw and ruin the threads.

Working with Other Types of Motors

Common shaft sizes for other motors you might work with (DC, DC gearhead, and
stepper) range from as small as 1/16 in to around 3/8 in for larger motors. The problem
here is that most gears and other components have inner diameters that are larger than
the motor shafts. You also might want to attach your motor shaft to a smaller or larger
shaft, and if you don’t get it perfectly centered, the whole thing will wobble.

When you attach something to a motor shaft, you are really asking for all the motor
torque to go from the motor into what you are attaching (gear, pulley, coupler, and so
on) without slipping. Hobby servos solve this problem for you by using a spline that
can bite into the mating piece to transfer torque. On the other hand, a smooth,
metal, circular shaft inserted into something with a smooth metal inner diameter is
just about the worst possible way to transfer torque, yet is often what we’re stuck
with when dealing with all motors other than hobby servos. Let’s look at these
common problems and how to solve them.

Using D-Shaped or Flatted Motor Shafts
An important rule of thumb is that any shape transfers torque better than a circle!
Many motors come with a D-shaped or flatted shaft (a circle with a flat on one side;
see Figure 7-17) or a shaft that’s flatted on both sides. Find and use these as often as
possible.