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Chapter 4 Forces, Friction, and Torque (Oh My!) 91

You can clamp a torque gauge onto a motor shaft or a screw head and read the
resulting torque in real time. However, at $580 for a gauge like McMaster’s
83395A29, this is not a very accessible option either.

Luckily, we can use the fact that torque = force × distance to our advantage here. All
we need to do is measure the force and the distance from the axis to the point where
it is being applied, and we get torque!

P roject 4-2: Measure Motor Torque

When you buy a motor, it will usually come with a list of specifications to tell you
everything you want to know about it. However, sometimes you’re stuck with a motor
that doesn’t have a data sheet. Here, we will use an adaptation of MakerBot’s Rack
(Figure 4-13) to measure motor torque indirectly by measuring motor force.

Shopping List:

• DC motor (Solarbotics GM9 shown)
• Shaft collar, gear, or other component that fits the shaft and has a set screw
hub; replace the set screw with a regular long screw
• Luggage scale, spring scale, or fish scale

• Two C-clamps to hold the motor and scale
• Ruler
• Optionally, epoxy putty and small hook (like one for hanging pictures)

Recipe:

1. Fix the screw and shaft collar or gear to your motor shaft.

2. Fix the motor to the edge of your work table with a clamp (see Figure 4-14).

3. If necessary, use the epoxy putty and small hook to create an attachment on
the scale that can hook around the screw.

4. Fix the luggage scale to your work table with a clamp.