As I mentioned in my previous post, “Gears & Torque: Crash Course,” using the available VEX gears, you can achieve a 7x increase in speed (or increase in torque) by combining a 12-tooth gear and an 84-tooth gear in between your motor and your moving part. But what if you need more than 7x faster? Or what if you need a really strong moving part? You can achieve this by stacking the gears in a certain way, called compound gears.

Below is a slightly-modified diagram of our Nothing But Net flywheel, which used compound gears to get the flywheel spinning fast enough to launch the NBN balls:

Compound Gears

The drive axle on the left had 2 motors, top and bottom, with a 60-tooth gear; the output axle on the right has a 12-tooth gear and the flywheel. The magic happens on the axle(s) in the middle of the sandwich. The key to compound gears is putting 2 gears of different sizes on the same axle, as shown above (a 12-tooth gear on top, being driven by the 60-tooth gear, and an 84-tooth gear below it, driving the 12-tooth gear on the flywheel axle). The formula is explained below, but it’s easiest to think about how things are turning, as described in the colored labels in the diagram.

The formula for simple gears, from my previous post, is:

Gear Ratio  =  Teeth on Driven Gear
(gear connected to moving part)
Teeth on Driving Gear
(gear connected to motor)

For compound gears, we just calculate this same gear ratio for each pair of gears (however many pairs you have), and then multiply them together, achieving the same answer as shown in the example above:

Gear Ratio  =  12
(connected to middle axle)
 x  12
(connected to flywheel axle)
 =  1  x  1  =  1
(connected to motor)
(connected to middle axle)
5 7 35

The nifty thing about compound gears is that you can stack them up in many different combinations, and by using even more than 3 axles, you can achieve very high speed or very high torque.

Notice I said high speed OR high torque. Just because we’re using compound gears doesn’t mean we get to break the laws of physics (sad face). And, as I mentioned in my previous post about how motors work, even when you pick one of these, there is a limit to how far you can go. Compound gears don’t come with magic unicorns included in the box (again, sad face). What will happen if you try to push the motors too far? Motor Overload (aka Motor Stall). They will just stop, or they will work for a short period of time, and then fail. My next post is going to be about motor overload, so stay tuned!