If you’ve ever purchased VEX high-strength gears, you know that they come with a great big rectangular hole in the center and 2 different types of inserts to shove in: dark-grey metal ones with a square hole in the middle, and green plastic ones with a round hole.

VEX high-strength gear inserts

Sometime last season it finally hit me what these green inserts are for. Up until then, our robots only had the need for the square metal inserts, and we ended up with a giant pile of green, seemingly-useless ones. Then the light-bulb went on: these green ones are for idler gears!

What Are Idler Gears?

When you’ve got, say, a wheel that needs power up at the front of your robot, but no where right there to attach a motor, you move the motor backward to wherever your design permits, and then transmit power from the motor to the wheel via a string of gears, known as a gear train.

When you have 3 or more gears in the train, the gear(s) in the middle are called idler gears; the one connected to the motor is the driving gear and the one connected to the wheel or moving part is called the driven gear.

Gear train

Putting It Together

As shown in the graphic above, the idler gear’s role is solely to transmit power from the motor to the wheel; it is not moving a part of the robot on its own (wheel, arm, etc.).

Here’s where those green plastic inserts come in. When you put one of these inserts into the gear, you end up with a round hole in the center of your gear. Stick a shaft through it and spin the shaft…nothing happens! Huh? Why would I want a gear that’s not really connected to the axle running through it?


Friction, most of the time, is not your friend, and a good robot design keeps that in mind, reducing it wherever possible. You want to get every last bit of performance—however small—from your robot.

In a gear train, if the idler gear uses the square metal insert—which does indeed turn the shaft going through it—the idler gear will be vampire-sucking a small amount of energy from the motor in order to make its axle turn (there’s friction wherever that turning axle goes through a bearing flat). Thus it will not transmit the highest amount of energy possible to the wheel; it will be using some of it for no purpose at all. Turning that idler axle accomplishes nothing.

With the round green insert—which does not turn the shaft running through it—all of the turning energy from the driving gear (attached to the motor) is being transferred to the driven gear (attached to the wheel). The shaft through the idler is just keeping the gear even and in place. Of course there’s some friction involved here too, where the gears mesh, but that’s a necessary cost of having a gear train. Turning an axle on an idler gear is not.

Use on a Lift

The green inserts are not just for gears in the middle of a train! This year my team’s Robot v1.0 had a double-reverse 4-bar (DR4B) lift in which idler gears feature prominently. For each side of the lift, there is one motor connected to the bottom vertical segment, and a second connected to the upper vertical segment. Every gear except the 12-tooth gears on the motor shafts are using the green idler insert:

Idler gears on a DR4B

How can this work? Can you have an idler gear on the last one in the gear train? For something like a lift, you can. On a chassis, the final gear in the train is on an axle that also holds the wheel being turned. The final gear in the train must turn that axle, or else the wheel wouldn’t move.

In a lift such as the one in the photo, the lift sections are screwed onto the large gears: when the gear turns, the lifting arm moves, so the only thing that’s actually necessary is to turn the gear. Turning the axle is not necessary.

So for a lift, these green inserts become really handy, again, because reducing unwanted friction is a really good thing.

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Take a look at the gears on your robot—where could idler inserts be used? Happy building!