About the Activity
In this activity, kids will assemble and test different gear sets to see how small actions can create big movements. Along the way, they will learn how to determine the direction of gear rotations, how to build compound gear trains, and the mathematics and importance of gear ratios.
Topic: STEM, Engineering, Crafting
Estimated Time: 40 minutes
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To complete this activity, you’ll need to have or buy a pack of plastic gears (like those included in our Junk Drawer Robotics Kit); they can be simple plastic gears meant for smaller kids (that often come with magnetic whiteboards) more builder-minded products like Legos, or anywhere in between. If you want to build your own gears out of cardboard, download our guide, ‘How to Make Cardboard Gears.’
Either way, you’ll need the gears, and something that enables them to turn – that can be a whiteboard, or axles that run through the gears and are suspended in an architecture.
For this activity, we will assume you are not using a magnetic whiteboard.
Part 1: Drive Gears and Driven Gears
- Using an empty cardboard box, line up one small gear and one large gear so their teeth fit within one another in the center of the box side. Mark on the box the spaces at the center of each gear.Did you know? The interlocking parts of a gear are called teeth.
- Next, cut or punch a small hole where you marked on the box (Note: For safety reasons, you may need an adult to help with this step). This is where the axle for each gear will slide through, so be sure that the hole is no larger than the circumference of the axle.Did you know? The perimeter, or outside edge, of a circle is called the circumference.
- Place the two gears back where you had them on the box originally, and push the axels through the center of each gear and through the cardboard, so the gears are held in place, suspended from the side of the cardboard box.
- Twisting the axle of the smaller gear, turn the smaller gear and observe how it, in turn, turns the larger gear. This is a gear train – a mechanical system formed by two or more gears mounted on a frame so the teeth engage with one another!Did you know? The gear that is the source of the power or rotation is called the drive gear. The gear that is turned by the drive gear is called the driven gear – in this case, since you twisted the small gear, that made the small gear the drive gear, and the large gear the driven gear.
Part 2: Gear Ratios
- Now, let’s determine the gear ratio (the difference between the speed at which the first and last gear rotate). To do that, first count the number of teeth on the larger gear, and the number of teeth on the smaller gear.
- Using your pen and paper, do the math: Divide the number of teeth of the larger gear by the number of teeth of the smaller gear. For instance, if the larger gear has 16 teeth and the smaller gear has 8 teeth, the ratio would be 16/8, which simplifies to 4/1 – that’s your gear ratio, which is expressed as 4:1.
- So what does this mean, exactly? It means that the drive gear (in our case, the small gear) has to turn four times to turn the driven gear (the large gear)!
Part 3: Idler Gears
- Now, pull the larger gear out of its place in the box. On the opposite side of the small gear, line up and mark a space for a medium-sized gear to fit.
- Punch the hole where you marked, and slide an axle through the gear and that hole.
- As you did earlier, mark a space beside the medium-sized gear where a large gear will fit – when you’re done, the small gear should turn the medium gear, which in turn should turn the large gear. Notice that the small gear turns the medium gear in the opposite direction, and the large gear is turned by the medium gear in the same direction as the small gear.Did you know? The middle gear of this gear train is called an idler gear, which is a gear placed between two gears to transfer motion without changing the direction.
Part 4: Compound Gears
- Now, let’s build a compound gear – two gears that are fixed together and rotate at the same speed. To do this, pull your large gear out of the box. First, thread a smaller-sized gear through the axle of the medium-sized gear, so that it sits on top of the medium-sized gear. These two gears attached to each other are your compound gear.
- Now, find and use a larger gear (either a little smaller or larger than the one you used previously), and hold it over the compound gear so the teeth of the large gear intersect with the teeth of the smaller component of the compound gear.
- Mark the center of the large gear on the box, punch a hole there, and thread the axle through the large gear so the large gear rests on the compound gear with its teeth intersecting with the smaller components of the compound gear.
- Now, turn the small gear, your drive gear, and watch as it interacts with the compound gear and your large gear, the driven gear – you’ve created a compound gear train! Amazing!
Bonus Activity (Optional)
Kids with more developed math skills can learn how to calculate the gear ratio of their compound gear.
- Let’s calculate the compound gear ratio.
- For example, let’s say the small gear has 7 teeth and the larger component of the compound gear has 21 teeth, and the smaller component of the compound gear has 9 teeth and the large gear has 30 teeth.
- The first gear ratio is 7:21 and the second ratio is 9:30.
- To calculate the compound ratio, you need to divide the second ratio by the first ration, which would be expressed as (7x9)/(21x30) = 63/630, which works out to 1/10 – expressed in ratio form as 1:10! This means that the small gear has to turn one time to turn the large gear 10 times.
Bonus questions to inspire wonder.
- How did the compound gears make a difference in the gear train?
- What was the hardest part of using the compound gears?
- Can you design a regular straight (non-compound) set of gears to get the same ratio and direction in your gear train?
Investigate and Explore
Take what you've learned to the next level to learn more and explore the possibilities.
Human civilization was made possible by the advent of six simple machines – the wheel and axle, the lever, the inclined plane, the pulley, the screw, and the wedge (though the second three are really just extensions or combinations of the first three).
Gears are a type of wheel and axle, and are central to all kinds of machinery around the world, including cars, factories, and various tools. In your day-to-day life, you may experience gears most directly in bicycles, which uses a gear mounted on the rear wheel and connected to a chain that is connected to the bike’s pedals, to move. And more advanced bikes can switch between various compound gears to go faster or slower.
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