Challenge 1
Make the shooter throw deliberately weakly: a shot that travels less than half the distance of your best one, and does it three times in a row within a hand-span of the same spot. Weak is easy. Weak and repeatable is the challenge.
What you are building 3 min
The Gear Shooter: a motor, a train of gears, and a heavy wheel that spins up to speed and throws a gear across the table.
Last lesson your Ferris Wheel turned slowly and strongly, and the reason was a small gear driving a big one. This model does the exact opposite with the same parts, and gets the opposite result. Same trade, run backwards.
By the end you will be able to look at any two meshed gears and say which way the trade is going — before switching anything on.
In the real world 5 min
Where you have seen it
Gears are how machines change their minds about speed and force. A bicycle is the clearest example you can actually feel: the low gear that gets you up a hill has your legs turning fast and the wheel turning slowly, and the high gear on the flat does the reverse.
Why it is built that way
When two gears mesh, their teeth have to move past each other at the same rate — they have no choice, they are touching. So a gear with few teeth must turn many times to drag a gear with many teeth round once.
You never get something for nothing. Gain speed and you lose turning force. Gain turning force and you lose speed. The teeth decide which.
What would go wrong without it
A motor has one speed it is good at. Bolt it straight to a fairground wheel and it is far too fast and far too weak to lift the cars; bolt it straight to a launcher and it is far too slow to throw anything. Gears are what let one ordinary motor do both jobs — which is why your kit has one motor and a bag full of gears.
The main concept — you cannot gain both 6 min
One rule, and it has no exceptions anywhere in engineering.
Small gear driving a big one: slower, stronger — that is gearing down. Big gear driving a small one: faster, weaker — that is gearing up.
The demo below makes it countable rather than something you have to take on trust. Pick which gear drives which, and watch both the turns and the load.
0.00turns in0.00turns out×3turning force
strongerslowerslow and strong — a lifting winch
Two gears, meshed. The one on the left is being turned; the one on the right is along for the ride.
The number of teeth is the whole story. 8 teeth driving 24 means 8 ÷ 24 of a turn out for every turn in — and the force changes by exactly the opposite amount.
Your last two models are the same rule, pointed opposite ways:
| Model | Driver → driven | What the machine gets |
|---|---|---|
| Ferris Wheel (Lesson 4) | small gear → 40-tooth gear | Geared down. A slow wheel with enough force to lift four loaded cars. |
| Gear Shooter (today) | big gear → small gear | Geared up. A shooting wheel spinning far faster than the motor — and easy to slow down by touching it. |
Gearing up and down, and which way things turnThe full reference for both — open it if the trade, or the direction two meshed gears turn, is not clear yet.Show meHide
Gearing up and gearing down
Two meshed gears are a bargain. Whatever one of them gains in turns it loses in force, and whatever it loses in turns it gains in force. There is no arrangement of gears that gives you more of both, and no amount of building will find one.
The ratio is the teeth
Count the teeth on the gear you are turning (the driver) and the teeth on the one being turned (the follower). That is the whole calculation:
turns out = turns in × driver teeth ÷ follower teeth
An 8-tooth gear driving a 24 gives ⅓ of a turn out for every turn in — and three times the turning force. Turn the pair round and you get three turns out and a third of the force. Count the turns yourself below.
0.00turns in0.00turns out×3turning force
strongerslowerslow and strong — a lifting winch
Two gears, meshed. The one on the left is being turned; the one on the right is along for the ride.
The number of teeth is the whole story. 8 teeth driving 24 means 8 ÷ 24 of a turn out for every turn in — and the force changes by exactly the opposite amount.
Gearing down, gearing up
| Arrangement | Turns | Force | What it is for |
|---|---|---|---|
| Gearing down — small driving large | fewer | more | winches, lifts, robot arms, anything that has to move a weight |
| Gearing up — large driving small | more | less | fans, spinners, launchers, wheels on a light fast robot |
| 1 : 1 — same size | same | same | moving the drive to a different axle, or fixing a direction |
The names are worth getting right because they are backwards from what people expect: gearing down makes the output slower, not smaller, and it is the setting that makes a weak motor able to lift things.
Gear trains
Put several pairs in a row and the ratios multiply. Two 1:3 reductions in series give 1:9 — which is how a Medium Motor ends up able to raise something it could never shift directly. This is also how a gearbox with a sensible number of parts reaches a ratio that a single pair never could: a 40-tooth gear driven by an 8 is 1:5, and doing it twice is 1:25.
Only the first and last gear affect the ratio. Anything in the middle passes the motion along and changes nothing but the direction — which is a whole idea of its own, in Changing the direction of a turn.
What it costs
Every mesh loses a little to friction, so a long train is less efficient than a short one. Gearing down far enough to lift a heavy load also makes the mechanism slow, and slow is not always acceptable. And a gear train that is geared down hard is very hard to turn backwards by hand, which is either a useful brake or a nuisance depending on what you are building.
Why it matters
A bicycle is the same idea with a chain instead of teeth in mesh: the low gear that gets you up a hill is turning the back wheel slowly and pushing hard, and the high gear you use going downhill does the opposite. Cars, drills, cranes and clocks are all making the same trade.
Changing the direction of a turn
Gears do two jobs and they are worth keeping apart. One is the trade between turns and force — that is Gearing up and gearing down. The other is geometry: which way the far end turns, and about which axis.
Meshed gears turn opposite ways
This follows from what a tooth actually does. Where two gears touch, one tooth is pushing the other sideways — so they cannot possibly be going the same way round. Every mesh in a train flips the direction again.
- Two gears — one mesh — the output turns backwards.
- Three gears — two meshes — the output turns the same way as the input.
- An odd number of gears comes out the same way; an even number comes out reversed.
The idler gear
A gear dropped between two others changes the direction and nothing else. Its own size cancels out — it takes exactly as much as it gives — so the ratio is still first gear ÷ last gear however big or small the middle one is.
That makes an idler the normal, sensible way to fix a direction. It costs one gear and one hole in a beam, and it is a far better answer than rewiring a motor or writing counterclockwise in half your program and forgetting it in the other half.
1meshesoppositedirection out1 : 1ratio
even number of meshes → backwardsteeth set the ratio, meshes set the direction
Two meshed gears. Where they touch, one tooth is pushing the other — so they cannot possibly turn the same way.
Watch the red marks, not the gears. Two gears meshed together can never turn the same way — everything else follows from that one fact.
Turning the corner: 90° drives
Ordinary spur gears keep the two axles parallel. Some jobs need the output at right angles to the input — a motor lying flat in the chassis driving an upright turntable, or a horizontal axle driving a vertical drill.
- Crown gear (the one with its teeth standing up on the rim) meshed with an ordinary spur gear. Simple, common in the EV3 set, and fine at modest loads.
- Bevel gears, cut at an angle and used in pairs. Stronger and quieter than a crown pair, and the right choice if the corner is carrying real force.
- Worm gear driving a spur gear. Also turns the corner, gears down enormously in one step, and cannot be back-driven — the output cannot turn the input. That last property makes it the standard choice for a lifting arm that must hold its position when the motor stops.
The teeth still set the ratio in all of these. Changing the axis and changing the speed are separate decisions that happen to be made by the same pair of parts.
Direct connection — no gears at all
Two parts on the same axle turn together, at the same speed, in the same direction, with no losses. It is the simplest answer and worth reaching for first: if the motor can already do the job at its own speed and force, gears add friction, backlash and parts for nothing.
Gears earn their place when you need a different speed, a different force, a different direction, or the drive to come out somewhere the motor cannot reach.
Why it matters
A car’s differential is a set of bevel gears turning the drive shaft’s rotation through 90° to the wheels. A hand drill turns your horizontal cranking into vertical drilling with the same trick. Once you can see the corner, you start seeing it everywhere.
Say this back before moving on: “Gears do not create anything. They trade speed for force, and I choose the direction of the trade by choosing which gear drives which.”
What’s in this build 4 min
Two electronic parts again — and a great many gears.
EV3 Brick × 1
Large Motor × 1Trace the gear train with a finger
Start at the motor and follow the gears one by one to the big wheel with the tyre. At every mesh, ask the same question: is the driver bigger or smaller than the one it turns?
- Count the teeth on the gear fixed to the motor’s axle.
- Count the teeth on the gear it meshes with.
- Keep going to the end of the train, writing each pair down.
- Now turn the motor’s axle exactly once by hand and count how many times the shooting wheel goes round.
That last number is your gear ratio, measured rather than calculated. Keep it — Challenge 3 and the mission both need it.
The heavy wheel is doing a job too. It is not just a wheel; it is a flywheel. Once spinning it holds its speed, which is what gives the shot its punch. A light wheel spun just as fast would throw far less.
Ports — and the rule 4 min
Sensors go in ports 1, 2, 3, 4. Motors go in ports A, B, C, D. They are not interchangeable, and nothing will tell you politely if you swap them.
| Part | Port | Why this one |
|---|---|---|
| Large Motor (drives the gear train) | A | A single working motor conventionally takes A. Every program on this page says A. |
Safety, and it is not a formality. This model throws a gear hard enough to sting. Point it down the table, never at a person or a face, and keep fingers away from the spinning wheel — a geared-up flywheel will not stop politely for a finger.
Check your own build now:
- Motor cable in port A — a letter, not a number.
- Turn the motor axle by hand. Does every gear in the train turn?
- Is the shooting wheel free to spin without rubbing the frame?
- Is the model pointing somewhere safe?
Connect the Brick 4 min
Two routes, and either is fine. USB is the reliable one and the one to fall back on when a room’s Bluetooth is busy; Bluetooth leaves the robot free to move, which some models need.
How to connect the BrickUSB and Bluetooth, step by step, with a photograph of every screen. Open it if you have not done this before — or if pairing is not working.Show meHide
USB — the reliable one
- Switch the Brick on with the dark grey centre button.
- Cable into the Brick’s PC port — the small square socket beside the numbered ports, not one of the numbered ones.
- Other end into the computer.
Bluetooth — name it first
Do these in order. Naming the Brick after you go looking for it in the list is how groups end up driving each other’s robots.
- Name your Brick. On the Brick: Settings (the spanner) → Brick Name. Type something nobody else will pick, then press the tick. Every Brick is called
EV3until somebody changes it. - Turn Bluetooth on. Settings → Bluetooth. Tick Bluetooth and Visibility. Leave iPhone/iPad/iPod unticked.
- Connect from EV3 Classroom. Click the Brick icon at the top of the programming area, find your Brick by name, and click Connect.
- Say yes on the Brick. It asks “Connect?” with the computer’s name — choose the tick, then accept the passkey, which is already 1234.
EV3.The two failures, every class, every time. The Brick has gone to sleep while you were building — press the centre button to wake it. Or you have paired with the group at the next table, which is why the name matters.
The long version, including Port View and how to read the port tiles, is in the Brick & Bluetooth guide.
Bluetooth is the better choice today if you have it. A model that throws things is easier to aim safely when it is not tethered to a laptop by a short cable.
Confirm the connection 2 min
- Check the Brick icon: connected, or not.
- Check the port tiles. A motor on A, and nothing anywhere else.
- Turn the shooting wheel by hand and watch the degrees counted number race up.
That third step is the gear ratio showing itself again, and this time in your favour: a small turn of the shooting wheel makes a big change in the counter, because the counter is on the motor and the motor is the fast-turning end of a gear-up.
If the number does not move at all, stop here. The cable is in the wrong port or not pushed fully home.
Make it move 10 min
A flywheel needs time to reach speed. So the program is the Ferris Wheel’s shape — start, wait, stop — with the numbers turned all the way round.
when program starts :: events hat [A v] set speed to (100) % :: motors [A v] start motor [clockwise v] :: motors wait (2) seconds [A v] stop motor :: motors
Walk it in the order the Brick runs it:
- set speed to 100 % — a launcher wants everything the motor has. A ride wanted 25 %; the block is the same, the job is not.
- start motor — the train begins turning and the program runs straight on.
- wait 2 seconds — the spin-up. The flywheel is still gaining speed through the whole of this.
- stop motor — ends the run.
What success looks like: a rising whirr as the wheel gains speed, and the gear thrown clear down the table. If it dribbles out instead, the wheel had not reached speed — give it longer before blaming the mechanism.
Change it and test 8 min
One change at a time, predict before each run, and measure how far the gear lands each time. Same loading spot every run, or you are measuring your hands.
- Change the wait to 0.5 seconds. Predict whether a shorter spin-up throws less far, and by how much.
- Now 4 seconds. Does it keep improving, or does it stop making a difference once the wheel is up to speed?
- Back to 2 seconds, and drop the speed to 50 %. Predict first.
- Hold the shooting wheel gently with a finger while it spins up at 50 %, then at 100 %. It is easy to slow — that is the price of gearing up, and it is the same trade the Ferris Wheel bought the other way.
If the gears grind or the train jams, a gear has worked loose along its axle and is only half meshed. Push each gear firmly against its bush and check the train turns freely by hand before running the motor again.
