Challenge 1
Work out your gear ratio two ways. Calculate it from the teeth counts, then measure it by turning the motor one full turn and seeing how far the drum moves. The two answers must agree.
What you are building 3 min
The Strong Lifter: a hoist geared down so far that one EV3 motor can raise a load it could not begin to lift directly.
The motor in this model is the same motor that was in the Cyclist and the Hammer Bot. Nothing about it has changed. It has not been given more power and it is not being asked for more.
The gears did it — and they charged you for it. Today you find out what the price was, and measure it.
In the real world 5 min
Where you have seen it
The steering gear of a sailing ship: a wheel a person can turn by hand, connected through a train of gears to a rudder that the whole ocean is pushing against.
Why it is built that way
A sailor cannot out-push the sea. So the gearing is arranged so that many turns of the wheel produce a small movement of the rudder — and in exchange, a modest push on the wheel becomes an enormous force at the rudder.
Nothing is created. The sailor does the same total amount of work either way; the gears only spread it out. Turn the wheel four times instead of once and you can push four times as hard. That is the bargain, and it is always the same bargain — you get force by giving up distance, or speed by giving up force.
You see the same trade on a bicycle. Low gear climbs the hill but your legs spin fast and you crawl; high gear is fast on the flat and hopeless uphill. Same rider, same legs, different exchange rate.
What would go wrong without it
Connect a small motor straight to a big load and it does not move slowly — it does not move at all. It stalls, draws current, gets hot, and eventually gives up. Gearing is not a nicety; it is what makes a small motor useful at all.
Gears never give you something for nothing. They let you choose what to pay with.
The main concept — speed out, force in 6 min
A small gear driving a large one turns force up and speed down, by exactly the ratio of their teeth.
| Driver → driven | Ratio | Output speed | Output force |
|---|---|---|---|
| Same size | 1 : 1 | Unchanged | Unchanged |
| Small → large (8 to 24) | 1 : 3 | Three times slower | Three times stronger |
| Large → small (24 to 8) | 3 : 1 | Three times faster | Three times weaker |
Read the last two columns together. They always move in opposite directions and always by the same factor. If somebody tells you a gear train made something stronger, the next question is “how much slower?” — and there is always an answer.
Trains multiply
Put two 1:3 reductions in a row and you do not get 1:6, you get 1:9. Ratios multiply along a train, which is how a hoist can end up hundreds of times stronger and hundreds of times slower than the motor driving it.
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.
Measuring the trade
You already have the instrument. Lesson 18 turned degrees and seconds into speed; Lesson 19 established how to compare two builds fairly. Put them together and the trade stops being a claim and becomes a number:
when program starts :: events hat [A v] reset degrees counted :: motors reset timer :: sensors [A v] run for (3) [rotations v] at (50) % speed :: motors write [Masa:] at line (1) :: display write (timer) at line (2) :: display
Every gear train is a decision about what you need more of. Say which one before you build it.
Which way a gear turnsMeshed gears turn opposite ways, and what that means for a train with several stages.Show meHide
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: “Stronger and slower, by the same amount.”
What’s in this build 4 min
| Part | What it is doing here |
|---|---|
| EV3 Intelligent Brick | Times the lift. It is the measuring instrument, and it must not change between tests. |
| Large Motor — the hoist | The constant. Same motor, same speed setting, every single test. Anything it does differently is the gears’ doing. |
| The gear train (not electronic) | The variable. Count the teeth on every gear in the chain — the numbers are moulded on, and you need them for your table. |
| The load (not electronic) | Something you can add to in equal steps — identical bricks are ideal, because “six bricks” is a measurement and “heavy” is not. |
Work out your ratio before you run anything
- Start at the motor and follow the chain to the drum, writing down the teeth on every gear in order.
- For each meshed pair, divide the driven gear’s teeth by the driver’s. That is one stage’s ratio.
- Multiply the stages together. That is your overall ratio — write it at the top of your results sheet.
- Turn the motor one full turn by hand and count how far the drum goes. Does it match the ratio you calculated? Two ways to the same number is how you know it is right.
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 |
|---|---|---|
| Hoist motor (Large) | A | One job motor, and it stays in A across every version of the gear train or the test is not identical. |
| Sensors | none | The timer is inside the Brick and the count is inside the motor. |
Check your own build now:
- Motor in A.
- Set the load on the floor, not hanging, before each run. A test that starts with the rope already tight is measuring something different.
- Nothing under the load. A geared-down hoist holds well and drops hard if something slips.
- Check the gears mesh cleanly and nothing is rubbing. Friction is a cost you did not choose, and it will show up in your times as noise.
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.
Keep it the same all lesson. As in Lesson 19, the connection is one of the things you are promising not to change — swap halfway through and your morning and afternoon numbers stop being comparable.
Confirm the connection 2 min
- Check the Brick icon: connected, or not.
- One Large Motor tile on A.
- Turn the motor by hand and check the count climbs — and notice how far the drum moves for a whole motor turn. That is the ratio, felt rather than calculated.
- Run the test program once with no load and record the time. That is your baseline for this gear train.
Make it move 10 min
Lift a load a fixed amount and time it. Then add weight until it will not lift any more.
when program starts :: events hat clear display :: display [A v] set motor to [hold position v] at stop :: motors [A v] reset degrees counted :: motors reset timer :: sensors [A v] run for (3) [rotations v] at (50) % speed :: motors write [Masa:] at line (1) :: display write (timer) at line (2) :: display write [Darjah:] at line (4) :: display write ([A v] degrees counted) at line (5) :: display play beep (65) for (0.2) seconds :: sound
- hold position at stop — the load stays up when the lift finishes. On a hoist this is not optional.
- 3 rotations at 50% — the fixed effort. Neither number changes today, whatever the gears are.
- The time on line 2 — the result that goes in your table.
- The degrees on line 5 — the check. Well short of 1080 means it stalled, and a stalled run is thrown away rather than recorded.
What success looks like: with a light load, three runs that agree closely and a degrees reading near 1080. Then start adding weight, one identical brick at a time, running three times at each weight, until the degrees reading finally comes up short. The weight just before that is what this gear train can lift.
Stop as soon as it stalls. A stalled motor is drawing full current and turning it into heat. Take the load off, let it rest, and record the last weight that worked.
Change it and test 8 min
One thing changes: the gears. The program, the motor, the speed and the load steps all stay exactly as they are.
- Record your current train fully: overall ratio, no-load time, and the heaviest load it lifted. That is version one.
- Gear it down further — add another reduction stage. Work out the new ratio on paper first and predict both numbers before you test: it should be slower, and it should lift more.
- Gear it up — remove a stage, or reverse one so a large gear drives a small one. Predict again. Faster, and it should give up at a lighter load.
- Plot your three versions: ratio against time, and ratio against maximum load. Two simple graphs, and the shape of them is the whole lesson.
- Multiply time by maximum load for each version. The numbers will not be identical — friction takes a cut — but they should be much closer to each other than either column is on its own. Say what that tells you.
Step 5 is the point. Speed and force are not two separate things you can improve independently; they are two ways of spending the same motor. The gears decide the exchange rate and nothing decides the total.
When somebody says a machine is stronger, ask what it gave up. If the answer is “nothing”, they have not measured it.