part · Level 9 · Robotics · output
What TT gear motors do for a robot
Gear motors + wheels (pair) — Slow, strong spin — the legs of every robot you will build.
What it is
The yellow plastic motors that come with almost every robot kit are TT gear motors: a small DC motor — the same kind you met at Level 4 — with a plastic gearbox bolted on. The motor inside spins fast and weakly; the gears slow it down and multiply its force, so the wheel on the end can push a chassis along a floor.
They come as a pair with plastic wheels and rubber tyres. Two motors, one on each side, is all a robot needs to go forward, back, and turn: run both forward to drive, one forward and one back to spin on the spot. That is called differential drive, and it is how vacuum robots and rovers move.
They are cheap, tough, and loud. The shaft comes out both sides so you can add an encoder disc later to count turns.
How it works
The common gearbox ratio is 1:48: the motor turns 48 times for one turn of the wheel, so the wheel has 48 times the torque and 1/48 of the speed. At 6 V the motor spins roughly 10 000 rpm, giving about 200 rpm at the wheel; a 65 mm wheel covers 0.2 m per turn, so that is about 0.6 m/s — a brisk walk. At 3 V everything halves.
Current is the number that matters for your design. Running free, one motor draws about 120–170 mA. Pushing a load it climbs; stalled — wheel held still — it can pull 1 A or more at 6 V. Two motors stalled at once can exceed 2 A, which is why they get their own battery pack and go through an L293D (600 mA continuous per channel — enough for driving, marginal when both stall; the L298N is the next size up).
The torque rating is about 0.8 kg·cm at 6 V: enough to move a light chassis on a hard floor, not enough for carpet with a heavy battery.
The numbers
| Voltage | 3–6 V — 4×AA (6 V) is the sweet spot |
|---|---|
| Gear ratio | 1:48 — 1:90 and 1:120 versions exist — slower, stronger |
| Wheel speed | ≈ 200 rpm at 6 V — about 0.6 m/s on 65 mm wheels |
| Current | ≈ 150 mA free, up to 1 A+ stalled — per motor |
| Torque | ≈ 0.8 kg·cm at 6 V — manufacturer figure |
Mistakes everyone makes
- Robot curves instead of driving straight. No two motors are identical. Fix: trim the slower side with PWM in code, or add encoders later.
- Runs for a minute then dies. 3×AA cannot supply two motors for long. Fix: 4×AA or a 2-cell lithium pack, separate from the logic.
- Wheels spin, robot doesn't move. Not enough torque or tyres slipping. Fix: lighter chassis, hard floor, or a 1:90 gearbox.
- Terminal tabs snap off. The solder tabs are fragile. Fix: strain-relieve the wires; solder gently.
Questions people ask
- What voltage do TT gear motors use?
- 3 to 6 V. On 3×AA (4.5 V) they run; on 4×AA (6 V) they are noticeably faster and stronger. Above 6 V they wear out quickly.
- Can an Arduino drive a gear motor directly?
- No. A pin gives 40 mA at most; the motor wants 150 mA running and up to 1 A stalled. Use an L293D or a transistor, and power the motors from their own battery pack.
- How fast will my robot go?
- About 200 rpm at the wheel on 6 V with a 1:48 gearbox; on 65 mm wheels that is roughly 0.6 metres per second, before losses and battery sag.
Projects that use this part
No project uses it yet — it arrives at Level 9, and the projects for that level are still being written and verified.
verified against: Adafruit — DC Gearbox "TT Motor" (product page & specs) · SparkFun — Motors and Selecting the Right One