Brick Gear Building: Ratios, Gear Trains and Shafts
The short version: start with ratio. A small gear driving a large one slows the output and multiplies torque; a large gear driving a small one speeds things up. The three families you will actually use are standard spur gears (8 to 40 teeth), crown gears and worm gears — all of them mounted on cross axles and held in place with bushes.
Meet the gear family
Almost every builder hits the same moment: a box of colorful gears gets tipped out and none of it looks obvious. In practice brick gears fall into just three families. Standard spur gears are the common ones, made in 8, 12, 16, 20, 24, 36 and 40 teeth, and the tooth count is what sets the ratio. Crown gears convert horizontal rotation into vertical rotation, which is why they turn up in the steering of car models. Worm gears produce a very large reduction in a single stage and are self-locking, so they suit any mechanism that has to hold its position and not run backwards. Get comfortable with these three and the rest of the build stops being intimidating.
Why gear ratios matter
The core of gear building is ratio. When two gears mesh, speed and tooth count are inversely proportional: a small gear driving a large one reduces speed and increases torque, which is what you want for climbing and for mechanisms that need force; a large gear driving a small one increases speed and reduces torque, which suits anything that needs high rpm. For example, a 24-tooth gear paired with an 8-tooth gear gives a 3:1 ratio — the motor turns three times for every single turn of the output shaft, while torque goes up roughly threefold. A beginner's rule of thumb: want force, drive big with small; want speed, drive small with big. Two gears meshing directly spin in opposite directions, but put a gear between them and the input and output turn the same way — do not overlook that when you lay out a train.
Common drivetrains: direct mesh, reduction sets and differentials
Once ratio makes sense, you can build real transmissions. The simplest is a direct mesh, two gears touching, which is ideal for seeing how rotation direction works. One step up is a two-stage reduction: two gear pairs drop the speed in successive steps, for instance a motor driving an 8-tooth gear, then a 24-tooth driving an 8-tooth, and finally a 40-tooth output, for an overall reduction of 15:1 and plenty of pulling power. Car MOCs almost always need a differential, which lets the left and right wheels turn at different speeds in a corner and avoids slipping and wear. When you build one, seat the differential housing squarely and get the planet gears fully in place, or it will bind as soon as the model turns.
Axles and gears: cross axles and bushes
A gear only transmits anything once it is on a shaft, and the workhorse here is the cross axle. The cross hole in the centre of a gear has to line up exactly with the cross axle, and once it is on you limit it from both sides with a bush or half bush so it cannot slide along the axle. The usual mistakes: an axle that is too short so the gear works its way off, or too many bushes stacked up so the gear is clamped solid. Slide the gear along the axle by hand before you fix it — if it runs smoothly, lock it down. If you want overload protection, a friction bush will slip under load and protect the motor, a trick that pays off in any motorised MOC.
FAQ
Q: How do I get started with brick gears? A: Learn the three families first: standard spur gears (8, 12, 16, 20, 24, 36 and 40 teeth are the common ones), crown gears and worm gears. Then practise the ratio rule — small driving big for reduction and torque, big driving small for speed.
Q: How do I track down a gear train that binds? A: First check whether the gears are meshed too tightly or an axle is not pushed fully home, then leave a tiny bit of play between the cross axle and the gear. Those two causes account for most seized gears — turn everything by hand once before you close up the assembly.
Q: Which drivetrain should I choose? A: Direct mesh is the simplest and teaches basic ratio; a reduction set is for trading motor rpm for torque; a differential is for steering and for letting two wheels run at different speeds, which suits vehicle MOCs. Work up through them as you need them.
Q: How should axles and gears fit together? A: Use cross axles with bushes to fix the gear's position, and always leave a little axial clearance so nothing is crushed. Where loads are high, use friction bushes or support the axle at two points so the gear cannot drift under load.
The two failures that catch most gear builds are seizure and free-spinning. Seizure is nearly always a spacing problem: gears meshed too tight or too loose both misbehave, and the most reliable way to set standard spacing is to use the length of one axle as your gauge. Free-spinning usually means an axle that is not seated, a gear that never engaged, or mismatched tooth counts. Another frequent complaint is noise, which is generally a gear interfering with the structure around it — check for any part poking into the gear's path of rotation. Turn each stage by hand as you finish it and only move on when it runs smoothly; finding the problem at the end of a full build costs far more time.
A practice order for new builders
To work through gear building systematically, take it in four steps. First, build a small official Technic set and study how the designers lay out the gears. Second, run a minimal transmission test: one motor, one gear pair, and watch how the output speed changes. Third, try a modification, such as converting an ordinary car model to differential steering. Only then start designing from scratch. It helps to keep a sorting tray of gears with the common tooth counts separated out, so you can find parts quickly. Remember that gear building is logic, not force — once ratio and structure are second nature, your MOCs will genuinely move.