Scissor mechanisms
A scissor mechanism is pairs of bars crossed like an X and pinned through their middles. Squeeze the bottom corners together and the whole thing grows taller; pull them apart and it folds back down. It turns a small sideways movement into a large movement at right angles to it.
How the geometry works
Each X contributes a height, and stacking them adds those heights up. For links of length L at an angle θ above horizontal:
height = stages × L × sin θ · base span = L × cos θ
So more stages means more height for exactly the same squeeze — which is why an extending arm gets dramatic very quickly. Each stage costs a pair of links, four pins and a little more wobble.
The force bar is the part builders meet in real life: a scissor lift that will not start from flat is not underpowered, it is being asked at its worst angle.
The part that catches people out
A scissor lift is hardest to start. When the links are nearly flat, a big sideways push produces only a small sideways movement of the pins but a huge rise — and the force needed to hold the load goes up correspondingly. As it rises and the links steepen, the job gets easier and easier.
This is why a LEGO scissor lift so often sits there straining at the bottom and then flies up once it is moving. The motor is not underpowered; it is being asked at the worst possible angle. Three ways out, in order of how well they work:
- Never start from flat. Build it so the links rest at 15–20° rather than fully folded. This is by far the easiest fix and costs nothing.
- Gear it down harder — see Gearing up and gearing down. Slower, and strong enough at the bottom.
- Help it at the start with a spring or band that pulls upwards, so the motor only has to do the rest.
Driving one
Something has to pull the bottom corners together. A lead screw or a worm drive is the usual answer, because both are strong and cannot be back-driven — the lift holds its height when the motor stops instead of sinking under the load.
Whatever drives it, the movement ends against something: fully folded, or fully extended. Drive to either of those with a timed motor block, never with degrees. A measured block waits for a shaft that has stopped turning, and the program hangs there for ever.
Building one that works
- Every pin is a little bit of play, and the errors add up along the stack. Three stages is usually the practical limit before an EV3 scissor lift becomes a wobble.
- Build it in pairs — two scissor stacks side by side, joined across — or it will fold sideways as readily as it folds down.
- One end of each bottom link must be free to slide. Pin both ends of both bottom links and the mechanism cannot move at all.
Why it matters
Scissor lifts raise people to work at height, lift cars in a garage, and sit under the beds of tipping trucks. The same linkage extends a folding gate, a lazy-tongs grabber, and the arm on a magic-trick boxing glove.
More mechanics tutorials
- Mechanisms without motors — How to investigate a build with no electronics in it, and where each idea lives.
- Gearing up and gearing down — Trade turns for force, or force for turns — you cannot have both.
- Changing the direction of a turn — Reverse a turn, restore it with an idler, or send it round a 90° corner.
- Pulley systems — Fixed, movable and combined — how rope pulled trades against force needed.
- Levers — effort, load and pivot — Where the pivot sits decides the force you need and the distance you get.
- Elastic and stored energy — Stretch a band, store energy, let it go — and find where more stops helping.
- Oscillatory motion — Turning a rotation into a back-and-forth, and what sets the rhythm.
- Centre of gravity — Why a robot tips, and how to build one that does not.
- Biomimetic mechanisms — Linkages that copy how animals move — turning a rotation into a step.