Mechanisms

104 mechanisms worth carrying in your head, each drawn from its real geometry and running. They are grouped by the job they do, because that is how you reach for one when a robot or a print needs it.

the part you drive
the motion you get out
links in between
fixed frame

Spin into slide

Turn a motor’s rotation into straight-line travel.

Crank and slider

rotation → back-and-forth

DOES: A spinning crank pushes and pulls a block along a straight guide.
TEACHES: The stroke is twice the crank length, and the slider is slowest at each end.
USE: Piston pumps, a punching or stamping head, a gripper jaw driven by one small motor.

Scotch yoke

rotation → smooth sine motion

DOES: A pin on a wheel rides in a slot, shoving the slotted bar side to side.
TEACHES: It gives a pure sine wave with no connecting rod, at the cost of a sliding pin that wears.
USE: Compact reciprocating pumps and shakers where there is no room for a long rod.

Cam and follower

rotation → any lift you shape

DOES: An off-round disc lifts and drops a follower once per turn.
TEACHES: The outline of the cam is the motion program: change the shape and you change the timing.
USE: Valve trains, automata, a latch that must open at one exact point in a cycle.

Rack and pinion

rotation ↔ straight travel

DOES: A gear rolls along a toothed bar, so turning one moves the other.
TEACHES: Travel per turn equals the gear’s circumference, and it works in both directions.
USE: Steering, sliding doors, a linear axis that needs speed more than force.

Lead screw and nut

rotation → slow, strong push

DOES: Turning a threaded rod drives a nut along it.
TEACHES: One turn moves the nut one thread pitch, so a fine thread trades speed for a lot of force and usually holds position with the power off.
USE: 3D-printer Z axes, vises, a lift or actuator that must not drop when unpowered.

Quick-return slotted lever

steady rotation → slow push, fast return

DOES: A crank pin slides in a slotted lever, swinging it so a ram goes out slowly and comes back fast.
TEACHES: Where you put the pivot sets how lopsided the timing is: the working stroke gets more of the turn than the return.
USE: Shapers and saws, or any tool that works in one direction and wastes time coming back.

Belt-driven carriage

rotation → long, fast travel

DOES: A carriage is clamped to one side of a belt loop running between a motor pulley and an idler.
TEACHES: Travel per turn is the pulley’s circumference, and the stroke can be as long as the belt.
USE: 3D-printer X and Y axes, plotters, a shoulder that rides up and down a mast.

Winch drum

rotation → pull along a rope

DOES: A drum winds a rope in or out to raise and lower a load.
TEACHES: Each turn takes up one drum circumference of rope, and a rope can only pull, never push.
USE: Hoists, cable-driven lifts, tendon drives where the motor sits far from the joint.

Cardan straight-line gears

rotation → an exact straight stroke

DOES: A gear half the size of the ring it rolls inside carries a point on its rim back and forth along a perfectly straight line.
TEACHES: When the rolling circle is exactly half the fixed one, the curved path collapses to a straight line.
USE: A crank-free piston drive, and a striking demonstration print.

Trade speed for force

Give up speed or distance to gain torque or push.

Gear pair

fast and weak → slow and strong

DOES: A small gear turns a big one, which turns more slowly and the opposite way.
TEACHES: The ratio is just the tooth counts. Here 12 teeth drive 36, so three turns in give one turn out with about three times the torque.
USE: Any reduction where two shafts can sit side by side.

Planetary gear set

big reduction on one axis

DOES: A centre gear drives three planets that walk around inside a fixed ring, carrying the output with them.
TEACHES: Ratio = 1 + ring teeth ÷ sun teeth. With 75 and 15 that is 6 : 1, shared across three meshes at once.
USE: Robot joints and wheel hubs, where the motor and output must stay in line.

Belt or rope drive

reduction across a distance

DOES: A small pulley pulls a belt that turns a larger one some way off.
TEACHES: The ratio is the pulley diameters. A rope wrapped on drums (a capstan drive) does the same job with almost no backlash.
USE: Putting a heavy motor near the body and driving a far joint, as in robot-dog legs.

Worm and wheel

huge reduction, turned 90°

DOES: A screw-shaped worm turns a gear one tooth for every turn of the worm.
TEACHES: The ratio is the wheel’s tooth count, and most worm drives cannot be driven backwards, so they hold their load.
USE: Tuning pegs, hoists, a joint that must stay where it is when the motor stops.

Lever

long light push → short strong lift

DOES: A bar on a pivot lifts a load on the short side when you push the long side.
TEACHES: Force gain is the ratio of the two arm lengths, paid for with the same ratio in distance.
USE: Pliers, brake handles, pry bars, and the starting point for every linkage.

Wedge

sideways push → strong lift

DOES: A ramp pushed sideways drives a block straight up.
TEACHES: The shallower the slope, the bigger the force gain. A screw thread is a wedge wrapped around a rod.
USE: Levelling feet, clamps, cam locks, anything that must hold by friction.

Block and tackle

long pull → heavy lift

DOES: A rope looped through a moving pulley halves the pull needed to lift the load.
TEACHES: Count the rope segments holding the load: that number is the force gain, and you pull that many times further.
USE: Hoists and tensioners, or doubling a small winch’s lifting force.

Cycloidal drive

high reduction, very stiff

DOES: An off-centre cam wobbles a lobed disc around a ring of pins; the disc creeps round one lobe per input turn.
TEACHES: With ten pins and nine lobes the ratio is 9 : 1 in a single flat stage, and many lobes share the load at once.
USE: Robot joints that need high torque and low backlash from printed parts.

Strain-wave (harmonic) drive

very high reduction, no backlash

DOES: An oval plug spins inside a thin flexible toothed cup, pressing it into a rigid ring that has two more teeth.
TEACHES: Each input turn walks the cup round by only the two-tooth difference, so the ratio is the cup’s tooth count divided by two.
USE: Precise robot joints; it is the gearbox inside most industrial robot arms.

Turnbuckle

twist the body → both ends draw in

DOES: A body with a right-hand thread at one end and a left-hand thread at the other pulls both rods in as it turns.
TEACHES: Opposite threads double the travel per turn and let you tension something without twisting its ends.
USE: Tensioning cables and belts, adjustable-length links in a linkage.

Guide a path

Make a point follow a line or curve using only bars and pins.

Four-bar linkage

rotation → a custom curve

DOES: Four rigid bars pinned in a loop. Spin the short one and a point on the middle bar draws a closed curve.
TEACHES: Bar lengths alone decide the path. It is the most useful single linkage to understand.
USE: Wiper arms, folding mechanisms, a leg or finger that must follow a set path from one motor.

Parallelogram linkage

swing → lift that stays level

DOES: Two equal, parallel arms carry an end plate that never tilts as the arms swing.
TEACHES: Equal opposite sides keep the far end at the same angle as the base, with no motor or sensor doing it.
USE: Desk-lamp arms, tool trays, keeping a gripper or cup upright while an arm raises it.

Scissor lift

short push → tall, straight lift

DOES: Crossed bars pinned at their middles fold flat and stand tall as their feet are pushed together.
TEACHES: A small sideways travel becomes a large vertical one, and the force needed is highest when it is nearly flat.
USE: Lift tables, a robot body that rises, compact extendable arms.

Chebyshev straight-line linkage

rotation → a nearly straight stroke

DOES: Three bars with special proportions make a point travel in an almost straight line for half of each turn.
TEACHES: You can get straight motion from pin joints alone, with no rail or slide to keep clean.
USE: Walking-robot legs, where the straight part is the foot on the ground and the curved part is the step.

Watt’s linkage

two swinging arms → a straight middle

DOES: Two arms pivoting from opposite sides are joined by a short link whose midpoint moves in a nearly straight line.
TEACHES: The two arcs pull in opposite directions and cancel each other’s sideways drift.
USE: Car rear axles, and guiding a carriage straight without a rail.

Peaucellier–Lipkin linkage

rotation → an exact straight line

DOES: Seven bars turn the arc of a small crank into a perfectly straight line.
TEACHES: A truly straight line is possible from pin joints alone; it just takes more bars than the approximate versions.
USE: Precision guides, and a good test print for pin-joint tolerances.

Jansen walking leg

rotation → a foot’s walking stride

DOES: One crank drives eleven bars so the foot glides flat along the ground, then lifts and swings forward.
TEACHES: Carefully chosen bar lengths can produce a whole gait from a single motor.
USE: Strandbeest-style walkers; pair several legs out of phase on one shaft.

Bell crank

push → push round a corner

DOES: An L-shaped lever turns a pull in one direction into a push at right angles.
TEACHES: The angle between the arms sets the change of direction, and their lengths set the ratio.
USE: Routing a servo’s motion around a corner, bike brakes, aircraft control runs.

Trammel of Archimedes

two sliders → an ellipse

DOES: A rod pinned to two blocks sliding in crossed slots sweeps its free end around an ellipse.
TEACHES: Two straight constraints at right angles are enough to generate an exact ellipse.
USE: Ellipse-drawing jigs, and a classic do-nothing desk toy to print.

Pantograph

trace a shape → copy it larger

DOES: A hinged parallelogram makes one point copy the path of another at a fixed scale.
TEACHES: Similar triangles inside the linkage set the scale, here two to one.
USE: Copying and engraving jigs, scaling a small actuator’s motion up to a bigger reach.

Five-bar parallel arm

two motors → a point anywhere in a plane

DOES: Two motor-driven arms on the base are joined by two more links that meet at the tool point.
TEACHES: Both motors stay on the base, so the moving parts are light; the two angles together set the position.
USE: Pen plotters, pick-and-place heads, a fast two-axis arm.

Ackermann steering

one tie rod → two wheels at different angles

DOES: Angled steering arms joined by a tie rod turn the inside wheel more sharply than the outside one.
TEACHES: Angling the arms inward makes both wheels roll around the same turning centre, so neither scrubs.
USE: Car-style steering on a rover or cart.

Step and one-way

Advance in fixed steps, or allow motion in one direction only.

Geneva drive

steady rotation → turn, pause, turn

DOES: A pin on a spinning disc flicks a slotted wheel a quarter turn, then leaves it parked until it comes round again.
TEACHES: It indexes to exact positions mechanically, with no sensor and no motor stopping and starting.
USE: Tool changers, turret feeders, film projectors, anything that advances one station at a time.

Ratchet and pawl

rocking → one-way stepping

DOES: A rocking lever pushes a toothed wheel forward one tooth; a spring catch stops it slipping back.
TEACHES: Sloped teeth let a catch ride over one way and jam the other, which turns back-and-forth effort into steady progress.
USE: Winches, tie-down straps, a lift that holds its height with the motor off.

Anchor escapement

steady push → evenly timed ticks

DOES: A rocking anchor lets a driven wheel advance half a tooth on each swing of a pendulum.
TEACHES: The pendulum sets the timing and the wheel only supplies energy, which is how a clock keeps a steady rate.
USE: Clocks, timers, and metering out motion at a fixed pace with no electronics.

Linear ratchet

pull → tightens, never loosens

DOES: A sprung pawl rides up the sloped teeth of a strap as it is pulled through and drops behind each one.
TEACHES: The same one-way idea as a ratchet wheel, laid out in a straight line.
USE: Cable ties, ski-boot buckles, a height adjuster that only needs a release tab.

Lock, hold, flex

Stay put without power, click into place, or bend instead of pivot.

Toggle clamp

light push → huge squeeze that stays

DOES: Two links are pushed toward a straight line; as they straighten, the pad is driven down hard, then the knee snaps just past centre and stays.
TEACHES: Force multiplies without limit as a linkage approaches straight, and going over centre makes it self-locking.
USE: Hold-down clamps, latches, locking a folded leg or a tool in place with no power.

Quarter-turn cam lock

a twist → two panels pulled tight

DOES: A stud screwed into one panel is hooked by a round cam in the other; turning the cam draws the stud in and closes the joint.
TEACHES: A spiral ramp turns rotation into a short, strong pull, and friction on the ramp keeps it shut. This is the flat-pack furniture fitting.
USE: Printed enclosures and robot skins that come off with a screwdriver and no loose nuts.

Quick-release cam lever

flip a lever → clamp shut

DOES: A lever with an off-centre round cam pulls a rod tight as it is flipped over.
TEACHES: A small offset gives a short, strong pull, and the cam passes its high point so the lever stays closed.
USE: Bike seat posts and wheels, folding joints, tool-free clamps on a robot.

Ball detent

slide → click into set positions

DOES: A spring-loaded ball drops into notches as a bar slides past.
TEACHES: A spring and a ramp make repeatable stops you can feel, and push past when you want to.
USE: Selector knobs, adjustable arms, holding a drawer or cover shut.

Snap-fit hook

push together → latched

DOES: A springy hook bends over a ledge as the parts are pushed together, then springs back behind it.
TEACHES: The ramp angle sets how hard it is to assemble and the back face sets how hard it is to pull apart.
USE: Printed lids, battery doors and covers with no screws.

Bistable snap-through

push past centre → stays flipped

DOES: A bowed strip held at both ends rests in one of two shapes and snaps to the other when pushed past the middle.
TEACHES: Stored spring energy gives two stable positions and nothing in between, with no latch.
USE: Switches, valves, a gripper that stays open or closed without holding current.

Flexure hinge

bending instead of a pin

DOES: A thin neck in a single piece of material bends to act as a hinge.
TEACHES: One part can replace a pin, two brackets and their slop, as long as the bend stays small and the material tolerates flexing.
USE: One-piece printed grippers and snap latches. Print in PETG, nylon or TPU; PLA cracks when flexed repeatedly.

Sprung belt tensioner

slack → taken up automatically

DOES: An idler on a sprung arm presses on the slack side of a belt.
TEACHES: A spring keeps tension constant as the belt stretches or the load changes, where a fixed adjuster would not.
USE: Belt drives on robots and printers that must stay tight without re-adjusting.

Bayonet mount

push and twist → locked

DOES: A pin slides down a slot, along, and back up into a seat where a spring holds it.
TEACHES: A J-shaped slot and a spring make a fast lock that cannot shake loose, with no threads.
USE: Camera lenses, light bulbs, quick-change tool heads and covers.

Spring-balanced arm

spring → weight cancelled at every angle

DOES: A spring from above the pivot to a point on the arm holds the arm’s weight wherever it is left.
TEACHES: With the right spring and anchor point, the spring’s pull matches gravity through the whole swing.
USE: Desk lamps and monitor arms; taking the static load off a robot shoulder motor.

Fold and deploy

Pack small and open out, the way umbrellas and camp furniture do.

Umbrella linkage

slide a collar → ribs spread

DOES: A collar sliding up the shaft pushes short struts that swing the long ribs outward.
TEACHES: One sliding part can open many arms at once, all in step.
USE: Deployable canopies, antennas, a gripper with several fingers on one actuator.

Folding leg brace

swing open → braced rigid

DOES: A two-piece brace unfolds with the leg and goes straight when the leg is fully open.
TEACHES: A straight brace carries load along its length, so the joint is stiff exactly when it is deployed.
USE: Folding tables, ladders, stowable legs and stands.

Telescoping tube with spring button

pull out → locks at set lengths

DOES: An inner tube slides inside an outer one, and a sprung button pops through a hole to lock it.
TEACHES: Nested sections give long reach from a short package, and a pin through a hole is the simplest positive lock.
USE: Tent poles, crutches, tripod legs, adjustable masts.

Lazy tongs

squeeze → long reach

DOES: A chain of crossed bars extends far out when the handles are squeezed together.
TEACHES: Each pair of crossed bars adds its own travel, so a small input stacks into a long stroke.
USE: Extending grabbers, retractable gates, a compact reach arm.

Cable-linked telescoping stages

push one stage → the next goes twice as far

DOES: A cable anchored to the base runs over a pulley on the first stage and pulls the second stage out as the first extends.
TEACHES: A moving pulley doubles travel, so every stage extends together from one actuator.
USE: Forklift masts, camera cranes, a robot mast or arm that must reach far and pack short.

Ball-bearing drawer slide

pull → smooth, supported extension

DOES: Balls held in a cage roll between a fixed rail and a moving rail.
TEACHES: A rolling ball travels half as far as the surface riding on it, so the cage always trails the drawer by half.
USE: Drawers, sliding trays, a linear guide built from cheap hardware.

Open and shut

Hinges, latches, locks and valves, from kitchen cabinets to aircraft.

Concealed cabinet hinge

swing → door moves out and round the edge

DOES: Two short links between the cabinet side and a cup in the door carry the door outward as it turns, so it clears the cabinet edge.
TEACHES: A four-bar hinge has no fixed pivot point: the door’s centre of rotation moves as it opens.
USE: Kitchen cabinets; hiding a hinge completely behind a flush printed panel.

Soft-close damper

slam → gentle last inch

DOES: The door swings freely until it meets a small piston, which lets it close only as fast as oil can pass through.
TEACHES: A damper resists speed, not position, so it removes the slam without holding the door open.
USE: Cabinet doors, drawers, toilet seats, a lid that must not drop.

Gas strut lid stay

lift a little → strut does the rest

DOES: A gas-filled strut between the cabinet and the lid pushes with nearly constant force and holds the lid up.
TEACHES: Where the strut is anchored sets how much of the lid’s weight it carries at each angle.
USE: Car tailgates, overhead bins, lift-up cabinet flaps, heavy enclosure lids.

Magnetic catch

nearly shut → pulled shut and held

DOES: A magnet in the frame pulls a steel plate on the door through the last few millimetres and holds it.
TEACHES: Magnetic pull rises very sharply at close range, so it acts only at the end of travel.
USE: Cabinet doors, access panels, printed lids with press-in magnets.

Spring latch bolt

push shut → latches itself

DOES: The bolt’s angled face rides over the strike plate and springs into the hole; the handle pulls it back to open.
TEACHES: A bevel on one side lets the bolt be pushed in from that direction only.
USE: Room doors, gate latches, printed enclosures that click shut.

Pin tumbler lock

right key → cylinder free to turn

DOES: Spring-loaded pin pairs block the plug until a key lifts every pair so the gaps line up along its edge.
TEACHES: The cuts on the key match the pin lengths, which is the whole secret.
USE: Door locks and padlocks; a classic see-through print.

Shoot bolts

one handle → bolts out top and bottom

DOES: A turning disc pushes two rods in opposite directions into the frame above and below.
TEACHES: Two slider-cranks on one disc lock a door at several points from a single handle.
USE: French doors, lockers, aircraft and vault doors, large enclosure panels.

Door closer

opening winds a spring → closes itself, slowly

DOES: Opening the door turns a pinion that drives a piston against a spring; the spring pushes back through a small oil passage.
TEACHES: A spring supplies the closing force and a restriction sets the closing speed.
USE: Self-closing doors and lids, slow-return levers.

Bifold door

pull → two panels fold flat

DOES: Two hinged panels fold against each other as the outer edge runs along a track.
TEACHES: It is a crank and slider: the fixed pivot is the crank and the track guide is the slider.
USE: Closet doors, folding screens, compact covers.

Sectional overhead door

lift → panels travel up and overhead

DOES: Hinged panels on rollers follow a track that curves from vertical to horizontal.
TEACHES: Breaking the door into hinged sections lets it turn a corner, and a spring-wound drum carries the weight.
USE: Garage doors, roll-top desks, tambour covers.

Over-centre draw latch

flip the lever → two halves pulled tight

DOES: A wire loop on a lever hooks the other half; swinging the lever flat draws the loop back and closes the gap.
TEACHES: Just past the straight-line point the loop’s tension holds the lever down, so it stays shut by itself.
USE: Toolboxes, flight cases, removable robot covers.

Rotary door latch

slam → fork catches the striker

DOES: The striker pin turns a forked disc as the door closes, and a sprung pawl drops in behind to stop it turning back.
TEACHES: The fork does the capturing and the pawl does the holding, so a light pull on the pawl releases a heavy door.
USE: Car doors, bonnets and boots.

Window regulator

small crank → glass up and down

DOES: A small gear turns a toothed sector whose long arm lifts the glass, with a roller sliding in a channel under it.
TEACHES: A sector gear gives a big reduction over a limited swing without needing a full wheel.
USE: Car windows, lift mechanisms with limited travel.

Butterfly valve

quarter turn → open or shut

DOES: A disc on a shaft across the pipe turns edge-on to let flow past or face-on to block it.
TEACHES: A quarter turn takes it from fully shut to fully open, and it works as a throttle in between.
USE: Carburettor throttles, ducts, large water lines.

Gate valve

many turns → gate raised clear

DOES: A screw lifts a flat gate out of the flow path into the housing above.
TEACHES: Fully open, nothing is left in the flow; the screw makes it slow to operate but easy to hold against pressure.
USE: Water mains and shut-offs.

Plug door

swing on parallel arms → out, then aside

DOES: Two equal arms carry the door outward and then sideways along the fuselage, keeping it parallel to the wall.
TEACHES: A parallelogram moves a panel without turning it, so it can sit flush when closed and clear the opening fully.
USE: Aircraft, bus and train doors; flush access panels.

Landing gear with folding brace

actuator → leg swings up; brace locks it down

DOES: An actuator swings the leg up into its bay while a two-piece brace folds; with the leg down the brace is straight.
TEACHES: The straight brace carries the landing loads, so the actuator only has to move the leg, not hold it.
USE: Retractable undercarriage; any fold-away leg that must be rigid when deployed.

Fowler flap

extend → wing gets bigger and more curved

DOES: The flap slides back along a track and then tilts down.
TEACHES: Moving back adds wing area and tilting adds curvature, which is two changes from one motion along a shaped track.
USE: Airliner wings; track-guided panels that must both move and tilt.

Grip

Close on an object from one actuator.

Parallel-jaw gripper

one motor → two jaws closing evenly

DOES: One gear between two racks moves both jaws the same distance in opposite directions.
TEACHES: Meshing both racks with the same gear keeps the jaws centred without any control.
USE: Robot grippers that must pick up parts of different widths squarely.

Tendon-driven finger

pull a cord → finger curls

DOES: A cord running along the underside of three hinged segments curls them all when pulled.
TEACHES: One actuator can drive several joints, and the finger wraps to whatever shape it touches.
USE: Robot hands and soft grippers, with the motor kept back in the palm or forearm.

Whiffletree

one pull → shared evenly across many points

DOES: A tree of pivoting bars splits one pull among four pads, each stopping where it meets the surface.
TEACHES: A free-pivoting bar divides force equally whatever the positions of its two ends.
USE: Adaptive grippers that wrap an uneven object, windscreen wipers, load-spreading clamps.

Parallel-linkage gripper

one pull → two pads closing square

DOES: Each finger is a parallelogram, so its pad swings inward while staying upright.
TEACHES: A parallelogram keeps the pad face parallel, which grips flat parts without tipping them.
USE: Servo grippers for boxes and flat-sided parts.

Connect and disconnect

Clutches and couplings: pass the drive, limit it, or let go.

One-way roller clutch

drives one way, freewheels the other

DOES: Rollers sit in tapered pockets between a hub and a ring; one direction wedges them tight, the other lets them roll free.
TEACHES: A wedge grips instantly with no teeth, so there is no click and no lost motion.
USE: Bicycle freewheels, starter motors, turning a rocking input into one-way rotation.

Centrifugal clutch

speed → engages by itself

DOES: Sprung shoes on the driven hub fly outward as it speeds up and grip the inside of a drum.
TEACHES: Spring force against spin sets the engagement speed, so the load connects only once the motor is up to speed.
USE: Chainsaws, go-karts, soft-starting a load.

Oldham coupling

joins two shafts that are slightly out of line

DOES: A floating middle disc with two crossed tongues slides in slots on the two shaft ends.
TEACHES: Two sliding joints at right angles absorb sideways offset while passing rotation at exactly the same speed.
USE: Coupling a motor to a lead screw; common on 3D-printer Z axes.

Dog clutch gear selector

slide a collar → pick a gear

DOES: Two gears spin freely on the shaft until a sliding collar keyed to the shaft locks onto one of them.
TEACHES: Every gear pair stays meshed all the time; shifting only chooses which one is connected to the shaft.
USE: Manual gearboxes, two-speed drives, connecting and disconnecting an output.

Switch, sense, pump

Turn a level, a temperature or a small signal into an action.

Relay

small current → a switch thrown

DOES: An energised coil pulls an iron arm that closes one contact and opens another; a spring pulls it back.
TEACHES: A small signal can switch a large, separate circuit.
USE: Switching motors and lights from a microcontroller pin.

Snap-action switch

slow press → instant click

DOES: A plunger presses on a sprung blade that suddenly flips from one contact to the other.
TEACHES: The blade snaps over centre, so the contacts change quickly however slowly you press, and it releases at a different point than it trips.
USE: Limit switches and end stops on printers and robots.

Bimetal strip

heat → bending

DOES: Two metals bonded together expand by different amounts, so the strip curls as it warms and makes a contact.
TEACHES: Temperature can be turned straight into movement with no electronics.
USE: Thermostats, kettle switches, thermal cut-outs.

Float valve

water level → valve closes itself

DOES: A float on a lever rises with the water and pushes the inlet shut.
TEACHES: This is feedback control: the thing being controlled operates its own valve.
USE: Toilet cisterns, livestock troughs, automatic top-up tanks.

Check valve

flow one way only

DOES: Forward pressure pushes a ball off its seat; reverse pressure presses it back on.
TEACHES: The fluid’s own pressure operates the valve.
USE: Pumps, pneumatic grippers, keeping a line primed.

Peristaltic pump

rotation → fluid pushed along a tube

DOES: Rollers squeeze a flexible tube and sweep the trapped fluid round.
TEACHES: The fluid only ever touches the tube, and the squeezed point stops it flowing back.
USE: Dosing pumps, 3D-printed liquid handlers, anything sterile or corrosive.

Inside old electronics

The clever bits from tape decks, VCRs, radios and cameras.

Capstan and pinch roller

steady rotation → steady tape speed

DOES: A rubber roller presses the tape against a thin, flywheel-steadied shaft that pulls it along.
TEACHES: Tape speed is set only by the shaft’s surface speed, not by how full the reels are.
USE: Tape decks and VCRs. The same idea feeds filament, paper and wire at a known rate.

Swinging idler

motor direction → which reel turns

DOES: An idler wheel on a loose arm is dragged toward one reel or the other depending on which way the motor spins.
TEACHES: Friction can do the switching: reversing the motor is the only control needed.
USE: Play and rewind in cassette decks; choosing between two outputs with one motor.

Slip clutch

drive → limited torque

DOES: A felt pad between two discs passes the drive until the output is held, then lets it slip.
TEACHES: Friction sets a torque ceiling, which keeps tension even and protects the parts when something jams.
USE: Take-up reels, screwdriver clutches, protecting a gearbox or a finger from a stalled motor.

Push-push latch

push once to latch, push again to release

DOES: A pin follows a one-way track around a heart-shaped island, parking in its notch on the first push and escaping on the second.
TEACHES: A shaped track can remember a state, so one button does two jobs with no electronics.
USE: Cassette doors, click pens, memory-card slots, touch-latch cabinet doors.

Interlocking push buttons

press one → the others pop up

DOES: Each button is held down by a shared sprung bar; pressing another button nudges the bar aside and frees the first.
TEACHES: One sliding latch shared between several buttons guarantees only one can be down at a time.
USE: Radio band selectors, tape-deck keys, any selector where two choices must never be active together.

Dial cord drive

turn a knob → pointer slides, tuner turns

DOES: A loop of cord wrapped round the knob shaft runs over pulleys to turn a large drum and carry a pointer along the scale.
TEACHES: A thin shaft driving a big drum gives fine control, and a cord can route motion anywhere there is room for a pulley.
USE: Radio tuning dials; cable drives that carry motion round corners.

VCR tape loading

two posts → tape wrapped round the drum

DOES: Two guide posts pull a loop of tape out of the cassette and wrap it around the spinning head drum.
TEACHES: The cassette stays simple because the machine does the threading, tracing an M with the tape.
USE: Video recorders. The pattern suits any machine that must thread a flexible strip automatically.

Grooved face cam

one gear → a programmed sequence

DOES: A pin rides in a winding groove cut into the face of a gear, moving a lever in and out as the gear turns.
TEACHES: The groove holds the lever still where its radius is constant and moves it where the radius changes, so one motor can run a whole sequence.
USE: The mode gear in VCRs and CD changers; sequencing several motions from a single motor.

Partial-tooth gear

steady rotation → move, then wait

DOES: A gear with teeth on only part of its rim turns its partner for a while, then leaves it standing.
TEACHES: Removing teeth is the simplest way to build a pause into a gear train.
USE: Auto-reverse and eject sequences, counters, timed feeds.

Slide-then-drop slot

one push → in, then down

DOES: Pins on a carrier follow L-shaped slots, so it travels straight in and then drops at the end.
TEACHES: The shape of a slot sets the path, so a single actuator can make a two-part move.
USE: Front-loading cassette and disc trays; docking a part and then seating it.

Cam stack sequencer

one shaft → switches fire in order

DOES: Several cams on one shaft each lift their own follower at a different point in the turn.
TEACHES: Timing is stored as the angle between the lobes, so the sequence can never get out of order.
USE: Washing-machine timers, music boxes, repeating a fixed routine without a controller.

Centrifugal governor

speed → position

DOES: Spinning weights swing outward as speed rises and pull a collar up the shaft.
TEACHES: It turns speed into a movement that can throttle the drive, giving speed control with no electronics.
USE: Record players, wind-up motors and old tape drives; a mechanical speed limiter.

Spring-cam hinge

swing past halfway → snaps the rest

DOES: A sprung plunger presses on a cam in the hinge, pushing the lid toward fully open or fully shut.
TEACHES: A cam with two low spots and a spring makes two rest positions and a snap between them.
USE: Flip phones, glasses cases, laptop and cabinet hinges that hold themselves.

Iris diaphragm

turn a ring → opening grows or shrinks

DOES: Overlapping blades each pivot a little as a ring turns, changing the size of the hole they leave in the middle.
TEACHES: Many small identical moves add up to a round opening that scales smoothly.
USE: Camera apertures, adjustable nozzles, a gripper that closes evenly around a round part.

Rim drive with stepped pulley

shift the idler → change speed

DOES: A rubber idler carries the drive from a stepped motor shaft to the inside of a platter rim.
TEACHES: The idler’s own size does not change the ratio; only the motor step it touches and the rim do.
USE: Record-player speed selection; a cheap, quiet reduction with a built-in gear change.

Counter wheels

turns → a running count

DOES: Each numbered wheel nudges the next one on by a tenth of a turn as it passes from nine to zero.
TEACHES: A carry is a once-per-turn kick, the same idea as carrying a digit in arithmetic.
USE: Tape counters, odometers, mechanical tally counters.

Helical cam barrel

twist a ring → tube slides out

DOES: A pin on the inner tube sits in both a straight slot and an angled slot; turning the sleeve with the angled slot drives the pin along.
TEACHES: The slot angle sets how far the tube moves per turn, and the straight slot stops it from rotating.
USE: Zoom and focus rings on lenses, twist-to-extend mechanisms.

Music-box comb

slow rotation → plucked notes

DOES: Pins on a turning drum lift and release tuned steel teeth, which ring.
TEACHES: The pin layout is the stored program, and tooth length sets the pitch.
USE: Music boxes; a pinned drum is also a simple way to sequence taps or triggers.

Film pull-down claw

rotation → grab, pull, let go, return

DOES: A crank drives an arm through a pivoting guide so its tip dips into a film hole, pulls down one frame and lifts out.
TEACHES: A point on a linkage can trace a D-shaped loop: a straight working stroke and a curved return.
USE: Movie cameras and projectors; indexing any perforated strip.

Sprocket feed

rotation → strip moved with no slip

DOES: Teeth on a wheel engage a row of holes in the strip and carry it along.
TEACHES: Teeth in holes give exact, repeatable feed where a friction roller would creep.
USE: Film, tractor-feed paper, timing belts and chains.

Focal-plane shutter

two curtains → a timed slit of light

DOES: One curtain uncovers the frame and a second follows it across to cover it again.
TEACHES: Exposure time is the delay between the two curtains, not how fast they travel.
USE: Film and digital cameras.

Diagrams are schematic: proportions are chosen to show the motion clearly, and none of them is a dimensioned part to print as drawn.