A ratchet lets something turn one way and stops it turning back. Turn the crank of a jack and you hear *click, click, click*: a small catch called a pawl is riding over the teeth of a gear. Let go and the load starts to sink, until the pawl drops into the next gap and stops it. That "until" is the part most people miss: a ratchet always gives back up to one tooth before it holds.
This guide explains how a ratchet works, then goes through the faults builders most often hit, with a fix for each. The Ratchet & Pawl page has the one-minute version.
How it works
The Ratchet Car Jack (level 3, about 35 minutes) is the clearest example in the set. A crank turns an 8-tooth gear, which turns a 24-tooth gear on the same axle as a tiny 8-tooth pinion. The pinion climbs a toothed rack fixed to the lifting post. Its build notes the trade: three turns of the crank make one turn of the pinion, so the crank is easy to turn.
The pawl is a black 4-hole beam hanging on a pin at the top of the left guide, with a red axle pin at its far end as the tooth. The pawl's own weight keeps that tooth resting on the 24-tooth gear. Lifting turns the gear the way the pawl can ride over. Sinking would turn it the other way, and the pawl's tip jams against a tooth.
Try it before you build. Change the ratchet gear, move it to the crank axle, flip the pawl, then crank and let go.
Interactive: Ratchet and pawl lab. A ratchet laid out like the Ratchet Car Jack: change the ratchet gear's teeth, move it to the crank axle, flip the pawl or lift it off, then crank and let go. Predict the clicks per turn, how far the load drops back before the pawl catches, and how hard it pushes on the pawl.
Fault 1: the load drops a little before it holds
Why: the pawl was partway between two gaps when you let go. With the jack's 24-tooth gear, the pinion can turn back up to 360° ÷ 24 = 15° before the pawl catches.
Fix: more teeth means smaller gaps. Or put the ratchet on a faster shaft: on the crank axle, one tooth is only a third as much pinion turn. Don't use fewer teeth: an 8-tooth ratchet gear lets the pinion turn back up to 45°.
Fault 2: the pawl doesn't catch at all
Why: nothing is holding it on the teeth. In the car jack the pawl hangs so its own weight rests its tooth on the gear. Tip the model on its side or build the pawl upside down and it can swing away.
Fix: check that the pawl hangs freely on its pin and falls back onto the teeth when you lift it with a finger. If the model has to work at any angle, a small rubber band can pull the pawl onto the teeth.
Fault 3: you can't crank it up at all
Why: the pawl is facing the wrong way. A pawl blocks one direction only, and flipped, it blocks lifting and lets the load fall instead.
Fix: turn the gear by hand with the pawl resting on it. It should click freely in the lifting direction and jam the other way. If not, hang the pawl on the other side of the gear.
Fault 4: the pawl pops out under load
Why: real ratchets have saw-shaped teeth, but a Technic-style ratchet uses an ordinary gear, whose teeth are slanted on both sides. If the pawl meets a tooth at a shallow angle, the load can push it up and out instead of jamming it in.
Fix: place the pawl's pivot so its tip points almost straight at the gear's centre when it is caught, not along the teeth. Make sure the pawl pin is fully pushed in, so the pawl can't lean away from the gear.
Fault 5: the pawl gets pushed too hard
Why: the pawl takes all the load's twist on the shaft it sits on. On the slow pinion shaft that twist is three times bigger than on the crank shaft.
Fix: put the ratchet on the faster shaft, like many real winches do. In the lab, moving a 24-tooth ratchet to the crank axle cuts the push on the pawl to a third and the drop-back to 5°.
Other ratchets and catches in the set
- The Launch-Lock Racer (level 3, about 40 minutes) puts a pawl on a 24-tooth gear on its back axle. Rolling backwards winds its pull-back motor and clicks; the pawl blocks it going forwards until you press the launch button, which lifts the pawl like a see-saw.
- The Spring Catapult (level 2, about 25 minutes) uses a simpler cousin: a latch whose tooth holds the cocked arm down until you pull it back.
- A worm gear holds a load without any pawl, because the wheel can't turn the worm. See how worm gears work for when to choose one over a ratchet.
Check yourself
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- What does the pawl in a ratchet do?
- You let go of a ratchet jack. How far can the load slip back before the pawl catches?
- In the Ratchet Car Jack, how many clicks do you hear per turn of the crank?
- Why might an engineer put the ratchet on the fast crank shaft instead of the slow pinion shaft?
