Ratchet and Pawl

The tooth shape is the entire engineering. Draw it before cutting it.

1. Scribe a 120 mm disc on 6 mm aluminium flat bar and mark 24 divisions, one every 15 degrees.
2. For each tooth, draw the WORKING face along a radius, or leaned back from radial by no more than about 5 degrees.
3. Draw the BACK of each tooth as a shallow ramp running from the tip of one working face to the root of the next.
4. Mark the centre and drill 8.0 mm for a 608 bearing.

Why radial matters. With the working face on a radius, the load line passes through the pawl pivot and simply presses the pawl into the tooth — it cannot lever itself out. Lean that face the other way, even slightly, and the load develops a component that lifts the pawl. That is a ratchet that slips under exactly the load it was fitted to hold.

Slow work, and the working faces are what deserve the care.

1. Rough out the disc with the hacksaw or jigsaw and file the rim true.
2. Saw down each working face first, on the waste side of your line.
3. File each ramp back from the tip of one face to the root of the next.
4. Finish every working face with a flat file, checking it stays on the radial line.
5. Deburr both sides.

Where the pawl pivots matters as much as the tooth shape.

1. Cut a pawl 70 × 18 mm from 6 mm aluminium flat bar, with a nose filed to match the tooth root.
2. Drill its pivot hole 4.2 mm.
3. Mount the ratchet to an 18 mm ply base through its bearing, using an M5 × 40 socket head cap screw, M5 flat washer × 2 and M5 hex nut × 1.
4. Position the pawl pivot so that a line from the pivot to the contact point runs roughly ALONG the working face when engaged.
5. Fix it with an M4 × 25 socket head cap screw, M4 flat washer × 2 and an M4 nylon insert lock nut set for free swing.

The spring's only job is to return the pawl. It should not be holding anything.

1. Fit a light compression spring from the base to the pawl's tail, so it presses the nose toward the teeth.
2. Turn the ratchet in the free direction — the pawl should click over each ramp and drop cleanly into each root.
3. Turn it in the locked direction and load it by hand. It must not move.
4. Now remove the spring entirely and load it again in the locked direction.

Measure what the tooth count costs you.

1. Your 24-tooth wheel can only be held every 15 degrees — that is its resolution.
2. Work out what 48 teeth would give, and 72.
3. Then consider the tooth ROOT area at each count, on the same diameter.

Finer teeth mean less lost motion before the pawl catches, and a weaker tooth. That is the whole design tension in a ratchet, and it is why a cheap socket wrench has a coarse loud action and an expensive one has 72 or 90 teeth in hardened steel — the fine-toothed version needs better material to survive the same torque on a smaller tooth.

History. The ratchet is ancient — it appears in Greek and Roman crossbow windlasses and in medieval cranes, where a load must never be allowed to run back. There is no single inventor and no patent to point at; it is one of the mechanisms humanity arrived at repeatedly because the need is universal.

What it does that nothing else here does: every other mechanism in this batch transforms motion. The ratchet PERMITS or FORBIDS it. That is a different category of job, and it is why ratchets appear inside other mechanisms rather than competing with them — in a winch alongside a worm drive, in a clock alongside an escapement, in a jack alongside a screw.

Its honest limits: it holds only at discrete positions, it makes noise by design, and it is a one-way device — reversing means a second pawl or a reversible one, which is exactly what the reversing lever on a socket wrench does. A friction brake holds anywhere but can slip; a worm drive holds continuously but only if it is self-locking. Three ways to stop something running backwards, each with a different failure mode.