A crown-ratchet — also called a rag wheel — is a face-toothed wheel with sawtooth-shaped teeth cut into its end face rather than its rim, engaged by a spring-loaded pawl that drops axially into each tooth. The form traces back to medieval European tower-clock striking trains documented by Henry de Vick around 1370, where rag wheels released the count for the hour strike. It allows free rotation in one direction, locks instantly in the other, and is the simplest way to convert oscillation into stepped one-way motion. You still see it in clock count wheels, hand-winch capstans, and tally counters today.
Crown-Ratchet Rag-Wheel Interactive Calculator
Vary tooth count, click count, ramp angle, and stop-face angle to see index resolution and pawl locking geometry.
Equation Used
The tooth count sets the angular resolution of a crown-ratchet. One click advances one tooth pitch, so the index angle is 360 degrees divided by the number of teeth. The total indexed angle is that pitch multiplied by the number of clicks.
- Each click advances exactly one tooth pitch.
- Tooth count is treated as an integer.
- Stop face is best when close to 90 deg.
- Ramp angle is measured in the free-running climb direction.
The Crown-ratchet (rag-wheel) in Action
A crown-ratchet works by axial engagement instead of radial engagement. The teeth sit on the flat face of a disc, each one shaped like a right triangle with a steep stop face on one side and a long ramp on the other. A pawl — which is just a small lever with a hardened tip — rides on those teeth under spring pressure. When the wheel turns in the free direction, the pawl tip slides up the ramp, lifts axially, and drops into the next tooth with a click. Try to turn the wheel the other way and the pawl tip jams against the steep face. Motion stops cold.
The geometry that matters is the ratio between the ramp angle and the stop-face angle. A typical rag wheel uses a ramp around 20-30° and a stop face within 5° of perpendicular to the disc. If you cut that stop face leaning outward — even 8 or 10° past vertical — the pawl can cam itself up under load and pop out. We have seen exactly this on poorly fabricated replacement count wheels for antique clocks: the wheel runs fine until the strike train sees full mainspring torque, then the pawl jumps a tooth and the clock strikes 13. The other classic failure is tooth pitch error. If the pawl tip width is greater than the gap at the root of the tooth, it bottoms on two teeth at once and never seats properly, so the click spring takes the load instead of the stop face.
Tooth count sets your resolution. A 60-tooth rag wheel gives you 6° per click — fine enough for a tally counter or a clock count wheel. Drop to 12 teeth and you get 30° per click, which is what you want on a hand winch where each click should equal a meaningful amount of rope take-up. Pawl mass and click-spring force also matter: too light and the pawl bounces off the ramp at speed, too heavy and the pawl skips on light loads.
Key Components
- Crown wheel (rag wheel): The face-toothed disc that carries the sawtooth profile on its flat end. Tooth count typically runs 8 to 72. Material is usually hardened steel for hand-winch service or brass for clockwork, with tooth flanks finished to Ra 0.8 µm or better to keep the pawl from gouging on entry.
- Pawl (click): A pivoted lever with a hardened tip that rides axially against the tooth face. Tip width must be 0.05-0.10 mm narrower than the tooth root gap so it seats fully against the stop face. Pivot clearance must stay under 0.05 mm or the pawl rocks and chatters at speed.
- Click spring: Maintains constant axial force on the pawl tip. Typical force is 50-200 grams for clock work, 2-5 N for hand winches. Too soft and the pawl floats off the wheel face above 60 RPM, too stiff and ramp wear accelerates.
- Pawl pivot: Carries the pawl on a stationary post offset from the wheel axis. Offset distance sets the angle at which the pawl tip meets the stop face — aim for 85-90° contact, never less than 80°, otherwise the pawl cams out under load.
- Stop face: The steep side of each tooth. Should be within 5° of true axial — perpendicular to the disc face. This is the surface that takes the full reverse-load reaction, so it gets case-hardened on steel wheels to 55-60 HRC.
Who Uses the Crown-ratchet (rag-wheel)
Crown-ratchets show up wherever you need cheap, axial, one-way locking with a clear audible click. The audible click matters more than people realise — it is the operator's confirmation that an event has occurred, which is why you see rag wheels in counters and capstans long after radial ratchets became cheaper to mass-produce.
- Horology: Count wheel on the striking train of a Comtoise (Morbier) French wall clock, where the rag wheel releases one tooth per hour-strike to set the count of strokes.
- Marine deck gear: Hand-cranked anchor capstan pawl on a Lewmar Ocean 30ST winch, where the rag wheel locks the drum against load while the operator repositions the handle.
- Industrial counting: Veeder-Root mechanical tally counter, where the input lever advances a 10-tooth crown ratchet by one click per actuation to drive the units digit wheel.
- Heritage machinery: Striking-train release on a Seth Thomas No. 2 regulator clock, where a 12-tooth rag wheel sets the hour count via pinned cam pickoff.
- Hand tools: Carpenter's brace ratcheting chuck — the Stanley No. 945 used a face-cut rag wheel and twin pawls so the user could choose forward, reverse, or locked drive.
- Mining and material handling: Pawl on a hand-operated chain block holding load between lift strokes, where axial pawl engagement keeps the housing thin compared to a radial ratchet.
The Formula Behind the Crown-ratchet (rag-wheel)
The single most useful number when designing a crown-ratchet is the angular resolution per click — the smallest backlash you can get before the pawl drops into the next tooth. At the low end of the practical tooth-count range (around 8 teeth), each click is 45° and the pawl is loud and slow but bombproof under shock load. At the high end (60-72 teeth), each click is 5-6° and you get clock-grade precision but the teeth get small enough that pawl tip width and stop-face perpendicularity become critical. The sweet spot for general hand-winch and tool work is 12 to 24 teeth — fine enough to feel smooth, coarse enough that machining tolerances do not eat the design alive.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θclick | Angular advance per pawl click — the maximum reverse backlash before the pawl re-engages | degrees | degrees |
| Nteeth | Total tooth count cut into the face of the crown wheel | count (dimensionless) | count (dimensionless) |
| ptooth | Tooth pitch measured along the pitch circle, used to size pawl tip width | mm | inches |
| Dpitch | Diameter of the pitch circle on which the teeth are cut | mm | inches |
Worked Example: Crown-ratchet (rag-wheel) in a hand-cranked grain-mill ratchet stop
You are designing the crown-ratchet stop for a hand-cranked stone grain mill — the kind built by Country Living Mill in Idaho. The crank turns a 90 mm pitch-diameter rag wheel that holds the millstone against back-driving when the operator releases the handle to reposition. You need to pick a tooth count that gives smooth clicking at typical cranking speeds while keeping reverse backlash low enough that the operator does not feel the stone slip backward between strokes.
Given
- Dpitch = 90 mm
- Nteeth (candidate) = 24 count
- Pawl tip width = 3.0 mm
- Operator cranking speed = 30-90 RPM
Solution
Step 1 — compute the nominal angular click at 24 teeth:
Step 2 — convert that to tooth pitch along the 90 mm pitch circle so we can check pawl tip width fits:
A 3.0 mm pawl tip seats comfortably in an 11.78 mm pitch with about 8 mm of tooth root to spare. Good.
Step 3 — check the low end of the tooth-count range. Drop to 12 teeth:
30° of backlash on a grain mill means the operator feels the stone clunk back almost a quarter-turn every time they release the handle. Functional, but the stone unloads hard against the pawl and you hear it across the kitchen.
Step 4 — check the high end. Push to 60 teeth:
ptooth,high = π × 90 / 60 = 4.71 mm
6° backlash feels almost continuous to the operator — exactly what you want on a precision count wheel. But the tooth pitch is now 4.71 mm and your 3.0 mm pawl tip leaves only 1.7 mm of root clearance. Any pawl wear or pivot slop above 0.1 mm and the tip starts riding two teeth at once. At 90 RPM cranking speed, that is 90 clicks per second — the click spring needs serious force to keep the pawl seated, and ramp wear accelerates.
Result
Nominal answer at 24 teeth is θclick = 15° per click with an 11. 78 mm tooth pitch. That feels like a firm, definite click every quarter-turn of the crank — the operator senses the lockup immediately when they release the handle, with maybe 4 mm of perceived stone movement at the rim. Compare to the 30° backlash at 12 teeth (clunky and audible across the room) and the 6° at 60 teeth (silky but tooth-pitch-fragile), and 24 teeth lands in the sweet spot for a hand-cranked mill. If you build it and the pawl skips teeth under cranking load, the three usual culprits are: (1) stop-face angle leaning past 90° because the cutter walked during fly-cutting — check with a small machinist's square against the disc face; (2) click-spring force below ~3 N letting the pawl float off above 60 RPM cranking; or (3) pawl pivot clearance over 0.1 mm letting the pawl tip wander sideways and bottom on the next tooth's ramp instead of dropping into the root.
Crown-ratchet (rag-wheel) vs Alternatives
The crown-ratchet competes against radial ratchets, sprag clutches, and roller clutches for one-way locking duty. Each one wins on a different axis — pick wrong and you either pay too much or run into a lifespan wall.
| Property | Crown-ratchet (rag wheel) | Radial ratchet & pawl | Sprag clutch |
|---|---|---|---|
| Backlash per engagement | 5-45° (set by tooth count) | 3-30° (set by tooth count) | <0.5° (line-contact engagement) |
| Max practical RPM | ~120 RPM before pawl float | ~300 RPM with light pawl | 10,000+ RPM continuous |
| Load capacity (typical) | 50-5,000 N axial reaction | 100-50,000 N tangential | 500-100,000 N radial |
| Cost per unit (small batch) | $5-30 fabricated | $3-20 fabricated | $40-200 sealed unit |
| Audible feedback | Loud click (often desired) | Loud click | Silent |
| Axial vs radial space | Compact radially, deep axially | Deep radially, thin axially | Compact both axes |
| Best application fit | Counters, capstans, clock count wheels | General winches and reels | High-speed indexing, automotive starters |
Frequently Asked Questions About Crown-ratchet (rag-wheel)
The hand-pressure test only loads the pawl tip with a few newtons. Under real load, the reaction force tries to cam the pawl up the stop face — and if that face is even 5-8° past true vertical, the pawl will lift and skip. Put a small square against the stop face and check it against the disc surface. The face must be within 5° of perpendicular, ideally undercut by 2-3° toward the wheel centre so load actually pulls the pawl deeper into engagement.
The other cause is pawl pivot offset. If the line from the pivot to the pawl tip does not cross the stop face at 80-90°, you have built in a camming moment. Move the pivot post until that contact angle is right, then re-test.
No, that is correct and it is the whole point of a striking-train count wheel. The irregular spacing encodes the strike count: 1 tooth gap for one o'clock, 2-tooth gap for two, and so on, up to 12. The pawl-equivalent (the count lever) drops into each gap and stops the strike train after the right number of strokes. If you fabricate a replacement with even spacing, the clock will strike the same count every hour. Measure the original gaps carefully before you cut a new one — the cumulative gap arithmetic must total 360°.
Depends which axis is tight. A crown-ratchet is shallow radially but eats axial depth equal to the tooth height plus pawl swing — typically 8-15 mm. A radial ratchet is the opposite: thin axially, but adds 2× the tooth height to the wheel diameter. On a Lewmar-style sailboat winch where the drum is wide but diameter-limited, the crown-ratchet wins. On a flat hand-crank reel where you want a thin profile, the radial ratchet wins. Sketch both on the actual envelope before you commit.
The pawl is bouncing off the ramp before it can drop into the root. At high actuation rates the pawl tip rides up the ramp, gains kinetic energy, and at the top of the ramp it launches axially instead of falling cleanly. The click spring is then asked to decelerate, reverse, and seat the pawl all within a few milliseconds — usually it cannot.
Two fixes: reduce pawl mass (drill it out, or remake from aluminium with a hardened tip insert) so its time-constant drops, or increase click-spring force by 30-50% so the seating motion happens faster. Do not increase both — you will just chew the ramp.
Different design drivers. A clock count wheel needs to encode the hour sequence (1+2+3+...+12 = 78 tooth-units around the wheel for a 12-hour strike), so the count is set by the strike pattern, not by ergonomics. A hand capstan needs each click to equal a useful rope advance — usually 10-30 mm at the drum surface — so you back-calculate teeth from drum diameter. For a 100 mm drum and 20 mm-per-click target, you want π × 100 / 20 ≈ 16 teeth.
You need a mismatch. Same-hardness sliding contact galls — the surfaces transfer material between each other and the stop face roughens until the pawl no longer seats. Standard horology practice is hardened steel pawl tip on brass wheel, with the pawl 30-40 HRC harder than the wheel. The brass wears slightly and the steel essentially does not, but the wear pattern is uniform and predictable. If you must run brass-on-brass for cost reasons, expect 10-20% of the service life and plan for replacement.
References & Further Reading
- Wikipedia contributors. Ratchet (device). Wikipedia
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