The Pin-wheel Escapement is a pendulum-clock escapement that uses cylindrical pins set perpendicular to the escape wheel face — instead of cut radial teeth — to drive the pallets. It replaces the Graham deadbeat's pointed wheel teeth with hardened steel pins, which makes it cheaper to manufacture, less sensitive to wheel-tooth damage, and dead-beat by nature. Clockmakers use it to deliver clean impulse to a heavy pendulum without recoil, which is why you find it in turret clocks, French precision regulators, and many 19th-century longcase movements running 8-day to 30-day trains.
Pin-wheel Escapement Interactive Calculator
Vary escape-wheel torque, pin radius, impulse arc, and contact efficiency to see impulse energy and pendulum drive strength.
Equation Used
The calculator follows the article relation for impulse energy: wheel torque times pin pitch radius times impulse arc times pin-on-pallet efficiency. Radius is entered in millimetres, impulse arc in degrees, and efficiency as a percent before conversion for the calculation.
- Uses the article impulse-energy relation directly.
- Pin radius is converted from mm to m.
- Impulse arc is converted from degrees to radians.
- Amplitude estimate is normalized to 2.5 deg at 100 uJ.
How the Pin-wheel Escapement Works
The Pin-wheel Escapement, also called the Lantern-wheel escapement in older English horological texts, drives the pendulum through cylindrical pins planted axially into one face of the escape wheel. As the wheel turns, a pin meets the entry pallet, slides along its locking face, then transfers onto the impulse face which gives the pendulum its kick. The pendulum swings, the exit pallet drops onto the next pin, and the cycle repeats. Two pins per pendulum beat — one on each pallet — and you get one tooth-by-tooth advance every full pendulum period.
Why build it this way? Because pins are easier to make and replace than cut teeth. If you damage a pin on a Graham deadbeat escape wheel, you scrap the wheel or have a watchmaker re-cut and re-harden the tooth. On a Pin-wheel Escapement, you knock the bad pin out and press a new hardened steel pin in its place. The pallet faces still need to be polished and angled correctly — typically 1° to 2° of impulse angle on each face, with locking faces concentric to the pendulum pivot to give true dead-beat action — but the wheel itself becomes a much simpler part. The pins are usually 0.8 mm to 2.5 mm diameter depending on clock size, and they must be perpendicular to the wheel face within roughly 0.05 mm runout or the pallet will see uneven impulse from beat to beat.
Get the tolerances wrong and the symptoms are unmistakable. If pin diameter varies by more than about 0.02 mm across the set, the clock develops a limping beat — tick-TOCK, tick-TOCK — that you can hear from across the room. If the pallet impulse angle is too steep, you lose amplitude and the clock stops on cold mornings when oil thickens. Too shallow and the pendulum runs hot, gaining 30 to 60 seconds a day, because the impulse is delivered too late in the swing. The most common failure mode in the field is pin wear flats — after 50 to 80 years of service, each pin develops a small flat where it kisses the pallet, and the clock's amplitude drops until you re-pin the wheel.
Key Components
- Escape Wheel with Axial Pins: A flat brass or bronze wheel carrying 15 to 30 hardened steel pins pressed perpendicular to its face. Pin diameter typically 0.8–2.5 mm, set on a pitch circle held to ±0.03 mm. Pin perpendicularity must be inside 0.05 mm runout or you get uneven impulse.
- Entry Pallet: Fixed to the pallet arbor on the side where pins arrive. The locking face is a circular arc concentric with the pallet pivot — that's what makes the action dead-beat with no recoil. The impulse face sits at 1° to 2° to the locking face.
- Exit Pallet: Mirror partner of the entry pallet, catching pins as they leave the wheel. Both pallets are usually hardened steel, polished to a mirror finish. Drop between pallets is set to roughly 1° of wheel rotation — too much drop wastes amplitude, too little and the pin jams the locking face.
- Pallet Arbor and Crutch: Carries both pallets and connects to the pendulum through a slotted crutch. Side-shake on the arbor pivots must be under 0.02 mm or the pallets walk laterally and you get audible beat error.
- Pendulum: Provides the timekeeping reference. Pin-wheel Escapements typically drive 1-second (1 m) or 1.5-second (2.25 m) pendulums in turret and regulator work. Bob mass is usually 5–25 kg in turret installations.
Where the Pin-wheel Escapement Is Used
You find the Pin-wheel Escapement wherever a clockmaker needed clean dead-beat impulse on a heavy pendulum without paying for a perfectly cut Graham wheel. It dominated French precision horology from about 1750 onward — Lepaute used it in the Paris Observatory regulator — and became the default in British and American turret clocks because a parish clock pin can be replaced by the local blacksmith if it shears.
- Public Tower Clocks: The Westminster turret clock movement and many JB Joyce of Whitchurch turret clocks used Pin-wheel Escapements (Lantern-wheel escapements in their period catalogues) because field repair of a sheared pin is trivial compared to re-cutting a damaged wheel tooth.
- Astronomical Regulators: Lepaute's Paris Observatory regulator and many 19th-century French precision regulators by Robin and Berthoud used the Pin-wheel Escapement for its low-recoil, dead-beat behaviour driving heavy 1-second pendulums.
- Longcase Domestic Clocks: Mid-19th century French Comtoise (Morbier) longcase movements used a vertical-axis Pin-wheel Escapement variant — cheap to produce, robust, easy for a country clockmaker to service.
- Railway Station Clocks: Several pre-1900 European railway platform clocks built around heavy 1.5-second pendulums used Pin-wheel Escapements for rate stability across a wide temperature range, since the geometry tolerates oil viscosity changes better than pointed-tooth deadbeats.
- Heritage Clock Restoration: Restorers at workshops like Smith of Derby still re-pin original wheels rather than fabricating replacements — pulling worn 1.2 mm pins, reaming the seats to 1.21 mm, and pressing new hardened pins gives another 80 years of service.
- Horological Education: BHI (British Horological Institute) and WOSTEP training programmes use cutaway Pin-wheel Escapements to teach impulse-and-locking geometry because the action is more visible than a Graham deadbeat at slow demonstration speed.
The Formula Behind the Pin-wheel Escapement
The number you most often need to compute is the impulse energy delivered to the pendulum per beat — that's what determines whether your pendulum will maintain its target amplitude under the friction load of the pallets and pivots. At the low end of the typical operating range, with light driving torque from a near-spent weight, the impulse barely overcomes pivot friction and amplitude collapses below 1.5°. At the nominal design point you want around 2° to 3° pendulum half-amplitude. Push too high and the pendulum amplitude exceeds 4°, the pallet starts unlocking on the impulse face, and rate stability falls apart. The sweet spot for a 1-second turret pendulum sits around 2.5° half-amplitude, driven by a wheel torque that delivers roughly 80–120 µJ per impulse.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Eimp | Energy delivered to pendulum per impulse | J (joules) | ft·lbf |
| Tw | Torque at the escape wheel arbor | N·m | oz·in |
| rp | Pin pitch-circle radius (centre of wheel to pin centre) | m | in |
| θi | Impulse arc swept by the wheel during pin contact | rad | rad |
| η | Mechanical efficiency of pin-on-pallet contact (typically 0.55–0.75) | dimensionless | dimensionless |
Worked Example: Pin-wheel Escapement in a heritage tower clock restoration
A heritage timekeeping cooperative in Quebec City is recommissioning an 1872 turret clock built by JB Joyce of Whitchurch for a stone bell tower. The movement has a Pin-wheel Escapement with 30 pins on a pitch-circle radius of 60 mm, driving a 1.5-second pendulum with a 14 kg bob. Wheel torque at the escape arbor measures 0.0042 N·m with a fresh weight, the impulse arc is 0.052 rad (about 3°), and pin-on-pallet efficiency is estimated at 0.65. They need to know whether the impulse energy is enough to maintain a healthy 2.5° pendulum half-amplitude across the full 8-day winding cycle, where torque drops as the cable unwinds.
Given
- Tw = 0.0042 N·m (fresh weight)
- rp = 0.060 m
- θi = 0.052 rad
- η = 0.65 dimensionless
- Pendulum period = 1.5 s (so 0.75 s per impulse)
Solution
Step 1 — at the nominal fresh-weight torque of 0.0042 N·m, compute impulse energy:
That's the energy delivered to the pendulum each time a pin meets a pallet. Step 2 — at the low end of the 8-day torque cycle, the weight has dropped about 75% and barrel friction has eaten roughly 30% of remaining torque, so effective Tw falls to around 0.0028 N·m:
At this impulse level the pendulum half-amplitude drops from 2.5° toward 1.8° on day 7 — visible to the eye if you watch the bob, and audible because the beat softens. Step 3 — at the high end, freshly wound with cold thick oil reducing pivot losses temporarily, effective torque can spike to 0.0050 N·m:
That pushes amplitude toward 3.2°, which is acceptable but flirts with the upper limit where the pallet locking face starts seeing impulse-side wear. Across an 8-day cycle this gives an impulse energy range of roughly 5.7 to 10.1 µJ, with the design centred near 8.5 µJ.
Result
Nominal impulse energy is 8. 5 µJ per beat, sufficient to hold the 14 kg pendulum at roughly 2.5° half-amplitude with a fresh weight. The range across the 8-day cycle runs from 5.7 µJ on day 7 (amplitude near 1.8°, soft beat) up to 10.1 µJ immediately after winding (amplitude approaching 3.2°), so the sweet spot is firmly mid-week. If you measure amplitude below 1.5° even with a fresh weight, suspect three failure modes in this order: (1) glazed pallet impulse faces — 50+ years of pin contact polishes a hard glaze that drops η from 0.65 to under 0.45, (2) bent pallet arbor causing one pallet to drop deeper than the other so half the impulses are wasted on locking-face slide, or (3) the click spring on the maintaining-power detent has weakened and the train is losing torque during the strike train release.
Choosing the Pin-wheel Escapement: Pros and Cons
The Pin-wheel Escapement competes mainly with the Graham deadbeat and the recoil anchor in pendulum work. Each one trades manufacturability against precision against serviceability. The Lantern-wheel escapement (the same mechanism by an older name) wins on field repairability and cost — it loses to the Graham on absolute precision in observatory-grade work.
| Property | Pin-wheel Escapement | Graham Deadbeat | Recoil Anchor |
|---|---|---|---|
| Rate stability (s/day) | ±2 s/day typical | ±0.5 s/day typical | ±10 s/day typical |
| Pendulum amplitude range | 1.5°–3.5° half-amplitude | 1°–2° half-amplitude | 3°–6° half-amplitude |
| Manufacturing cost (relative) | Low — pins pressed in | High — cut & hardened teeth | Medium — cut teeth, looser tolerance |
| Field repairability | Excellent — replace individual pins | Poor — re-cut wheel or scrap | Fair — file teeth in situ |
| Tolerance to oil thickening | Good — pin geometry forgiving | Poor — pointed teeth jam | Excellent — recoil masks drag |
| Typical service life before re-pinning | 50–80 years | 80–150 years (no re-cut) | 30–50 years |
| Best application fit | Turret clocks, French regulators | Observatory regulators, precision longcase | Domestic 30-hour clocks, lantern clocks |
Frequently Asked Questions About Pin-wheel Escapement
Yes — they're the same mechanism. Older English horological texts, particularly pre-1900 turret-clock catalogues from makers like JB Joyce and Smith of Derby, called it the Lantern-wheel escapement because the wheel with its row of axial pins resembles a lantern pinion turned on its face. Modern usage settled on Pin-wheel Escapement after Britten's standard horological dictionary normalised the term.
That's almost always pendulum thermal expansion combined with impulse timing. As the rod warms, the bob drops a fraction of a millimetre and the pendulum should slow — but on a Pin-wheel Escapement with a slightly shallow impulse angle, warmer oil also reduces pivot drag, raising amplitude and pulling the impulse delivery earlier in the swing. The two effects partially cancel, but if your impulse face is worn shallow, the amplitude effect dominates and the clock gains.
Diagnostic check: measure half-amplitude at noon and at midnight. If amplitude varies by more than 0.3°, the impulse-face geometry is your problem, not the pendulum compensation. Re-polishing the pallet impulse faces to a true 1.5° angle usually fixes it.
Depends on what you value. If you're chasing observatory-grade rate (better than ±1 s/day), build the Graham — its pointed teeth give crisper locking and tighter drop control. If you're building for a client who wants the clock to still be running in 150 years with simple servicing, build the Pin-wheel. A village clockmaker can knock out a worn pin and press a new one in 20 minutes. Re-cutting a damaged Graham wheel tooth is a specialist job that may take a week.
For private-collector regulator work where the clock will live in a controlled museum environment, Graham wins. For anything heading into a tower, a country house, or a public space, Pin-wheel is the safer long-term choice.
You used the wrong steel or the wrong heat treatment. Original pins were typically blue-tempered carbon steel hardened to roughly 58–62 HRC and then drawn back to relieve brittleness. If you used unhardened mild steel rod from a hardware shop, the pin work-hardens at the pallet contact point and then snaps under cyclic shock.
The correct material is silver steel (drill rod) hardened in oil and tempered to a light straw colour. Pin diameter should match the original within 0.01 mm — if you ream the seat oversized to fit a 1.3 mm pin where 1.2 mm originals lived, the pin sits proud of the wheel face and side-loads on every impulse.
Above about 4° half-amplitude you're entering territory where the pendulum's natural swing carries the pallet past the end of the impulse face and the pin starts hammering the locking corner. You'll hear it as a harder, sharper tick — and within a few months you'll see a witness mark on the locking face right at the impulse-corner transition.
If amplitude is climbing above 3.5°, reduce drive weight before doing anything else. A pin-wheel sized for a 14 kg bob should not need more than about 8 kg of drive weight on a typical 8-day movement; if a previous restorer added weight to compensate for some other fault, you're hammering your wheel for no reason.
Beat error. On the bench the clock is sitting level on a flat surface; in a stone tower the mounting board is rarely truly plumb, and a Pin-wheel Escapement is sensitive to beat error because uneven pallet drop on each side reduces effective impulse on the weak side. If one side gets 0.060 rad of impulse arc and the other gets 0.044 rad, the pendulum loses amplitude on alternating swings and eventually stalls.
Listen for the beat — tick-tock should be evenly spaced. If you hear tick-TOCK or TICK-tock, the crutch is mis-aligned. Adjust the crutch (most movements have a friction-fit beat-setting collar) until the beat is symmetric, then re-check after 24 hours.
You can, and it's been done in heritage turret-clock work where original Graham wheels were beyond economical repair. But it's not a drop-in swap. The pallet geometry differs — Pin-wheel pallets sit on a different impulse plane because pins contact the pallet face at a different angle than radial teeth. You'll need new pallets, a new escape wheel, and likely a re-set crutch length to keep beat correct.
If you go ahead, match the original tooth count with pin count (e.g. 30 teeth becomes 30 pins on the same pitch circle), keep the same wheel rotation rate, and have a horologist verify the pallet impulse angle on the bench before installing. Done right, the clock will outlive everyone in the room.
References & Further Reading
- Wikipedia contributors. Escapement. Wikipedia
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