A Two Escape-wheel Pallet Escapement is a horological escapement that uses two co-axial escape wheels acting on a single pallet fork mounted on the front plate of the movement. The pallet fork is the critical component — its two stones engage the two wheels in alternation, splitting the impulse between them so each tooth-pallet contact carries only half the load. This halves wear on the impulse faces and lets the clock run with a lighter mainspring or driving weight. Practical builds, like Riefler-style precision regulators, hold rates inside ±0.5 seconds per day.
Two Escape-wheel Pallet Escapement Interactive Calculator
Vary normalized impulse load, wheel count, tooth count, and wheel offset to see pallet contact load sharing and phase error.
Equation Used
The calculator uses the article principle that two escape wheels share the impulse load, so each tooth-pallet contact carries F_total / n. For correct phasing, the rear wheel is offset by half a tooth pitch, equal to 180 / tooth count degrees.
- Impulse load is split equally between active escape-wheel contacts.
- Wheel phase target is half a tooth pitch.
- Total load is entered as a normalized percentage for comparison.
Operating Principle of the Two Escape-wheel Pallet Escapement (front)
The mechanism works by stacking two escape wheels on the same arbor, usually offset rotationally by half a tooth pitch. The pallet fork carries two stones — one for each wheel — and as the pendulum swings, one stone receives impulse from the front wheel while the other stone locks the rear wheel. On the return swing the roles flip. You get one impulse per half-cycle of the pendulum, same as a Graham deadbeat, but the contact load is shared across two tooth-pallet interfaces instead of one.
Why bother? Two reasons. First, the impulse load on each pallet stone drops by roughly half, so wear on the impulse face slows dramatically. Second, the front-plate mounting puts the whole escapement visible behind the dial — useful for skeleton clocks and for service access without splitting plates. The pallet stones are typically polished agate or synthetic ruby, set into brass nibs, and the impulse angle on each wheel runs around 2° to 3° per tooth.
If you get the tolerances wrong, the symptoms are obvious and ugly. Drop lock — the depth the tooth falls onto the locking face after release — must sit between 0.5° and 1.0° measured at the pallet. Less than that and the wheel trips during the supplementary arc, giving you a gallop. More than that and you waste pendulum energy on each unlock. The two wheels must also be phased within ±0.05 mm at the tooth tips; if one wheel leads the other, one pallet stone takes nearly all the load and you've built a single-wheel escapement with extra parts. Common failure modes are pallet-stone chipping at the impulse corner, wheel-arbor end-shake letting the two wheels drift apart axially, and pivot wear in the pallet arbor causing inconsistent drop lock between left and right swings.
Key Components
- Twin Escape Wheels: Two thin steel or brass wheels mounted on a common arbor, offset by half a tooth pitch (typically 6° on a 30-tooth wheel). Each wheel carries 25 to 40 teeth depending on beat rate, with tooth tip thickness held to ±0.02 mm so impulse phasing matches between the pair.
- Pallet Fork: The front-mounted lever carrying both pallet stones. Its arbor pivots between front-plate cocks, and the entrance and exit stones engage opposite wheels. Pallet span is set so each stone clears its own wheel by the locking depth, usually 0.4 to 0.6 mm at the tooth tip.
- Pallet Stones: Polished agate or synthetic ruby blocks set into brass nibs, one per wheel. Impulse face angle runs 12° to 15°, and surface finish must hit Ra 0.05 µm — anything rougher and the wheel teeth gall the stone within months of running.
- Crutch and Pendulum Link: Connects the pallet fork arbor to the pendulum rod. Side-shake at the crutch pin must stay under 0.1 mm, otherwise the pendulum's natural arc decouples from the impulse timing and rate becomes load-dependent.
- Front-plate Cocks: Two small bridges screwed to the front plate carrying the pallet arbor and escape-wheel arbor pivots. They allow access to the escapement without dismantling the train, and their alignment sets the depthing between pallets and wheels.
Who Uses the Two Escape-wheel Pallet Escapement (front)
The two escape-wheel pallet escapement shows up wherever a builder wants the longevity of shared-load impulse without giving up the visual drama of a front-plate escapement. It's not common — most production clocks stick with a single deadbeat — but in precision regulators, exhibition skeleton clocks, and certain astronomical instruments it earns its keep. The double-wheel arrangement also pairs naturally with remontoire drives, since the lighter individual impulse load suits a constant-force input.
- Precision Horology: Riefler-style observatory regulators, where the divided impulse reduces pallet-stone wear over decade-long service intervals between overhauls.
- Skeleton Clockmaking: High-end skeleton mantel clocks by makers like Sinclair Harding, where the front-plate escapement is positioned for visibility through the open dial.
- Scientific Instruments: Astronomical sidereal-rate regulators in university observatories, including the older Shortt-Synchronome free-pendulum slave clocks where impulse consistency drives accuracy.
- Heritage Restoration: Late-19th-century turret clocks built by makers such as Gillett & Johnston, where dual-wheel escapements were occasionally specified for cathedral clocks running heavy hands.
- Bespoke Watchmaking: One-off table regulators commissioned through ateliers like George Daniels' workshop tradition, where novel escapement geometries are part of the brief.
- Horological Education: Teaching benches at the British Horological Institute, where the visible front-plate layout makes the impulse-and-lock cycle easy to demonstrate.
The Formula Behind the Two Escape-wheel Pallet Escapement (front)
The most useful number to compute is the impulse energy delivered per tooth, because that drives both pendulum amplitude and pallet-stone wear rate. At the low end of typical operating torque you'll find weak amplitude and risk of trip-out on a knock. At nominal torque the supplementary arc sits around 2° either side of the impulse arc — the sweet spot. Push torque too high and you over-bank the pendulum, the circular error climbs, and rate goes load-sensitive. The formula below gives impulse energy per tooth shared across the two wheels.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Eimp | Impulse energy delivered per tooth per pallet stone | J | ft·lbf |
| Tdrive | Torque at the escape-wheel arbor from the going train | N·m | lbf·in |
| θimp | Impulse angle swept by the escape wheel during pallet engagement | rad | rad |
| Nteeth | Tooth count on each escape wheel | — | — |
Worked Example: Two Escape-wheel Pallet Escapement (front) in a Berlin observatory regulator rebuild
A precision instrument workshop in Potsdam is rebuilding a 1908 Strasser & Rohde observatory regulator fitted with a two escape-wheel pallet escapement on the front plate. Each escape wheel carries 30 teeth, the impulse angle is 2.5° per tooth, and the going train delivers 0.0015 N·m at the escape-wheel arbor under nominal mainspring force. The workshop wants to know the impulse energy per tooth at nominal drive, at the low end where the going barrel approaches end-of-wind, and at the high end straight after winding.
Given
- Tdrive (nominal) = 0.0015 N·m
- θimp = 2.5° = 0.0436 rad
- Nteeth = 30 teeth per wheel
- Tdrive (low end, end-of-wind) = 0.0011 N·m
- Tdrive (high end, fully wound) = 0.0019 N·m
Solution
Step 1 — at nominal drive torque, compute impulse energy per tooth shared across both wheels:
That's roughly 1.1 microjoules per tooth per stone. In practical terms, the pendulum receives 2.2 µJ per full beat (two impulses), which holds a 1-metre seconds pendulum at about 2° peak amplitude — exactly what Strasser & Rohde specified for their observatory regulators.
Step 2 — at the low end of the operating range, going barrel near end-of-wind:
Impulse drops by 27%, which sounds catastrophic but isn't — the pendulum amplitude only falls by about 13% because amplitude scales with √E. You'd see the pendulum swing tighten from 2° to roughly 1.7°, and the rate would shift by perhaps 0.3 seconds per day from circular error. This is why regulators get wound on a strict schedule.
Step 3 — at the high end, straight after winding with full mainspring or weight torque:
Amplitude climbs to around 2.3°, which sounds harmless but pushes the pendulum into the region where circular error grows quadratically with arc. Rate now runs fast immediately after winding and slow before. The fix in Strasser's original design was a remontoire that re-armed every 30 seconds, capping torque variation to within 5% — a luxury you only get with the dual-wheel layout because the lighter per-stone load makes a small remontoire spring viable.
Result
Nominal impulse energy per tooth comes out to 1. 09 × 10-6 J, or 1.1 µJ. That figure means the pendulum sits comfortably at 2° amplitude with the supplementary arc clear of the locking faces — the sweet spot for a 1-second observatory regulator. At the low end of typical operating torque you drop to 0.80 µJ and 1.7° amplitude, still safely above trip-out; at the high end you hit 1.38 µJ and 2.3° amplitude where circular error starts costing you accuracy. If your measured amplitude differs from these predictions, look at three things in order: (1) one of the two pallet stones sitting 0.05 mm proud of the other, which throws all the load onto a single wheel and halves effective impulse, (2) escape-wheel arbor end-shake above 0.08 mm letting the wheels drift axially out of pallet engagement, or (3) crutch-pin slop above 0.1 mm decoupling the pendulum from the fork on the return swing.
Choosing the Two Escape-wheel Pallet Escapement (front): Pros and Cons
Choosing a two escape-wheel pallet escapement over a conventional Graham deadbeat or a gravity escapement is a deliberate engineering decision, not a default. Here's how the three stack up on the dimensions that actually matter to a clockmaker specifying a regulator build.
| Property | Two Escape-wheel Pallet Escapement | Graham Deadbeat (single wheel) | Double Three-legged Gravity |
|---|---|---|---|
| Typical rate accuracy | ±0.5 sec/day | ±1 to 2 sec/day | ±0.2 sec/day |
| Pallet stone wear interval | 20-30 years between re-polish | 10-15 years between re-polish | N/A — gravity arms instead of stones |
| Drive torque sensitivity | Moderate — benefits from remontoire | High — needs constant force | Very low — gravity decouples train |
| Build complexity | High — twin wheels must phase within 0.05 mm | Low — well-documented, forgiving | Very high — beat fly and arms must be tuned |
| Visual access for service | Excellent — front-plate mounted | Moderate — between plates | Excellent — usually exposed at top of movement |
| Suitability for turret clocks | Rare but workable | Common up to medium turrets | Standard for cathedral-scale clocks |
| Cost to build (one-off) | £8,000-£15,000 | £3,000-£6,000 | £10,000-£20,000 |
Frequently Asked Questions About Two Escape-wheel Pallet Escapement (front)
You don't trust eyeballing it — set the phase optically. Mount the pair on a depthing tool with both wheels visible, rotate to bring the front wheel's tooth tip to the entrance pallet contact point, then rotate the rear wheel on its keyway or pinning collar until its tooth contacts the exit pallet at the symmetric position. The two contacts must occur at the same pallet-arbor angle within ±0.1°.
The reason this matters: if one wheel leads by even 0.5°, that wheel's pallet takes nearly the full impulse load while the other coasts. You've effectively built a single-wheel escapement carrying twice the wear it should. A common workshop trick is to scribe a witness line across both wheel rims before final pinning so any future drift is immediately visible.
The dual-wheel layout shares load between pallets but does nothing to isolate the escapement from going-train torque variation. You're seeing classic circular-error response to amplitude change — high torque after winding pushes amplitude up, larger arcs run slow due to circular error, and the opposite happens before the next wind.
The fix is a remontoire between the going train and the escape wheels, which re-arms a small spring every 30 to 60 seconds and feeds the escapement constant torque regardless of mainspring state. Strasser & Rohde, Riefler, and later Shortt all relied on this. Without one, expect a 0.5 to 1 second daily variation tied to your wind cycle.
Three conditions tip the decision: you need exceptionally long service intervals (decades not years), the clock will be on display so the front-plate layout has aesthetic value, and the budget supports the extra build cost — typically two to three times a comparable Graham. If any one of those three is missing, build a Graham.
The dual-wheel arrangement does not improve raw accuracy over a well-built Graham deadbeat. What it buys you is wear life and visibility. For a turret clock running 24/7 in a cathedral, that wear life argument is real. For a private mantel regulator wound weekly, it's mostly cosmetic.
Asymmetric drop lock between left and right swings almost always points to pallet-arbor pivot wear, not pallet-stone misadjustment. As the pivot wears oval, the pallet fork sits off-centre at rest, so one stone falls deeper onto its locking face than the other.
Check the pivots with a 10x loupe — you're looking for the classic figure-of-eight wear pattern. If found, replace or re-bush the pivot holes. Don't try to compensate by re-setting the stones; you'll just transfer the asymmetry into the impulse phase and create a beat-error problem on top of the existing lock-error problem.
Technically yes, practically rarely worth it. You'd need a new escape-wheel arbor long enough to carry both wheels, a new pallet fork with twin stones at correct lateral spacing, modified front-plate cocks, and almost certainly a re-cut depthing because the pallet span changes. By the time you've done all that you've built a new escapement and reused the plates and train.
The exception is a well-documented historical conversion — for example a turret clock that originally had a dual-wheel arrangement and was simplified during a 20th-century overhaul. In that case you're restoring rather than retrofitting, and the original cock locations and arbor dimensions usually survive.
Cold air is denser, so pendulum aerodynamic drag rises. With a fixed impulse energy, higher drag means lower steady-state amplitude. Between a 25°C summer day and a 0°C winter night, air density rises by roughly 9%, and amplitude can drop by 0.1° to 0.2° on a typical 1-metre regulator pendulum — visible on a beat-recording instrument.
The dual-wheel escapement doesn't change this; it's a pendulum-environment problem, not an impulse problem. Observatory regulators historically lived in temperature-stabilised cellars or, like the Shortt clocks, sealed evacuated tanks specifically to remove this variable.
References & Further Reading
- Wikipedia contributors. Escapement. Wikipedia
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