Lantern-wheel Stops are a rotation-limiting mechanism that uses a fixed pin or finger to block one of the trundles (rod-shaped teeth) of a lantern wheel after a set number of turns. They show up in clockmaking and mechanical counters, where you need a wheel to rotate a defined count and then stop dead. The stop pin enters the gap between two trundles and arrests the wheel. The result is a hard, repeatable mechanical limit — no electronics, no slip, no drift across thousands of cycles.
Lantern-wheel Stops Interactive Calculator
Vary the pitch diameter, trundle count, trundle diameter, and desired side clearance to size the trundle gap and stop pin.
Equation Used
The lantern-wheel stop is sized from the clear arc between neighboring trundles. First calculate the center-to-center arc spacing on the pitch circle, subtract the trundle diameter to get the nominal gap, then subtract twice the desired side clearance to estimate a practical stop pin diameter.
- Trundles are evenly spaced on the pitch circle.
- Stop pin seats in the clear arc between adjacent trundles.
- Recommended side clearance is 0.1-0.3 mm; default uses 0.2 mm per side.
- Negative gap or pin diameter indicates the geometry will jam.
The Lantern-wheel Stops in Action
A lantern wheel is the old-style pinion built from two end discs joined by a ring of cylindrical rods called trundles. Stops for a Lantern Wheel work by adding either a fixed pin on the frame or a pivoting finger that drops into the trundle gap and physically blocks rotation. When the wheel turns, the trundles sweep past the stop position. As soon as the relevant trundle arrives, it strikes the stop, and the wheel halts within a fraction of a degree. You get the same final angular position every cycle because the geometry is mechanical, not timed.
Why build it this way? Lantern pinions tolerate dirt, wear, and misalignment far better than cut-tooth gears, which is exactly why 18th-century clockmakers used them in striking trains. The trundles can rotate slightly in their sockets, so contact with a stop pin doesn't gall or burr the way a gear tooth would. The stop finger usually sits on a pivot with a light spring or counterweight, so it can be lifted out of the way to release the wheel for the next cycle.
Get the geometry wrong and you'll know within minutes. If the stop pin diameter exceeds the trundle gap minus about 0.2 mm of clearance, the pin jams instead of seating cleanly. If it's too small, the wheel overruns by 1-3° and your count drifts. Worn trundles — anything more than 0.1 mm of diameter loss on a 3 mm trundle — let the stop finger slip past under load. The classic failure is a striking clock that strikes 13 instead of 12, which traces back to a worn count wheel trundle every time.
Key Components
- Lantern Wheel: The driven wheel built from two parallel end discs joined by a ring of trundles. Trundle diameter typically runs 2-5 mm in clockwork, with 6-12 trundles spaced evenly. The discs are press-fit or riveted to the arbor with no more than 0.05 mm runout.
- Trundles: The cylindrical rod-teeth that engage the driving pinion or stop finger. Hardened steel, ground to ±0.01 mm on diameter. Each trundle should rotate freely in its end-disc socket — a seized trundle wears flat on one side and shortens service life by an order of magnitude.
- Stop Pin or Finger: The fixed obstacle that arrests the wheel. Sized to fit the trundle gap with 0.1-0.3 mm clearance on each side. In striking clocks the finger pivots on a frame-mounted post and is lifted by a cam at the start of each strike sequence.
- Lifting Cam or Lever: Releases the stop finger from the trundle path so the wheel can rotate. Cam profile must lift the finger at least 1 mm clear of the trundle radius before the wheel begins to move, otherwise the finger drags and chatters.
- Count Wheel (in clock applications): A separate notched wheel that determines how many trundle steps occur per cycle. The notch depth and spacing set the strike count for each hour. A worn or out-of-true count wheel is the most common cause of miscounted strikes.
Industries That Rely on the Lantern-wheel Stops
Stops for a Lantern Wheel show up wherever a mechanism needs to count a discrete number of rotations and then halt without an encoder or limit switch. The mechanism is cheap, silent, and survives in dusty or oily environments that would kill an optical sensor. You see it most often in clockwork, but it also appears in mechanical counters, music boxes, and antique calculators.
- Horology: Striking trains in longcase clocks like the Thomas Tompion regulator family, where the lantern wheel and count wheel together meter out the correct number of hammer strikes per hour.
- Mechanical Counters: Veeder-Root rotary counters used on industrial production lines from the 1920s onward. The lantern pinion advances a digit wheel and a stop finger arrests it after each click.
- Music Boxes: Reuge cylinder music boxes use a lantern-style pinion driven by the spring barrel, with a stop finger that halts the cylinder at the end of a tune to prevent overrun and tooth damage.
- Antique Calculating Machines: The Brunsviga pinwheel calculator family used lantern-pinion-style elements with stops to limit register rotation to one decimal step per crank turn.
- Textile Machinery: Older Jacquard loom card-advance mechanisms used lantern wheels with stop pins to index the card chain by exactly one step per pick.
- Turret Clocks: Public clock movements like those in St Paul's and many cathedral towers use heavy lantern wheels with iron stop pins in their quarter-strike trains, where cut gears would foul with dust and bird debris.
The Formula Behind the Lantern-wheel Stops
The key calculation for sizing a Stops for a Lantern Wheel arrangement is the trundle gap — the open arc between two adjacent trundles where the stop pin must seat. At the low end of the typical trundle count (6 trundles), the gap is generous and the stop pin tolerance is forgiving, but the angular resolution of your count is coarse at 60° per step. At the high end (12 trundles), you get 30° resolution but the gap shrinks and pin clearance becomes critical. The sweet spot for most clockwork applications sits at 8 trundles — 45° resolution with about 1.5 mm of working gap on a typical 20 mm pitch-circle wheel.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| g | Trundle gap (clear arc length between adjacent trundles at the pitch circle) | mm | in |
| Dpc | Pitch-circle diameter of the lantern wheel (centre-to-centre across the trundle ring) | mm | in |
| Nt | Number of trundles | count | count |
| dt | Trundle diameter | mm | in |
| dp | Stop pin diameter (must be less than g, with 0.1-0.3 mm clearance per side) | mm | in |
Worked Example: Lantern-wheel Stops in a brass lantern-wheel stop for a kitchen wall clock strike train
You are restoring a 1890s German kitchen wall clock and need to fabricate a replacement lantern wheel and stop pin for the strike-train release. The original wheel has a 20 mm pitch-circle diameter, 8 trundles each 3 mm in diameter, and you need to size a stop pin that seats cleanly with the right clearance for a worn-but-serviceable mechanism.
Given
- Dpc = 20 mm
- Nt = 8 trundles
- dt = 3.0 mm
Solution
Step 1 — compute the pitch-circle circumference and divide by trundle count to get the centre-to-centre arc spacing:
Step 2 — subtract the trundle diameter to get the nominal clear gap:
Step 3 — at the low end of the typical clearance range, allow 0.1 mm per side. The maximum stop pin diameter becomes:
A 4.6 mm pin at this clearance gives crisp seating with minimal lost motion — the wheel arrests within about 0.5° of nominal. At the high end of the clearance range, 0.3 mm per side, the pin shrinks to roughly 4.25 mm and the wheel can overrun by up to 2° before the pin contacts. That sounds small, but on a strike count it's the difference between a clean stop and an audible double-tick. The sweet spot for a worn wall-clock train sits at 0.2 mm per side, giving a 4.45 mm pin — enough slop to tolerate trundle wear of around 0.1 mm without the pin binding when the train winds down with reduced spring force.
Result
Use a 4. 45 mm stop pin in a 4.854 mm nominal gap. At nominal clearance the wheel stops within 1° of the target position, which is inaudible on a domestic strike train. With tight 0.1 mm clearance you get sub-half-degree repeatability but the pin binds the moment a trundle wears past 3.05 mm; with loose 0.3 mm clearance the strike count drifts noticeably as the mainspring weakens through the week. If your measured stop position drifts more than 2°, check three things in this order: (1) trundle diameter — anything below 2.9 mm on a 3 mm spec means the trundle has work-hardened and shed material at the contact face; (2) stop finger pivot wear — a sloppy pivot lets the finger rise off the trundle path under impact; (3) end-disc runout — more than 0.05 mm wobble on the arbor throws the trundle path off-centre and the pin catches on alternating trundles unevenly.
Lantern-wheel Stops vs Alternatives
Stops for a Lantern Wheel compete with two other rotation-limiting approaches: hard-stop pins on a cut-gear or plain wheel, and Geneva-style intermittent indexers. Each suits a different combination of cost, accuracy, and operating environment.
| Property | Lantern-wheel Stops | Cut-gear with Stop Pin | Geneva Indexer with Stop |
|---|---|---|---|
| Angular accuracy at stop | ±0.5° to ±2° | ±0.1° to ±0.5° | ±0.05° to ±0.2° |
| Typical operating speed | 1-60 RPM | 1-200 RPM | 10-300 RPM |
| Tolerance to dirt and contamination | Excellent — trundles self-clear debris | Poor — debris jams between cut teeth | Moderate — depends on slot geometry |
| Manufacturing cost (low volume) | Low — turned trundles, drilled discs | Medium — gear cutting required | High — precision slot milling |
| Lifespan in clock-train service | 100+ years with periodic trundle replacement | 30-50 years before tooth wear becomes audible | Not used in clockwork — overkill |
| Repair difficulty | Easy — replace individual trundles | Hard — recut or replace whole gear | Hard — full mechanism replacement |
| Best application fit | Clock striking trains, antique counters | High-precision indexing tables | Production-line rotary indexers |
Frequently Asked Questions About Lantern-wheel Stops
This is almost always a stop pin that's slightly too large for the trundle gap. The pin is contacting the leading face of the previous trundle before the target trundle arrives at the stop position. Measure your gap with a feeler gauge or pin gauges — if you've installed a pin sized for the nominal gap without accounting for trundle wear, the effective gap is now smaller than your calculation suggests.
Pull the pin and turn it down by 0.1-0.2 mm. The clock should pick up the missing strike immediately.
Trundle count sets your angular resolution and your gap geometry. 6 trundles gives 60° per step with a generous 7-8 mm gap on a 20 mm pitch circle — forgiving for hand-fabrication but coarse for counting. 12 trundles gives 30° per step but the gap shrinks below 2 mm on the same wheel, which is below the usable size for a hand-fitted stop pin in a worn mechanism.
8 trundles is the practical sweet spot for clockwork. If you need finer resolution, increase the pitch-circle diameter rather than packing in more trundles.
Chatter means the lifting cam isn't fully clearing the finger from the trundle path before the wheel begins to rotate, or the finger's return spring is too stiff and it's bouncing on contact. Watch the cam-to-finger relationship through one full cycle. The finger should lift at least 1 mm above the trundle outer radius before the wheel moves, and it should drop back under gravity or a very light spring — anything stiffer than about 50 g of return force will bounce.
A second cause is a worn finger tip. If the contact face has rounded over by more than 0.2 mm, replace or re-square the tip.
For a counter that sees less than about 5 N·cm of torque and runs at under 30 RPM — yes, with caveats. PETG or nylon end discs hold up well, but the trundles themselves should be steel pins press-fit into the discs. Plastic trundles deform under stop-pin impact within a few hundred cycles and your count starts drifting.
Also keep the stop pin steel. A plastic pin against steel trundles wears the pin into a flat in days.
Variable stop position with no visible wear usually traces to inconsistent driving torque. The lantern wheel arrives at the stop at different speeds depending on mainspring state-of-wind, friction in the upstream train, or temperature changes affecting lubricant viscosity. Higher arrival speed means more elastic deflection in the stop finger and pivot before the wheel actually halts.
Diagnose by running the mechanism at full wind versus quarter wind and measuring the stop position with a degree wheel on the arbor. If the difference is more than 1°, stiffen the stop finger pivot or add a lightweight buffer to the finger contact face.
Yes — they refer to the same mechanism. 'Lantern-wheel Stops' is the modern catalog name; 'Stops for a Lantern Wheel' is the older horological term you'll find in 19th-century clockmaking texts and parts lists. Both describe the same arrangement of a lantern pinion arrested by a fixed pin or pivoting finger.
One-direction jamming points to either a tapered trundle (one end of the trundle is slightly larger than the other, so it wedges in the stop gap on reverse) or a stop finger that's been ground asymmetrically. Lantern wheels are fundamentally bidirectional, but a stop finger filed for forward operation often has a relieved leading face and a square trailing face. Reversing the wheel means the trundle now hits the square face at an angle and binds.
If you need genuine bidirectional operation, dress the finger symmetrically and verify trundle diameter consistency to ±0.02 mm end-to-end.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
