Spur-gear Stops are paired spur gears with one or more teeth removed and replaced by a raised boss or pin, so the assembly can only rotate through a fixed number of turns before the boss meets the boss on the mating gear and locks the train. Unlike electrical limit switches, the stop is purely mechanical and cannot fail open. The purpose is to cap travel on a shaft that must never overrun — safe-dial mechanisms, hand-cranked winches, and watch fusee chains all use the principle. Outcome: a 6:1 stop pair caps a shaft at exactly 6 turns, every time, for the life of the gear.
Spur-gear Stops Interactive Calculator
Vary the input and output gear tooth counts to see the mechanical stop turn limit and the boss collision position.
Equation Used
The stop limit is set by the gear ratio. With one boss on the output gear, the bosses collide after the output gear completes one revolution, so input turns to lock equal output teeth divided by input pinion teeth.
- One stop boss is used on each gear.
- External spur gears mesh without slip.
- The output gear locks once per output revolution.
- Tooth counts are treated as exact design values.
The Spur-gear Stops in Action
Spur-gear Stops, also called Stops for a Spur Gear in older horology and locksmith texts, work by mutilating one tooth on each of two meshing spur gears and replacing the gap with a raised boss that is taller than the tooth height. As long as the bosses pass each other in the mesh, rotation continues normally. When the input shaft has turned through the designed number of revolutions, the two bosses arrive at the mesh point simultaneously and physically collide — face-to-face — locking the gear train solid. You hear a hard clack and the crank refuses to move further. That hard stop is the whole point.
The geometry that makes this work is unforgiving. The boss height must clear the addendum circle of the mating gear by at least 0.3 mm but no more than 0.6 mm — too short and the bosses ride over each other under torque, too tall and they foul the next tooth in normal running. The boss face must sit on a true radial line through the gear centre; even 1° of skew turns a face-to-face stop into a glancing impact that hammers the boss flat over a few hundred cycles. The most common failure mode is exactly this: a sloppy boss orientation peens itself round, and after maybe 500 cycles the stop slips a tooth and the count drifts. The second failure mode is using soft material — a brass boss against a steel one will mushroom by 0.1 mm per thousand cycles. Match the materials, and harden both bosses to at least 45 HRC if the train sees more than 50 N·m of stop torque.
The ratio between the two gears sets the number of turns the input is allowed before lockout. A 1:6 ratio gives 6 input turns per single output turn, and since the output gear only carries one boss, the system locks once per output revolution — meaning 6 input turns total before the hard stop.
Key Components
- Input (mutilated) spur gear: Carries one boss in place of one tooth. Boss height typically tooth height + 0.4 mm, boss width matches face width of the gear. Made from the same hardened steel as the mating gear to prevent peening.
- Output (mutilated) spur gear: Carries the matching boss. The ratio between input and output teeth determines how many input revolutions occur before the bosses coincide. A 12-tooth input meshing with a 72-tooth output gives a 6-turn stop.
- Stop boss: The raised radial face that takes the impact. Must sit on a true radial line within ±0.5°, with a flat striking face at least 2 mm × the gear face width to spread the impact load below 200 MPa contact stress.
- Centre distance and shaft support: Bearings must hold the centre distance to within ±0.05 mm. Any wobble lets the bosses glance instead of meeting flat, which is the single biggest cause of premature stop failure.
- Backstop or return spring (optional): Some designs add a light torsion spring on the output shaft to prevent the bosses settling against each other under vibration, which can preload the train and make the next start sticky.
Industries That Rely on the Spur-gear Stops
Spur-gear Stops show up wherever a shaft must be physically prevented from exceeding a known number of turns and an electrical limit switch is either unreliable, illegal, or too slow. The mechanism is cheap, silent until it stops, and requires no power. You will find Stops for a Spur Gear in safes, hand winches, fusee clocks, vintage rifle scopes, and the wind-up mechanism of older mechanical timers.
- Security & safes: The dial spindle on a Sargent & Greenleaf Group 2 combination lock uses a spur-gear stop pair to cap dial rotation, preventing manipulation tools from spinning past the wheel pack.
- Lifting equipment: Hand-cranked boat trailer winches like the Fulton F2 series use a 6-turn spur-gear stop to prevent the operator from over-spooling the cable past the drum flange.
- Horology: The fusee-and-chain stopwork on a traditional English marine chronometer (Earnshaw pattern) uses a mutilated spur gear to halt winding before the chain over-rides the cone.
- Optical instruments: The elevation turret on a Unertl 8x target scope uses a spur-gear stop to cap travel at 60 MOA, so the shooter cannot wind past the erector tube limit and bind the spring.
- Industrial actuators: Some legacy valve actuators — including older Limitorque SMB units before electronic torque sensing — used spur-gear stops as a mechanical backup to the limit-switch pack on quarter-turn valves.
- Mechanical counters: Veeder-Root predetermining counters used a spur-gear stop on the reset shaft to prevent rotation past the zero position during manual reset.
The Formula Behind the Spur-gear Stops
The core question is: how many input turns can I get before the bosses collide? That depends only on the gear ratio and how many bosses you put on each gear. At the low end of the typical range — ratios near 2:1 with single bosses — you get 2 turns of allowed travel, which suits short-throw applications like a quarter-turn valve override. At the nominal range (5:1 to 8:1) you cover most safe-dial and winch applications. Push the ratio above 12:1 and the gear train starts to need a third idler to keep the centre distance practical, which is where designers usually switch to a worm-gear stop or a multi-turn potentiometer-style approach.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Nturns | Allowed input shaft revolutions before hard stop | revolutions | revolutions |
| Zoutput | Tooth count on the output gear | teeth | teeth |
| Zinput | Tooth count on the input gear | teeth | teeth |
| Boutput | Number of bosses on the output gear (usually 1) | count | count |
Worked Example: Spur-gear Stops in a vintage gun-safe dial retrofit
You are retrofitting a spur-gear stop onto the dial spindle of a 1970s Browning ProSteel gun safe to cap dial rotation at 4 full turns, preventing a curious teenager from spinning the dial endlessly and wearing out the wheel pack. The input pinion has 15 teeth and you can fit an output gear up to 80 mm pitch diameter on the available shelf inside the door.
Given
- Zinput = 15 teeth
- Boutput = 1 boss
- Nturns target = 4 revolutions
Solution
Step 1 — rearrange the formula to solve for the output tooth count needed for a nominal 4-turn stop:
Step 2 — at module 1.0 mm, a 60-tooth output gear gives a pitch diameter of 60 mm, which fits the 80 mm shelf with 10 mm clearance on each side. This is the nominal sweet spot for the design.
Step 3 — at the low end of the typical design range, suppose you only want 2 turns (a more secure setup that limits brute-force dialling):
A 30 mm output gear is small enough that the boss face width drops below 6 mm, and the contact stress at the stop face climbs above 250 MPa for a typical 8 N·m dial torque. You will see the boss start to peen within a few hundred cycles unless you harden to 50 HRC. At the high end, suppose you want 8 turns to match a commercial dial:
That is too big for the 80 mm shelf. Your options are to drop the module to 0.5 mm (giving dp = 60 mm but halving the tooth strength), add a 2:1 idler stage, or add a second boss on the output gear (Boutput = 2) to halve the required tooth count back to 60.
Result
The nominal design uses a 15-tooth input meshing with a 60-tooth output, module 1. 0 mm, single boss, giving exactly 4 turns of dial travel before hard stop. In practice this feels like a positive, decisive clack at the end of travel — no spongy bottom-out, no slip. The 2-turn variant feels harsher because the impact energy is spread over a smaller boss face, while the 8-turn variant needs a second boss or an idler to fit the available space. If you build it and measure 4.1 or 3.9 turns instead of clean 4, the most likely causes are: (1) the boss on either gear is offset from true radial by more than 1°, which shifts the lock point by a fraction of a tooth, (2) backlash between the gears is above 0.15 mm, letting the stop hunt under reverse torque, or (3) the centre distance is off by more than 0.05 mm and the bosses are catching on the tooth tips before reaching face-to-face contact.
Spur-gear Stops vs Alternatives
Spur-gear Stops are one of three common ways to physically cap shaft travel. The other two are worm-gear stops and pin-and-slot stops. Each has a clear sweet spot.
| Property | Spur-gear Stops | Worm-gear Stops | Pin-and-slot Stops |
|---|---|---|---|
| Maximum allowable turns | 2 to 12 turns practical | 10 to 100+ turns | Less than 1 turn |
| Stop impact load capacity | Up to 50 N·m at the boss with hardened steel | Up to 200 N·m, the worm self-locks | Up to 500 N·m, pin shears in shear not bending |
| Cost (production) | Low — two stock spur gears + boss machining | Medium — worm/wheel sets cost 3-5× spur pairs | Very low — single pin and slotted plate |
| Backlash at stop | 0.05-0.15 mm typical, depends on gear quality | Near zero, worm self-locks under load | Zero — pin contacts slot end face directly |
| Lifespan at rated load | 10,000+ cycles if hardened to 45 HRC | 50,000+ cycles, worm geometry spreads load | 1,000-5,000 cycles, pin work-hardens and breaks |
| Best application fit | Multi-turn dials, winches, safe locks | High-turn-count actuators, valve drives | Quarter-turn valves, single-rev limits |
Frequently Asked Questions About Spur-gear Stops
Almost always centre-distance creep, not boss wear. Each hard stop sends a radial impact through the bearings, and if the bearing housings are aluminium or unsupported sheet metal, they yield microscopically each cycle. After 200-500 cycles the centre distance has grown by 0.1-0.2 mm, the bosses no longer meet face-to-face, and one rides up over the other.
Diagnostic check: measure centre distance with a bore gauge before and after a stress test of 100 hard stops. If it has grown by more than 0.03 mm, switch to steel bearing blocks or add a tie-plate between the two shaft centres.
You get fewer turns, not more. Each additional boss on the output gear divides the allowed turns by the number of bosses, because the bosses now coincide at the mesh more frequently. Two bosses on the output gear of a 4-turn design gives you 2 turns of travel.
The reason to add bosses is the opposite — to fit a higher allowed turn count into a smaller package. Six turns with a single boss needs a 6:1 ratio; six turns with two bosses on the output and one on the input needs a 12:1 ratio but only a 3-turn pure ratio if you arrange the boss positions correctly. This trick is how Sargent & Greenleaf got 4-turn stops into a 35 mm-diameter dial mechanism.
Worm-gear stop, almost always. At 20 turns you need a 20:1 spur ratio, which means either a tiny pinion that strips under stop impact, or a large output gear that bloats the package. Worm gears handle 20:1 in a single stage with a 30 mm centre distance, and the worm naturally self-locks so the stop impact is shared along the worm thread instead of concentrated on a single boss.
The crossover is around 10-12 turns. Below that, spur-gear stops win on cost and ease of manufacture. Above that, worm-gear stops win on package size and durability.
The rule we use: boss height = tooth addendum + 0.4 mm ±0.1 mm, and boss circumferential width = 0.6 × tooth thickness at the pitch line. That puts the boss tip clearly above the addendum circle of the mating gear (so it cannot ride over the boss in the opposing gear) but well below the clearance circle (so it does not foul the root of the next tooth).
If your module is below 0.8 mm, scale the +0.4 mm clearance proportionally — at module 0.5 mm use +0.25 mm, otherwise the boss starts dragging on the tooth tips during normal rotation and you get a gritty crank feel.
The bosses are settling into each other under residual torque and slightly cold-welding or galling at the contact face. This is common when the two bosses are the same material with no surface treatment, and especially common when the operator leans on the crank at the stop.
Two fixes: (1) add a light torsion spring to the output shaft, around 0.1-0.2 N·m, so the bosses separate by 1-2° as soon as input torque is removed. (2) Surface-treat one boss differently — black oxide on one, hard chrome on the other — so the contact pair will not gall. Both fixes cost cents and eliminate the sticky-start complaint.
Design for at least 3× the expected operator torque, because a frustrated user who hits the stop unexpectedly will reflexively yank the crank with peak torque well above their steady-state effort. For a typical 100 mm crank handle, steady-state torque is around 8-12 N·m but reflexive peak can hit 35-40 N·m.
Sized for 40 N·m at a 20 mm boss radius gives a tangential force of 2,000 N. Spread over a boss face of 4 mm × 6 mm (24 mm²), contact stress is 83 MPa — comfortably below the 250 MPa fatigue limit for hardened tool steel. Drop the boss face area below 15 mm² and you are into the regime where the boss peens visibly within a year.
References & Further Reading
- Wikipedia contributors. Gear. Wikipedia
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