A Single-tooth Small Driver with Rim Lock is an intermittent indexer where a driver disc carrying one tooth meshes with a notched count wheel for a brief portion of each input revolution, while the remainder of the driver's circular rim sits flush against a matching concave land on the count wheel and locks it stationary. Unlike a Geneva drive, which needs slotted plates and a centre crank pin, this mechanism uses only two flat-faced discs. It exists to add one count per input revolution with positive dwell between counts. You see it on tally registers, web-press impression counters, and odometer-style drum counters logging tens of millions of cycles.
Single-tooth Small Driver Rim Lock Interactive Calculator
Vary notch count and engagement angle to see index advance, dwell lock angle, and the intermittent rim-lock motion.
Equation Used
The count wheel advances one notch for each full revolution of the single-tooth driver, so the index angle is 360 deg divided by the number of notches. The remaining driver phase after tooth engagement is the rim-lock dwell period.
FIRGELLI Automations - Interactive Mechanism Calculators.
- One driver tooth advances the count wheel one notch per input revolution.
- Notches are evenly spaced around the count wheel.
- Dwell is the non-engagement portion of the 360 deg driver cycle.
- Rim and land radii are assumed correctly matched for positive lock.
How the Single-tooth Small Driver, Lock by Rim Actually Works
The driver is a disc with one gear tooth standing proud of an otherwise circular rim. The count wheel has a matching set of notches around its perimeter, and between each notch the rim of the count wheel is shaped as a concave arc that matches the convex rim of the driver. As the driver spins, the single tooth enters the next notch, sweeps the count wheel forward by exactly one pitch, then exits. For the rest of the input revolution — typically 270° to 320° of driver rotation — the smooth rim of the driver rides against the smooth concave land of the count wheel, and the count wheel cannot move. That is the rim lock. No external pawl, no spring detent, no separate locking plate.
The geometry only works if three things are right. The driver rim radius and the count-wheel land radius must be cut from the same nominal arc — typically held to ±0.02 mm on a brass counter wheel — or the count wheel will either bind during dwell or develop free play and miscount. The single tooth must be timed so it enters the notch exactly as the rim relief on the driver clears the count wheel land; if entry is early, the tooth crashes into the side of the notch and you get a characteristic tick-and-skip symptom. And the notch count on the count wheel sets the index angle directly: a 10-notch wheel advances 36° per input rev, a 100-notch wheel advances 3.6°. There is no gear reduction beyond that — one input revolution equals one count, full stop.
The common failure modes are predictable. Worn driver tooth flanks (look for a rounded leading edge) cause late engagement and double-counts under vibration. A rim-land scuff that you can feel with a fingernail means the dwell radii were never matched and the wheel is creeping. And if the count wheel bore opens up beyond about 0.05 mm clearance on its post, the wheel rocks during dwell and the lock isn't really locking — you'll see the digit drum drift between counts on a postage meter or pinball score reel.
Key Components
- Single-tooth Driver Disc: A circular disc with one gear tooth on its rim. Driven continuously at the input speed. The rim radius is typically held to ±0.02 mm because it doubles as the locking surface for the rest of each revolution.
- Notched Count Wheel: The output wheel with N evenly spaced notches around its rim, each separated by a concave land matched to the driver rim radius. For a 10-count wheel the notch pitch is 36°; for a 100-count score reel it's 3.6°. Notch flank angle is usually 20° pressure angle, same as a standard spur tooth.
- Concave Locking Land: The arc of count-wheel rim between adjacent notches, cut to the same radius as the driver. This is what gives the mechanism its rim lock — during 270°–320° of driver rotation, this land sits flush against the driver rim and the count wheel cannot rotate either direction.
- Tooth-Notch Engagement Zone: The 40°–90° of driver rotation during which the single tooth is actually engaged with a notch and pushing the count wheel forward. The shorter this zone, the longer the dwell, but the higher the tooth-flank stress at the moment of engagement.
- Count Wheel Bore and Post: A press-fit bushing or jewel bearing on the count-wheel hub. Radial clearance must stay below 0.05 mm — anything looser lets the wheel rock during the dwell phase and the lock no longer holds position positively.
Who Uses the Single-tooth Small Driver, Lock by Rim
You find this mechanism wherever someone needs to add exactly one count per input revolution, hold the digit firmly between counts, and do it cheaply with two stamped or machined parts. It shows up most often as the units-digit driver in mechanical counters, where the count wheel is itself printed with digits 0–9 and reads through a window. It also shows up as the carry mechanism between digit drums on odometers — the units drum has a single carry tooth that advances the tens drum once per ten counts. The reason engineers reach for it over a Geneva or a ratchet-and-pawl is the rim lock: there's no spring to fatigue, no pawl to wear, and the locked position is positively constrained in both directions.
- Mechanical Counters: Veeder-Root Series 1500 stroke counters used on punch presses — the single-tooth driver advances the units drum one digit per ram cycle and the rim lock holds the reading steady while the operator records it.
- Postage and Office Equipment: Pitney Bowes impression counters on franking machines, where the units digit must advance once per print stroke and stay rock-steady through the inking cycle.
- Coin-Operated Amusement: Gottlieb and Williams electromechanical pinball score reels from the 1960s–70s, where each reel is a single-tooth-driven 10-notch count wheel reading 0 through 9 in the backbox.
- Automotive Instrumentation: Pre-electronic odometer drums on Smiths and VDO speedometers — the units drum carries a single tooth that indexes the tens drum once per kilometre on the carry-over.
- Industrial Production Logging: Hengstler tally counters on injection-moulding machines logging shot counts, where the unit-revolution-to-count ratio must be exact across millions of cycles.
- Utility Metering: Older Sangamo and Sensus water-meter register dials, where a single-tooth pinion advances the next-higher digit wheel each time the lower wheel completes 10 counts.
The Formula Behind the Single-tooth Small Driver, Lock by Rim
The core relationship is the index angle per count. At the low end of the typical operating range — say a 100-notch score reel — each input revolution advances the count wheel only 3.6°, which feels almost imperceptible to the eye and is what you want for a fine-resolution display. At the nominal range of 10 to 24 notches, common in unit-digit drums, the wheel ticks visibly between counts. Push to the high end of small-N — a 4-notch carry wheel on an odometer — and you're advancing 90° per count, which is a hard, audible snap and demands more careful tooth-flank geometry to absorb the impact. The sweet spot for most counters sits at 10 notches: clean digit display, manageable engagement shock, and 36° of stride that the human eye reads as a clear digit change.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θindex | Angle the count wheel advances per single input revolution of the driver | degrees (°) | degrees (°) |
| Nnotch | Number of notches around the count wheel rim | count (integer) | count (integer) |
| ωout | Average output angular velocity of the count wheel | rad/s | deg/s |
| Nin | Driver input speed | rev/s | RPM |
Worked Example: Single-tooth Small Driver, Lock by Rim in a coffee-roaster batch counter in a Vancouver micro-roastery
Specifying the single-tooth driver and rim-locked count wheel for a batch counter on a Probat P12 sample roaster at a small Vancouver coffee roastery. The roaster's drum-discharge lever rotates a stub shaft once per batch dump, and the operator wants the counter to log batches up to 999 per shift on a three-drum mechanical display. The units drum needs one count per lever cycle, and you need to know the index angle, the dwell ratio, and how the system behaves across the realistic batch-rate range of the roastery (slow morning sample roasts at one batch every 12 minutes, nominal production at one batch every 4 minutes, fast cupping sessions at one every 90 seconds).
Given
- Nnotch = 10 notches on units drum
- Engagement arc = 60 degrees of driver rotation
- Batch rate (low) = 5 batches/hour
- Batch rate (nominal) = 15 batches/hour
- Batch rate (high) = 40 batches/hour
Solution
Step 1 — compute the index angle per count for a 10-notch units drum:
That's the nominal stride. Each lever cycle rotates the digit drum exactly 36°, which is what carries the printed digit window from one number to the next.
Step 2 — compute the dwell ratio. The tooth is engaged for 60° of driver rotation; the rim lock holds for the remaining 300°:
That high dwell fraction is exactly why this mechanism beats a ratchet-and-pawl for display counters — the digit is positively held more than 5/6 of the time and the operator can read it without any wobble.
Step 3 — at the low end of the typical operating range, 5 batches/hour:
This is essentially static motion — the operator sees a clean digit flip every 12 minutes and the rim lock is doing all the work. Engagement shock is negligible.
Step 4 — at the nominal 15 batches/hour rate:
Still trivial average speed, but each individual index event happens at the lever's hand-driven speed — typically 60 to 120°/s during the engagement window. Tooth-flank stress is well within a 0.5-module brass tooth's capacity.
Step 5 — at the high end, 40 batches/hour during cupping:
Average rate is fine, but at this cadence the operator slams the lever quickly and the engagement-window speed climbs above 200°/s. The digit drum can overshoot and rebound off the rim land — you'll hear a double-click. If you see this in testing, add a light wave-spring detent on the drum hub to bleed off the rebound energy.
Result
The nominal index angle is 36° per count with an 83. 3% dwell ratio. In practice that means the digit window snaps cleanly from one number to the next in about 0.3 seconds of operator lever pull, then sits dead still for the next several minutes — exactly what a roaster wants when they're walking past the panel logging shift totals. Across the operating range, the low-end 5 batches/hour rate looks identical to nominal because everything is dominated by dwell, while the high-end 40 batches/hour rate starts exposing rebound noise that wasn't visible at lower speeds. If your built unit miscounts or sticks, the three most likely causes are: (1) the units drum bore has worn past 0.05 mm clearance on its post and the drum rocks during dwell, drifting the digit window; (2) the leading flank of the single driver tooth has rounded over from repeated impact and now engages 2°–3° late, producing intermittent skips under fast lever strokes; or (3) the concave locking land radius was machined 0.05 mm undersized relative to the driver rim, so the lock has measurable backlash and the digit creeps between counts.
Single-tooth Small Driver, Lock by Rim vs Alternatives
The single-tooth-with-rim-lock isn't the only way to add one count per input revolution. Geneva drives and ratchet-and-pawl mechanisms cover overlapping ground, and the right choice depends on speed, load, accuracy, and how much you want to spend on parts. Here's how they stack up on the dimensions that actually matter when you're picking one off the bench.
| Property | Single-tooth Driver, Rim Lock | Geneva Drive (4–8 slot) | Ratchet and Pawl |
|---|---|---|---|
| Typical operating speed | Up to ~300 RPM driver speed for digit counters | Up to ~600 RPM, higher with cycloidal acceleration profile | Up to ~150 RPM before pawl bounce becomes a problem |
| Indexing accuracy (positional) | ±0.1° at the count wheel with matched-radius rim lock | ±0.02° with hardened slotted plate and ground crank pin | ±0.5° typical, drifts with pawl wear |
| Dwell ratio | 75–90% (set by engagement arc) | 67% for 4-slot, 83% for 6-slot, fixed by geometry | Variable, depends on stroke geometry, no positive lock between strokes |
| Part count and cost | 2 parts, lowest cost, stampable in brass or steel | 3+ parts, slotted plate is hardest to make | 3–5 parts including spring, lowest mechanical cost but highest spring-fatigue risk |
| Lifespan in cycles | 10–50 million cycles in brass counter applications | 20–100 million cycles in hardened steel turret indexers | 1–10 million cycles, pawl spring usually fails first |
| Best application fit | Display counter digit drums, low-load indexing, carry-over between drums | High-speed turret indexing, automated assembly, packaging machinery | Backstops, hand tally counters, ratcheting tools where reverse-lock is the primary need |
| Mechanical complexity | Low — two flat discs | Medium — slotted plate and crank pin assembly | Low to medium — depends on pawl spring design |
Frequently Asked Questions About Single-tooth Small Driver, Lock by Rim
The tooth tip must reach the notch centreline at the same instant the driver-rim relief clears the count-wheel land. In practice you set this by indexing both parts on a fixture: rotate the driver until the tooth points at the notch centre, then verify the relief gap on the rim is at the trailing edge of the locking land, not still over it. A rule of thumb is 1°–2° of clearance between rim relief exit and tooth contact — any tighter and thermal expansion will cause crash; any looser and the count wheel kicks free during the handoff and you get position drift.
Almost always rebound. The single tooth pushes the count wheel forward, exits, and the wheel's own inertia carries it slightly past the notch centre before the rim lock catches it. If the rim-land radius is even 0.03 mm small relative to the driver, that overshoot finds clearance and the wheel slips into the next notch. Diagnostic check: rotate the driver by hand at very low speed — if double-counts disappear, it's an inertia/rebound problem and you need to either tighten the rim-land match or add a light drag washer on the count-wheel hub.
Depends on whether you're indexing a digit display or doing real work. For a counter drum carrying nothing but printed digits and turning at low speed, single-tooth-with-rim-lock wins on cost and part count — two parts versus three or four. For a turret carrying actual tooling or workpieces with measurable inertia, the Geneva is the right call because its slot-and-pin engagement transmits torque through a rolling contact rather than a sliding tooth flank, and the acceleration profile is gentler. Cutoff in our shop is roughly 200 g·cm² of indexed inertia: below that, single-tooth; above that, Geneva.
Two things drift score reels: the count-wheel bushing wears oval over a few million cycles, and the rim-land surface polishes down to a slightly smaller radius than the driver. Either one introduces clearance during dwell, and the reel gravity-drops a fraction of a degree until something stops it. If you're restoring a Gottlieb backbox, the fix is to replace the hub bushing first, then check rim contact with engineer's blue — you want a continuous contact band across the locking land, not a polished spot in the middle.
The limiting factor is the time the tooth is in contact with the notch — the engagement arc divided by the driver speed. Below about 5 ms of engagement time, the count-wheel inertia hasn't fully accelerated to the tooth's tangential velocity by the time the tooth exits, and you get partial indexing. For a typical 10-notch brass counter wheel that ceiling is around 300 driver RPM. Past that, you either widen the engagement arc (which cuts dwell) or move to a Geneva.
That 1° shortfall is almost always the count wheel not seating fully into the rim-lock position. Likely causes in order of probability: tooth flank geometry is off — the trailing flank is steeper than the notch flank and the tooth disengages before the count wheel has finished its stride; or the rim-land arc on the count wheel was cut on a slightly wrong centre and the wheel hangs up 1° early during entry to the lock. Check by marking the driver and count wheel with a Sharpie and stepping through one engagement cycle by hand — you'll see exactly where motion stops short.
References & Further Reading
- Wikipedia contributors. Geneva drive. Wikipedia
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