A driving disk with rim and stops is an intermittent-motion device where a rotating disk carries a partial rim (or finger) that engages slots or pins on a driven wheel for part of each revolution, then a concentric stop arc on the same disk locks the driven wheel stationary for the rest of the cycle. The geometry forces the driven wheel to advance one fixed step per input revolution and dwell the rest of the time. We use it in mechanical counters, register wheels, and clock striking trains where each input rotation must produce exactly one indexed output step — billions of these run worldwide in odometers and gas meters.
Driving Disk with Rim and Stops Interactive Calculator
Vary the driving arc, driven-wheel pin count, and input speed to see dwell ratio, stop arc, step angle, and indexing speed.
Equation Used
The calculator uses the article dwell-ratio equation: the driven wheel is locked for the stop-arc portion of each input revolution. A smaller drive arc increases dwell time but forces the same one-step index into a shorter active interval, raising the average indexing speed.
- One driven-wheel index step occurs per input revolution.
- The stop arc fills the remainder of a 360 deg disk revolution.
- Average driven speed assumes uniform motion during the active driving arc.
Operating Principle of the Driving Disk with Rim and Stops
The disk has two functional zones machined onto a single rotating part. One zone is a driving feature — typically a single tooth, finger, or short rim segment — that engages the driven wheel only across a small arc of the input rotation, often 30° to 60°. The other zone is a stop arc, a concentric circular rim with the same radius as the gap between the driven wheel's stop pins, which sits inside that gap and physically blocks the driven wheel from rotating during the dwell phase. As the disk rotates, the driving feature meshes, kicks the driven wheel through one tooth-pitch of advance, and then the stop arc immediately slides into the next pin-gap to lock the driven wheel until the cycle repeats.
Why designed this way? Because intermittent motion needs two things — positive advance during the active phase and positive lock during the dwell. A plain gear gives you continuous motion. A Geneva gives you the same idea with a slot-and-pin geometry. The rim-and-stops disk is the cheap, robust cousin: a single stamping or moulding does both jobs. The dwell-to-advance ratio is set purely by the angular extent of the rim cutaway. If you cut 30° of driving rim and leave 330° of stop arc, the driven wheel dwells for 11/12 of every input revolution.
What goes wrong if tolerances slip? The stop arc radius must match the driven wheel's pin-gap radius to within roughly 0.05 mm on a small mechanism. Too tight and the disk binds during dwell — you'll feel ratcheting and hear chatter. Too loose and the driven wheel can backdrive under load, causing miscounts. The transition from rim-cutaway to stop arc must be timed so the driving feature fully disengages before the stop arc engages — otherwise both zones grab simultaneously and the mechanism jams. Lantern-pinion-style driven wheels, common in older counter designs, are particularly sensitive to this because the pins are small and bend rather than shear when overloaded.
Key Components
- Driving Disk: The main rotating input, typically 15-60 mm diameter in counter mechanisms, carrying both the driving feature and the concentric stop arc machined or stamped on a single body. Concentricity between the two features must hold to within 0.05 mm or the driven wheel will alternately bind and lash.
- Driving Finger or Rim Segment: The active feature that engages the driven wheel — usually a single tooth, pin, or short arc subtending 30° to 60° of disk rotation. Its leading edge sets the start of advance and its trailing edge must clear the driven wheel before the stop arc engages.
- Stop Arc (Lock Rim): A concentric circular surface on the disk that fills the gap between two adjacent pins on the driven wheel during the dwell phase. The arc's radius equals the driven-wheel pin-circle radius minus the pin radius, with a working clearance of typically 0.02-0.10 mm.
- Driven Wheel: Carries the indexed output — often a lantern pinion with 6, 8, or 10 pins, or a slotted disk on Geneva-style variants. Each input revolution advances it by one pin-pitch (360°/N where N is the pin count).
- Stop Pins or Slots: The features on the driven wheel that the stop arc bears against. Pin diameter, surface finish, and hardness determine wear life — Ra below 0.4 µm and case-hardened steel are typical for high-cycle counter applications.
Where the Driving Disk with Rim and Stops Is Used
You find this mechanism wherever a single input rotation must produce exactly one indexed output step, and where cost and part count matter more than ultra-fast indexing. The cutaway-rim disk is the workhorse of mechanical counting and register wheels — it shows up in places you'd never inspect because it just keeps working for decades.
- Utility metering: Sensus and Itron residential gas meters use stacked driving disks with rim cutaways to advance each digit wheel after the previous one completes a full revolution — the classic carry mechanism in cubic-foot register heads.
- Automotive: Pre-2000 Stewart-Warner and VDO mechanical odometers use a driving disk with a single tooth and stop arc to advance the tenths-of-a-mile wheel exactly once per worm-shaft revolution.
- Horology: The strike train of a 19th-century Junghans or Black Forest wall clock uses a driving disk with rim cutaway to release the count wheel one step per hour, controlling how many times the hammer strikes.
- Industrial counting: Veeder-Root mechanical predetermining counters on packaging lines advance the units wheel one step per input pulse using a stamped driving disk with integral stop rim.
- Textile machinery: Pick counters on Picanol and older Saurer weaving looms use a rim-and-stops disk to log each weft insertion — driven directly off the loom's main shaft.
- Mechanical calculators: Felt-Tarrant Comptometer and Odhner pinwheel calculator carry mechanisms use a driving disk with rim cutaway to propagate tens-carry from one column wheel to the next.
The Formula Behind the Driving Disk with Rim and Stops
The key design number is the dwell ratio — the fraction of each input revolution during which the driven wheel is locked stationary. At the low end of the practical range (around 50% dwell) you get fast indexing but minimal hold time, which works for light register wheels but fails under any external torque. At the high end (95%+ dwell) you get rock-solid lock but the driving phase becomes a violent kick because all the indexing motion compresses into a tiny arc. The sweet spot for most mechanical counters sits at 80-92% dwell — long enough to resist backdriving, short enough that peak driving torque stays manageable.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Rdwell | Dwell ratio — fraction of input revolution during which the driven wheel is locked | dimensionless | dimensionless |
| θdrive | Angular extent of the driving rim cutaway on the disk | degrees | degrees |
| ωdriven | Average angular velocity of the driven wheel during the active phase | rad/s | deg/s |
| N | Number of pins or slots on the driven wheel (steps per input revolution = 1) | count | count |
Worked Example: Driving Disk with Rim and Stops in a vintage parking-meter coin counter restoration
You are restoring a 1962 Duncan Miller parking meter and need to verify the driving disk and stop-arc geometry on the nickel-counter wheel. The driven wheel has 10 pins on a 12 mm pitch circle, and you measure the existing driving rim cutaway at 36°. The input shaft turns once per coin drop. You need the dwell ratio at the spec design and at the worn-rim and over-cut extremes you might see across a batch of restored mechanisms.
Given
- θdrive,nom = 36 degrees
- N = 10 pins
- Pitch circle diameter = 12 mm
- Input speed = 1 rev per coin drop —
Solution
Step 1 — at the nominal 36° driving cutaway, calculate dwell ratio:
That means the nickel wheel is locked for 90% of every input revolution and advances during just 10%. For a coin counter this is the sweet spot — the wheel resists accidental backdriving from vibration but the driving kick is gentle enough that the lantern pins don't bend over time.
Step 2 — at the low end of the worn-disk range, suppose 60 years of wear has rounded the rim and effectively widened the cutaway to 60°:
83% dwell still functions, but you'll notice the wheel becomes susceptible to over-travel — a hard coin drop can advance the wheel 1.5 steps instead of 1, causing miscounts. This is the classic symptom of a worn Duncan disk you see on parts-bin units.
Step 3 — at the high end, suppose someone has reground a replacement disk too aggressively and the cutaway is only 18°:
95% dwell sounds great, but the active phase now has to deliver a full 36° step in only 18° of input rotation — the driven wheel must accelerate at 2× the nominal angular velocity, which doubles the impact load on the lantern pins. On a 0.8 mm pin diameter you'll see bending within a few hundred cycles.
Result
The nominal dwell ratio is 0. 90 (90%), meaning the nickel wheel sits locked for 90% of each input revolution and indexes during the remaining 10%. At 0.83 the wheel is loose enough to over-travel under shock loading; at 0.95 the indexing kick becomes violent enough to bend the lantern pins. If you measure miscounts on a restored mechanism, the most common causes — distinct from over-travel and pin bending — are: (1) stop-arc radius machined 0.10 mm undersize, letting the wheel rattle in dwell and creep under spring tension, (2) burrs on the cutaway transition causing the stop arc to engage before the driving finger fully disengages, which jams the mechanism intermittently, or (3) a bent driven-wheel arbour throwing pin-pitch off by more than 0.2 mm so the stop arc binds at one position and slips at another.
Choosing the Driving Disk with Rim and Stops: Pros and Cons
The rim-and-stops disk is one of three classic intermittent-motion families. Each one solves the same advance-then-dwell problem with different geometry, different cost, and different failure modes. Pick based on indexing speed, accuracy, and how much torque you need to hold during dwell.
| Property | Driving Disk with Rim and Stops | Geneva Drive | Ratchet and Pawl |
|---|---|---|---|
| Indexing speed (max practical RPM) | 20-200 RPM input | 60-400 RPM input | 30-300 RPM input |
| Dwell-to-advance ratio range | 50-95% (set by rim cutaway angle) | Fixed by slot count: 67% (4-slot), 75% (6-slot) | 0-100% (independent of input) |
| Indexing accuracy | ±0.5° typical, depends on pin-arc clearance | ±0.1° with hardened pin and slot | ±1-3° due to pawl tooth pitch |
| Part cost (single-stage indexer) | Lowest — single stamped disk | Medium — precision-ground slot wheel | Low — but more parts (pawl, spring, pivot) |
| Holding torque during dwell | High — full stop-arc contact area | Very high — pin in slot bottom | Medium — depends on pawl spring force |
| Typical lifespan (cycles) | 10⁶-10⁷ in counters and odometers | 10⁷-10⁸ with proper lubrication | 10⁵-10⁶ before pawl tooth wear |
| Best application fit | Counters, registers, clock strike trains | High-speed indexing turrets, film advance | Hoists, escapements, allow-reverse stops |
Frequently Asked Questions About Driving Disk with Rim and Stops
You're seeing momentum overshoot. When the driving finger releases, the driven wheel still has angular velocity and wants to keep moving. The stop arc is supposed to catch the next pin immediately, but if the stop-arc leading edge is chamfered too aggressively or the radial clearance exceeds about 0.10 mm, the wheel coasts past the lock position before the arc engages.
Fix it by checking the stop-arc-to-pin radial clearance with feeler gauges. On a 12 mm pitch circle you want 0.02-0.05 mm. If the clearance is correct and you still see double-stepping, the driven wheel inertia is too high for the dwell geometry — add a light drag spring on the driven arbour to bleed off the residual energy.
It comes down to torque margin during dwell versus pin stress during advance. Wider arc (lower dwell) means the driving phase spreads the indexing motion over more input rotation, so peak angular acceleration of the driven wheel drops — easier on the pins. But you give up dwell time, which is what resists backdriving.
Rule of thumb: if the driven wheel sees external torque during dwell (a clock strike train, a coin counter with spring-loaded display), bias toward 85-92% dwell. If the load is purely inertial and the driven wheel coasts free during dwell (a totalizer with no return spring), you can drop to 60-70% dwell and get a much gentler driving kick.
Probably not. The rim-and-stops disk works best for low-mass driven wheels — counter wheels weighing grams, not kilograms. A 2 kg payload at 4 stations means 0.5 kg·m² range of inertia, and the abrupt engagement of a single driving finger will hammer that mass into the stop arc on every cycle.
Geneva geometry distributes the acceleration over a 90° input arc with a smooth velocity profile that starts and ends at zero. That matters enormously for payload indexing. Use the rim-and-stops disk for register wheels and counters where the driven inertia is negligible compared to the input torque.
Check the timing transition between the driving finger releasing and the stop arc engaging. If both features are active simultaneously for even 1-2° of input rotation, the disk tries to drive the wheel forward while the stop arc holds it back, and the whole stack locks up.
Put dye on the disk, run it slowly by hand, and inspect the wear pattern on both the finger trailing edge and the stop-arc leading edge. You should see contact on one OR the other at any given input angle, never both. If you see overlap, file the stop-arc leading edge back by 1-2° and re-test.
Because dwell ratio is purely a function of input geometry — how much of the disk's rotation has the cutaway exposed versus the stop arc filled. The pin count on the driven wheel sets the step size (360°/N) but doesn't change what fraction of the input revolution is dwell.
That said, pin count does affect peak angular velocity of the driven wheel during the active phase. With more pins, each step is smaller, so the wheel accelerates and decelerates faster within the same driving arc. A 20-pin wheel at 36° drive arc spins twice as fast during advance as a 10-pin wheel — relevant for impact stress on the pins, not for the dwell ratio itself.
The stop-arc leading edge wears first, almost without exception. Every cycle the driven wheel slams its pin into that edge as the wheel decelerates from indexing motion to dwell. Over 10⁶+ cycles the edge rounds off, which effectively widens the cutaway and reduces dwell ratio.
The diagnostic symptom is gradual onset of miscounts under vibration — a meter or counter that worked fine for 40 years and started skipping in the last few. Inspect the stop-arc leading edge under magnification; if you see a radius greater than 0.2 mm where the original was sharp, the disk is at end of life.
References & Further Reading
- Wikipedia contributors. Intermittent mechanism. Wikipedia
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