The jumping intermittent rotary, also called drop-and-pawl, is a counter mechanism that advances a toothed wheel one tooth per input cycle by lifting a spring-loaded pawl over a ramped tooth and letting it drop into the next gap. You see it inside Veeder-Root mechanical tally counters and the digit drums of a Jaeger taximeter. It converts a continuous or oscillating input into discrete unit-by-unit rotation so a count can be read directly off a numbered drum. Each drop is positive, audible, and repeatable to within a single tooth pitch.
How the Jumping Intermittent Rotary (drop-and-pawl) Actually Works
A drop-and-pawl indexer has three parts that matter — a ratchet wheel with asymmetric teeth, a pivoted pawl, and a return spring. The pawl rides up the shallow flank of the tooth as the input lever swings, then falls off the steep flank under spring tension and slams into the next gap. That fall is what gives the mechanism its name — the pawl literally jumps from tooth to tooth. The wheel locks in the drop position because the steep flank presents a near-vertical face the pawl cannot climb backwards over.
Why build it this way instead of using a Geneva or a cam-and-disk indexer? Because you need single-unit increments driven by an input that may not be perfectly timed — a coin drop, a key press, a wheel rotation. The pawl drop angle, typically 15° to 30° on the steep flank, sets how positively the wheel locks. Go shallower than 15° and the wheel can creep backwards under vibration. Go steeper than 30° and the pawl tip starts to chip after a few hundred thousand cycles, especially in stamped steel pawls running on a hardened ratchet wheel.
If the tooth pitch and pawl throw are mismatched the failure is immediate and obvious. Throw too short — pawl doesn't clear the tooth crest, wheel doesn't advance, you get a missed count. Throw too long — pawl skips two teeth and you get a double count. The carry pawl on a multi-digit totaliser must engage exactly when the units wheel passes 9-to-0, and a 0.2 mm misalignment of the carry cam is enough to skip the tens carry on a Veeder-Root style drum. We see this in restored gas pumps all the time — the gallon digit advances cleanly, the dime digit lags by one count.
Key Components
- Ratchet wheel: A toothed disk with asymmetric teeth — a shallow lifting flank around 20° and a steep locking flank around 75° to 85° from the radial. Tooth count sets the count resolution. A 10-tooth wheel gives one digit (0-9) per revolution, which is why almost every mechanical totaliser uses 10 teeth per drum.
- Drive pawl: The lever that pushes the wheel forward one tooth per input stroke. Pivot point sits roughly 1.5 to 2 tooth pitches away from the wheel centre. The pawl tip radius must be smaller than the tooth root radius — typical values are 0.3 mm tip on a 1.5 mm root — or the pawl bottoms out before fully engaging.
- Holding pawl (detent): A second spring-loaded pawl that drops into each tooth gap to prevent backlash and over-travel. Without it the wheel coasts past the intended position when driven fast. Spring force is typically 0.05 to 0.2 N — enough to hold position, light enough not to fight the drive pawl.
- Return spring: Torsion or leaf spring that pulls the drive pawl back to its rest position after each stroke. Spring force must overcome pawl friction plus any cam load — a rule of thumb is 3× the calculated friction force to keep return time under 50 ms in fast counters.
- Carry pawl (multi-digit only): Engages when the lower digit wheel rotates from 9 to 0, advancing the next-higher digit by one tooth. Must time within ±2° of the 9-to-0 transition or the carry skips. This is the single most common failure point in restored mechanical counters.
Industries That Rely on the Jumping Intermittent Rotary (drop-and-pawl)
Drop-and-pawl indexing shows up wherever you need a discrete, audible, mechanically positive count from an input that is irregular in timing or amplitude. The mechanism doesn't care if the input stroke is fast or slow, big or small, as long as it exceeds one tooth pitch of pawl throw — the wheel still advances exactly one tooth. That is why it dominates legacy counter, totaliser, and metering hardware that predates electronic counting.
- Public transport metering: The fare drum in a Jaeger taximeter advances one unit per distance pulse from the cable drive, with carry pawls cascading the digits up to the dollar wheel.
- Industrial counting: Veeder-Root Series 7000 mechanical tally counters use a drop-and-pawl on each digit wheel, rated for 10 million cycles before pawl-tip wear becomes measurable.
- Firearms: The round counter on a Browning M2 belt-fed test fixture uses a drop-and-pawl driven off the bolt cycle, one count per round fired.
- Textile machinery: Pick counters on Picanol and Sulzer weaving looms log every weft insertion via a cam-driven pawl onto a 10-tooth units wheel.
- Vintage fuel dispensers: Tokheim and Wayne gas pump gallon totalisers from the 1940s through 1970s use stacked drop-and-pawl drums with carry pawls between digits.
- Postage and ticket printers: Pitney Bowes mechanical postage meters used drop-and-pawl indexing on the value wheels to register each franking impression.
The Formula Behind the Jumping Intermittent Rotary (drop-and-pawl)
The core sizing question is how far the drive pawl must throw to guarantee one full tooth advance with margin against wear and slop. At the low end of the typical operating range — small counters with 20+ teeth on a 12 mm wheel — the pawl throw is only 1.5 to 2 mm and any pivot wear shows up fast. At the high end — 10-tooth digit drums on a totaliser — the pawl throw is 6 to 10 mm and you have plenty of margin, but the steep flank wears faster because the pawl drops harder. The sweet spot for most counters lands around a 10-tooth wheel of 15 to 25 mm pitch diameter, where pawl throw, drop energy, and tooth strength balance cleanly.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θthrow | Required angular throw of the drive pawl per input stroke | degrees | degrees |
| Z | Number of teeth on the ratchet wheel | teeth | teeth |
| kmargin | Throw margin factor — typically 1.10 to 1.25 to absorb pivot slop, spring sag, and tooth wear | dimensionless | dimensionless |
| spawl | Linear throw of the pawl tip = θ<sub>throw</sub> × R<sub>pitch</sub> × π / 180 | mm | in |
| Rpitch | Pitch radius of the ratchet wheel at the pawl tip contact | mm | in |
Worked Example: Jumping Intermittent Rotary (drop-and-pawl) in a vintage pinball machine score reel
You are rebuilding the 100-point score reel from a 1965 Gottlieb Sing Along pinball backbox. The reel uses a 10-tooth ratchet wheel of 22 mm pitch diameter, driven by a solenoid-actuated drive pawl. You need to verify the pawl throw is enough to guarantee single-tooth advance every time the score solenoid fires, with margin against the 50+ year-old pivot bushings that now have measurable slop.
Given
- Z = 10 teeth
- Dpitch = 22 mm
- kmargin = 1.15 (nominal) dimensionless
- Solenoid stroke = 8 mm
Solution
Step 1 — compute the nominal angular throw needed for one tooth at kmargin = 1.15:
Step 2 — convert that to linear pawl-tip travel at the pitch radius (Rpitch = 11 mm):
Step 3 — at the low end of the typical margin range, kmargin = 1.10 (a fresh build with tight bushings):
That 6.91 mm needs only 87% of the 8 mm solenoid stroke — comfortable, the reel snaps over cleanly with energy to spare. Step 4 — at the high end, kmargin = 1.25 (worn pivot, sagged spring, oxidised tooth flanks):
That exceeds the 8 mm solenoid stroke. In practice this means the reel intermittently fails to advance — you bump the score and nothing happens. The nominal 7.95 mm sits right at the edge of the 8 mm solenoid envelope, which tells you this reel was designed with almost no margin for a worn machine. That matches what restorers actually report on Gottlieb wedgeheads of this era.
Result
Nominal pawl-tip throw lands at 7. 95 mm against an 8 mm solenoid stroke — workable on a fresh machine, marginal on a 50-year-old one. At the low-end fresh-build case (6.91 mm) the reel cycles crisply with a sharp click. At the high-end worn case (8.64 mm) the solenoid runs out of stroke and you get intermittent skipped counts — the symptom every pinball restorer recognises. If your measured advance is short of one full tooth, the most likely causes are: (1) drive-pawl pivot bushing worn elliptical, eating 0.3 to 0.5 mm of effective throw, (2) return spring sagged below 0.15 N so the pawl doesn't fully retract before the next stroke, or (3) ratchet tooth crests rounded over from decades of use, lowering the effective lift profile and forcing the pawl to ride higher than the geometry predicts.
Choosing the Jumping Intermittent Rotary (drop-and-pawl): Pros and Cons
Drop-and-pawl is one of three classic ways to get unit-by-unit indexing — the others being Geneva drives and electronic stepper-driven counters. Each one wins on different axes. Pick based on input-timing tolerance, count rate, and whether the readout has to be mechanical.
| Property | Drop-and-pawl | Geneva drive | Electronic stepper counter |
|---|---|---|---|
| Max indexing rate | ~20 counts/s before pawl bounce sets in | 300+ indexes/min on a 4-slot | 1000+ steps/s easily |
| Indexing accuracy | ±1 tooth (no fractional positions) | ±0.05° with quality slot/pin fit | ±1 microstep, sub-arc-minute |
| Input-timing tolerance | Very forgiving — any stroke ≥ θ<sub>throw</sub> works | Strict — driver pin must engage at exact slot angle | Very strict — pulse train timing critical |
| Cost per indexer | $2 to $20 in stamped steel | $30 to $300 machined | $50 to $400 stepper + driver + controller |
| Reliability / lifespan | 5 to 20 million cycles with pawl-tip wear the limit | 10+ million cycles, slot wear the limit | 100+ million cycles, electronics outlast mechanics |
| Best application fit | Counters, totalisers, taximeters, pinball reels | Production indexing turrets, capping carousels | CNC, robotics, anywhere a digital readout exists |
| Mechanical complexity | Low — 3 to 5 parts per stage | Medium — driver disk, pin, slotted wheel | High — motor, driver, controller, encoder |
Frequently Asked Questions About Jumping Intermittent Rotary (drop-and-pawl)
You are seeing pawl bounce. When the drive pawl drops off the steep flank too hard, it rebounds off the tooth root and the rebound momentum carries the wheel one extra tooth past the holding-pawl detent. It happens almost exclusively when input stroke speed exceeds the design rate — a fast solenoid hit instead of a hand-cranked stroke.
Fix it by stiffening the holding pawl spring (try 1.5× the original force) or by adding a soft elastomer bumper to the drive-pawl rest position to damp the rebound. On Veeder-Root counters this shows up as occasional double-counts at high input rates and disappears below about 10 counts per second.
Pick 10 teeth if you want one digit (0-9) of readout per revolution and you can spare the diameter — you need roughly 15 to 25 mm pitch diameter to get reasonable tooth strength at 10 teeth. Pick 20 teeth if you need higher count resolution per revolution or you are space-constrained on diameter, but accept that pawl throw drops to 18° and any pivot slop becomes a much larger fraction of the throw budget.
The rule of thumb: tooth count above 24 makes the pawl throw so short that pivot wear becomes the dominant failure mode within a few hundred thousand cycles. Stay at or below 20 teeth for anything you want to last.
The carry pawl has to engage during a narrow angular window as the units wheel passes 9-to-0, typically 8° to 12° of wheel rotation. If the carry cam is misaligned by more than about ±2° relative to that window, the pawl arrives early or late and only catches when the timing happens to drift back into the window — hence the intermittent skip.
Check two things: first, that the carry cam set screw hasn't loosened on the units shaft (this is the most common cause on restored gas pumps), and second, that the carry pawl spring still has enough force to drop the pawl quickly into engagement. A sagged spring extends the engagement time and pushes you outside the window even if the cam is correctly positioned.
Strictly one-way as a single mechanism. The asymmetric tooth profile — shallow lifting flank, steep locking flank — is what makes the pawl drop positive in one direction and hard-locked in the other. Reverse direction and the pawl jams against the steep flank.
For bidirectional counting you need two opposed pawls each on its own ratchet wheel, with a clutching arrangement that engages whichever pawl matches the current direction. Most bidirectional mechanical counters from Hengstler and Kübler use exactly this two-pawl arrangement, and the clutching adds enough complexity that electronic counters won this market by the 1990s.
The tip radius must be smaller than the tooth root radius by at least 30%, otherwise the pawl tip bottoms out on the root fillet before fully seating and you lose effective throw. For a typical 1.0 mm-pitch ratchet with a 0.4 mm root radius, target a 0.25 mm tip radius. For coarser 2 mm-pitch teeth with 0.8 mm root radii, target 0.5 mm.
Going too sharp (below 0.15 mm) is its own problem — the tip work-hardens, then chips after 100,000 to 500,000 cycles. The sweet spot is a tip radius about 60-70% of the root radius, made from hardened tool steel running on a cyanide-hardened brass or steel wheel.
Solenoid actuation peaks at much higher pawl velocity than hand cycling — often 5 to 10× faster. At those speeds two things go wrong that don't show up in slow hand testing. First, the return spring may not have time to fully retract the pawl before the next solenoid pulse, leaving the pawl mid-stroke when the new stroke begins. Second, the holding-pawl detent doesn't fully drop into the tooth gap before the wheel coasts past it.
Diagnostic check: cycle the solenoid at half its operating voltage. If the count is now correct, you have a velocity-dependent timing problem and the cure is either a stiffer return spring or a small mechanical damper on the drive pawl to slow the return stroke into a controlled timing window.
References & Further Reading
- Wikipedia contributors. Ratchet (device). Wikipedia
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