A mutilated wheel-pinion intermittent is a gear pair where teeth have been deliberately removed from one or both members so the driven wheel only advances during the toothed engagement and dwells when the smooth arcs face each other. It solves the problem of producing timed, repeatable indexing from a continuously rotating shaft without a separate clutch or stop pin. The locking arcs hold the driven wheel stationary between engagements, giving a precise dwell-to-index ratio. Counters, decade carry drums, and slow-speed indexers rely on this geometry — a Veeder-Root totaliser carry uses exactly this trick.
Inside the Mutilated Wheel-pinion Intermittent
The idea is simple — start with a normal gear mesh, then cut away the teeth you don't want engaging. The driving member keeps spinning, but its pinion only carries teeth across part of its circumference. The rest of the rim is a smooth locking arc, machined to the wheel's root circle so it slides past the driven gear's tooth tips without rotating it. When the toothed sector comes around, normal involute action takes over, the driven wheel advances by however many teeth pass through engagement, then the locking arc returns and the wheel sits still until the next cycle.
Design this mechanism wrong and the failures are loud and obvious. If the locking arc radius is even 0.1 mm oversize, the arc binds against the driven tooth tips and the input shaft stalls. Undersize the arc by the same amount and the driven wheel free-wheels during dwell — you'll see the digit drum drift past its index position when a finger flicks it. The transition tooth, the first tooth that re-engages after the dwell, takes the entire impact load of the start-up. That tooth is where mutilated pinions fail. On a worn Veeder-Root unit, you'll see the leading flank of the carry tooth peened over and the digit jumping two positions instead of one.
The dwell ratio is set by how much rim you remove. Strip 270° of teeth off a pinion and you get 75% dwell, 25% index. Leave a single tooth on a 360° pinion and you get a single-tooth pinion intermittent — the classic carry gear that drives the next-higher digit on a mechanical counter once per revolution of the units wheel.
Key Components
- Driving pinion (mutilated): Continuously rotating input member with one or more teeth machined off, replaced by a smooth locking arc. The remaining toothed sector typically spans 30° to 120° of arc depending on the desired index distance. Tooth count on the active sector follows standard involute geometry — module and pressure angle must match the driven wheel exactly.
- Driven wheel: The intermittently advanced output. Carries full teeth all the way around since it must mesh whenever the pinion's toothed sector comes into position. Tooth count sets the index resolution — a 10-tooth driven wheel paired with a single-tooth pinion gives you a decade counter that advances 36° per input revolution.
- Locking arc: The smooth section of the pinion rim that holds the driven wheel during dwell. Its radius must equal the driven wheel's tip circle minus a running clearance of 0.05 to 0.10 mm — too tight and you get binding, too loose and the driven wheel drifts. Surface finish matters here, Ra below 0.8 µm to avoid scoring the tooth tips during dwell.
- Locking concavity on the driven wheel: On precision units, the driven wheel teeth get a matching concave profile cut into their tip lands so the locking arc nests against two adjacent teeth at once. This kills the rotational slop that a pure radial-arc design suffers from. Found in better-grade decade counters and odometer drums.
- Transition tooth: The first and last teeth of the toothed sector. These take impact loading at engagement and disengagement and are usually case-hardened or made slightly thicker than the rest. When this tooth fails, the symptom is a counter that occasionally skips or double-indexes.
Real-World Applications of the Mutilated Wheel-pinion Intermittent
You'll find mutilated wheel-pinion intermittents wherever continuous rotation needs to be chopped into discrete steps without the cost or bulk of a Geneva drive or pawl-and-ratchet. They live inside cheap counters, totalisers, and slow-speed indexers — anywhere the dwell time exceeds the index time and the loads are modest. The mechanism doesn't handle reverse direction and it doesn't handle high indexing torque, but for unidirectional metering at modest speed it's hard to beat for parts count.
- Mechanical counters: Veeder-Root Series 1500 totaliser carry mechanism — single-tooth pinion drives the next-decade wheel once per units-wheel revolution
- Vintage cash registers: National Cash Register Class 5 carry drums use mutilated pinion intermittents between digit shafts to propagate the carry
- Industrial production counting: Hengstler stroke counters on stamping presses, where the operator-visible digit drum advances one position per ram cycle
- Gas pump totalisers: Tokheim and Wayne pre-electronic gallon registers, where the tenths-of-a-cent wheel mutilates a carry pinion to step the cents wheel
- Watchmaking: Date-change mechanism on traditional Swiss calendar movements, where a single-tooth wheel on the hour pipe steps the 31-tooth date ring once per 24 hours
- Slow-speed indexing fixtures: Bench-top engraving rotaries where a hand-cranked pinion with two active teeth indexes a 36-position dial
The Formula Behind the Mutilated Wheel-pinion Intermittent
The core relationship you need is the dwell ratio — the fraction of one input revolution during which the driven wheel sits still. Get this wrong at the low end of the typical operating range, say 50% dwell, and the mechanism behaves more like a slow continuous gear than an indexer, with very little time for the downstream operation to complete. At the high end, push past 95% dwell and the toothed sector becomes so narrow that the transition tooth carries punishing impact loads and chips inside a few thousand cycles. The sweet spot for most counter and totaliser work sits between 80% and 92% dwell, which gives the downstream wheel time to settle but leaves enough engagement arc to transmit the indexing torque without hammering the lead tooth.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Dr | Dwell ratio — fraction of input revolution during which driven wheel is stationary | dimensionless | dimensionless |
| θt | Angular span of the toothed sector on the mutilated pinion | degrees | degrees |
| zactive | Number of teeth remaining on the active sector of the pinion | teeth | teeth |
| zw | Total tooth count on the driven wheel | teeth | teeth |
| Nidx | Angular index step of the driven wheel per pinion revolution | degrees | degrees |
Worked Example: Mutilated Wheel-pinion Intermittent in a postage-meter impression counter
You are designing the impression counter for a refurbished Pitney Bowes Model 5300 postage meter where the units digit drum must advance exactly 36° per print cycle and remain locked during the 0.6 second print stroke. The drum has 10 teeth and you plan a single-tooth mutilated pinion turning at 60 RPM driven by the print-cam shaft.
Given
- zw = 10 teeth
- zactive = 1 tooth
- θt = 36 degrees
- np = 60 RPM
Solution
Step 1 — compute the index step at nominal geometry. With a single active tooth on the pinion and 10 teeth on the driven wheel, each pinion revolution advances the wheel by one tooth pitch:
Step 2 — compute the nominal dwell ratio with a 36° toothed sector:
That is 90% dwell, 10% index. At 60 RPM the input takes 1.0 s per revolution, so the drum dwells for 0.90 s and indexes in 0.10 s. The 0.6 s print stroke fits inside the dwell with 0.30 s of margin — good engineering headroom.
Step 3 — check the low end of the typical operating range. If the operator hand-cranks the meter for service at roughly 20 RPM, the cycle stretches to 3.0 s with a 2.7 s dwell and a 0.3 s index. The drum still locks cleanly, the print stroke fits with room to spare, and you can watch the digit step over visibly. This is the regime where the mechanism is most forgiving.
Step 4 — check the high end. If a service tech jumpers the cam motor and runs the meter at 180 RPM for a print-rate test, cycle time drops to 0.333 s. Dwell shrinks to 0.30 s and the index window collapses to 33 ms. The print stroke no longer fits — the drum is still moving when the platen lands, and you'll see smeared digits on the tape.
Result
Nominal dwell time is 0. 90 s with a 36° index step at 60 RPM input. That feels right on a desk-top meter — you hear a crisp tick as the drum advances, the digit lands square in its window, and the platen has settled time before it strikes. At 20 RPM hand-crank speed the dwell stretches to 2.7 s and the mechanism is bulletproof; at 180 RPM the dwell collapses to 0.30 s and the print smears because the drum hasn't fully locked when the platen hits. If you measure index steps that come out at 33° or 39° instead of 36°, the usual culprits are: (1) a worn locking arc letting the drum drift during dwell — check radial play with a dial indicator, anything above 0.05 mm is suspect; (2) a transition tooth that has peened or chipped and is grabbing the wheel late; or (3) backlash in the cam-shaft coupling causing the pinion to lag its commanded position by a few degrees.
When to Use a Mutilated Wheel-pinion Intermittent and When Not To
The mutilated wheel-pinion intermittent competes with the Geneva drive, the pawl-and-ratchet, and the cam-and-roller indexer for slow-speed indexing duty. The right choice depends on indexing torque, accuracy demand, cost target, and whether the mechanism needs to handle reverse motion. Here's how the mutilated pinion stacks up on the dimensions that actually drive the decision.
| Property | Mutilated wheel-pinion | Geneva drive | Pawl-and-ratchet |
|---|---|---|---|
| Practical input speed | Up to ~300 RPM before transition-tooth impact damages the lead tooth | Up to ~1000 RPM in 4-slot configurations, higher with proper roller pin | Limited to ~100 RPM by pawl bounce and inertia |
| Index accuracy at rest | ±0.5° typical, ±0.1° with concave locking flats | ±0.05° with hardened roller pin and ground slot | ±1° to ±3°, depends heavily on pawl spring and ratchet wear |
| Relative cost | Low — one modified gear plus a stock gear | Medium — driver, slotted disc, roller pin, and accurate centre distance | Lowest — stamped pawl and ratchet from sheet stock |
| Lifespan at typical duty | 1–10 million cycles depending on transition-tooth hardening | 10–50 million cycles with hardened pin and ground slot | 100k–1 million cycles, pawl tip wear dominates |
| Indexing torque capacity | Modest — limited by single-tooth engagement | High — full involute mesh during index | Modest — limited by pawl tip contact area |
| Reverse direction capability | No — locking arc prevents reverse | Yes — fully bidirectional | No — pawl is directional by design |
| Best application fit | Counters, totalisers, decade carry drums | Tool-changer turrets, packaging indexers | Hand-cranked dispensers, ratchet wrenches |
Frequently Asked Questions About Mutilated Wheel-pinion Intermittent
This is almost always inertia overrun on the driven wheel. When the toothed sector engages under sudden acceleration, the driven wheel picks up rotational kinetic energy, and if the locking arc has even a few thousandths of clearance the wheel coasts past its index position before the arc catches it. You'll see a digit jump from 4 straight to 6.
The fix is either to reduce the driven wheel's moment of inertia — drill lightening holes in the digit drum web — or to add a friction detent that brakes the wheel as it approaches the index position. On Veeder-Root units the factory cure was a leaf-spring drag against the drum face, set to 0.2 to 0.3 N of preload.
Look at three numbers — required dwell ratio, indexing torque, and cycle rate. A 4-slot Geneva is locked into a 75% dwell ratio by its geometry. If you need 90% dwell, the mutilated pinion wins by default because you can simply remove more rim. If you need under 75% dwell, the Geneva wins.
For indexing torque above roughly 2 N·m, switch to the Geneva — the mutilated pinion's single-tooth engagement during the transition is the weak link and you'll fatigue the lead tooth. Cycle rate above 200 RPM also pushes you toward Geneva because the mutilated pinion's transition tooth takes a hammer blow at every engagement.
The calculation assumes the driven wheel begins moving the instant the first pinion tooth contacts it and stops the instant the last tooth disengages. In reality you have backlash on entry and a small overrun on exit, and both eat into dwell.
Measure the backlash with a dial indicator on the driven wheel face while rocking the input shaft. If you find more than 0.5° of free motion, the toothed sector is engaging slightly late on each cycle. The other common cause is a chamfer on the leading edge of the transition tooth that's been ground too aggressively — the tooth doesn't bite until it's a few degrees into the engagement arc.
Only if you add a positive lock. The locking arc holds the driven wheel against accidental rotation but it's a friction-and-form fit, not a hard stop. A reaction torque of more than a few tenths of a N·m will skid the wheel against the arc and you'll lose position.
For real holding torque, pair the mutilated pinion with a separate detent — a spring-loaded ball into a notched face on the driven wheel works well, or a sprung lever into the wheel teeth. Plan for the detent to handle the entire reaction load and treat the locking arc as a positioning aid only.
Two causes show up most often. First, thermal expansion — if the assembly heats up during run, the pinion and wheel grow at different rates depending on materials. A steel pinion and brass driven wheel, common in counters, will lose 0.04 mm of clearance per 50°C rise. Spec the cold clearance accordingly.
Second, the locking arc isn't actually circular. If it was turned in a lathe with the pinion blank centred on its bore, fine. If it was hobbed and then the locking arc was cut as a separate operation, runout between the two features can easily exceed 0.05 mm and you'll get scoring on the high side of the rotation. Check arc concentricity to the bore on a comparator before blaming the clearance spec.
For a brass or mild-steel pinion driving a brass digit wheel of roughly 10 g·cm² inertia, expect transition-tooth fatigue cracks somewhere between 200 and 400 RPM continuous duty. The failure mode is a hairline crack at the tooth root on the leading flank, propagating until the tooth shears off entirely.
The cure is either case-hardened steel for the pinion (Rockwell C58 or better on the tooth flanks) or shaping the lead tooth with a slightly relieved profile so engagement starts gradually rather than all-at-once. Watchmakers solve this with a chamfered impulse face on the single-tooth carry pinion — same idea, smaller scale.
References & Further Reading
- Wikipedia contributors. Intermittent mechanism. Wikipedia
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