Intermittent motion is the class of mechanisms that converts continuous input rotation into a repeating step-and-hold output, where the driven member advances briefly then stays locked stationary for a defined dwell. Common implementations like a 4-slot Geneva drive run input shafts at 30-300 RPM while the output dwells for 75% of every cycle. Designers use it to index workpieces under tools that need a stationary target — heat-sealers, cap-tighteners, film advance pawls. You see it inside the Ferguson cam indexers feeding pharmaceutical blister lines and inside every mechanical film projector built before 1980.
Intermittent Motion Interactive Calculator
Vary Geneva slot count and drive pins to see step angle, index time, dwell ratio, and locking hold angle.
Equation Used
This calculator uses the Geneva drive phase relationship shown in the worked example: a star with N slots advances 360/N degrees per engagement. With P equally spaced drive pins, the indexing share of one driver revolution is P/N and the remaining angle is dwell, where the locking arc holds the output stationary.
- External Geneva drive with evenly spaced slots.
- Drive pins are equally spaced and do not overlap engagement.
- One slot advances the output by one index step.
- Clearance, inertia, and acceleration torque are not included.
How the Intermittent Motion Actually Works
Intermittent motion works by interrupting power flow between a continuously turning input and a driven output, so the output only moves during a small fraction of the input cycle. In a Geneva drive, a driver wheel carries a single pin that engages a slot in the star wheel, swings it through a fixed angle (90° for 4 slots, 60° for 6 slots), then disengages — and a circular locking arc on the driver keeps the star wheel rigidly stationary until the next pin entry. In a ratchet and pawl, the pawl drops into a tooth on each input stroke and the back-stop tooth profile prevents reverse rotation between strokes. In a cam-driven indexer like the Ferguson parallel-shaft series, a barrel cam rolls a follower turret through a programmed rise-dwell-rise profile.
The geometry has to be exact or the dwell breaks. On a Geneva, the pin centre must enter the slot tangent to the slot centreline — if the centre distance between driver and star is off by more than about 0.1 mm on a 50 mm Geneva, you get a step-in jolt that hammers the slot mouth and chips the corner within a few thousand cycles. The locking arc radius and the star wheel locking flat must clear by 0.05-0.10 mm of running fit, no more. Too tight and the mechanism binds when it warms up; too loose and the indexed position drifts under inertia.
The stop-start motion failure modes you actually see in the field are slot-mouth wear from misaligned pins, pawl bounce on ratchets running too fast (above roughly 200 cycles/minute the pawl skips teeth), and follower lift on cam indexers where the preload spring isn't sized to the inertia of the load being indexed. Intermittent motion (form) covers all of these — they are different physical implementations of the same underlying step-and-hold cycle.
Key Components
- Driver (input member): The continuously rotating element that delivers the motion pulse — a Geneva crank with drive pin, a ratchet rocker arm, or a barrel cam shaft. Typically runs 30-300 RPM in industrial indexers. Concentricity to the bearing axis must hold within 0.02 mm or you get cyclic position error every revolution.
- Driven member (indexed output): The element that actually steps and holds — Geneva star wheel, ratchet wheel, or cam follower turret. Carries the workload (parts, fixtures, film). Mass moment of inertia about its axis sets the peak torque demand during acceleration; oversize this and the input shaft sees torque spikes 3-5× the nominal running torque.
- Locking element: The geometry that holds the output stationary during dwell — a circular locking arc on a Geneva, the back face of the pawl tooth on a ratchet, or the dwell arc machined into a barrel cam. Must overlap engagement by at least 5° of input rotation to absorb shock without unlatching.
- Engagement feature: The pin, tooth, or follower that actually transmits motion during the active step. Hardened to 58-62 HRC on metal-on-metal designs. Edge break of 0.2 mm chamfer reduces stress concentration at entry.
- Anti-backlash preload: Spring, gravity, or geometric preload that keeps the engagement feature seated. On ratchets a 2-5 N pawl spring is typical for sub-100 mm wheels; on cam indexers a 200-800 N follower preload is common. Underpreloaded systems lose position under vibration.
Who Uses the Intermittent Motion
Intermittent motion shows up wherever a tool has to act on a stationary workpiece while a continuous prime mover keeps running. It is the cheapest way to convert smooth rotation into precise, repeatable indexing without servo control, and the dwell built into the geometry gives free time for secondary operations. The same step-and-hold cycle drives everything from antique watch escapements to modern rotary indexing tables on automotive assembly lines.
- Packaging: Geneva indexers in carousel cap-tighteners on Krones and Arol filling lines — the bottle dwells under the capping head while the chuck spins down the closure.
- Film and projection: The Maltese cross (4-slot Geneva) in 35 mm film projectors like the Bell & Howell Filmosound, advancing one frame per 24th of a second with 75% dwell for the shutter to clear.
- Pharmaceutical: Ferguson barrel-cam indexers feeding blister-pack turrets on Marchesini and Uhlmann lines, holding each blister stationary for heat-seal dwell of 200-400 ms.
- Watchmaking: Lever escapements in mechanical watches — a stop-start motion that releases the gear train one tooth per oscillation of the balance wheel, typically 18,000-28,800 beats per hour.
- Textile and sewing: Ratchet-and-pawl feed-dog drives in industrial Juki and Brother lockstitch machines, advancing fabric one stitch length per needle stroke.
- Automotive assembly: Camco rotary indexing tables stepping fixtures through weld, drill, and inspection stations on Tier 1 supplier lines for Magna and Linamar.
The Formula Behind the Intermittent Motion
The fundamental design number for any intermittent motion system is the dwell ratio — the fraction of each cycle that the output sits stationary. This is what tells you whether the secondary process (heat-seal, weld, fill, capture) actually fits inside the available stop time. At the low end of typical operation (slow shafts, long dwell ratios above 80%) you get generous process windows but low throughput. At the high end (fast cycling, dwell ratios below 60%) throughput climbs but acceleration spikes and the mechanism wears fast. The sweet spot for most production indexers sits around 65-75% dwell, which is exactly where a 4-slot Geneva naturally lands.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| DR | Dwell ratio — fraction of each cycle the output is stationary | dimensionless | dimensionless |
| tdwell | Time the output is stationary per cycle | s | s |
| tcycle | Total time of one input revolution | s | s |
| θindex | Input angle during which the output is moving | degrees | degrees |
| Nin | Input shaft speed | RPM | RPM |
Worked Example: Intermittent Motion in a coffee-pod sealing carousel
You are sizing a 6-slot Geneva indexer for a single-serve coffee-pod sealing carousel running on a Keurig-style filling line. Each station has to hold the pod stationary long enough for the foil heat-seal head to descend, dwell at 180°C, and retract — total seal cycle 350 ms. The carousel must hit a target throughput of 80 pods per minute, and you want to know whether the geometry actually gives you the dwell window the seal head needs.
Given
- Slots in star wheel = 6 —
- Target throughput = 80 pods/min
- Required seal dwell = 350 ms
- θindex for 6-slot Geneva = 120 degrees
Solution
Step 1 — calculate the input shaft speed needed to hit 80 pods per minute. A 6-slot Geneva advances one slot per input revolution, so input RPM equals pods per minute:
Step 2 — compute the dwell ratio for a 6-slot Geneva. The pin engages the slot for 120° of input rotation, so:
Step 3 — compute the actual dwell time at nominal 80 RPM:
That gives you 500 ms of stationary time per station — comfortable margin over the 350 ms seal cycle. Now check the operating-range edges. At the low end, 50 RPM (slow start-up or a derated line):
That is plenty of seal time but throughput drops to 50 pods/min — fine for commissioning, not for production. At the high end, push to 120 RPM to chase a 120 pod/min target:
That is below the 350 ms seal requirement. The seal head won't complete its cycle before the carousel starts moving — you'd see partial seals on every pod. The hard ceiling for this geometry sits at roughly 114 RPM, which is where tdwell equals exactly 350 ms.
Result
At nominal 80 RPM the 6-slot Geneva delivers 500 ms of dwell per station — a comfortable 43% margin over the 350 ms seal requirement. The range tells the design story: at 50 RPM you have 800 ms of dwell but only 50 pods/min throughput, while at 120 RPM dwell collapses to 333 ms and seals fail. The sweet spot sits at 80-100 RPM where you keep at least 100 ms of margin. If you measure short dwells in the field — say 420 ms instead of the predicted 500 ms — the most common causes are: (1) the input drive belt slipping under acceleration torque, dropping effective shaft speed only during the indexing phase; (2) the locking arc clearance opening up beyond 0.10 mm from wear on the driver hub, letting the star wheel creep early into the next index; or (3) a worn drive pin with measurable side flat that starts engaging the slot 3-5° before tangency.
When to Use a Intermittent Motion and When Not To
Intermittent motion comes in three dominant flavours and the choice between them is rarely about whether the mechanism works — they all work — but about throughput, accuracy, cost, and how much abuse the mechanism takes. Geneva drives, ratchet-and-pawl systems, and cam-driven indexers each occupy a different corner of the design space. The intermittent motion (form) family also includes escapements, mutilated gears, and star wheels, but the three below cover 90% of practical industrial choices.
| Property | Geneva drive (this mechanism) | Ratchet and pawl | Cam-driven indexer (barrel cam) |
|---|---|---|---|
| Typical input speed | 30-300 RPM | 20-200 cycles/min | 60-1200 RPM |
| Indexing accuracy (positional repeatability) | ±0.05° at output | ±0.5° (tooth pitch limited) | ±0.005° (precision Ferguson class) |
| Dwell ratio range | 50-75% (geometry-fixed) | Variable, typically 50% | 10-95% (cam-programmable) |
| Relative cost | Low — 2 machined parts | Lowest — stamped pawl + wheel | High — precision-ground cam |
| Maintenance interval | 10-50M cycles before slot wear shows | 1-5M cycles before pawl tip rounds | 50-200M cycles, sealed oil bath |
| Load capacity | Medium — limited by slot stress | Low to medium — pawl tooth shear | High — full cam-follower contact |
| Best application fit | Bottle/pod carousels, film advance | Sewing feeds, counters, hoists | Pharma blister, automotive welding |
Frequently Asked Questions About Intermittent Motion
The slot is almost certainly running with too much clearance on the drive pin. Geneva slots wear from the inside out — the entry mouth stays sharp but the inner contact face polishes and opens up by 0.05-0.15 mm over the first few million cycles. When the pin disengages at the end of the index, the star wheel is still carrying angular momentum, and a worn slot lets it overshoot before the locking arc catches it. Measure pin-to-slot side clearance with feeler gauges; anything above 0.08 mm on a 50 mm star is past replacement.
The fix is either replacing the star wheel or, on serviceable designs, installing an oversize hardened pin to take up the wear.
It comes down to dwell budget vs acceleration. A 4-slot gives you 75% dwell but the star wheel sees roughly 1.6× the peak angular acceleration of a 6-slot at the same input RPM, because the same 360° of input has to push the star through 90° instead of 60°. If your indexed load is heavy (loaded fixtures, full bottle carriers), the 6-slot is easier on bearings and motor. If your secondary process is the bottleneck (long heat-seal, slow fill), the 4-slot's extra dwell wins.
Rule of thumb: if peak indexed torque exceeds 50% of your motor's continuous rating on the 4-slot calculation, switch to a 6-slot.
You're almost certainly losing dwell time to settling. The calculated dwell ratio assumes the output is stationary the moment the pin disengages, but in reality the star wheel rings against the locking arc for 20-80 ms after the geometric end of indexing. The seal head can't start its descent until that ringing damps out, or you get a moving target during touchdown.
Confirm with a high-speed camera or accelerometer on the star hub. The fix is usually adding a viscous damper or torsional pre-load on the output shaft — sometimes as simple as a felt washer with light spring preload to bleed off the residual energy.
Geometrically yes, mechanically no — not without redesign. Standard Genevas are cut with the slot entry tangent to the pin path on one side only. Run it the other way and the pin slams into the slot wall at finite velocity instead of entering tangentially, hammering the slot mouth and the pin equally. You'll see chipping inside 10,000 cycles.
If you need bidirectional indexing, look at a double-pin Geneva or switch to a cam indexer where the cam profile can be programmed for symmetric forward/reverse motion.
Pawl bounce. The pawl has its own mass and a return spring with a finite natural frequency. When the input drives the ratchet faster than roughly half that natural frequency, the pawl can't settle into one tooth before the next tooth arrives, and it floats above the wheel skipping engagements. The threshold is usually 150-300 cycles/minute on a stamped pawl with a 2-3 N spring.
Three fixes, in order of cost: stiffen the pawl spring (raises natural frequency), reduce pawl mass (same effect), or add a damping pad under the pawl tail to absorb the bounce. Don't just increase spring force without checking — too much preload accelerates tooth tip wear.
Three conditions push you to a cam indexer: throughput above ~150 indexes/min, indexed load above ~50 kg, or required positional repeatability tighter than 0.05°. Genevas hit a wall on all three because the slot-and-pin geometry concentrates contact stress at a single point and the motion profile is fixed by geometry — you can't tune the acceleration curve.
Cam indexers from Ferguson, Camco, or Sankyo cost 5-20× more but give you a programmable rise-dwell-rise profile, distributed cam-follower contact, and sealed oil baths that run hundreds of millions of cycles. The break-even point on cost is usually around 24-month service life at 2-shift operation.
References & Further Reading
- Wikipedia contributors. Geneva drive. Wikipedia
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