An intermittent transmission is a mechanical drive that converts continuous input rotation into a sequence of timed motion-and-dwell cycles at the output. It solves a specific problem: many machines need the workpiece to hold dead still while a tool acts on it, then move precisely one step before stopping again. The mechanism delivers the index motion through cams, Geneva wheels, or ratchet pawls, and holds the dwell through geometric lock-up. You see it everywhere from rotary bottle fillers running 600 cycles per minute to 35 mm film projectors pulling 24 frames per second.
Intermittent Transmission Interactive Calculator
Vary Geneva slot count and input speed to see index angle, motion/dwell split, and index time.
Equation Used
For an ideal external Geneva drive with N slots, each driver revolution advances the output by 360/N degrees. The driver spends ((N - 2)/(2N)) of the cycle in the indexing motion and the remaining time in dwell, where the locking arc holds the output stationary.
- External Geneva drive with one indexing pin.
- One output index occurs per input revolution.
- Ideal geometry; backlash, shock, and acceleration limits are ignored.
Inside the Intermittent Transmission
Every intermittent transmission has the same job — take a shaft that spins steadily and produce an output that moves, stops, moves, stops, on a strict schedule. The input keeps turning at constant RPM because that's what motors do well. The output cycles through a motion period and a dwell period, and the ratio between those two is fixed by the geometry. A 4-slot Geneva drive, for example, spends 25% of each input revolution moving the output and 75% locked stationary. A 6-slot Geneva splits it 33% / 67%. The dwell isn't held by a brake or a clutch — it's held by a circular locking arc on the driver that mates with a matching concave on the driven wheel, so the output physically cannot rotate while the driver is in the dwell zone.
The reason designers reach for this over a servo is reliability and cost. A cam indexer or Geneva wheel runs millions of cycles with nothing more than periodic grease, and the index position is set by hard geometry — not by encoder counts that can drift. The trade-off is that the motion profile is baked in at design time. You can't reprogram a Geneva drive to do 5 stations instead of 4. If you machine the slot angle wrong by even half a degree on a 6-slot wheel, the driver pin will hit the slot wall before it engages cleanly and you'll hear a sharp clack at every index — that's the symptom of bad slot geometry, and it eats pins fast.
Failure modes cluster around three things. First, dynamic shock at engagement and disengagement: pin-and-slot mechanisms ideally enter and exit the slot tangentially so acceleration ramps from zero, but worn pins or sloppy slot tolerances break that tangency and you get impact loading. Second, the locking arc wears: any radial play here lets the output station drift while the tool is acting, ruining position accuracy on indexing tables. Third, lubrication starvation on cam indexers — a Sankyo or CAMCO unit running dry will pit the cam follower roller in under 100,000 cycles versus the rated 10+ million.
Key Components
- Driver (input crank or cam): The continuously rotating member coupled to the motor. On a Geneva drive it carries the indexing pin and a circular locking disc; on a cam indexer it's a barrel cam with a rib machined to a precise rise-dwell-rise profile. Runout on the driver shaft must stay under 0.02 mm TIR or the locking action chatters.
- Driven wheel or output turret: The intermittently moving output. For a Geneva wheel this has 3 to 12 radial slots; for a cam indexer it's a turret carrying cam followers spaced at exact angular pitch. Slot or follower spacing tolerance typically holds to ±0.01° at the index position.
- Locking arc / dwell surface: The geometric feature that holds the output stationary during dwell. On a Geneva drive it's a convex disc on the driver mating with a concave cutout on the driven wheel. Wear here directly translates to angular backlash at the indexed station.
- Indexing pin or cam follower: The element that transmits motion during the active phase. Geneva pins are typically hardened ground steel running in bronze-bushed slots; cam indexers use needle-bearing rollers. The pin diameter must match the slot width within 0.05 mm to avoid impact at entry.
- Driven shaft and station plate: Carries the actual workpiece, tool, or filling head. Mass and inertia of this assembly directly determine peak torque demand during the index phase — a heavy turret means a longer index period or a stiffer drive.
Who Uses the Intermittent Transmission
Intermittent transmissions show up wherever a process needs precise positioning followed by a working dwell. They beat servo systems on cost and reliability when the index pattern is fixed and the duty cycle is high. The classic question — why use a Geneva drive instead of a stepper? — answers itself once you see the same machine running 24/7 for a decade with nothing but grease changes.
- Packaging machinery: Rotary bottle fillers like the Krones Modulfill use cam indexers with 12 to 60 stations to dwell each bottle under a fill nozzle for 200-400 ms while the turret holds dead still.
- Film and motion picture: 35 mm cinema projectors built by Bell & Howell and Simplex use a 4-slot Geneva drive to advance one frame at a time, holding each frame stationary for ~75% of the cycle while the shutter is open.
- Watchmaking and precision assembly: Hauser and Mikron rotary transfer machines index small turrets through 6 to 16 stations for drilling, tapping, and milling watch components, with positional repeatability under 5 µm.
- Pharmaceutical tablet pressing: Fette and Korsch rotary tablet presses use cam-driven turret indexing to advance dies through fill, compression, and ejection stations at up to 1 million tablets per hour.
- Automotive assembly: Sankyo and CAMCO roller-gear indexers drive welding fixture rotary tables on Toyota and Honda body-in-white lines, holding ±30 arc-second positional accuracy under 500 kg payloads.
- Textile machinery: Multi-needle quilting machines use ratchet-and-pawl drives to advance the fabric one stitch length per needle stroke, synchronised to the main shaft by a chain or timing belt.
The Formula Behind the Intermittent Transmission
The most useful number for sizing an intermittent drive is the motion-period ratio — the fraction of each input revolution spent actually moving the output. For a Geneva wheel this is set entirely by the slot count n. At the low end of typical builds, a 3-slot Geneva spends 16.7% of its cycle indexing and 83.3% in dwell — useful when you need a long working window per station, but the angular acceleration spikes are brutal. At the high end, a 12-slot Geneva spends 41.7% indexing — gentler accelerations, shorter dwell. The sweet spot for general-purpose indexing sits at 4 to 6 slots, which is why you see those counts on 90% of commercial Geneva drives.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Ï„motion | Time spent indexing the output (motion period) | s | s |
| Ï„cycle | Total cycle time of one input revolution | s | s |
| n | Number of slots in the Geneva wheel (must be ≥ 3) | — | — |
| ωin | Input shaft angular velocity | rad/s | RPM |
Worked Example: Intermittent Transmission in a glass ampoule labelling carousel
Sizing a 6-slot Geneva drive for a glass ampoule labelling carousel at a Boehringer Ingelheim packaging line in Ingelheim, Germany. The line targets 90 ampoules per minute, the labeller needs at least 450 ms of stationary dwell per ampoule for the wraparound applicator to work cleanly, and the turret carries 6 ampoule cradles weighing 1.2 kg each. You need to confirm that a 6-slot Geneva at the chosen input RPM gives enough dwell time, and check what happens if production rate is dialled up or down.
Given
- n = 6 slots
- Production rate (nominal) = 90 ampoules/min
- Required dwell = ≥ 450 ms
- Turret stations = 6 —
Solution
Step 1 — at the nominal 90 ampoules/min, each ampoule corresponds to one input revolution of the Geneva driver, so the cycle time is:
Step 2 — compute the motion-period fraction for a 6-slot Geneva:
So motion takes 0.333 × 0.667 = 0.222 s and dwell takes the remaining 0.445 s. That's right on the edge of the 450 ms dwell spec — close enough that the labeller will start missing if the line speeds up at all.
Step 3 — at the low end of the typical operating window, slow the line to 60 ampoules/min for fragile product runs:
That's a comfortable 667 ms dwell — the labeller has margin, and the ampoules stop cleanly with no rocking. Step 4 — at the high end, push the line to 120 ampoules/min for fast SKUs:
That's only 333 ms — well below the 450 ms the wraparound applicator needs, and you'll see labels going on crooked or the head crashing into a still-moving cradle. To run 120/min you'd need to switch to an 8-slot Geneva (dwell fraction 0.625) or a cam indexer with a custom 70° motion / 290° dwell profile.
Result
At the nominal 90 ampoules/min the 6-slot Geneva gives 445 ms of dwell per station — just barely meeting the 450 ms labeller spec. Drop the line to 60/min and you have 667 ms of dwell with comfortable margin; push to 120/min and dwell collapses to 333 ms, which is where you start seeing crooked labels and applicator-head collisions. If you measure dwell time with a high-speed camera and read 380 ms when the calculation predicts 445 ms at 90/min, the usual suspects are: (1) the input motor slipping above nominal RPM under load — check the VFD frequency setting, (2) Geneva slot wear opening up the engagement window so the locking arc engages late, or (3) excessive turret inertia causing the output to overshoot and oscillate before settling, which a stiffer locking arc and a flywheel on the driver shaft will fix.
Intermittent Transmission vs Alternatives
Intermittent transmission isn't a single mechanism — it's a family. Geneva drives, cam indexers, and ratchet-and-pawl systems all deliver stop-go motion but trade off speed, accuracy, and cost very differently. Pick the wrong family for your duty cycle and you'll either burn through parts or pay for capability you don't need.
| Property | Geneva Drive | Cam Indexer (e.g. Sankyo, CAMCO) | Ratchet and Pawl |
|---|---|---|---|
| Typical max indexing speed (cycles/min) | 300 | 1200+ | 60 |
| Positional accuracy at index | ±2 arc-min | ±30 arc-sec | ±10 arc-min |
| Cost (relative) | Low | High (5-10× Geneva) | Very Low |
| Lifespan (cycles before rebuild) | 1-5 million | 10-50 million | 0.5-2 million |
| Load capacity (turret payload) | Up to 50 kg | Up to 2000 kg | Up to 10 kg |
| Motion profile flexibility | Fixed by slot count | Fully customisable cam | Fixed by tooth pitch |
| Best application fit | Mid-speed packaging, projectors | High-speed precision assembly | Low-speed manual or stitching feeds |
Frequently Asked Questions About Intermittent Transmission
That clack means the driver pin is not entering the slot tangentially — it's hitting the slot wall sideways instead of sliding in along the slot axis. Two things cause it. First, the centre distance between driver and driven shafts is wrong. For a Geneva drive the centre distance must equal R / sin(180°/n), where R is the driver crank radius. Get that distance off by even 0.5 mm on a small drive and the pin enters at an angle.
Second, the driver and driven shafts aren't square to each other. Any axial misalignment means the pin contacts the upper or lower slot edge first instead of the slot floor. Put a dial indicator on both shafts and verify parallelism within 0.05 mm over 100 mm before you blame the parts.
Three questions decide it. What's your cycle rate? Below 200 cycles/min a Geneva drive is fine and saves you money. Above 400, you basically need a cam indexer because Geneva acceleration spikes get brutal and pin life collapses.
What positional accuracy does the downstream process need? Geneva drives hold a few arc-minutes; cam indexers hit arc-seconds. Filling a 50 ml bottle? Geneva is plenty. Drilling a 0.5 mm hole at station 7? You want a Sankyo or CAMCO unit.
Last, do you need a custom motion profile — a long dwell with a fast index, or a slow ramp at engagement? Geneva geometry is fixed by slot count. Cam indexers let you machine any profile you want. Pay the premium when the application demands it.
Because at high RPM the output turret's inertia starts to matter. The closed-form ratio (n − 2) / (2n) assumes the output reaches the locking arc instantly at the end of the motion phase. In reality, the turret overshoots slightly, oscillates against the locking arc, and that settling time eats into your effective dwell.
The cure is either reducing turret inertia (lighter station plates, lighter fixtures) or adding a flywheel to the driver shaft so the input speed stays rock-steady through the impact. If you can't do either, derate the design dwell by 10-15% above 60% of rated speed.
Yes. Geneva wheels work for any integer slot count from 3 upward, and 5-slot and 7-slot units are standard. The motion-period ratio scales smoothly: 5-slot gives 30%, 7-slot gives 35.7%. The reason you mostly see 4, 6, and 8 in catalogues is convention — even slot counts let designers symmetrically place tooling around the turret.
The one gotcha with low slot counts is angular acceleration. A 3-slot Geneva has peak acceleration roughly 4× higher than a 6-slot at the same input RPM, which means dynamic loads on the pin go through the roof. Below 4 slots, expect short pin life unless you derate input speed.
Almost always it's radial loading on the output during dwell — something is trying to rotate the turret while the locking arc holds it still. A common culprit is downstream tooling that grabs the workpiece and applies a tangential force during the working stroke, or a brake on the output that drags continuously.
The locking arc is designed for geometric containment, not for resisting steady torque. Even small loads of 5-10 Nm applied repeatedly will galling the arc surface fast. If your application has any holding torque requirement at the station, add a separate cam-actuated brake at each station and let the Geneva arc do nothing but locate.
Skipping happens when the pawl lifts off the ratchet tooth before the next tooth catches. Check three things in order. First, pawl spring force: if the spring has fatigued, the pawl floats above the ratchet at high cycle rates. Replace the spring and verify with a fish scale that pawl lift force matches the original spec.
Second, tooth profile wear. The leading face of each tooth should be sharp and square; if it's rounded over from wear, the pawl will cam out under load. Third, drive-link compliance — if the link pushing the pawl flexes under load, the pawl stroke shortens and may not clear a full tooth pitch. Stiffen the link or shorten its length.
It matters more than people expect. The torque demand on the motor isn't constant — it spikes during the indexing phase and drops to almost zero during dwell. Without a flywheel, the motor speeds up during dwell and slows down during index, which means your actual dwell time and motion time both vary cycle-to-cycle.
Rule of thumb: size the flywheel so its rotational inertia is at least 5× the reflected inertia of the loaded turret at the driver shaft. That keeps speed variation under 2% and makes the indexing geometry behave the way the math predicts.
References & Further Reading
- Wikipedia contributors. Geneva drive. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
