An intermittent gear is a gear with teeth cut around only part of its circumference, so a full input revolution produces one period of driven rotation followed by a stationary dwell. Typical industrial indexers run 30 to 200 cycles per minute with positional accuracy inside ±30 arc-seconds. The mechanism exists to convert continuous shaft rotation into precise start-stop motion without clutches or servos. You'll see it inside film cameras, rotary bottle fillers, and watch escapement trains.
Intermittent Gears Interactive Calculator
Vary the toothed sector and locking arc angles to see the motion/dwell split for one intermittent gear cycle.
Equation Used
The toothed sector angle sets the part of the driver revolution where the follower indexes. The locking arc sets the dwell portion where the follower is held stationary. With a 120 deg toothed sector and 240 deg locking arc, the cycle is 1/3 motion and 2/3 dwell.
- One toothed sector and one locking arc define one driver cycle.
- Driver speed is constant through the cycle.
- Follower moves only while the toothed sector is engaged.
- Follower is held stationary during the locking arc dwell.
The Intermittent Gears in Action
An intermittent gear — sometimes called a mutilated gear or partial-tooth gear — is the simplest way to turn a constantly spinning input shaft into a follower that moves, stops, holds position, and then moves again. The driver carries teeth on only a fraction of its circumference. The rest is a smooth concentric arc, machined to the same radius as a matching concave arc on the follower. While the toothed sector engages, the follower rotates by a fixed angular increment. While the smooth arc rides against the follower's locking face, the follower is geometrically pinned in place — it cannot rotate even if external torque is applied. That locking action is the whole reason this mechanism exists, and it's why you see it on machines that need a precise dwell with no brake.
The geometry has to be tight or it falls apart fast. The locking arc radius and the follower's mating radius must match within roughly 0.05 mm on a 50 mm part — any looser and the follower drifts during dwell, any tighter and the surfaces gall. Tooth engagement timing matters too. The first driver tooth has to enter the follower mesh exactly as the locking arc disengages; if the timing is off by even half a tooth pitch, you get a hard impact, chipped tooth tips, and a shock load that propagates back into the gearmotor. Most failures we see in customer rebuilds are not gear-tooth wear — they're locking-arc wear, where decades of dwell-load contact has galled the smooth surface and the follower now hunts a few arc-minutes during what should be a hard stop.
Compared to a Geneva drive, which uses a pin and slot to achieve a similar partial-tooth gear effect, intermittent gears can deliver more than one indexing motion per driver revolution and they handle higher continuous loads because torque transfers through full involute tooth contact rather than a single pin. The trade is impact at engagement — Geneva drives ease in through the slot geometry, while a mutilated gear engages cold. Designers usually add a soft entry tooth or a spring-loaded first tooth to absorb the strike.
Key Components
- Driver Gear (Mutilated Gear): The continuously rotating input. Teeth occupy only a defined arc — commonly 60° to 180° depending on the indexing ratio required. The remaining circumference is a precision-ground locking arc, typically held to within 0.02 mm of nominal radius on a 100 mm gear.
- Follower Gear (Driven Wheel): Carries full teeth around its circumference, interrupted by concave locking faces machined into the gaps. Each locking face matches the driver's locking arc radius. The follower indexes by one tooth-set per driver revolution and holds rigid during dwell.
- Locking Arc: The toothless portion of the driver. Acts as a circular cam riding against the follower's locking face. Surface finish must be Ra 0.4 µm or better — rougher surfaces wear quickly under the dwell-load Hertzian contact stress and cause positional drift over time.
- Entry / Exit Teeth: The first and last teeth on the driver's toothed sector. These are often modified — relieved, chamfered, or spring-loaded — to soften the impact at engagement. A standard involute profile here will chip within tens of thousands of cycles on a high-speed indexer.
- Indexing Plate or Coupling: Mounts to the follower shaft and carries the actual workpiece, dial, or carousel. Inertia of this load drives the sizing of the entry tooth and the gearmotor torque rating, because the follower has to accelerate from rest to full speed inside one tooth pitch.
Industries That Rely on the Intermittent Gears
Intermittent gears live in any machine that needs a shaft to rotate, stop dead, do something useful during the stop, then rotate again — all driven from a single continuously turning input. The mechanism is cheaper than a servo-driven indexer, more compact than a cam-driven Ferguson indexer, and unlike a Geneva drive it can carry heavy loads through full tooth contact. You'll see it in packaging lines running 60 to 120 cycles per minute, in mechanical counters, in film advance mechanisms, and in any rotary station where the dwell phase is when the actual work happens — capping, labelling, filling, inspecting.
- Packaging Machinery: Rotary bottle filler turrets on Krones and KHS lines, where the bottles must stop precisely under each fill nozzle for 0.4 seconds before indexing forward.
- Photographic Equipment: Film advance and shutter timing inside Bell & Howell 16 mm cine projectors, where each frame must hold stationary for the exposure interval then jump exactly one frame pitch.
- Horology: Calendar-wheel advance in mechanical watches like the ETA 2824-2 movement — once per 24 hours the date wheel jumps one position, then locks for the rest of the day.
- Industrial Counters and Meters: Mechanical odometers and totalising counters where each input pulse advances the units wheel by one digit, and the carry-over to the tens wheel uses a partial-tooth gear so the tens wheel only moves once every ten units.
- Assembly Automation: Indexing dial tables on small-scale assembly cells, similar to the Weiss TC series, where 6, 8, or 12 stations rotate into position in sequence for pick-and-place operations.
- Pharmaceutical Capping: Rotary capping turrets on machines like the Groninger KFL series, where the capping spindle must hold each bottle stationary for the torque-controlled cap application.
The Formula Behind the Intermittent Gears
The core sizing question for an intermittent gear is the ratio of motion time to dwell time per driver revolution. At the low end of typical operating ranges — say a driver spinning at 30 RPM with teeth on 60° of arc — the follower moves for only 56 ms out of every 2 second cycle, leaving a long 1.94 second dwell. That's the regime for inspection stations or slow-fill operations. At the high end, a 200 RPM driver with teeth on 180° gives a 150 ms motion phase and a 150 ms dwell — the sweet spot for high-speed packaging where the work head needs roughly equal time to engage and release. Push past 200 RPM with a heavy follower inertia and the entry-tooth impact loads scale with the square of speed, which is what shears teeth.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| tmotion | Duration of follower motion per driver revolution | s | s |
| tcycle | Total driver revolution period (60 / RPM) | s | s |
| θteeth | Arc of driver occupied by teeth | degrees | degrees |
| Ndriver | Driver rotational speed | RPM | RPM |
| iindex | Follower angular advance per driver revolution | degrees | degrees |
Worked Example: Intermittent Gears in a 6-station rotary capper
You're sizing the intermittent gear that drives the 6-station indexing turret on a small-batch cosmetics capping machine, similar in scale to a CVC Technologies CCS-300. The driver runs at a nominal 90 RPM off a gearmotor. Each driver revolution must advance the turret by 60° (one station out of six) and dwell long enough for the capping head to engage, torque, and retract. The toothed sector occupies 120° of the driver — the remaining 240° is locking arc.
Given
- Ndriver = 90 RPM
- θteeth = 120 degrees
- iindex = 60 degrees per driver rev
- Stations = 6 —
Solution
Step 1 — find the driver cycle time at nominal 90 RPM:
Step 2 — split the cycle into motion and dwell phases using the toothed-arc fraction:
So at 90 RPM nominal, the turret indexes in 0.22 seconds and holds stationary for 0.44 seconds — a 1:2 motion-to-dwell ratio that gives the capping head plenty of time to engage and torque before the next move.
Step 3 — at the low end of the typical operating range, drop the driver to 45 RPM:
That's 0.89 seconds of dwell — comfortable for any capping torque profile and ideal for slow startup or sample runs. The downside is throughput drops to 45 caps per minute.
Step 4 — at the high end, push the driver to 180 RPM:
Throughput climbs to 180 caps per minute but you only have 0.22 seconds of dwell — barely enough for a fast pneumatic capping head, and the entry-tooth impact load is now 4× the nominal value because impact energy scales with the square of approach velocity. Above this speed you'll hear the engagement strike from across the room.
Result
Nominal output is 0. 22 s motion and 0.44 s dwell per cycle, giving 90 caps per minute. At 45 RPM the dwell stretches to nearly 0.9 seconds — the right setting for thick-thread closures or torque-controlled validation runs — while at 180 RPM you barely clear 0.22 seconds, which is the practical ceiling for a pneumatic capping spindle. The sweet spot for most cosmetics and pharma capping work sits between 75 and 110 RPM. If your measured dwell is shorter than the predicted 0.44 s, check three things: (1) the locking arc has worn or the follower's locking face has galled, allowing the follower to rotate slightly during dwell and shifting the apparent motion window, (2) the gearmotor is overshooting on shutdown because the load inertia exceeds the brake spec, or (3) the entry tooth has chipped, advancing the engagement event by 2-4° of driver rotation.
Intermittent Gears vs Alternatives
Intermittent gears are one of three common ways to generate indexed rotary motion from a continuous input. The other two are Geneva drives and cam-driven indexers (sometimes called Ferguson or roller-gear indexers). Each has a clear application window — pick the wrong one and you'll either pay too much or wear the mechanism out in months.
| Property | Intermittent Gear | Geneva Drive | Cam-Driven Indexer |
|---|---|---|---|
| Typical operating speed | 30-200 RPM input | 30-300 RPM input | 60-1200 RPM input |
| Indexing accuracy | ±2-5 arc-min | ±5-15 arc-min | ±15-30 arc-sec |
| Load capacity (rotary inertia) | High — full tooth contact | Medium — single pin contact | Very high — roller cam contact |
| Engagement smoothness | Hard impact unless modified | Smooth — slot geometry eases in | Smooth — cam profile shaped |
| Cost (small-batch industrial) | Low — standard gear cutting | Low to medium | High — precision cam machining |
| Lifespan at nominal load | 10-50 million cycles | 5-20 million cycles | 100+ million cycles |
| Best application fit | Counters, slow indexers, calendars | Film projectors, light indexers | High-speed packaging, assembly cells |
Frequently Asked Questions About Intermittent Gears
The first tooth takes the full impact load when the locking arc disengages and the toothed sector enters mesh. The follower is sitting at zero velocity and the driver is at full angular velocity — the kinetic energy difference dumps into the first tooth contact in microseconds.
Two fixes work in practice. First, relieve the entry tooth by 0.1-0.2 mm and chamfer the leading edge so initial contact is smaller and the force ramps up over a few degrees of rotation. Second, check the angular timing between locking-arc disengagement and tooth engagement — if there's even a 1-2° gap, the follower can creep before the tooth strikes, which makes the impact worse not better.
Cut multiple toothed sectors around the driver, separated by locking arcs. Two sectors at 180° apart give two indexes per revolution; three sectors at 120° give three. The sum of all toothed arcs plus all locking arcs must equal 360°, and each locking arc must be long enough that the follower fully decelerates and locks before the next sector engages.
Practical limit is usually 4 sectors per revolution. Beyond that the locking arcs get too short to guarantee positional hold against follower inertia, and you'll see the follower hunt during what should be dwell.
Pick the intermittent gear when the follower has significant rotary inertia or carries a working load during indexing. Tooth contact distributes torque across multiple teeth, while a Geneva drive concentrates everything on a single pin-slot contact. For a 6-station capping turret carrying 2 kg of bottles, an intermittent gear will outlast a comparable Geneva by 3-5×.
Pick the Geneva drive when engagement smoothness matters more than load capacity — film projectors are the classic case, where any impact at engagement would jitter the image.
Drift during dwell almost always traces to one of three sources, and locking-arc surface wear is only one of them. Check the follower-shaft bearing clearance first — a worn bushing can let the follower walk radially, which translates to angular drift at the indexing-plate radius. Then check the keyway or pin connecting the follower hub to the indexing plate; a 0.05 mm keyway slop reads as several arc-minutes at a 200 mm plate radius.
If both of those are tight, then yes — the locking arc and follower locking face have galled and you need to regrind both surfaces to nominal radius. Don't just polish one; the radii must match within 0.02-0.05 mm or the contact patch concentrates and re-galls quickly.
Mechanically yes, but only if the entry and exit teeth are symmetric — many designs relieve only the entry tooth, which means running backwards puts the impact on an unrelieved tooth and you'll chip it within hundreds of cycles.
If reverse operation is a real requirement, specify symmetric tooth modifications on both ends of the toothed sector and confirm the locking arc surface finish is identical at both transition points. Most off-the-shelf intermittent gears from indexer suppliers are unidirectional by design.
Roughly 150-200 input RPM is where intermittent gears start losing to cam indexers in real applications. Above that the engagement impact energy — which scales with the square of approach velocity — starts shearing tooth tips even with relieved entry teeth, and the locking arc Hertzian contact stress during dwell deceleration causes pitting.
Cam-driven indexers like the CAMCO or Sankyo units profile the acceleration and deceleration smoothly, so they comfortably run 600-1200 input RPM. The trade is cost — a precision cam indexer is 5-10× the price of an intermittent-gear setup for the same station count.
The torque demand peaks at the start of the motion phase, when the follower has to accelerate from zero to full angular velocity within roughly one tooth pitch. Compute the angular acceleration from the motion time and the index angle (α = 2 × iindex / tmotion2), multiply by follower-plus-load inertia, and add a 2× safety factor for the entry-tooth impact spike.
A common mistake is sizing on average torque — the gearmotor will run fine at average load but stall or trip thermally during the peak. If your motor trips intermittently right at the engagement event, that's the signature.
References & Further Reading
- Wikipedia contributors. Geneva drive. Wikipedia
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