A continuous to alternating crank gear is a motion-control mechanism that takes a steady one-way input rotation and produces an output shaft that rotates first one way, then the other, in a repeating cycle. The defining component is a partially-toothed driver gear — often called a mutilated or sector gear — that meshes alternately with two opposed pinions on the output shaft. We use this when a job needs reversing motion from a single unidirectional motor without a clutch or electrical reversing controller. You will see it driving paddle agitators, fabric dye-vat oscillators, and small machine-tool reciprocating tables.
Continuous to Alternating Crank Gear Interactive Calculator
Vary sector angle, input speed, and gear tooth counts to see output stroke, dwell, cycle rate, and reversal frequency.
Equation Used
The toothed sector angle is converted to output shaft swing by the gear tooth ratio. With two equal sectors, the remaining driver rotation is split into two dwell gaps, and one input revolution gives one complete out-and-back cycle.
- Two equal toothed sectors are placed 180 deg apart on the driver.
- Exactly one pinion is engaged at a time.
- One input revolution produces one full out-and-back output cycle.
- Gear tooth counts represent pitch diameter ratio.
How the Continuous to Alternating Crank Gears Actually Works
The trick is geometry, not electronics. A continuous to alternating crank gear works by giving the driver a tooth set that only covers part of its circumference — typically a 120° to 180° arc — with smooth dwell zones between the toothed sectors. As the driver rotates steadily, its toothed arc engages one pinion on the output shaft, drives it through a defined angle, then disengages. The driver continues rotating through its dwell zone while the output sits still. Then a second toothed sector engages a second pinion mounted on the same output shaft but oriented to spin the shaft the opposite way. The output reverses. One full input revolution gives you one full out-and-back cycle.
The geometry has to be right or the mechanism eats itself. The pitch circles of the driver and pinion must match within roughly 0.05 mm centre-distance error on a Module 1 gear, otherwise the leading tooth on the sector clashes with the pinion as it engages. The angular position where the sector first touches the pinion sets the reversal point — get that wrong and you lose stroke symmetry, so one direction overshoots and the other undershoots. Most designs use a locking arc, a smooth concave surface on the driver that rides against a matching convex surface on the pinion during the dwell, holding the output stationary and preventing back-driving.
Failure modes are predictable. If the sector entry tooth wears or chips, you get a hard knock at every reversal as the gears clash before meshing properly. If the dwell-arc clearance opens up beyond about 0.1 mm, the output shaft drifts during dwell and the next engagement fights that drift, accelerating wear on both flanks. Mutilated gear drives don't tolerate shock loads on the output — the entry and exit teeth carry all the impact, and they're the first parts to fail.
Key Components
- Mutilated driver gear (sector gear): The input gear with two opposed toothed arcs, typically 90° to 180° each, separated by smooth dwell zones. Its arc length sets the output stroke angle, and the arc start/end positions set the dwell duration. Pitch tolerance on the entry tooth must hold to roughly 0.02 mm to avoid clash.
- Output pinion pair: Two small gears fixed to the output shaft, oriented so each engages the driver during opposite halves of the input cycle. The pair must be phased so that exactly one pinion is engaged at any time — overlap causes a lock-up, gap causes free-wheel and lost stroke.
- Locking arc / dwell surface: Concave-convex surface pair on the driver and pinion that rides during dwell, holding the output stationary. Surface clearance should sit at 0.03 to 0.08 mm — tighter and you get drag, looser and the output drifts and clashes on re-engagement.
- Output shaft: Carries the two pinions and delivers the alternating motion to the load. Must resist reversing torque shock at every cycle, so we typically size it 1.5× the steady-state torque requirement and use a keyway rather than a setscrew on anything above 5 Nm.
- Reversal cam timing pin: On precision builds, a hardened pin or keyed feature ensures the two pinions stay in correct angular relationship — a 1° phase error between pinions translates to 1° of lost stroke at every reversal.
Where the Continuous to Alternating Crank Gears Is Used
Continuous to alternating crank gears appear wherever a designer needs reversing output from a unidirectional motor and doesn't want the cost or complexity of a reversing drive. They suit slow-cycle applications — typically under 60 cycles per minute — where the impact at reversal is manageable. Machine builders pick them over electrical reversing because there's no contactor wear, no inrush current, and no software to debug. The trade-off is fixed stroke and fixed timing — you can't change the cycle profile without swapping the driver gear.
- Textile finishing: Beck dyeing machines such as the Thies Roto-Plus use alternating crank drives to oscillate the fabric reel, ensuring even dye penetration without reversing the main drive motor.
- Food processing: Hobart commercial dough mixers in the H600 series use mutilated gear drives on the planetary agitator arm to alternate scraping direction across the bowl floor.
- Machine tools: Older South Bend and Atlas shaper rams use a variant of this drive on the auxiliary tool-feed mechanism to step the tool sideways at the end of each stroke.
- Toy and animatronic mechanisms: Disney's classic Audio-Animatronics figures used alternating crank gear sets in head-shake and arm-wave subassemblies driven from a single 24V DC gearmotor.
- Agricultural equipment: Kuhn rotary tedders and older John Deere side-delivery rakes used sector-gear reversing drives on the reel-tine pitch mechanism to alternate tine angle through the work cycle.
- Industrial mixing: Lightnin and Chemineer reactor agitators sometimes use alternating crank drives on auxiliary baffle-cleaning arms to scrape vessel walls in both directions from a single drive shaft.
The Formula Behind the Continuous to Alternating Crank Gears
The key number to compute is the output stroke angle per cycle — how far the output swings each direction before reversing. This depends on the driver sector arc, the gear ratio between driver and pinion, and the number of toothed sectors on the driver. At the low end of typical builds, you'll see 60° output stroke from short sector arcs and conservative ratios — useful when you want a shaker action. Nominal designs land around 120° to 180° stroke, which suits agitator and oscillator work. At the high end, beyond 270° output stroke, the dwell zone shrinks to nothing and the mechanism loses its self-locking behaviour, so reversals get violent. The sweet spot for most industrial machines sits between 120° and 200° stroke per direction.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θout | Output shaft swing angle per direction per cycle | degrees (°) | degrees (°) |
| αsector | Toothed-arc angle on the driver (one sector) | degrees (°) | degrees (°) |
| Ndriver | Tooth count of driver gear at full circumference (theoretical) | teeth | teeth |
| Npinion | Tooth count of the output pinion | teeth | teeth |
Worked Example: Continuous to Alternating Crank Gears in a paint-pigment ribbon blender
You are designing the reversing drive on a 200 L laboratory ribbon blender for an automotive paint-pigment trial line at a coatings R&D centre in Stuttgart. The job is to oscillate an inner stripper paddle ±150° to keep pigment off the vessel wall, driven from a single 25 RPM gearmotor through a continuous to alternating crank gear. Driver pitch diameter is 120 mm at Module 2, theoretical full-circumference tooth count is 60. Each toothed sector covers 150°. Output pinion has 25 teeth.
Given
- αsector = 150 degrees
- Ndriver = 60 teeth
- Npinion = 25 teeth
- Input speed = 25 RPM
Solution
Step 1 — at nominal sector arc 150°, compute the output swing angle per direction per cycle:
That's a full revolution per direction, which is too much — the paddle would over-rotate and slap the opposite vessel wall. We need to drop the sector arc. Try 60° on the driver:
Step 2 — recompute with a sector arc that targets 150° output:
So the driver needs a 62.5° toothed arc per sector, with two such arcs 180° apart, leaving 117.5° dwell zones between them. At 25 RPM input, each cycle takes 2.4 seconds, with the output swinging 150° one way, dwelling, swinging 150° back, dwelling.
Step 3 — check the high-end behaviour. If a junior engineer pushes the input to 60 RPM hoping for faster mixing, the cycle time drops to 1.0 second. The reversals now hit at 1 Hz, and the entry tooth on each sector takes a measurable shock load. On a Module 2 steel-on-steel pair we'd expect entry-tooth pitting within 200 hours at that speed.
At the low end, 10 RPM input gives a 6.0 second cycle — slow and gentle, almost no impact at reversal, but the pigment starts settling between strokes. The sweet spot for this blender is 20 to 30 RPM input.
Result
The driver needs a 62. 5° toothed arc per sector to deliver the required 150° output swing per direction. At nominal 25 RPM you get a 2.4 second full cycle — slow enough to keep reversal shock manageable, fast enough to keep pigment moving. At the 10 RPM low end the cycle stretches to 6 seconds and pigment settles between strokes; at the 60 RPM high end the cycle drops to 1 second but you'll see entry-tooth pitting on Module 2 steel within roughly 200 hours. If your measured output stroke comes back below 150°, check (1) sector-to-pinion phasing, since a 2° error on the driver shows as a 5° loss of stroke at the output, (2) backlash on the locking-arc surfaces — clearance above 0.1 mm lets the output drift during dwell so the next sector engages mid-position, and (3) keyway slop on the output pinion, which absorbs angle during impact reversal and shows up as a softening of the stroke ends.
Continuous to Alternating Crank Gears vs Alternatives
You have three real options for getting alternating output from a continuous input, each with different cost, speed, and lifespan profiles. Pick based on cycle rate, stroke accuracy, and how much shock the load can tolerate at reversal.
| Property | Continuous to alternating crank gear | Scotch yoke + rocker arm | Electrical reversing motor |
|---|---|---|---|
| Practical cycle rate | 10 × 60 cycles/min | 30–300 cycles/min | Limited by contactor life, 1–20 cycles/min |
| Stroke accuracy / repeatability | ±0.5° once worn-in | ±0.2° | ±2° depending on stop method |
| Stroke adjustability | Fixed — swap driver to change | Adjustable via crank radius | Fully programmable |
| Reversal shock load | High at entry tooth | Low — sinusoidal motion | High — full motor torque reverses |
| Component cost (small machine) | $80–$250 for gear pair | $120–$400 with linkage | $300–$900 with VFD or H-bridge |
| Expected lifespan | 5,000–20,000 hours | 20,000–50,000 hours | Motor 10,000+ hrs, contactor 100,000 cycles |
| Maintenance interval | Inspect entry teeth every 1,000 hrs | Re-lubricate slot every 2,000 hrs | Replace contactor every 100k cycles |
| Best application fit | Slow agitators, oscillators, dwell-required jobs | High-speed reciprocating tables | Variable-stroke programmable jobs |
Frequently Asked Questions About Continuous to Alternating Crank Gears
Almost always this is a phasing error between the two output pinions, not gear wear. If pinion A and pinion B are clocked on the output shaft so they're not exactly 180° opposed, one direction's stroke gets clipped by the angle of the phase error and the other gets extended by the same amount.
Quick check — mark the output shaft at zero, run the mechanism, mark both end positions. If the two angles aren't symmetric about zero, pull the second pinion and re-key it. Setscrew-mounted pinions are the usual culprit; the setscrew lets the pinion creep under reversal shock and the phase walks over time.
Symmetric 180°/180° gives you equal stroke time both directions and equal dwell — pick this when the load is symmetric, like an agitator paddle. Asymmetric splits give you a long stroke one way and a fast snap-back the other, which suits indexing applications where you want slow controlled motion forward and rapid return.
Rule of thumb: if the load does work in only one direction (a scraper, a wiper), use asymmetric and put the long sector on the working stroke. If both directions do work, stay symmetric or you'll wear one entry tooth twice as fast as the other.
The knock is almost always the locking arc, not the gear teeth. As the dwell-arc surfaces wear, clearance grows past about 0.1 mm and the output shaft drifts a few degrees during dwell. When the next sector engages, it has to slam through that gap before meshing, and you hear it.
Stick a feeler gauge between the locking arc and the pinion's arc surface during dwell — if it's over 0.1 mm on a Module 1 or 2 gear, replace or shim. Don't ignore this. Once the knock starts, entry-tooth pitting follows within a few hundred hours because every reversal is now an impact instead of a roll-on engagement.
You can, but you're fighting the wrong battle. The limit isn't tooth strength — it's reversal shock. At 100 RPM input the output swings and reverses 100 times a minute, and every reversal is a near-instantaneous direction change of the entire output inertia. The shock load scales with the square of speed, not linearly.
A Scotch yoke gives you smooth sinusoidal reversal with zero acceleration at the endpoints, and that's the right answer above 60 RPM. Use the alternating crank gear where dwell matters or where the load itself can't tolerate continuous motion at the endpoints.
One direction is carrying more load than the other. Three usual causes: the driven load is gravity-biased (a weighted arm that helps one direction and resists the other), the output bearing has more drag in one rotation sense due to seal lip orientation, or the two pinions weren't installed with matched backlash so one engages with more clash than the other.
Measure backlash at both pinions individually with a dial indicator on the output shaft. If they differ by more than 0.05 mm at the pitch line, that's your answer. Shim the offending pinion's bearing carrier to centre the mesh.
Not if the locking arc is intact and the load is balanced. The concave-convex dwell surfaces are designed to hold the output stationary — that's why we call them locking arcs. Adding an external brake usually does more harm than good because it loads the dwell surface unevenly.
Where you do need a detent: if the output drives a back-drivable load like a vertical lift or an unbalanced lever, the locking arc alone may not hold against that bias. In that case, add a wrap-spring brake on the output shaft sized to the bias torque, not a friction brake — wrap springs release cleanly when the next sector engages and don't fight the drive.
References & Further Reading
- Wikipedia contributors. Mechanism (engineering). Wikipedia
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