A two crank-disks variable alternating traverse is a slider-crank variant that drives a reciprocating output from two coaxial crank disks whose relative angular phase you can change on the fly. Unlike a fixed-throw single crank, where stroke is locked at twice the crank radius, the resultant stroke here is the vector sum of two cranks — so shifting one disk relative to the other smoothly tunes stroke from zero up to the geometric maximum. That lets one driveline deliver multiple traverse lengths without a gearbox swap, which is why yarn winders, magnet-wire spoolers and cable layers use it.
Two Crank-disks Variable Alternating Traverse Interactive Calculator
Vary the relative phase angles between the two crank pins and see the normalized traverse stroke for each setting.
Equation Used
For equal crank radii, the output traverse stroke normalized by radius is S/r = 2 cos(phi/2). A 0 deg phase gives 2r, 90 deg gives about 1.41r, and 180 deg cancels to zero.
- Both crank disks have equal radius r.
- Stroke is normalized to crank radius, so outputs are multiples of r.
- Ideal summing linkage with no pin slop or radius mismatch.
- Phase angle phi is measured between the two crank pins.
The Two Crank-disks Variable Alternating Traverse in Action
Two crank disks sit on a common axis. Each carries a crankpin at the same radius r. A connecting link from each crankpin joins a common output slider — either through a summing yoke, a differential link, or a shared slotted crosshead — so the slider's stroke is the projection of the vector sum of the two crank vectors. When the disks run locked at 0° relative phase, the cranks add and you get the maximum stroke of 2r on either side of centre. Lock them at 180° and the cranks cancel — zero stroke, the slider sits still even though both disks keep spinning. Anywhere in between gives a continuously adjustable stroke, set by the included angle φ between the two crankpins.
The disks rotate together once the phase is set, usually through a planetary or worm differential that lets an operator (or a servo) shift the phase under load. That shift is what makes it a *variable* alternating traverse — the slider's reciprocation amplitude changes without altering shaft speed. Frequency stays at the disk RPM, but stroke length tracks the cosine of half the phase angle: S = 2r·cos(φ/2).
Tolerance discipline matters. If the two crankpins are not at exactly equal radius — say one is 0.05 mm longer than the other on a 25 mm crank — you cannot reach true zero stroke at 180° phase, you'll see a residual oscillation of about 0.1 mm peak-to-peak that shows up as a faint ridge on a wound package. Loose pins in the disk (more than 0.02 mm radial slop) cause the slider to chatter at end-of-stroke, and worn link bushings let the phase wander under reversing load, which is the most common failure mode in retrofitted machines.
Key Components
- Primary crank disk: The driven disk, keyed to the input shaft. Carries crankpin A at radius r (typical r = 15-50 mm on textile traverses). Pin fit must be H7/p6 — any radial slop above 0.02 mm shows up as stroke jitter.
- Secondary crank disk: Coaxial with the primary, but free to be rotated relative to it through a phase-shift mechanism. Carries crankpin B at the same radius r. Equal radius within 0.05 mm is mandatory — mismatched radii prevent true zero-stroke at the 180° phase position.
- Phase-shift differential: A worm-and-wheel, planetary gear, or hand-cranked screw that re-indexes the secondary disk relative to the primary while both rotate. Self-locking worm sets are preferred — back-driving torque from the slider would otherwise drift the phase under reversing load.
- Summing linkage or slotted crosshead: Combines the two crankpin motions into a single slider output. Either two connecting rods feeding a common yoke, or a slotted bar engaged by both pins. The slot width must match pin diameter to within 0.03 mm or you get end-stroke chatter.
- Output slider and traverse rod: Carries the working tool — a yarn guide, wire eyelet, or cable shoe. Mass should be kept low; at 200 RPM with a 50 mm stroke the slider sees roughly 22 g acceleration peaks, and inertia loads bend the connecting rods if mass exceeds about 0.5 kg without rebalancing.
Where the Two Crank-disks Variable Alternating Traverse Is Used
Any process that lays a continuous strand back and forth across a rotating package benefits from a traverse where stroke is adjustable without stopping the line. Operators get to change package width to fit a different bobbin, or correct edge build-up, without tearing down the gear train. The mechanism dominates older textile and wire machinery for that reason — the phase-shift differential is cheap, robust, and tolerant of dirty environments where electronic servo-traverse systems struggle.
- Textile machinery: Yarn traverse on a Saurer Allma TC2 two-for-one twister — phase-shift adjusts package width between 150 mm and 250 mm without changing cams.
- Magnet wire winding: Layer-wound transformer bobbin winders at ABB's Brilon plant use a two-disk traverse to hold layer alignment within 0.1 wire diameter across 32 mm and 60 mm bobbins.
- Submarine cable layup: Helical armour layup machines at Nexans Halden run a slow traverse where stroke must shift mid-run to taper the cable end — the differential phase-shift handles it without stopping the catenary.
- Heritage rope-making: Restored ropewalk laying machines use the configuration to vary lay length on hemp cordage of different diameters from a single jackshaft.
- Glass fibre roving winders: Owens Corning Type 30 forming-package winders use phase-adjustable traverse to vary package width between 165 mm and 195 mm to match downstream creel spacing.
- Industrial sewing: Heavy-canvas zigzag heads on Singer 132K-class sailmaker machines use a small two-disk traverse to set zigzag width from 0 to 9 mm via a phase knob.
The Formula Behind the Two Crank-disks Variable Alternating Traverse
The core sizing question is: for a given crank radius and a chosen phase angle, what's the resultant slider stroke? At the low end of the phase range (φ near 0°) you're operating near the geometric maximum stroke of 2r — small phase changes barely move the stroke, so fine width control is poor. At the high end (φ near 180°) you're near zero stroke and a 1° phase error can swing the stroke by several percent of r — sensitive, but unstable to back-driving load. The sweet spot for usable, predictable adjustment sits between roughly 30° and 150° of phase, where stroke responds linearly enough that an operator can dial in a target package width without overshooting.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| S | Resultant slider stroke (peak-to-peak / 2, i.e. amplitude from centre) | mm | in |
| r | Crank radius (same on both disks) | mm | in |
| φ | Relative phase angle between crankpin A and crankpin B | degrees | degrees |
| N | Crank disk rotational speed (both disks share this) | RPM | RPM |
Worked Example: Two Crank-disks Variable Alternating Traverse in a glass-fibre roving package winder
You are sizing the two crank-disks variable alternating traverse on an Owens Corning Type 30 forming-package winder retrofit at a composites plant in Chambéry, France. Both disks carry a crankpin at r = 100 mm. The line runs at 180 RPM and the operator needs to shift package width from 195 mm down to 165 mm without stopping. Find the phase-angle settings and check stroke behaviour across the operating range.
Given
- r = 100 mm
- N = 180 RPM
- Starget,wide = 97.5 (half of 195 mm) mm
- Starget,narrow = 82.5 (half of 165 mm) mm
Solution
Step 1 — solve the stroke formula for phase angle so you can set the operator dial:
Step 2 — at the nominal wide setting (195 mm package width, S = 97.5 mm):
That's a comfortable middle-of-the-range phase setting. Stroke responds smoothly to small phase changes here — about 1.5 mm of stroke change per degree of phase shift, which is exactly the resolution a manual differential handwheel handles well.
Step 3 — at the narrow end of the operating range (165 mm width, S = 82.5 mm):
So the operator only needs to shift the differential by about 9.6° to step package width down by 30 mm. That feels like a quarter-turn of a typical 36:1 worm handwheel — fast and intuitive.
Step 4 — check the geometric maximum at φ = 0° (both pins aligned):
And the low end at φ = 170° (near full cancellation):
Below about φ = 150° the stroke gets twitchy — a 1° phase error swings the stroke by 5% or more, and any backlash in the differential shows up as visible package edge wander. Above φ = 30° you hit the opposite problem: stroke barely responds to phase changes, so fine width tuning becomes impossible. The 30°–150° band is the usable sweet spot.
Result
Set φ ≈ 121. 7° for the 195 mm wide package and φ ≈ 131.3° for the 165 mm narrow package, with both disks running at 180 RPM. In practice that puts the slider traversing at 0.585 m/s peak velocity at the wide setting — fast enough to hear a clear rhythmic snap at end-of-stroke, slow enough that the yarn guide doesn't whip. At the geometric maximum (φ = 0°) the slider would hit 1.2 m/s peaks and shake the frame; near full cancellation (φ > 170°) the package builds up as a near-stationary ring and you'll see a hard ridge form in under a minute. If your measured stroke is 5–10 mm short of the predicted value, check first for unequal crank radii (the secondary disk pin sometimes ships at r = 99.95 mm from the supplier — measure both with a height gauge), then for differential worm backlash above 0.3° (a common wear symptom on Saurer-style traverse heads), and finally for connecting-link bushing wear that lets the summing yoke lag the cranks under load.
Two Crank-disks Variable Alternating Traverse vs Alternatives
The two-disk variable traverse competes with three other ways to get adjustable reciprocation: a fixed slider-crank with mechanical stroke-adjuster on the crankpin, a Scotch yoke with adjustable throw, and a fully electronic servo-traverse. Each wins on different axes.
| Property | Two crank-disks variable alternating traverse | Adjustable-throw slider-crank | Servo-driven linear traverse |
|---|---|---|---|
| Stroke adjustment under load | Yes — phase shift while running | No — must stop machine to re-set crankpin | Yes — programmable on the fly |
| Typical stroke range | 0 to 2r continuously | 0.2r to r, in stepped settings | 0 to mechanical limit, software-defined |
| Maximum practical RPM | 300-400 RPM (mass-limited) | 200-300 RPM | 1000+ RPM with linear motor |
| Stroke repeatability | ±0.1 mm with H7/p6 pins | ±0.3 mm typical (locking screw drift) | ±0.01 mm with closed-loop encoder |
| Capital cost (relative) | 1.0× | 0.6× | 4-8× |
| Failure modes | Worn link bushings, differential backlash | Crankpin lock loosening, stroke creep | Drive electronics, encoder faults |
| Best application fit | Continuous winding with mid-run width changes | Set-and-forget single-product runs | Recipe-driven multi-product lines |
Frequently Asked Questions About Two Crank-disks Variable Alternating Traverse
You're not actually at zero phase — you're at zero resultant stroke, but only if the two crankpins sit at exactly equal radii. A 0.05 mm radius mismatch on a 100 mm crank leaves about 0.1 mm of residual oscillation, which is enough to lay a visible ridge on a tightly wound package over a few thousand turns.
Pull both disks, mip the pins on a height gauge against the disk centre, and grind or shim until they match within 0.02 mm. This is the single most common warranty issue we see on retrofitted traverse heads.
The decision pivots on three things: stroke change frequency, accuracy demand, and environment. If you change package width less than once per shift and need ±0.1 mm repeatability, the two-disk traverse wins on cost and reliability — no electronics to fail in a humid, fibre-laden room. If you change stroke recipe-by-recipe, every few minutes, or need ±0.01 mm, go servo.
The hidden tradeoff is service life. A well-built two-disk traverse with hardened pins runs 20+ years on bushings alone. A servo traverse needs encoder, drive, and motor service every 5-8 years, and the spare parts chain dries up faster than you'd think.
That's geometric sensitivity, not a mechanical fault. Stroke = 2r·cos(φ/2), and the cosine derivative steepens as φ approaches 180°. Near φ = 160°, one degree of handwheel rotation changes stroke by roughly 1.7% of r — about three times more than the same one degree near φ = 90°.
If operators need fine control at narrow strokes, fit a higher-ratio worm (60:1 instead of 36:1) on the differential. That mechanically stretches out the sensitive region without changing the kinematics.
Yes, and the result is a beat-frequency stroke envelope — stroke amplitude oscillates at the difference frequency between the two disks. It's used deliberately on some yarn winders to break ribboning patterns. But it's not what this mechanism is normally specified for, and you lose the closed-form S = 2r·cos(φ/2) relationship.
If you want anti-ribboning, a small (1-3%) speed difference between the disks works well. Going beyond that produces stroke modulation deep enough that the package geometry changes layer-to-layer, which most processes don't want.
If you've already checked unequal crank radii and differential backlash, the next suspect is the summing linkage geometry. If you're using two connecting rods feeding a common yoke, the rod length-to-crank-radius ratio (L/r) introduces a second-order error — at L/r = 4, stroke is short by about 3% versus the pure cosine formula; at L/r = 6, the error drops to under 1.5%.
Check yoke pivot wear too. A worn yoke pin lets each connecting rod take up part of the stroke as pivot travel rather than slider travel. 0.1 mm of pin clearance on a yoke can eat 1-2 mm of slider stroke at the extreme positions.
Mass and reversing inertia set the ceiling. At 50 mm amplitude and 400 RPM, peak slider acceleration hits about 88 g. A typical yarn-guide assembly of 0.3 kg sees roughly 260 N of reversing force at end-of-stroke, which is well within link strength but starts loading the bushings hard enough to halve service life versus a 200 RPM duty.
The rule of thumb on textile traverse heads: keep peak slider acceleration under 50 g for 24/7 duty, under 100 g for intermittent duty. For 50 mm amplitude, that pegs continuous-duty RPM at about 300 and intermittent peak at 420.
References & Further Reading
- Wikipedia contributors. Slider-crank linkage. Wikipedia
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