A Variable Crank is a crank disc or arm with a spiral slot crossing a radial slot, letting the crank pin slide along both slots so you can change the throw radius — and therefore the stroke length — without swapping parts. The two slots index the pin position by simple relative rotation between disc and arm. It solves the problem of fixed-stroke cranks needing tear-down to retune. Production presses, shaker tables, and metering pumps use it to tune output from zero to full stroke in seconds.
Variable Crank Interactive Calculator
Vary the relative slot rotation and stroke range to see the crank pin throw, resulting stroke length, and spiral pitch update live.
Equation Used
The calculator treats the spiral as an Archimedean slot, so the crank pin radius changes linearly as the spiral disc is rotated relative to the radial slot arm. The resulting slider-crank stroke is twice the throw radius.
- Spiral slot is Archimedean, so throw radius changes linearly with relative rotation.
- Slider-crank stroke is twice the crank throw radius.
- Relative rotation is clamped to the usable adjustment range.
- Slot clearance, wear, and elastic deflection are not included.
Operating Principle of the Variable Crank (spiral and Radial Slots)
The mechanism is two plates stacked on a common shaft. One plate carries a straight radial slot. The other carries a spiral slot — usually an Archimedean spiral so pin travel is linear with relative rotation. The crank pin passes through both slots at once. Where the slots cross, the pin sits. Rotate one plate relative to the other and the crossing point slides up or down the radial slot, which changes the throw radius — call it r — from near zero out to the full plate radius. Lock the two plates together, and the assembly rotates as a rigid crank with the new throw.
Why build it this way? Because a single slot only constrains the pin in one axis. You need two slots crossing at an angle to pin down a unique position. The spiral gives you a smooth, calibrated mapping between angular adjustment and stroke. The radial slot keeps the pin aligned with the crank centreline so the connecting rod always pulls clean. If the spiral pitch is too steep, fine adjustment becomes twitchy. Too shallow, and you run out of plate before reaching full throw. We typically target a spiral that delivers full-throw range over 270° to 330° of relative rotation.
Tolerances bite hard here. The pin-to-slot clearance must stay tight — typically 0.02 to 0.05 mm diametral — or you get backlash that shows up as stroke jitter at top-dead-centre. If the locking clamp slips, the pin walks under load and the stroke creeps shorter every cycle. The most common failure mode is slot wear where the spiral and radial slots cross: that crossing point sees the highest contact stress because the pin only touches the slot edges along a short arc. Once you see ovaling at the crossing, replace the plate. Don't try to dress it.
Key Components
- Spiral Slot Disc: The driving plate machined with an Archimedean spiral slot, typically 6 to 10 mm wide. Spiral pitch is sized to give full throw range over roughly 270° to 330° of relative rotation between the two plates. Slot edges must be hardened to 55 HRC minimum or wear at the crossing point will halve service life.
- Radial Slot Arm: The driven plate carrying a straight radial slot through which the pin slides. Slot length sets the maximum stroke. Slot width matches the spiral within 0.02 to 0.05 mm diametral clearance — go tighter and the pin binds, go looser and stroke jitter shows up at every reversal.
- Crank Pin: The hardened pin that passes through both slots simultaneously. Typically ground to h6 tolerance, surface finish Ra 0.4 µm or better. The pin's effective length is set so it engages both slots fully without protruding to foul the connecting rod end.
- Locking Clamp or Bolt: Once you set the throw, the clamp locks the two plates so they rotate as a single rigid crank. Clamp force must hold against peak inertial torque — undersize the clamp and the plates index under load, and stroke drifts mid-run.
- Throw Scale or Vernier: An indexed scale machined or etched onto the disc edge so the operator can dial in stroke length repeatably. On well-built units the scale reads directly in mm of stroke, not degrees of rotation, so setup is one step instead of two.
Where the Variable Crank (spiral and Radial Slots) Is Used
The Variable Crank earns its place anywhere stroke needs to change between batches without tearing down the drive. It's mechanical, it's repeatable, and it doesn't need a servo. You see it most in legacy production equipment where the original engineers wanted runtime adjustability but couldn't justify electronic position control. Modern retrofits still favour it where dust, oil mist, or wash-down would kill an electromechanical alternative.
- Vibration Testing: Adjustable-stroke shaker tables for soil compaction labs, where the operator dials throw from 2 mm to 25 mm to match ASTM D4253 test conditions on a Humboldt H-3756 vibratory table.
- Pharmaceutical Dosing: Variable-displacement metering pumps on a ProMinent Sigma diaphragm dosing pump, where stroke adjustment sets dose volume from 10% to 100% of rated capacity without changing pump heads.
- Foundry Equipment: Sand-mould jolt-squeeze machines on a DISA MATCH 20 line where jolt amplitude is tuned per mould profile by adjusting crank throw between casting runs.
- Textile Machinery: Stroke-adjustable needle bars on a Karl Mayer HKS 3-M warp knitting machine, where pattern changes require small throw adjustments between production lots.
- Packaging: Variable-stroke film cut-off knives on a Bosch Pack 403 horizontal flow wrapper, letting the line run pouches from 80 mm to 220 mm long without changing the crank disc.
- Heritage Machine Tools: Adjustable-stroke shapers like the Atlas 7B metalworking shaper, where the spiral-and-radial slot crank sets cutting stroke between 50 mm and 175 mm depending on the workpiece.
The Formula Behind the Variable Crank (spiral and Radial Slots)
The output stroke of a Variable Crank is twice the throw radius — straightforward kinematics, but the practitioner's real question is what throw setting they need for a given stroke and what happens at the limits. At small throw radii (say below 10% of plate radius) the connecting rod angularity dominates and the slider motion gets noticeably non-sinusoidal. At full throw you hit the geometric limit of the radial slot and any further adjustment does nothing. The sweet spot for clean kinematics and low side-load on the slider is typically 30% to 80% of maximum throw.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| S | Output stroke length of the slider | mm | in |
| r | Current throw radius (distance from shaft centre to pin) | mm | in |
| rmax | Maximum throw radius set by the radial slot length | mm | in |
| θadj | Relative rotation between spiral disc and radial arm from zero-throw position | deg | deg |
| θfull | Relative rotation that produces full throw | deg | deg |
Worked Example: Variable Crank (spiral and Radial Slots) in a foundry sand-rammer retrofit
You are sizing the variable crank that drives the ram on a Hunter HMP-20 squeeze-jolt moulding machine retrofit at an iron foundry in Sheffield. The machine produces three different mould profiles, each needing a different ram stroke. The radial slot allows a maximum throw radius of 60 mm, full throw is reached at 300° of relative plate rotation, and the operator needs strokes of 40 mm, 80 mm, and 110 mm depending on the mould being run.
Given
- rmax = 60 mm
- θfull = 300 deg
- Snom = 80 mm
Solution
Step 1 — at the nominal stroke of 80 mm, calculate the required throw radius:
Step 2 — find the relative plate rotation that places the pin at 40 mm throw:
Step 3 — at the low end of the operating range, a 40 mm stroke for the shallow-cope profile, throw radius is 20 mm and adjustment angle is:
At this setting the pin sits at 33% of plate radius. Connecting rod angularity becomes pronounced — the ram acceleration profile skews toward the bottom of stroke, which actually helps mould compaction at the parting line. The operator will feel the jolt as a sharper top-end snap.
Step 4 — at the high end, the 110 mm stroke for the deep-flask profile:
This puts the pin at 92% of maximum throw — close enough to the slot end that you must verify the locking clamp is fully seated. Above 90% throw the leverage on the clamp goes up sharply and any clamp slip during the squeeze stroke walks the pin inward by 1 to 2 mm per shift, shortening the stroke noticeably between morning and afternoon production.
Result
The nominal 80 mm stroke needs the throw set to 40 mm radius, achieved by rotating the spiral disc 200° relative to the radial arm. At the 40 mm short-stroke setting (100°) the ram delivers a snappy, asymmetric jolt that's well-suited to shallow moulds, while the 110 mm long-stroke setting (275°) sits near the slot limit where clamp slip becomes the dominant risk. If you measure a stroke 3 to 5 mm short of predicted, the usual culprits are: (1) clamp bolt under-torqued so the plates index under inertial load — torque to 85 Nm minimum on an M12 clamp, (2) ovaling at the slot crossing point letting the pin migrate radially under cyclic load, visible as a teardrop wear pattern, or (3) spiral disc machined with pitch error so θfull isn't actually 300° — verify with a no-load throw measurement at 0° and at full adjustment before trusting the scale.
When to Use a Variable Crank (spiral and Radial Slots) and When Not To
The Variable Crank competes with a few alternatives whenever stroke needs to change between runs. The decision usually comes down to how often you change stroke, how repeatable the change must be, and what your maintenance environment looks like.
| Property | Variable Crank (Spiral & Radial Slots) | Fixed Crank with Swappable Discs | Servo-Driven Linear Actuator |
|---|---|---|---|
| Stroke adjustment time | 10-30 seconds | 10-20 minutes per swap | Under 1 second via control input |
| Stroke repeatability after re-set | ±0.2 mm with vernier scale | ±0.05 mm — fixed geometry | ±0.01 mm with encoder feedback |
| Max operating speed | Up to 600 RPM with balanced disc | Up to 1500 RPM — single rigid casting | Limited by actuator dynamics, typically 200 mm/s peak |
| Capital cost (relative) | Medium — precision slot machining | Low — simple discs plus spares | High — servo, drive, encoder, controller |
| Tolerance to dust, oil, wash-down | Excellent — fully mechanical | Excellent — fully mechanical | Poor without IP67 rating and sealed cabling |
| Service life before slot replacement | 10-15 million cycles at 60% throw | 30+ million cycles — no wear surfaces | Servo lifetime depends on duty cycle, 20,000+ hours typical |
| Best application fit | Frequent stroke changes, dirty environments | Fixed-stroke production with rare changeover | Programmable motion profiles, clean rooms |
Frequently Asked Questions About Variable Crank (spiral and Radial Slots)
The clamp is slipping under cyclic inertial load. Every reversal at top and bottom of stroke loads the locking clamp in shear, and on a Variable Crank near full throw the lever arm on the clamp is at maximum. If the clamp bolt was torqued cold or the clamp face has any oil contamination, the friction coefficient drops and the plates index a fraction of a degree per hundred cycles. After 4,000 cycles you'll measure 2-3 mm of lost stroke.
Fix: degrease the clamp faces with brake cleaner, torque to spec (85 Nm typical for M12), and add a Belleville washer stack to maintain preload as the joint settles. If slip persists, the clamp face is glazed and needs a light scuff with 240-grit.
Below about 8-10% of maximum throw the mechanism becomes unreliable for two reasons. First, the spiral-to-radial crossing angle gets shallow near the centre, which means the pin position is highly sensitive to small adjustment errors — a 1° rotation of the disc moves the pin further than it does at mid-throw. Second, any pin-to-slot clearance (even the 0.03 mm you specified) becomes a large fraction of the throw, so backlash dominates the actual stroke.
Rule of thumb: design your operating range to start at 15% of maximum throw minimum. If you genuinely need very small strokes, use a longer connecting rod and a smaller rmax rather than running a large crank near zero.
For twice-a-day changes in a clean environment, a servo wins on repeatability and operator effort — you change stroke from the HMI, not from a wrench. But if you're in a foundry, a paint line, or any wash-down environment, the Variable Crank wins on uptime. Servo cables and encoders are the first thing to fail in dirty plants, and a 30-second manual stroke change beats a 4-hour servo replacement every time.
The break-even rule we use: under 6 stroke changes per shift in a hostile environment, mechanical wins. Over 6 changes per shift or in a clean environment, servo wins.
That's connecting rod angularity, not a fault in the Variable Crank. When throw radius is small relative to connecting rod length, the slider motion approaches pure sine. But when r/L (throw radius over rod length) climbs above about 0.25, the rod's swing introduces a quick-return asymmetry — the slider takes more crank rotation to traverse one direction than the other.
If you don't want the asymmetry, lengthen the connecting rod so r/L stays below 0.2 across your full adjustment range. If you actively want the asymmetry (common in shapers and some compaction equipment), the Variable Crank lets you tune it by changing throw without changing the rod.
Run a two-point check. Set the crank to its zero-throw position (pin at shaft centre) and measure slider stroke — it should be zero or within 0.5 mm of zero. Then set to full-throw position and measure stroke against 2 × rmax. If both endpoints match the design values but mid-range readings are off, the spiral pitch is non-linear — common on cheaply machined discs where the spiral was approximated with a series of arcs rather than cut on a true Archimedean path.
If the endpoints themselves are off, you have either a slot-length error (measure rmax directly with the pin removed) or a scale-zero error (re-index the vernier). The two-point test takes 5 minutes and saves an afternoon of chasing the wrong root cause.
Full throw concentrates contact stress at the slot crossing point because the pin sits at the maximum-leverage end of the radial slot. Hertzian contact stress at that location can be 2-3× higher than at mid-throw. On hardened slot edges (55 HRC) you'll still get 5-8 million cycles at full throw before measurable ovaling. On soft slots (under 45 HRC) life drops to 1-2 million cycles.
Practical guidance: if your duty cycle calls for sustained operation above 90% of maximum throw, oversize rmax by 20% so your operating point sits at 75-80% of the slot capacity. The slot will last 3-4× longer for a small increase in plate diameter.
References & Further Reading
- Wikipedia contributors. Slider-crank linkage. Wikipedia
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