A Variable Crank Throw is a crank mechanism whose effective radius — the distance from the shaft centreline to the crank pin — can be changed to alter the stroke of a connected slider, ram, or rod. It works by relocating the crank pin along a radial slot or screw track, which directly scales the reciprocating stroke as 2 × r. The purpose is to tune stroke length without swapping parts, so one machine can run different products or different process strokes. Briquette presses, shapers, and bowl feeders use it to dial in stroke from full throw down to near-zero in seconds.
Variable Crank Throw Interactive Calculator
Vary crank throw radius, slot length, and leadscrew pitch to see the resulting reciprocating stroke and adjustment sensitivity.
Equation Used
The adjustable crank pin sets the effective throw radius r. For a simple slider-crank, the reciprocating stroke is S = 2r. If a leadscrew moves the pin radially by pitch p each turn, the stroke changes by 2p per turn.
- Slider stroke is twice the effective crank throw radius.
- Slot length sets maximum throw as r_max = L_slot / 2.
- Leadscrew pitch moves the crank pin radially with negligible backlash.
Operating Principle of the Variable Crank Throw
The Variable Crank Throw, also called a Variable Crank Pin in press and shaper terminology, replaces the fixed crank pin of a standard slider-crank with a pin that slides along a radial slot, T-slot, or leadscrew machined across the face of the crank disc or web. Move the pin outward and the throw radius r increases, so stroke S = 2r grows. Move it inward toward the shaft centreline and stroke shrinks toward zero. That's the whole idea — one drive, infinitely variable stroke between the mechanical limits.
The adjustment hardware is where it gets interesting. Most industrial designs use a captive leadscrew running through the crank web, with the crank pin riding on a square nut inside the slot. You turn the leadscrew — sometimes via a bevel gear stack accessible through a port in the flywheel guard — and the pin walks in or out. The crank radius adjustment is read from a graduated scale or a dial indicator on the leadscrew shaft. Tolerances matter here: if the slot has more than about 0.05 mm of side play on the pin block, you'll see stroke variation cycle-to-cycle and the connecting rod big-end will hammer. If you notice the stroke wandering 0.2 mm or more during a run, the gib screws on the slot are loose or the leadscrew thrust bearing has worn.
Dynamic balancing is the other catch. As you change r, you change both the unbalanced rotating mass and the radius at which it rotates, so the centrifugal force F = m × ω² × r shifts. Machines that run at any speed need a counterweight that also adjusts — usually slaved to the same leadscrew on the opposite side of the disc. Skip that, and at high RPM the bearings will warn you, loudly. Common failure modes are pin-block galling from missed lubrication of the slot, leadscrew thread stripping when operators force adjustment under load (always adjust at rest), and crack initiation at the slot ends if the disc is undersized.
Key Components
- Crank Disc or Web: The rotating element carrying the radial slot. Typically forged or cast steel, 4140 normalized is common, with the slot finish-ground to 0.02 mm flatness. Slot length sets the maximum throw — a 100 mm slot gives up to 50 mm crank radius and 100 mm stroke.
- Adjustable Crank Pin: Hardened steel pin (58-62 HRC) press-fit into a sliding block that rides in the slot. The pin diameter must match the connecting rod big-end bore within H7/g6 — typically 0.013 mm diametral clearance on a 25 mm pin. Anything looser and you get knock at top dead centre.
- Radial Leadscrew: Threaded shaft running through the slot, driving the pin block in and out. Usually a fine pitch — 1 mm or 1.5 mm — so one turn moves the pin block 1 mm and stroke changes 2 mm. Trapezoidal or ACME thread for load capacity, with a thrust bearing at each end.
- Pin Block / Slider: The square or rectangular carrier holding the crank pin and threading onto the leadscrew. Bronze or oil-impregnated material against the steel slot flanks. Side gibs adjusted to under 0.05 mm clearance keep the pin from chattering.
- Counterweight (slaved): Mass on the opposite side of the disc that moves outward as the pin moves inward, and vice-versa, to keep the rotating assembly balanced at any setting. Without it, a 5 kg pin block at 50 mm radius spinning at 300 RPM throws roughly 1,400 N of unbalanced force.
- Locking Mechanism: Clamping screws or a hydraulic clamp that locks the pin block to the disc once stroke is set. Critical — running with only the leadscrew holding the pin will eventually back-drive the screw under cyclic load and the stroke will drift mid-shift.
Who Uses the Variable Crank Throw
Anywhere you need to change reciprocating stroke without changing parts, the Variable Crank Throw earns its keep. Briquette presses, metal shapers, vibratory feeders, mechanical sieves, and laboratory test rigs all use the same core idea — adjust the throw, change the stroke. The Variable Crank Pin terminology shows up most often in older British and German shaper manuals and in vibration-test equipment specs.
- Metal Shapers: The Atlas 7B and South Bend 7-inch shapers use a Variable Crank Pin inside the bull gear to set cutting stroke from about 25 mm up to 175 mm, matched to workpiece length.
- Briquette and Tablet Presses: Komarek B-series and K&P briquetting presses use a Variable Crank Throw on the eccentric drive to tune compaction stroke between roll feeds — typical adjustment range 6 mm to 20 mm.
- Vibratory Feeders: Afag and RNA bowl feeder drive units use an adjustable eccentric drive — same mechanism, different name — to tune amplitude on the trough between 0.3 mm and 2.5 mm peak-to-peak.
- Sieve Shakers: Retsch AS 200 and W.S. Tyler RX-29 sieve shakers use a variable crank pin to adjust horizontal stroke from 0.1 mm to 3 mm depending on particle size and sample mass.
- Fatigue Test Rigs: MTS and Instron rotating-bending fatigue machines use an adjustable crank radius to set displacement amplitude on the specimen grip without swapping the drive crank.
- Reciprocating Pumps: Older positive-displacement metering pumps from LEWA and Milton Roy used a manually adjustable crank throw to set delivery volume per stroke from 10% to 100% of rated displacement.
The Formula Behind the Variable Crank Throw
Stroke is governed by a brutally simple relationship — stroke equals twice the crank radius. What matters is interpreting that across the range your machine sees. At the low end of typical operating range — say r near 5 mm — the slider barely moves and any slot backlash dominates the actual stroke seen at the rod. At the high end — r near the slot's maximum — bearing forces climb with r at constant ω, and the connecting rod angle deviation grows, distorting the slider's velocity profile away from pure sinusoidal. The sweet spot is usually 50-80% of maximum throw, where slot backlash is negligible relative to stroke and rotating loads are still inside the bearing's L10 envelope.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| S | Slider stroke length (peak-to-peak displacement) | m | in |
| r | Crank radius — distance from shaft centreline to crank pin axis | m | in |
| m | Effective rotating mass at the crank pin (pin block + half rod) | kg | lb |
| ω | Crank angular velocity | rad/s | rad/s |
| Fcentrifugal | Unbalanced rotating force at the crank pin | N | lbf |
Worked Example: Variable Crank Throw in a textile loom shedding cam rig
You are setting the Variable Crank Throw on the auxiliary shedding-motion drive of a Picanol OmniPlus 800 air-jet loom retrofit at a denim mill in Ahmedabad, India. The drive runs at 420 RPM and you need to tune the heald frame lift from a low-end 28 mm stroke for fine plain-weave shirting up to a high-end 96 mm stroke for heavy 3/1 denim twill, with a nominal 60 mm setting for the standard product. The pin block plus effective rod mass is 2.8 kg. You need to know the required crank radius at each setting and the centrifugal load the pin bearing will see at full speed.
Given
- Slow = 28 mm
- Snom = 60 mm
- Shigh = 96 mm
- N = 420 RPM
- m = 2.8 kg
Solution
Step 1 — convert speed to angular velocity, the same for all three settings:
Step 2 — compute crank radius at the nominal 60 mm stroke:
Step 3 — compute centrifugal force at the nominal setting:
At the low end, S = 28 mm, so rlow = 14 mm and Flow = 2.8 × 43.982 × 0.014 = 75.8 N. The heald frame barely flutters — perfect for fine plain-weave shirting where shed opening must stay tight to keep the pick straight, and the bearing is loafing at less than half nominal load.
At the high end, S = 96 mm, rhigh = 48 mm and Fhigh = 2.8 × 43.982 × 0.048 = 260.0 N. That's a 3.4× swing in bearing load between low and high settings on the same machine at the same speed, which is exactly why the pin bearing on this kind of drive is always sized to the high-end load, not the nominal.
Result
At the nominal 60 mm stroke setting, you need a crank radius of 30 mm and the pin bearing will see 162. 5 N of centrifugal load at 420 RPM. The low-end shirting setting at 14 mm radius is gentle on the drive (75.8 N) and the high-end denim setting at 48 mm radius pushes 260 N — sweet spot for bearing life sits around the nominal where you're at roughly 60% of peak load. If you measure the actual stroke and find it 1-2 mm short of predicted across all settings, the connecting rod big-end is wearing oversized — check for clearance above 0.05 mm. If stroke drifts during a shift but is correct at startup, the leadscrew thrust bearing has lost preload and the pin block is back-driving under load. If only the high-stroke settings show stroke shortfall, the slot ends are deforming or the disc is flexing — inspect for crack initiation at the slot terminations.
Variable Crank Throw vs Alternatives
Other ways exist to vary stroke — Scotch yokes with adjustable pivots, hydraulic stroke limiters, servo-driven linear actuators. Each has a place. The Variable Crank Throw, sometimes specced as a Variable Crank Pin in older drawings, wins where you want mechanical simplicity and rigid stroke control, and loses where you need on-the-fly adjustment under load or sub-millimetre repeatability.
| Property | Variable Crank Throw | Adjustable Scotch Yoke | Servo Linear Actuator |
|---|---|---|---|
| Maximum continuous speed | Up to 600 RPM with proper balancing | 200-400 RPM (yoke wear-limited) | Limited by ballscrew critical speed, typically ≤ 3000 RPM equivalent |
| Stroke setting repeatability | ±0.05 mm with leadscrew + lock | ±0.1 mm (pivot pin slop dominates) | ±0.01 mm or better (encoder feedback) |
| Adjustable under load | No — must stop machine | No — must stop machine | Yes — change setpoint anytime |
| Cost (relative) | 1× (baseline mechanical) | 1.2-1.5° | 5-10× |
| Maintenance interval | Slot lubrication every 500 h, leadscrew check yearly | Yoke surface re-machine every 2000-4000 h | Ballscrew preload check every 4000 h, drive electronics largely service-free |
| Typical lifespan | 20+ years on properly sized discs | 10-15 years before yoke surface fatigue | 30,000-50,000 hours on the actuator |
| Best application fit | Production machines with infrequent stroke changes (shapers, presses, looms) | Constant-velocity reciprocation needs (valve actuators) | Recipe-driven machines requiring frequent stroke change (test rigs, modern packaging) |
Frequently Asked Questions About Variable Crank Throw
Cyclic load on the connecting rod alternately pushes and pulls the pin block along the slot. If the only thing holding the pin block in position is the leadscrew threads, those threads see reversing axial load every revolution and any backlash lets the block walk. Trapezoidal threads aren't self-locking under vibration.
The fix is to use the dedicated locking screws or hydraulic clamp that should be part of the assembly — set the position with the leadscrew, then clamp the block solidly to the disc face. If your machine doesn't have a clamp, retrofit one. Don't rely on Loctite on the leadscrew nut, that fails under heat cycling.
Decision rule: how often does stroke change in a shift? If stroke is set once a week or once a product changeover, the Variable Crank Pin wins on cost (5-10× cheaper), simplicity, and rigidity. If stroke needs to change every batch or under recipe control, the servo wins because the mechanical version requires stopping the machine and unlocking the pin clamp every time.
The other deciding factor is force at top dead centre — a mechanical crank handles huge instantaneous forces (presses, shapers) without breaking a sweat, while a servo actuator capable of the same peak force costs significantly more and needs regenerative braking on the deceleration stroke.
Three places typically eat stroke between the crank pin and the slider, and they add up. First, connecting rod big-end clearance: 0.1 mm of bore wear gives roughly 0.2 mm of lost stroke. Second, slider gib clearance: a slider with 0.05 mm side play loses an arc-second of motion at each end of stroke. Third, and most often the culprit, the dial graduation references leadscrew rotation, not actual pin position — so leadscrew backlash (commonly 0.3-0.5 mm on worn ACME threads) translates to roughly twice that in stroke error.
Verify by mounting a dial indicator directly on the slider and turning the crank by hand through one revolution. If the indicator reads correct stroke but the dial doesn't match, the dial is the liar. If the indicator also reads short, the rod or slider is worn.
Counter-intuitive but real. At small throw, the pin spends more of each revolution near the shaft centreline where oil-slinger lubrication systems are designed to fling oil outward — the pin is too close to the centre to catch much. Large-throw settings sweep through the oil-rich zone of the crankcase and pick up better splash lubrication.
If you run mostly at small-stroke settings, switch to forced lubrication through a drilled passage in the leadscrew, or run a heavier oil grade. Pin temperatures above 70°C above ambient indicate inadequate oil film and you're hours from a galled pin.
The rod itself stays the same length, but the rod-to-crank ratio L/r changes dramatically as you adjust throw, and that changes the motion profile. At small throw (large L/r ratio, say 10:1 or higher), the slider motion is nearly pure sinusoidal — symmetric acceleration on both halves of the stroke. At large throw approaching the slot maximum, L/r might drop to 3:1 or 4:1, and the motion becomes noticeably asymmetric — faster on the half-stroke away from the crank, slower on the return.
For most applications this doesn't matter. For applications where slider velocity profile is critical (some machine tools, certain pumps), size the rod long enough that L/r stays above 4 even at maximum throw.
Don't. The slot length is sized against the disc's section modulus and fatigue stress at the slot ends — the highest-stress region on the entire crank disc. Lengthening the slot moves the slot terminus closer to the disc rim and dramatically reduces fatigue life. We've seen home-shop modifications to South Bend shapers crack the bull gear within a year when operators extended the slot to get more stroke.
If you genuinely need more stroke than the original design allows, the right answer is a larger-diameter disc, not a longer slot in the existing one. Or accept the design limit — it's there for a reason.
References & Further Reading
- Wikipedia contributors. Crank (mechanism). Wikipedia
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