A variable crank can locate a pin at the intersection of a straight radial slot and a spiral slot in a second coaxial disc. Relative disc rotation changes the intersection radius; after setting and locking, that radius is the crank throw. This calculator assumes an Archimedean spiral and interpolates between entered stroke endpoints.
Variable Crank Interactive Calculator
Enter relative disc rotation, an adjustment-angle span and minimum and maximum stroke values. Results are an assumed linear radial interpolation—not measured hardware geometry, locking capacity, force, wear or design guidance.
Equation Used
- The two discs are coaxial and one contains a straight radial slot.
- The second slot is assumed Archimedean over the entered angle span, giving linear radial change with relative angle.
- Entered relative angle is clamped to zero through the entered adjustment span.
- Stroke endpoints are entered ideal peak-to-peak values; throw equals half stroke.
- The animation shows a locked assembly rotating after the entered setting and does not model adjustment under load.
- Slot width, pin diameter, intersection angle, clearance, lock construction, connecting-rod length, forces, torque, friction, stress, wear, backlash, accuracy, speed, life and safety are omitted.
Spiral-and-Radial Slot Topology
The Digital Mechanism and Gear Library documents a setting mechanism in which a pin slides along an Archimedean spiral in one disc and a radial slot in another. Thomas Walter Barber's Engineer's Sketch-Book also documents a shifting eccentric whose throw is governed by a disc with a spiral slot and locking bolt.
The animation illustrates the two slots and their intersection using the assumed spiral. Slot widths, pin clearance and locking details are not specified.
What This Calculator Represents
The tool clamps an entered relative angle to an entered adjustment span and linearly interpolates between entered stroke endpoints. It reports half of that ideal stroke as throw radius and the radial change per full revolution implied by the Archimedean assumption.
Assumed Archimedean Setting Calculation
effective upper stroke s_hi = max(s_min, s_max)
setting fraction f = theta_c / theta_range
throw r = [s_min + (s_hi - s_min)f] / 2
ideal stroke S = 2r
assumed pitch = [(s_hi-s_min)/2] 360/theta_range
The twice-throw relation is the ideal peak-to-peak stroke of an in-line slider-crank. Connecting-rod length affects intermediate motion but not those two endpoint extremes.
Worked Entered-Value Example
With entered stroke endpoints 2 mm and 25 mm, an adjustment span of 300 degrees and relative rotation of 300 degrees, the clamped setting is 100%. Ideal throw is 12.50 mm, ideal peak-to-peak stroke is 25.00 mm and the assumed spiral pitch is 13.80 mm per revolution.
Model Boundary
- Documented and illustrated: a pin jointly constrained by spiral and radial slots.
- Assumed and calculated: Archimedean linear radial interpolation, twice-throw stroke and implied pitch.
- Not calculated: actual slot coordinates or intersection angle, lock design, adjustment force, running load, connecting-rod motion, contact stress, clearance, backlash, wear, material, service life, accuracy, operating limits or safety.
Frequently Asked Questions
Does every spiral slot produce linear throw versus angle?
No. This result specifically assumes an Archimedean spiral.
What happens above the entered angle span?
The angle is clamped to the entered span.
What if maximum stroke is below minimum stroke?
The minimum becomes the effective upper endpoint, producing zero span and zero pitch. The setting percentage is also displayed as zero when both effective stroke endpoints are equal.
Does the animation adjust while transmitting load?
No. It shows the entered setting rotating as an assembly.
Does twice throw describe every point in the slider motion?
No. It describes only peak-to-peak travel for an ideal in-line slider-crank.
Can this specify slots, a pin or a locking clamp?
No.
References
- Digital Mechanism and Gear Library: pin sliding in an Archimedean spiral slot and a radial slot in two discs.
- Thomas Walter Barber, The Engineer's Sketch-Book, no. 188: shifting eccentric governed by a disc with spiral slot and locking bolt.
- David H. Myszka, Machines and Mechanisms: Applied Kinematic Analysis: ideal in-line slider-crank stroke equals twice crank length.
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