An adjustable crank changes the radial distance from shaft axis to crank pin. For an ideal in-line slider-crank completing a full revolution, peak-to-peak slider stroke equals twice that radius. This calculator compares two independently entered radius states.
Variable Crank Motion Interactive Calculator
Enter crank-radius state A and state B. Results are ideal endpoint geometry—not an instantaneous motion profile, load, power, clamp, connecting-rod or machine design calculation.
Equation Used
- Both inputs are positive crank radii in millimetres.
- Each state is an ideal in-line slider-crank completing a full revolution.
- The two radii are independent entered comparison states, not minimum and maximum constraints.
- The generic animation uses a schematic radial slot and does not infer a specific adjustment or locking construction.
- Connecting-rod length affects intermediate position and force transmission but not ideal peak-to-peak endpoint stroke.
- Instantaneous motion, speed, acceleration, load, torque, power, clamp force, stress, fatigue, clearance, backlash, wear, accuracy, operating limits and safety are omitted.
Documented Adjustable-Radius Principle
U.S. Patent 3,672,242 documents a crank with a crank pin movable radially relative to the shaft axis, while U.S. Patent 3,703,839 documents a different variable-throw crankpin arrangement. These sources support the general adjustable-radius principle but not one universal slot, clamp or lock.
The animation illustrates a generic radial slot and two entered crank-radius states. The slot and clamp are not dimensioned for a particular machine.
What This Calculator Represents
The tool doubles each entered radius for ideal peak-to-peak stroke, subtracts state A from state B and divides B by A. It makes no claim about force, power, speed, adjustment method or machine suitability.
Ideal Endpoint Comparison
state B stroke S_B = 2r_B
signed difference = S_B - S_A
stroke ratio = S_B / S_A = r_B / r_A
For an ideal in-line slider-crank, the two extreme slider positions differ by twice the crank radius. Connecting-rod length is needed for intermediate position and angularity, neither of which is calculated here.
Worked Entered-Value Example
At entered radii 35 mm and 80 mm, ideal endpoint strokes are 70 mm and 160 mm. The signed difference B - A is 90 mm and the stroke ratio B/A is 2.29.
Model Boundary
- Calculated: two ideal twice-radius strokes, signed difference and ratio.
- Illustrated: generic radial adjustment and two crank states.
- Not calculated: instantaneous slider position, velocity, acceleration, rod angle, force, power, mechanical advantage, pin or slot design, locking, stress, fatigue, backlash, wear, accuracy, limits or safety.
Frequently Asked Questions
Are the inputs required to be short and long?
No. They are independent states A and B; the difference may be negative.
Does twice radius require a connecting-rod length?
Not for peak-to-peak endpoint stroke of an ideal in-line slider-crank.
Does the ratio describe force or power?
No. It is only a stroke ratio.
Does the drawing specify a clamp or lock?
No.
Are speed and acceleration included?
No.
Can this size or approve a real adjustable crank?
No.
References
- U.S. Patent 3,672,242, Crank with adjustable crank radius: radially movable crank pin and adjustment construction.
- U.S. Patent 3,703,839, Variable-throw crankpin crankshaft: another documented variable-eccentricity construction.
- David H. Myszka, Machines and Mechanisms: Applied Kinematic Analysis: ideal in-line slider-crank stroke equals twice crank length.
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