An adjustable crank pin can move radially relative to a shaft axis, changing ideal slider-crank stroke. This calculator treats entered slot length as a full symmetric radial span centered on the shaft, so the one-sided comparison radius is half that entered span.
Variable Crank Throw Interactive Calculator
Enter crank radius, a full symmetric adjustment-span convention and ideal radial screw pitch. Results are ideal stroke and adjustment calculations—not actual slot capacity, lock design, load, balance or machine guidance.
Equation Used
- Entered slot length L is defined here as a full symmetric span centered on the shaft; this is an input convention, not a universal slot definition.
- Entered radius is independent and is not clamped to the entered half-span.
- Ideal in-line slider-crank peak-to-peak stroke is twice radius.
- Entered screw pitch is treated as ideal radial pin travel per turn, so stroke changes by twice pitch.
- The drawing is a generic radial screw, pin block, crank and slider cue, not a specific construction.
- Rod geometry, instantaneous motion, forces, torque, power, balance, screw efficiency, lock, backlash, stress, fatigue, wear, accuracy, capacity and safety are omitted.
Documented Adjustable-Radius Constructions
U.S. Patent 3,672,242 documents a crank pin moved radially by adjustment hardware. U.S. Patent 678,546 documents another variable-stroke crank-wheel construction with a radial slot and screw-threaded stud. These sources support adjustable-radius topology, while also showing that construction details vary.
The animation is therefore generic. Its entered full-span convention is part of this calculator only.
What This Calculator Represents
The tool doubles entered radius for ideal stroke, treats half the entered full symmetric span as a comparison radius, doubles entered radial screw pitch for ideal stroke change per turn and reports radius divided by that half-span.
Ideal Stroke and Adjustment Calculations
entered one-sided comparison radius = L/2
twice entered half-span = L
ideal stroke change per screw turn = 2p
radius / entered half-span = 100r/(L/2)
The percentage is not clamped. A value above 100% simply means entered radius exceeds the calculator's separately entered half-span convention.
Worked Entered-Value Example
At 50 mm entered radius, ideal stroke is 100.0 mm. With entered full symmetric span 100 mm, the half-span comparison radius is 50 mm and the ratio is 100%. An ideal radial screw pitch of 1 mm/rev changes ideal stroke by 2.0 mm per turn.
Model Boundary
- Calculated: ideal twice-radius stroke, full/half-span comparison and twice-pitch sensitivity.
- Illustrated: generic radial screw and adjustable pin block.
- Not calculated: actual usable slot length, screw or lock capacity, instantaneous motion, connecting-rod geometry, force, torque, power, balance, stress, fatigue, backlash, wear, accuracy, operating limits or safety.
Frequently Asked Questions
Why is the comparison radius half the entered span?
Because this calculator explicitly defines L as a full symmetric span centered on the shaft.
Is every real slot specified that way?
No.
Can the percentage exceed 100%?
Yes. The entered radius is independent and the output is not a validity limit.
Does twice pitch include backlash?
No. It is ideal sensitivity only.
Are balance and centrifugal load included?
No.
Can this size a real crank, screw or lock?
No.
References
- U.S. Patent 3,672,242, Crank with adjustable crank radius: radially movable crank pin and mechanical adjustment.
- U.S. Patent 678,546, Speed-regulator for explosive-engines: variable stroke using a radial slot and screw-threaded stud.
- David H. Myszka, Machines and Mechanisms: Applied Kinematic Analysis: ideal in-line slider-crank stroke equals twice crank length.
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