An adjustable crank throw changes the radial distance between a shaft axis and crank pin. In an ideal in-line slider-crank, the slider's peak-to-peak stroke is twice that entered throw. This calculator reports that geometric relationship and where the entered throw lies within an entered interval.
Variable Adjustment Interactive Calculator
Enter a minimum throw, an adjustable throw and a maximum throw. Results are an ideal geometric comparison—not a linkage-dimension, load, accuracy, actuator, lock or machine-selection calculation.
Equation Used
- Throw values are nonnegative geometric inputs in millimetres.
- The output slider is ideal, in-line and completes a full crank revolution.
- If entered maximum throw is below entered minimum throw, the minimum is used as the effective upper bound.
- Interval use is clamped to 0–100%; an entered adjustable throw outside the interval is not silently changed for the stroke calculation.
- The drawing is generic and does not infer disc, slot, screw, rod, bearing or guide dimensions from entered throw.
- Connecting-rod angularity effects on instantaneous position, backlash, compliance, loads, balance, friction, locking, stress, fatigue, speed limits, tolerances and safety are omitted.
Documented Adjustable-Throw Topology
U.S. Patent 3,672,242 describes an adjustable-throw crank whose crank pin moves radially. U.S. Patent 3,703,839 describes another variable-throw crankpin arrangement whose eccentricity is changed by axial movement of an internal rod and inclined pin. These sources support the general idea of changing crankpin radius; they do not establish one universal adjustment mechanism.
The animation shows a generic radial slot, adjustment-screw cue, crank pin, connecting rod and in-line slider. The patent examples use their own adjustment arrangements.
What This Calculator Represents
The tool doubles each entered throw to report ideal peak-to-peak stroke. It also places the entered adjustable throw inside an entered minimum-to-effective-maximum interval and clamps that percentage to 0–100%.
Ideal Stroke and Range Calculations
minimum stroke = 2 r_min
effective upper bound r_hi = max(r_max, r_min)
maximum stroke = 2 r_hi
interval use = clamp[100(r_adj-r_min)/(r_hi-r_min), 0, 100]
For an ideal in-line slider-crank, the two extreme slider positions differ by twice the crank radius. This peak-to-peak result does not require a connecting-rod length, although rod length affects intermediate position, angle and force transmission.
Worked Arithmetic Example
With entered minimum throw 0 mm, adjustable throw 25 mm and maximum throw 25 mm, ideal peak-to-peak stroke is 50 mm, the entered minimum stroke is 0 mm, the effective maximum stroke is 50 mm and interval use is 100%.
Model Boundary
- Calculated: twice-throw strokes and clamped position within an entered interval.
- Illustrated: generic radial adjustment slot, crank pin, connecting rod and in-line slider.
- Not calculated: instantaneous slider position, rod angle, forces, torque, bearing load, balance, vibration, backlash, adjustment screw travel, locking, stress, fatigue, tolerances, accuracy, speed limits or safety.
Frequently Asked Questions
Does twice the throw always equal peak-to-peak stroke?
It does for the ideal in-line slider-crank completing a full revolution.
Does the percentage represent machine utilization?
No. It is only the entered throw's clamped position within the entered interval.
What happens if maximum throw is below minimum throw?
The calculator uses the entered minimum as the effective upper bound, making the interval span zero and the displayed interval percentage zero.
Is an out-of-range adjustable throw changed?
No. Its stroke still uses the entered value; only the interval percentage is clamped.
Does the drawing specify a real adjustment screw or lock?
No.
Can this size or approve a real mechanism?
No.
References
- U.S. Patent 3,672,242, Crank with adjustable crank radius: radially movable crank pin and mechanical adjustment.
- U.S. Patent 3,703,839, Variable-throw crankpin crankshaft: variable crankpin eccentricity and documented adjustment construction.
- David H. Myszka, Machines and Mechanisms: Applied Kinematic Analysis: ideal in-line slider-crank stroke equals twice crank length.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
