Question 1

What are the A and B von-Mises groupings in shaft design?

Accepted Answer

A and B are the distortion-energy (von Mises) combinations of the alternating and midrange loads. A = sqrt(4·(Kf·Ma)² + 3·(Kfs·Ta)²) groups the alternating bending moment Ma with the alternating torque Ta; B = sqrt(4·(Kf·Mm)² + 3·(Kfs·Tm)²) groups the midrange bending Mm with the midrange torque Tm. The factor 4 multiplies the bending term and 3 the torsion term. They give the von-Mises stresses σ′a = 16A/(πd³) and σ′m = 16B/(πd³). This is Shigley's Mechanical Engineering Design, Chapter 7, Eq. 7-5/7-6.

Question 2

Which shaft criterion should I use — DE-Goodman, DE-Gerber, DE-ASME-Elliptic or DE-Soderberg?

Accepted Answer

DE-Goodman (Eq. 7-8) is the common, slightly conservative default. DE-Soderberg (Eq. 7-14) is the most conservative because it uses the yield strength Sy. DE-Gerber (Eq. 7-10) and DE-ASME-Elliptic (Eq. 7-12) are less conservative and usually fit test data better; they give a smaller diameter. Conservatism ranks Soderberg > Goodman > Gerber ≈ Elliptic for typical inputs. A good workflow is to size with DE-Goodman and read the others for the spread — and always check first-cycle yield n_y.

Question 3

Why is the first-cycle yield check n_y important?

Accepted Answer

The fatigue diameter equations guard against crack-initiation fatigue, but a shaft can be fatigue-safe yet yield on the very first overload. The first-cycle yield factor n_y = Sy/σ′max (Shigley Eq. 7-15/7-16) uses the von-Mises of the peak (max) bending and torsion. If n_y is below your design factor n, yield governs and you must increase the diameter. This calculator always reports n_y and flags when it governs.

Question 4

Should Kf and Kfs be applied to the stress or to the endurance limit?

Accepted Answer

Apply the fatigue stress-concentration factors Kf and Kfs to the stress — i.e. multiply the moments Ma, Mm by Kf and the torques Ta, Tm by Kfs inside the A and B groupings — never divide Se by Kf. The Table 7-1 presets (sharp/well-rounded shoulder fillet, end-mill/sled-runner keyseat, retaining-ring groove) are first-iteration estimates; keep Kf and Kfs editable and refine them with the actual r/d once the diameter is known.

Question 5

Is imperial torque in lbf·in or lbf·ft?

Accepted Answer

This tool uses lbf·in (pound-force inch) for all moments and torques in US customary units, matching Shigley's worked examples — not lbf·ft. The moment and torque unit dropdowns are locked together so you cannot mix units. Mislabeling lbf·ft as lbf·in is off by a factor of 12 and will badly mis-size the shaft.

Question 6

What does the optional Marin endurance limit estimate do?

Accepted Answer

By default you enter the fully corrected endurance limit Se directly (recommended; cross-link to the fatigue-calculator). The optional 'estimate via Marin' path builds Se = ka·kb·kc·kd·ke·Se′ from Shigley Chapter 6, where the size factor kb depends on the diameter — so in design mode the calculator iterates the diameter and kb to convergence. This estimate is labeled approximate; verify it with the fatigue-calculator for any real design.

Shaft DesignCalculator

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