Shaft Key Design Calculator

How to use

1. Select Applied torque to screen key shear, key bearing pressure, and the reduced shaft section, or Shaft strength to estimate static and cyclic torque capacity.
2. Enter shaft diameter d in millimetres between 10 and 200. This selects b, h, t₁, and t₂ from the implemented parallel-key table.
3. In Applied torque mode, enter positive torque T in N·m and effective key length L in mm. Keep L at or below about 1.5d unless load distribution is justified.
4. In Shaft strength mode, enter verified static shear strength τₙ in MPa, dynamic/static ratio r, and stress concentration factor Kf. All are positive; 0 < r ≤ 1.
5. Select Calculate, or edit a field for an immediate update. Select Reset to restore d = 20 mm and the displayed mode defaults.
6. Read bearing pressure as the Applied torque result or cyclic torque capacity as the Shaft strength result. Supporting values identify the selected section and related stresses/capacities.
7. Correct any red warning before interpretation. Outputs are rounded to two decimals using the implemented source-style rule.
8. Do not use this screening model for tapered/Woodruff keys, splines, multiple keys, impact, plastic bearing, detailed fatigue spectra, fretting life, tolerance design, or safety-critical final design.

Worked example

1. Select Applied torque and enter d = 20 mm, T = 100 N·m, and L = 20 mm. 2. The selected section is b × h = 6 × 6 mm with t₁ = 3.5 mm, t₂ = 2.8 mm, and dₖ = 16.5 mm. 3. The 10.00 kN tangential force gives 83.33 MPa key shear, 166.67 MPa bearing pressure, and 113.38 MPa nominal torsional stress at dₖ. 4. Compare all three values with verified shaft, key, and hub allowables; the calculation does not itself declare the joint safe.

Equations used

Tangential force: Fₛ = Tₘₘ/(d/2), where Tₘₘ = 1000T

Key shear: τkey = Fₛ/(Lb)

Key bearing: p = Fₛ/[(h/2)L]

Critical diameter: dₖ = d − t₁

Shaft torsion: τshaft = 16Tₘₘ/(πdₖ³)

Torsion section modulus: S = (π/16)dₖ³

Dynamic shear strength: τₑ = rτₙ

Static capacity: Tstatic = Sτₙ

Cyclic capacity: Tcyclic = Sτₑ/Kf = Srτₙ/Kf

T is positive torque magnitude in N·m; Tₘₘ is N·mm; d, b, h, t₁, t₂, dₖ, and L are mm; Fₛ is N; τkey, p, τshaft, τₙ, and τₑ are N/mm² = MPa; S is calculated in mm³ and displayed as cm³; r and Kf are dimensionless. Capacity equations produce N·mm internally and are displayed in N·m. No load or stress sign is represented.

Theory and method

A parallel key transfers shaft torque as tangential force across the key width and through nominal bearing over half the key height. Shaft diameter selects one standardized nominal key section. The shaft keyway reduces the resisting core diameter to dₖ; the same solid-round torsion section modulus is used to estimate shaft stress and strength-based torque capacities.

Assumptions and limits: one centered parallel key, quasi-static torque, uniform shear over bL, uniform bearing over (h/2)L, elastic nominal shaft torsion, and adequate hub/key engagement. The dimensional table covers 10–200 mm shafts. It omits end effects, clearance, key tipping, nonuniform bearing, shaft/hub flexibility, local notch fields, fatigue spectrum, shock, wear, fretting, corrosion, temperature, residual stress, manufacturing tolerances, and failure interaction. Key length should generally remain below about 1.5d to limit load-distribution degradation as the shaft twists.

Reference: A. van Beek, Advanced Engineering Design: Lifetime Performance and Reliability, keys and keyways calculation collection. Verify dimensions and tolerances against the applicable ISO or DIN parallel-key standard and use certified material data for final design.

Continue in TriboSolver to refine keyway stress concentration, contact pressure, fatigue, and load-spectrum effects.

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Spline-Shaft Connection Calculator screens a splined core. Pinned Joint Calculator screens a transverse pin connection.