Key / Keyway Capacity Calculator

Inputs

Applied torque T

Used for stress & safety-factor checks

Shaft diameter d (mm)

Key width b (mm)

Key height h (mm)

Bearing uses h/2 (pressure on shaft side)

Key length L (mm)

Number of keys n

Allowable shear τ_allow (MPa)

Allowable bearing σ_allow (MPa)

Design safety factor (optional)

Divides allowable stresses

Capacity & Required Length

Model assumes a standard parallel key. With tangential force F at radius d/2, F = 2T/d. Shear: τ = F/(bL) → T_shear = τ_allow·d·b·L / 2.
Bearing (crushing) on shaft side uses h/2 contact: σ = F/((h/2)L) → T_bearing = σ_allow·d·h·L / 4. For multiple keys, capacity scales with n.

Check at Applied Torque (optional)

How to Use the Key / Keyway Capacity Calculator

A quick guide to check torque capacity of a parallel key and estimate the minimum key length for a given torque. Supports SI & Imperial units, multiple keys, optional safety factor, and applied-torque checks.

1) Pick Units

Select SI (mm, N·m, MPa) or Imperial (in, lbf·in, psi).
Tip: Presets switch units automatically.

2) Enter Geometry

d – shaft diameter
b × h – key width and height (bearing uses h/2)
L – engaged key length in the hub
n – number of keys (capacity scales with n)

3) Allowables & Safety Factor

τ_allow – allowable shear stress of the key (or limiting part)
σ_allow – allowable bearing (crushing) stress on the shaft/hub
FS (optional) – divides both allowables for design margin

4) Optional: Applied Torque Check

Enter T and select its unit to compute actual stresses & safety factors.
The tool also returns the minimum required length from shear and bearing.

5) Read the Results

T_max (shear) and T_max (bearing) plus the governing capacity.
If torque is provided: L_min from shear and bearing, actual τ, actual σ, and safety factors.

Notes & Formulas

Tangential force at the key: F = 2T / d
Shear stress: τ = F / (b · L · n) ⇒ capacity T_shear = τ_allow · d · b · L · n / 2
Bearing stress (on shaft side): contact height ≈ h/2
σ = F / ((h/2) · L · n) ⇒ capacity T_bearing = σ_allow · d · h · L · n / 4

Use standard key sizes; ensure L fits the hub. If shaft/hub materials are weaker than the key, base allowables on the limiting material.

Quick Checklist

Units correct (and torque units match)
Geometry matches the engaged length in the hub
Allowables reflect the limiting material & temperature
Multiple keys modeled with correct n and spacing
Governing capacity ≥ required torque with margin

FAQ & Tips

Which allowable controls? Often bearing controls for short, tall keys; shear can control for long, thin keys.
What if I use two keys? Set n = 2; capacity and required length scale with n (assuming balanced placement).
Why is required length huge? Increase key width/height, use more keys, raise allowables, or verify torque/diameter.
Does fillet or chamfer matter? This simple model ignores local stress raisers; keep generous hub wall and proper keyseat fillets.