Machine elements

Shaft–hub connections and parallel keys

A parallel key creates a positive-locking torque path between a shaft and hub. Correct selection requires more than choosing a standard width: the shaft, key, hub material, engaged length, fits and axial retention must work as one connection.

STANDARD SELECTION

Start with shaft diameter, then verify capacity

DIN 6885-1: defines standard parallel-key dimensions and forms.
Shaft diameter: selects the standard key width b and height h range.
Key length: must fit the hub and provide sufficient loaded contact area.
Designation example: Feather key DIN 6885 · A · 12 × 8 × 56.

LOAD PATH

Convert torque into tangential force

The transmitted torque produces a tangential force at the shaft radius. Use this force to screen the key for shear and the loaded keyway face for surface pressure.

F_t = 2T / d · τ ≈ F_t / (b·l_eff) · p ≈ F_t / A_bearing

The softer hub or shaft-keyway surface often governs permissible bearing pressure. Account for shock, reversing torque and load distribution when defining allowable values.

Design check	Practical decision
Key and keyway fit	Use the specified width tolerances for the required fit; Fischer lists P9/N9 for shaft keyways and P9/JS9 for hub keyways.
Surface pressure	Verify the effective loaded flank area, especially with cast-iron or aluminium hubs.
Stress concentration	The shaft keyway reduces the effective section and introduces a notch; include it in fatigue-sensitive shaft design.
Axial location	A parallel key transmits torque but does not by itself locate the hub axially; add a shoulder, nut, retaining ring or suitable clamping feature.

Sources

Source basis: Ulrich Fischer et al., Mechanical and Metal Trades Handbook, torque and stress fundamentals, pp. 35, 43–44; keys and splined joints, pp. 242–243.

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