Parallel Key
Check of a parallel key transmitting torque between shaft and hub. The DIN 6885 key section is selected from the shaft diameter; hub bearing pressure and key shear stress are calculated.
Hub bearing pressure p83MPa
- Key b × h
- 12 × 8 mm
- Shaft keyway depth t₁
- 5 mm
- Tangential force F = 2T / d
- 12.5 kN
- Key shear stress τ
- 21 MPa
- Minimum required bearing length
- 46.3 mm
- Bearing pressure: 93 % utilisation
- Shear: 35 % utilisation
- Key length ≤ 1.5·d — longer keys do not share load evenly
METHOD
How it is calculated
F = 2·T / d p = F / ((h − t₁) · l) bearing pressure, hub side τ = F / (b · l) shear l_req = max(F / (p_allow·(h − t₁)), F / (τ_allow·b))
ASSUMPTIONS & NOTES
- l is the bearing length; for round-ended (form A) keys subtract b from the overall length.
- Allowable bearing pressure depends on the hub material: 90–150 MPa for steel hubs, 50–90 MPa for cast iron (lower under shock).
- Bearing pressure usually governs; if it fails, use a second key at 180° (75 % effective) or a splined shaft.
Results are for preliminary design and checking; base the final design on the governing standard and safety factors.
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