From motor power to shaft diameter: calculate torque from power (kW/HP) + RPM, determine minimum diameter, DIN 6885 keyway, standard sizes, and critical speed.
T = 9550 × P(kW) / N(rpm) · Pure torsion diameter: d = ∛(16T / πτ)Limit note: this tool does not accept direct torque input — torque is always derived from power + RPM. Pure torque-from-T design is in tool #1509 angle-of-twist Tab 4. The τ_allow values above are typical design starting points; verify against material certificates/applicable codes.
| Ø poros (mm) | Key b×h (mm) | t1 poros | t2 hub |
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Boundary convention: a diameter exactly at a lower bound uses that row (e.g. 22.0 mm → 8×7 key, not 6×6).
Always round UP to the nearest standard size — never down.
| Metric standard sizes (mm) | |
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| Inch standard sizes | |
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Rule of thumb: operating speed should be below 70% of critical speed (subcritical) or well above it (supercritical) — do not operate near Nc.
First calculate the torque: T = 9550 × P(kW) / N(rpm). Then the minimum diameter from pure torsion: d = ∛(16T / πτ_allow). Example: a 15 kW motor at 1450 rpm gives T = 98.79 N·m; with τ_allow = 40 MPa, d_min = 23.26 mm, rounded up to the 25 mm standard size. Tab 1 computes this entire chain automatically.
Tool #1509 (Tab 4 “Shaft design”) calculates diameter from KNOWN torque — its input is T in N·m. This tool does NOT accept direct torque input: torque is always derived from motor POWER (kW/HP) + RPM, i.e. data shown on the motor nameplate. If you already know the torque magnitude, use #1509; if you know the motor power, use this tool.
Use the ASME shaft design equation: d³ = 16/(πτ) × √((Kb×M)² + (Kt×T)²), with M the bending moment and Kb/Kt shock factors. Check the bending option in Tab 1 and select the load condition. Example: T = 98.79 N·m, M = 150 N·m, Kb = 1.5, Kt = 1.0, τ_allow = 40 MPa → d_min = 31.51 mm (larger than the 23.26 mm pure-torsion value due to the bending contribution).
Use the DIN 6885-1 table: key size b×h is determined from shaft diameter. Example: Ø25 mm shaft → 8×7 mm key, with shaft groove t1 = 4.0 mm and hub groove t2 = 3.3 mm. This tool’s boundary convention: a diameter exactly at a lower bound uses that row — e.g. Ø22.0 mm uses the 8×7 key (row 22–30), while Ø21.99 mm uses 6×6. Tab 2 shows the full table plus a key length calculator.
Always round UP to the nearest standard size — never down. Example: calculated d_min = 23.26 mm → 25 mm metric standard or 1″ inch standard (25.4 mm). Tab 3 finds both at once and displays the metric and inch standard size lists.
Critical speed (Nc) is the rotation rate at which the excitation frequency equals the shaft’s natural bending frequency — operating near it causes damaging resonance/whirling. For a simply supported shaft with concentrated mass m at the center: Nc = 30/π × √(48EI/(mL³)). Example: 20 kg mass, 1 m span, Ø40 mm steel shaft → Nc = 2345 rpm. Tab 4 calculates it.
Yes, optionally. Self-weight is calculated with the Rayleigh method then combined with the concentrated-mass effect using the Dunkerley formula: 1/Nc² = 1/Nc₁² + 1/Nc₂². Example above: mass Nc = 2345 rpm, self-weight Nc = 4757 rpm → combined = 2104 rpm (always LOWER than each component).
Rule of thumb: operate below 70% of critical speed (subcritical region) or well above it (supercritical) — not around Nc. For the Nc = 2104 rpm example, the safe limit ≈ 1472 rpm. Tab 4 warns automatically if the operating speed you enter is within ±30% of Nc.
The Predator 212 (6.5 HP gasoline engine popular for go-karts/mini bikes) uses a straight keyed crankshaft of 3/4 inch diameter (19.05 mm) — enter 0.75 inch in Tab 3 to see its standard size and keyway per DIN 6885.
1 metric HP (PS) = 735.50 W, while 1 imperial HP = 745.70 W — a difference of about 1.4%. Example: a 10 imperial HP motor at 1750 rpm gives T = 40.69 N·m (30.01 lb·ft). Tab 1 provides both HP units to avoid conversion mistakes.
The key must be checked against two failures: crushing (surface pressure) L ≥ 4T/(σc·d·h) and shear L ≥ 2T/(τ·d·b) — use the longer one. Example: T = 98.79 N·m, Ø25 mm shaft, 8×7 key, σc = 90 MPa, τ = 60 MPa → needs 25.1 mm (crush) vs 16.5 mm (shear), so use at least 25.1 mm (pick the next standard length, usually ≈ hub length).
This tool is an early design aid, not a substitute for engineering standards: (1) it does not calculate fatigue/S-N curves or stress concentrations at keyways/fillets; (2) the built-in τ_allow values are typical starting points — verify with material certificates; (3) critical speed uses a simple simply-supported model (not full rotor-bearing analysis); (4) keyways follow DIN 6885-1 up to Ø500 mm. For
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