18/08/2026
22 kW motor at 960 RPM — what is the shaft torque? And what diameter shaft does it need? Two calculations in one worked example.
SHAFT TORQUE & POWER FORMULAS:
T (N·m) = P (kW) × 9,550 / N (RPM)
P (kW) = T (N·m) × N (RPM) / 9,550
d (m) = ∛(16T / π × τ_allow)
GIVEN:
P = 22 kW | N = 960 RPM | Material: mild steel (τ_allow = 40 MPa)
STEP 1 — Shaft Torque:
T = 22 × 9,550 / 960 = 210,100 / 960 = 218.9 N·m
T = 219 N·m
STEP 2 — Shaft Diameter:
d = ∛(16 × 219 / (π × 40,000,000))
d = ∛(3,504 / 125,663,706)
d = ∛(2.789 × 10⁻⁵)
d = 0.0306 m = 30.6 mm
Select d = 35 mm standard shaft (round UP — never down)
CONSTANT 9,550 EXPLAINED:
9,550 = 60,000 / (2π) = unit conversion from kW and RPM to N·m
For HP and RPM: T (lb·ft) = HP × 5,252 / N
CROSS-CHECK:
Power back-calculated: P = 219 × 960 / 9,550 = 22.03 kW ✓
Save this — the most fundamental relationship in mechanical power transmission.
