Petroff's Law Friction

Viscous friction torque on a concentric journal bearing: T = 4π²·η·N·R³·L / c (R, c in consistent units).

Lubrication

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Formula, assumptions, and limits

T = \frac{4 \pi^2 \eta N R^3 L}{c}

Symbol	Meaning	Unit
$η$	Dynamic viscosity	Pa·s
$N$	Rotation rate	rpm
$R, L, c$	Journal radius, length, clearance	mm

Source: Petroff (1883)

Viscous friction torque on a concentric journal bearing: T = 4\u03c0\u00b2\u00b7\u03b7\u00b7N\u00b7R\u00b3\u00b7L / c (R, c in consistent units).