The torque required to overcome viscous resistance of a footstep bearing is (where $$\mu $$ = Viscosity of the oil, N = Speed of the shaft, R = Radius of the shaft and t = Thickness of the oil film)

A. $$\frac{{\mu {\pi ^2}{\text{NR}}}}{{60{\text{t}}}}$$

B. $$\frac{{\mu {\pi ^2}{\text{N}}{{\text{R}}^2}}}{{60{\text{t}}}}$$

C. $$\frac{{\mu {\pi ^2}{\text{N}}{{\text{R}}^3}}}{{60{\text{t}}}}$$

D. $$\frac{{\mu {\pi ^2}{\text{N}}{{\text{R}}^4}}}{{60{\text{t}}}}$$

Answer: Option D