The solid steel shaft $D F$ has a diameter of $25 \mathrm{mm}$ and is supported by smooth bearings at $D$ and $E .$ It is coupled to a motor at $F,$ which delivers $12 \mathrm{kW}$ of power to the shaft while it is turning at 50 rev / s. If gears $A, B,$ and $C$ remove $3 \mathrm{kW}, 4 \mathrm{kW},$ and $5 \mathrm{kW}$ respectively, determine the maximum shear stress in the shaft within regions $C F$ and $B C$. The shaft is free to turn in its support bearings $D$ and $E$