An aluminum alloy cylindrical roller with diameter $1\mathrm{.}25$ in and length $2$ in rolls on the inside of a cast$-$iron ring having an inside radius of $6$ in$,$ which is $2$ in thick. Find the maximum contact force F that can be used if the shear stress is not to exceed $4000$ psi. $3-153\mathrm{.}1$ What is the maximum shearing stress to maximum pressure ratio, $?$