5. 活塞環的功用有：密封、散熱、刮油及_________ 。

Problem 4 (25%)A stepped shaft ACB is held against rotation at ends A and B and subjected to a torque T0 acting at section C. The two segments of the shaft have diameters dA and dB, respectively, and polar moments of inertia IpA and IpB, respectively. The shaft has length L and segment AC has length a. For what ratio a/L will the maximum shear stresses be the same in both segments of the shaft?