What is the one thing we cannot do when using a solid (rigid) coupling?

When using a solid (rigid) coupling, the primary limitation is in its inability to connect two misaligned or offset shafts. Rigid couplings are designed to transmit torque and power between two aligned shafts without any flexibility. They create a direct connection that does not accommodate any angular or parallel misalignment.

When shafts are misaligned, using a rigid coupling can lead to excessive stress and wear on the coupling and connected components, potentially causing failure. In contrast, other types of couplings, such as flexible couplings, are specifically designed to handle misalignment and can compensate for slight offsets between shafts. Thus, connecting misaligned shafts is not feasible with solid couplings, making this option the correct choice.