Rigid couplings and shaft adapters transmit power in applications without flexibility or misalignment. Rigid shaft couplings for round and keyed shafts firmly join two matching shafts together with set screws, one-piece slotted, or two-piece bodies. Keyless locking rigid couplings join two shafts together by applying pressure to a central compression fitting. Shaft adapters change the outside diameter of a shaft without replacing equipment.