Which of the following is a direct result when critical speed is reached?

When critical speed is reached, one of the primary concerns is the potential for structural damage. Critical speed refers to the speed at which the natural frequency of a rotating system matches the frequency of rotation itself, leading to significant resonance effects. In this state, vibration amplitudes can increase dramatically, resulting in excessive dynamic forces being exerted on the machine components.

These heightened vibrations can cause various issues, including loosening of assembly components, failure of bearings, and potentially catastrophic damage to the structure of the machinery itself. For instance, if a rotor spins at a critical speed, the stress concentrations can exceed the material limits, leading to fractures or other structural failures. Addressing this aspect is crucial in the design and operation of rotating machinery to prevent costly damage and ensure operational safety.

While the other options highlight important considerations in operation, they do not directly capture the specific and immediate risk posed by reaching critical speed, which is primarily structural compromise. Reduced efficiency and increased wear might arise indirectly due to excessive vibrations, but they aren't the most direct consequences compared to the risk of structural damage when critical speeds are approached. Increased output is generally not a consequence of reaching critical speed, as operational stability and safety are compromised at that point.