An object is said to reach critical speed when the speed of its rotation corresponds to one of its natural frequencies. This type of speed is studied in a branch of physics known as rotordynamics, which deals with rotational, or angular, motion. A rotating object, such as a propeller or a centrifugal pump, must often pass through one or more of its critical speeds as it accelerates or decelerates. While operating at critical speed, these objects vibrate at a high amplitude, which can cause damage.
All objects that are composed of an elastic material have one or more natural frequencies. The natural frequency of an object is the number of times it will move back and forth once it is set into motion. When an object is vibrating at one of its natural frequencies, it is said to have resonance, or a large vibrational amplitude. In a musical instrument, for example, this resonance is desirable because it causes a natural amplification of the instrument’s sound. In rotordynamics, however, this resonance is undesirable because it makes the mechanical pieces involved vibrate strongly, which can damage the system.
There are a number of stimuli that can cause resonance, one of which is rotational motion. When an object's rotational motion, also called angular velocity, causes resonance, it is at critical speed. Rotating mechanical objects must be designed to pass quickly through these speeds so the amplified vibration that occurs at this velocity does not result in damage.
A centrifugal pump or a propeller will move through different angular velocities as it accelerates or decelerates. While a moving system such as this will naturally have some vibration, the amplified vibration at the critical speed must be avoided or passed through quickly if the system is to hold up over time. Rotordynamics is thus greatly concerned with resolving the various critical speeds that may affect the life of a rotating machine.
The lowest rotational frequency that causes an object to vibrate at one of its natural frequencies is known as its first critical speed. An object may have an infinite number of critical speeds, but the most important one for engineers to consider is the first one. Some rotating objects are designed to operate below their first critical speed, but many rotate above this velocity. As long as the rotational speed passes quickly through the critical speed, this should not have a significant effect on the life of the rotating machine. Issues can occur, however, if an object is designed to rotate at its critical speed, as the resulting vibrations could cause the machine to break.