Critical Speeds of Rotating Shafts and Mass Review

Critical Speeds of Rotating Shafts or Mass: The center of a rotating mass is always offset from the center of rotation. This is due to the reality of imperfect geometry and uneven mass distribution. As the mass rotates, the offset mass will generate a centrifugal force caused by the heavier side of the mass. This will cause the mass or rotating shaft to deflect toward the heavier side and cause the rotating mass to rotate in a small circle. As rotating mass changes in rotational velocity it can become dynamically unstable and induce undesired vibrations as well as amplified deflections. This phenomenon or condition will become more apparent at higher rotational velocities, however there is a point or rotational velocity where the vibrations and amplitude increase significantly. As the rotational velocity increases beyond that speed the vibration and amplitude decrease significantly. The rotational velocity at which the vibration increases dramatically is called the "critical speed of the rotating mass. This characteristic for a shaft-mounted mass is called "settling of the wheel". The settling is at the velocity the axis of rotation and the shaft or mass begin to rotate around an axis through their center of gravity. The rotating mass is deflected so that for every revolution its geometrical center traces a circle around the center of gravity of the rotating mass. Critical speeds depends on the location of the unbalanced load rotating on the shaft, the length of the shaft, its diameter and the supporting bearing or bearings configuration.

Typically, the designed operating speed of a machine is less than the critical speed. This is done to prevent a machine from ever achieving the undesired critical speed vibration and possible resulting damage. However, some machines are designed to operate at a rotational velocity above the critical speed. This can work well if the machine passes or accelerates quickly through the critical speed or before the vibrations build up to an excessive amplitude.

The closer a machine operates near the critical speed, the alignment, balance and maintenance of the bearings, and general quality of assembly will require higher quality considerations.

The formulas illustrated on Engineers Edge to calculate critical speeds are adequate for general purposes. However, there are cases where the torque applied to the rotating shaft will dramatically effect the critical speed. Therefore, care should exercised when applying these calculations.

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