Keep the vibration of the centrifuge within the normal range.

Only when the centrifuge’s operating speed is not at its critical speed can the rotor rotate smoothly. For low-speed centrifuges with relatively low operating speeds, the critical speed is generally raised as much as possible, enabling the centrifuge to operate below its critical speed. Such centrifuges have thick shafts with high rigidity, and these shafts are referred to as rigid shafts. When very high rotational speeds are required, the critical speed is minimized as much as possible, and the shaft is made thinner with lower rigidity—these are called flexible shafts. Modern ultracentrifuges all employ flexible shafts.

The critical speed is related to both the centrifuge itself and the rotor it is equipped with. A single centrifuge can be fitted with several different types of rotors; therefore, when designing a centrifuge, it’s essential to take into account the vibration-damping design’s compatibility with various rotors. Centrifuges operate over a wide range of speeds—from several thousand to tens of thousands of revolutions per minute. The higher the rotational speed at which vibration occurs in a centrifuge, the greater the associated risk.

Once vibration occurs at high rotational speed, the centrifuge should be immediately brought to a stop using the fastest deceleration setting. All personnel must promptly evacuate the area to avoid injury in case of accidents such as rotor breakage. Under no circumstances should the power be cut off—once the power is interrupted, the braking mechanism (which provides rapid deceleration) will fail, and the rotor will come to a halt very slowly. Such accidents are typically caused by improper operation of the centrifuge—for example, if the rotor is not properly seated on the spindle, the rotor lid is not securely closed, or the basket for swinging out the rotor is not positioned correctly. These accidents are often accompanied by a loud bang; a Shanghai centrifuge weighing 300–400 kg may suddenly rotate more than 270 degrees. Upon opening the lid, you’ll find the rotor lying inside the chamber. When the rotor is removed, you’ll notice that its outer surface has been severely scratched and discolored. The inner walls of the centrifuge chamber can also be damaged, and in some cases, refrigerant may even leak out. Sometimes, the rotor and rotor lid separate, causing liquid from the tubes to spill into the chamber, and the spindle may break or bend severely. Accidents caused by operator error occur repeatedly. Modern centrifuges are generally equipped with an imbalance protection feature: when the rotor’s vibration exceeds a certain threshold upon startup, the centrifuge automatically slows down and shuts down. However, centrifuges with this feature can sometimes fail to trigger the alarm, leaving the risk of accidents unmitigated.

Both machining errors in the centrifuge rotor and installation errors in the centrifuge can cause the rotor's center of mass to deviate from the centrifuge’s main shaft to varying degrees. Consequently, vibrations are generated when the rotor spins. At certain specific rotational speeds, these vibrations may resonate with the natural frequency of the centrifuge’s drive system, thereby inducing intense vibrations throughout the entire system. The rotational speed at which this phenomenon occurs is referred to as the critical speed.

When a rotor spins at high speed, it invariably causes vibration. However, a flexible shaft has an “automatic centering effect.” The underlying principle is that as the rotor spins at high speed, it generates a disc effect, which creates an elastic restoring force when the flexible shaft bends. This force pulls the rotor—whose axis has deviated from the geometric centerline of the main shaft—back toward the main shaft’s geometric centerline, thereby eliminating the vibration. Of course, in this process, the imbalance present in the rotor must remain within certain limits; if the imbalance exceeds these limits, the vibration will not dissipate.