Humans must have any errors in processing anything. The higher the machining accuracy, the smaller the error, but the processing cost is greatly improved. The centrifuge is designed with a variety of knowledge to achieve a certain machining error, so that the vibration of the centrifuge is controlled within the normal range. However, if the user uses it improperly, it will destroy this design balance and lead to serious accidents.
Only when the working speed of the centrifuge is not at the critical speed, the rotor can move smoothly. For low-speed centrifuges with low operating speeds, the critical speed is generally increased as much as possible to allow the centrifuge to operate below the critical speed. The centrifuge has a very thick shaft and a high stiffness, called a rigid shaft. When the speed requirement is high, the critical speed is reduced as much as possible. The shaft is made thinner and the rigidity is small. It is called a flexible shaft. Modern ultracentrifuges use flexible shafts.
The critical speed is related to the centrifuge and the associated rotor. A centrifuge can be equipped with several kinds of rotors, so the design of the centrifuge should consider the containment of the various vibrations of the centrifuge. Centrifuges have a wide operating range, from a few thousand revolutions per minute to tens of thousands of revolutions per minute. The higher the rotational speed of the centrifuge, the greater the risk.
Once the vibration occurs at high speed, the centrifuge should be stopped immediately with the fastest speed reduction gear. All personnel should leave the scene quickly to avoid personal injury in the event of a broken shaft. Never power off, once the power is off, the brakes (fast speed reduction) are gone, and the rotor stops very slowly. The accident is usually caused by improper operation of the centrifuge. For example, if the rotor is not placed on the main shaft, the cover of the rotor is not covered, and the basket of the rotor is not placed. The structure of this accident is accompanied by a huge sound, and the 300-400kg Shanghai centrifuge suddenly rotates 270 degrees. Open the cover section and see that the rotor is lying in the cavity. When the rotor is removed, the outer surface of the rotor is severely scratched and the inner wall of the centrifugal cavity is damaged, and even the refrigerant leaks. Sometimes the rotor and rotor cover are separated, the liquid in the test tube is sprinkled inside the cavity, and the spindle is broken or severely bent. Due to the frequent occurrence of such accidents caused by improper operation of the user, modern centrifuges generally have an imbalance protection function. That is, once the vibration of the rotor is greater than a certain size when the centrifuge is started, the centrifuge automatically stops at a reduced speed. The centrifuge with such performance sometimes misses the report and cannot ensure that no accidents occur.
Centrifuge rotor machining error and centrifuge installation error will cause the rotor centroid to deviate from the centrifuge shaft to varying degrees. Therefore, vibration will occur when the rotor rotates. These vibrations will be inherent to the centrifuge drive system at certain speeds. The frequency resonates, causing the entire system to vibrate strongly. The speed at which this phenomenon occurs is called the critical speed.