What Is Critical Speed?

Critical speed is a concept with a wide meaning, which has different meanings for different objects. The main meaning is the critical speed between two states when an object switches. Regarding the critical speed, often discussed are: the critical speed of gas, the critical speed of gas in the fluid, the critical speed of the circular motion of the object and the free fall, the critical speed of the locomotive snake and the critical speed of the rotary axis. Beyond that, the concept of critical speeds studied for more complex systems will be relatively more complex.

When the gas flows adiabatically through a tube or nozzle, the flow velocity increases with a decrease in downstream pressure at a certain upstream pressure. However, when the flow velocity in the tube and the throat of the nozzle reaches the pressure wave propagation speed in this state, that is, the speed of sound, the flow velocity remains the same regardless of how the downstream pressure is reduced. The speed at which the flow reaches this critical situation is called the critical speed.

Critical velocity of gas in fluid

If an object falls in a fluid state from a stationary state and finally reaches a uniform falling speed, it is called the settling velocity of the object in the fluid. If an air current is given from below at this time, the object is blown upward to make it in a suspended state. The speed of the air current is called the levitation speed of the object. The sedimentation speed and the suspension speed are both called critical speeds, while the sedimentation speed is for objects, and the suspension speed is the speed of airflow. The values of the two speeds are equal. ^[1]

Object circular motion and critical speed of free fall

The critical speed of the circular motion of an object refers to the speed required for the object to pass through the highest point to complete the circular motion; and the speed when the air resistance equals its own gravity during the free fall of the object is the critical speed of the free fall.

Critical speed of locomotives

The wheel tread of a locomotive is tapered, and there is a gap between the rim and the rail. When the wheel pair runs on a straight track, the center of the wheel pair occasionally deviates from the center of the track. The left and right wheels roll on the rail with rolling circles of different diameters. The wheel pair is made to oscillate laterally while traveling, and to swing back and forth around a vertical axis passing through its center of gravity to perform a wave motion called a snake motion. This movement is unique to railway rolling stock.

It is necessary for the wheel tread to be tapered, so that the center can be automatically centered by the meandering motion of the wheel pair when it is away from the center of the track. The wheelset, the bogie frame and the vehicle body are also elastically connected in the horizontal plane. The wheelset serpentine motion caused by the silveriness of the tread surface will cause the serpentine motion of the locomotive. The severe meandering motion not only damages the smooth running of the locomotive, but also causes the wheel flange to hit the rails, damage the lines, damage the bearings, and even cause derailment accidents. Seriously hindered the increase of train speed.

The speed at which a locomotive begins to violently snake is called the snake's critical speed, or critical speed for short. ^[2]

Critical speed of rotary axis

At a certain speed, the rotary machine has strong vibration. When the rotation speed of the rotating machine coincides with the natural vibration frequency of the rotation cycle system, the above phenomenon occurs, and the rotation speed at this time becomes the critical speed of the rotation shaft. ^[3]