TAILIEUCHUNG - Vibration Fundamentals Episode 8

Tham khảo tài liệu 'vibration fundamentals episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 204 Vibration Fundamentals Figure Typical discrete natural frequency locations in structural members. line may be energized by the running speed of a roll. However it also can be made to resonate by a bearing frequency overhead crane or other such energy source. The resonant frequency depends on the mass stiffness and span of the excited member. In general terms the natural frequency of a structural member is inversely proportional to its mass and stiffness. In other words a large turbocompressor s casing will have a lower natural frequency than that of a small end-suction centrifugal pump. Figure illustrates a typical structural support system and the discrete natural frequency locations. Each of the arrows indicates a structural member or stationary machine component having a unique natural frequency. Note that each time a structural span is broken or attached to another structure the stiffness changes. As a result the natural frequency of that segment also changes. While most stationary machine components move during normal operation they are not always resonant. Some degree of flexing or movement is common in stationary machine-trains and structural members. The amount of movement depends on the spring constant or stiffness of the member. Types of Resonance 205 Figure Rotor support stiffness versus critical rotor speed. Dynamic Resonance When the natural frequency of a rotating . dynamic structure such as a bearing or a rotor assembly in a fan is energized the rotating machine element resonates. This phenomenon is called dynamic resonance and the rotor speed at which it occurs is the critical speed. Figure illustrates a typical critical speed or dynamic resonance plot. The graph shows the relationship between rotor-support stiffness X-axis and rotor speed Y-axis . Rotor-support stiffness depends on the geometry of the rotating element . shaft and rotor and the bearing-support structure. These are the two dominant factors that determine

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