Why do cardan shafts need to be dynamically balanced?

Due to various reasons, the center of mass or the main axis of inertia of the coupling does not coincide with the axis of rotation, and the phenomenon of unbalanced centrifugal inertial force, centrifugal inertial couple force and dynamic deflection (vibration shape) will occur during operation, which is called rotor imbalance. Phenomenon, this unbalanced phenomenon will inevitably cause the vibration of the shafting, thus affecting the normal operation and service life of the machine, so it must be paid attention to.

The degree of unbalance (unbalance U) is usually expressed by the product mr of the mass m of the rotor and the distance r from the center of mass to the rotor axis of rotation, which is called the mass-diameter product. It is also expressed by the mass-diameter product of unit mass, which is called eccentricity e (not eccentricity in the geometric sense). The mass-diameter product mr is a relative quantity related to the mass of the rotor, while the eccentricity e is an absolute quantity independent of the mass of the rotor.

The former is more intuitive and is often used in the balancing operation of a specific given rotor. The latter is used to measure the quality of the rotor balance or to detect the balance accuracy. The balance level standard of the coupling is evaluated by e. For flexible rotors, the vibration mode eccentricity (nth mode mode) en=Un/mn, where Un and mn are the nth mode mode and mode quality respectively.

In order to correct or minimize the unbalance of the coupling, the appropriate balance level should be selected according to the needs, and after the product is manufactured and installed on the machine, on the balance (correction) plane specified by the coupling, pass A method of adding or subtracting the appropriate mass to bring it up to the balance level requirement. This process is called balance correction, or balance for short.