Time:2021-06-11
pulverizers will vibrate in the manufacturing process, and the vibrations produced mainly include forced vibration and self-excited vibration (flutter).
(1) Forced vibration: vibration excited by external periodic interference forces (internal or external vibration sources in the process system). Its main features are:
The frequency of the forced vibration is the same as the frequency of the external periodic interference force, or an integer multiple of it.
Except for the forced vibration caused by the unevenness of the cutting process itself, the interference force is generally related to the cutting process. The interference force is eliminated and the vibration is forced to stop.
The amplitude of the forced vibration is related to the amplitude of the interference force, the stiffness and damping of the process system. In the case of constant interference force frequency, the greater the interference force amplitude and the smaller the rigidity and damping of the process system, the greater the forced vibration amplitude.
The ratio of the frequency of the interference force to a certain natural frequency of the process system is equal to or close to 1 o’clock. The system will resonate and the amplitude will reach its maximum.
(2) Self-excited vibration (flutter) during machining, without periodic external force (relative to the cutting process), the periodic vibration generated by the internal excitation feedback of the system. Its main features:
The frequency of self-excited vibration is equal to or close to the natural frequency of the system.
Whether self-excited vibration can be generated and its amplitude depends on the comparison between the energy obtained by the system and the energy consumed by system damping in each vibration period.
Since the disturbance force that maintains the self-excited vibration is excited by the vibration (cutting) process itself, once the vibration (cutting) is stopped, the disturbance force and the energy replenishment process disappear immediately.
Vibration source and diagnosis of forced vibration
(1) Vibration source of forced vibration
1) Periodic centrifugal force caused by unbalance of rotating parts on machine tools, such as motors, grinding wheels, belt wheels, chucks, and workpieces.
The inhomogeneity of the cutting process itself, such as milling, broaching, turning the surface of the workpiece with keyway, etc., causes the periodically changing cutting force.
Defects of machine tool transmission components, such as inaccurate manufacturing or poor installation of gears, uneven thickness of V-belts, flat belt joints, bearing rolling element size and shape errors, and periodic changes caused by hydraulic pressure pulsation of hydraulic pump work 4 Transmission force.
The inertial impact generated when the direction of movement of the reciprocating part changes.
Interference force from other external sources of vibration.
(2) Diagnosis of forced vibration
Pick up the vibration on site and perform frequency spectrum analysis. Under on-site processing conditions, use sensors (accelerometers, force sensors, etc.) to pick up the vibration signals in the process along the vibration sensitive direction near the processing part, perform spectrum analysis and draw a spectrum graph. There are as many obvious peak points on the spectrogram as there are vibration frequency components in the machining system.
Do an environmental test, find the external vibration source, pick up the vibration signal in the shutdown state, and perform spectrum analysis. The vibration frequency components obtained at this time are all frequency components of the external interference force source. Compare these frequency components with the vibration frequency components of the on-site processing. If the two are exactly the same, it can be determined that the vibration generated in the machining is forced vibration and the vibration source is outside the machine; for example, the main vibration frequency components of the on-site processing are compared with the machine. If the frequency of the external interference force is inconsistent, the dry run experiment shall continue.
Do a dry running test to find the vibration source inside the machine, and the machine tool will perform dry running according to the motion parameters used in the processing site, pick up the vibration signal and perform frequency spectrum analysis. Comparing the spectrogram obtained in the dry-running test and the on-site processing, except for the frequency components of the external interference force source that has been identified, if the two are exactly the same, it can be determined that the vibration generated in the on-site processing is the front vibration, and Interfering force sources also exist in the machine; if there are frequency components on the spectrogram during on-site processing that are different from those of the machine tool idling (to be an integer multiple of it), it can be determined that in addition to forced vibration, there is also There is self-excited vibration.
If the source of the interference force is inside the machine tool, the specific location should also be found. It is possible to drive each moving part of the machine tool separately to perform an dry run test to find the specific location of the vibration source, or for all the moving parts that may become a vibration source, calculate the interference force frequency of each moving part according to the motion parameters, and run it with the machine idling. Compare the test spectrum to determine the specific location of the vibration source inside the machine.